Add doxygen config and update formatting and doc comments

1 files changed, 1235 insertions, 1215 deletions

diff --git a/src/libnsgif.c b/src/libnsgif.c
index 87371f2..69c54ea 100644
--- a/src/libnsgif.c
+++ b/src/libnsgif.c
@@ -15,82 +15,39 @@
 #include "libnsgif.h"
 #include "utils/log.h"
 
-/*	READING GIF FILES
-	=================
-
-	The functions provided by this file allow for efficient progressive GIF
-	decoding. Whilst the initialisation does not ensure that there is
-	sufficient image data to complete the entire frame, it does ensure that
-	the information provided is valid. Any subsequent attempts to decode an
-	initialised GIF are guaranteed to succeed, and any bytes of the image
-	not present are assumed to be totally transparent.
-
-	To begin decoding a GIF, the 'gif' structure must be initialised with
-	the 'gif_data' and 'buffer_size' set to their initial values. The
-	'buffer_position' should initially be 0, and will be internally updated
-	as the decoding commences. The caller should then repeatedly call
-	gif_initialise() with the structure until the function returns 1, or
-	no more data is avaliable.
-
-	Once the initialisation has begun, the decoder completes the variables
-	'frame_count' and 'frame_count_partial'. The former being the total
-	number of frames that have been successfully initialised, and the
-	latter being the number of frames that a partial amount of data is
-	available for. This assists the caller in managing the animation whilst
-	decoding is continuing.
-
-	To decode a frame, the caller must use gif_decode_frame() which updates
-	the current 'frame_image' to reflect the desired frame. The required
-	'disposal_method' is also updated to reflect how the frame should be
-	plotted. The caller must not assume that the current 'frame_image' will
-	be valid between calls if initialisation is still occuring, and should
-	either always request that the frame is decoded (no processing will
-	occur if the 'decoded_frame' has not been invalidated by initialisation)
-	or perform the check itself.
-
-	It should be noted that gif_finalise() should always be called, even if
-	no frames were initialised.  Additionally, it is the responsibility of
-	the caller to free 'gif_data'.
-
-	[rjw] - Fri 2nd April 2004
-*/
-
-/*	TO-DO LIST
-	=================
-
-	+ Plain text and comment extensions could be implemented if there is any
-	interest in doing so.
-*/
-
-
+/**
+ *
+ * \file
+ * \brief GIF image decoder
+ *
+ * The GIF format is thoroughly documented; a full description can be found at
+ * http://www.w3.org/Graphics/GIF/spec-gif89a.txt
+ *
+ * \todo Plain text and comment extensions should be implemented.
+ */
 
 
-/*	Maximum colour table size
-*/
+/** Maximum colour table size */
 #define GIF_MAX_COLOURS 256
 
-/*	Internal flag that the colour table needs to be processed
-*/
+/** Internal flag that the colour table needs to be processed */
 #define GIF_PROCESS_COLOURS 0xaa000000
 
-/*	Internal flag that a frame is invalid/unprocessed
-*/
+/** Internal flag that a frame is invalid/unprocessed */
 #define GIF_INVALID_FRAME -1
 
-/*	Maximum LZW bits available
-*/
+/** Maximum LZW bits available */
 #define GIF_MAX_LZW 12
 
-/* Transparent colour
-*/
+/** Transparent colour */
 #define GIF_TRANSPARENT_COLOUR 0x00
 
-/*	GIF Flags
-*/
+/* GIF Flags */
 #define GIF_FRAME_COMBINE 1
 #define GIF_FRAME_CLEAR 2
 #define GIF_FRAME_RESTORE 3
 #define GIF_FRAME_QUIRKS_RESTORE 4
+
 #define GIF_IMAGE_SEPARATOR 0x2c
 #define GIF_INTERLACE_MASK 0x40
 #define GIF_COLOUR_TABLE_MASK 0x80
@@ -105,30 +62,28 @@
 #define GIF_BLOCK_TERMINATOR 0x00
 #define GIF_TRAILER 0x3b
 
-/*	Internal GIF routines
-*/
+/* Internal GIF routines */
 static gif_result gif_initialise_sprite(gif_animation *gif, unsigned int width, unsigned int height);
 static gif_result gif_initialise_frame(gif_animation *gif);
 static gif_result gif_initialise_frame_extensions(gif_animation *gif, const int frame);
 static gif_result gif_skip_frame_extensions(gif_animation *gif);
 static unsigned int gif_interlaced_line(int height, int y);
 
-
-
-/*	Internal LZW routines
-*/
+/* Internal LZW routines */
 static void gif_init_LZW(gif_animation *gif);
 static bool gif_next_LZW(gif_animation *gif);
 static int gif_next_code(gif_animation *gif, int code_size);
 
-/*	General LZW values. They are shared for all GIFs being decoded, and
-	thus we can't handle progressive decoding efficiently without having
-	the data for each image which would use an extra 10Kb or so per GIF.
-*/
+/* General LZW values. They are shared for all GIFs being decoded, and thus we
+ * can't handle progressive decoding efficiently without having the data for
+ * each image which would use an extra 10Kb or so per GIF.
+ */
 static unsigned char buf[4];
 static unsigned char *direct;
-static int maskTbl[16] = {0x0000, 0x0001, 0x0003, 0x0007, 0x000f, 0x001f, 0x003f, 0x007f,
-			  0x00ff, 0x01ff, 0x03ff, 0x07ff, 0x0fff, 0x1fff, 0x3fff, 0x7fff};
+static int maskTbl[16] = {
+        0x0000, 0x0001, 0x0003, 0x0007, 0x000f, 0x001f, 0x003f, 0x007f,
+        0x00ff, 0x01ff, 0x03ff, 0x07ff, 0x0fff, 0x1fff, 0x3fff, 0x7fff
+};
 static int table[2][(1 << GIF_MAX_LZW)];
 static unsigned char stack[(1 << GIF_MAX_LZW) * 2];
 static unsigned char *stack_pointer;
@@ -140,1181 +95,1246 @@ static int firstcode, oldcode;
 static bool zero_data_block = false;
 static bool get_done;
 
-/*	Whether to clear the decoded image rather than plot
-*/
+/* Whether to clear the decoded image rather than plot */
 static bool clear_image = false;
 
 
-
-/**	Initialises necessary gif_animation members.
-*/
-void gif_create(gif_animation *gif, gif_bitmap_callback_vt *bitmap_callbacks) {
-	memset(gif, 0, sizeof(gif_animation));
-	gif->bitmap_callbacks = *bitmap_callbacks;
-	gif->decoded_frame = GIF_INVALID_FRAME;
+/**
+ * Updates the sprite memory size
+ *
+ * \param gif The animation context
+ * \param width The width of the sprite
+ * \param height The height of the sprite
+ * \return GIF_INSUFFICIENT_MEMORY for a memory error GIF_OK for success
+ */
+static gif_result
+gif_initialise_sprite(gif_animation *gif,
+                      unsigned int width,
+                      unsigned int height)
+{
+        unsigned int max_width;
+        unsigned int max_height;
+        struct bitmap *buffer;
+
+        /* Check if we've changed */
+        if ((width <= gif->width) && (height <= gif->height)) {
+                return GIF_OK;
+        }
+
+        /* Get our maximum values */
+        max_width = (width > gif->width) ? width : gif->width;
+        max_height = (height > gif->height) ? height : gif->height;
+
+        /* Allocate some more memory */
+        assert(gif->bitmap_callbacks.bitmap_create);
+        buffer = gif->bitmap_callbacks.bitmap_create(max_width, max_height);
+        if (buffer == NULL) {
+                return GIF_INSUFFICIENT_MEMORY;
+        }
+
+        assert(gif->bitmap_callbacks.bitmap_destroy);
+        gif->bitmap_callbacks.bitmap_destroy(gif->frame_image);
+        gif->frame_image = buffer;
+        gif->width = max_width;
+        gif->height = max_height;
+
+        /* Invalidate our currently decoded image */
+        gif->decoded_frame = GIF_INVALID_FRAME;
+        return GIF_OK;
 }
 
 
-/**	Initialises any workspace held by the animation and attempts to decode
-	any information that hasn't already been decoded.
-	If an error occurs, all previously decoded frames are retained.
-
-	@return GIF_FRAME_DATA_ERROR for GIF frame data error
-		GIF_INSUFFICIENT_FRAME_DATA for insufficient data to process
-		          any more frames
-		GIF_INSUFFICIENT_MEMORY for memory error
-		GIF_DATA_ERROR for GIF error
-		GIF_INSUFFICIENT_DATA for insufficient data to do anything
-		GIF_OK for successful decoding
-		GIF_WORKING for successful decoding if more frames are expected
+/**
+ * Attempts to initialise the next frame
+ *
+ * \param gif The animation context
+ * \return error code
+ *         - GIF_INSUFFICIENT_DATA for insufficient data to do anything
+ *         - GIF_FRAME_DATA_ERROR for GIF frame data error
+ *         - GIF_INSUFFICIENT_MEMORY for insufficient memory to process
+ *         - GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the frame
+ *         - GIF_DATA_ERROR for GIF error (invalid frame header)
+ *         - GIF_OK for successful decoding
+ *         - GIF_WORKING for successful decoding if more frames are expected
 */
-gif_result gif_initialise(gif_animation *gif, size_t size, unsigned char *data) {
-	unsigned char *gif_data;
-	unsigned int index;
-	gif_result return_value;
-
-	/* 	The GIF format is thoroughly documented; a full description
-	 *	can be found at http://www.w3.org/Graphics/GIF/spec-gif89a.txt
-	*/
-
-	/*	Initialize values
-	*/
-	gif->buffer_size = size;
-	gif->gif_data = data;
-	
-	/*	Check for sufficient data to be a GIF (6-byte header + 7-byte logical screen descriptor)
-	*/
-	if (gif->buffer_size < 13) return GIF_INSUFFICIENT_DATA;
-
-	/*	Get our current processing position
-	*/
-	gif_data = gif->gif_data + gif->buffer_position;
-
-	/*	See if we should initialise the GIF
-	*/
-	if (gif->buffer_position == 0) {
-
-		/*	We want everything to be NULL before we start so we've no chance
-			of freeing bad pointers (paranoia)
-		*/
-		gif->frame_image = NULL;
-		gif->frames = NULL;
-		gif->local_colour_table = NULL;
-		gif->global_colour_table = NULL;
-
-		/*	The caller may have been lazy and not reset any values
-		*/
-		gif->frame_count = 0;
-		gif->frame_count_partial = 0;
-		gif->decoded_frame = GIF_INVALID_FRAME;
-
-		/* 6-byte GIF file header is:
-		 *
-		 *	+0	3CHARS	Signature ('GIF')
-		 *	+3	3CHARS	Version ('87a' or '89a')
-		 */
-		if (strncmp((const char *) gif_data, "GIF", 3) != 0)
-			return GIF_DATA_ERROR;
-		gif_data += 3;
-
-		/*	Ensure GIF reports version 87a or 89a
-		*/
-/*		if ((strncmp(gif_data, "87a", 3) != 0) &&
-				(strncmp(gif_data, "89a", 3) != 0))
-			LOG(("Unknown GIF format - proceeding anyway"));
-*/		gif_data += 3;
-
-		/* 7-byte Logical Screen Descriptor is:
-		 *
-		 *	+0	SHORT	Logical Screen Width
-		 *	+2	SHORT	Logical Screen Height
-		 *	+4	CHAR	__Packed Fields__
-		 * 			1BIT	Global Colour Table Flag
-		 * 			3BITS	Colour Resolution
-		 * 			1BIT	Sort Flag
-		 * 			3BITS	Size of Global Colour Table
-		 *	+5	CHAR	Background Colour Index
-		 *	+6	CHAR	Pixel Aspect Ratio
-		 */
-		gif->width = gif_data[0] | (gif_data[1] << 8);
-		gif->height = gif_data[2] | (gif_data[3] << 8);
-		gif->global_colours = (gif_data[4] & GIF_COLOUR_TABLE_MASK);
-		gif->colour_table_size = (2 << (gif_data[4] & GIF_COLOUR_TABLE_SIZE_MASK));
-		gif->background_index = gif_data[5];
-		gif->aspect_ratio = gif_data[6];
-		gif->loop_count = 1;
-		gif_data += 7;
-
-		/*	Some broken GIFs report the size as the screen size they were created in. As
-			such, we detect for the common cases and set the sizes as 0 if they are found
-			which results in the GIF being the maximum size of the frames.
-		*/
-		if (((gif->width == 640) && (gif->height == 480)) ||
-				((gif->width == 640) && (gif->height == 512)) ||
-				((gif->width == 800) && (gif->height == 600)) ||
-				((gif->width == 1024) && (gif->height == 768)) ||
-				((gif->width == 1280) && (gif->height == 1024)) ||
-				((gif->width == 1600) && (gif->height == 1200)) ||
-				((gif->width == 0) || (gif->height == 0)) ||
-				((gif->width > 2048) || (gif->height > 2048))) {
-			gif->width = 1;
-			gif->height = 1;
-		}
-
-		/*	Allocate some data irrespective of whether we've got any colour tables. We
-			always get the maximum size in case a GIF is lying to us. It's far better
-			to give the wrong colours than to trample over some memory somewhere.
-		*/
-		gif->global_colour_table = calloc(GIF_MAX_COLOURS, sizeof(unsigned int));
-		gif->local_colour_table = calloc(GIF_MAX_COLOURS, sizeof(unsigned int));
-		if ((gif->global_colour_table == NULL) || (gif->local_colour_table == NULL)) {
-			gif_finalise(gif);
-			return GIF_INSUFFICIENT_MEMORY;
-		}
-
-		/*	Set the first colour to a value that will never occur in reality so we
-			know if we've processed it
-		*/
-		gif->global_colour_table[0] = GIF_PROCESS_COLOURS;
-		
-		/*	Check if the GIF has no frame data (13-byte header + 1-byte termination block)
-		 *	Although generally useless, the GIF specification does not expressly prohibit this
-		 */
-		if (gif->buffer_size == 14) {
-			if (gif_data[0] == GIF_TRAILER)
-				return GIF_OK;
-			else
-				return GIF_INSUFFICIENT_DATA;
-		}
-
-		/*	Initialise enough workspace for 4 frames initially
-		*/
-		if ((gif->frames = (gif_frame *)malloc(sizeof(gif_frame))) == NULL) {
-			gif_finalise(gif);
-			return GIF_INSUFFICIENT_MEMORY;
-		}
-		gif->frame_holders = 1;
-
-		/*	Initialise the sprite header
-		*/
-		assert(gif->bitmap_callbacks.bitmap_create);
-		if ((gif->frame_image = gif->bitmap_callbacks.bitmap_create(gif->width, gif->height)) == NULL) {
-			gif_finalise(gif);
-			return GIF_INSUFFICIENT_MEMORY;
-		}
-
-		/*	Remember we've done this now
-		*/
-		gif->buffer_position = gif_data - gif->gif_data;
-	}
-
-	/*	Do the colour map if we haven't already. As the top byte is always 0xff or 0x00
-		depending on the transparency we know if it's been filled in.
-	*/
-	if (gif->global_colour_table[0] == GIF_PROCESS_COLOURS) {
-		/*	Check for a global colour map signified by bit 7
-		*/
-		if (gif->global_colours) {
-			if (gif->buffer_size < (gif->colour_table_size * 3 + 12)) {
-				return GIF_INSUFFICIENT_DATA;
-			}
-			for (index = 0; index < gif->colour_table_size; index++) {
-				/* Gif colour map contents are r,g,b.
-				 *
-				 * We want to pack them bytewise into the 
-				 * colour table, such that the red component
-				 * is in byte 0 and the alpha component is in
-				 * byte 3.
-				 */
-				unsigned char *entry = (unsigned char *) &gif->
-						global_colour_table[index];
-
-				entry[0] = gif_data[0];	/* r */
-				entry[1] = gif_data[1];	/* g */
-				entry[2] = gif_data[2];	/* b */
-				entry[3] = 0xff;	/* a */
-
-				gif_data += 3;
-			}
-			gif->buffer_position = (gif_data - gif->gif_data);
-		} else {
-			/*	Create a default colour table with the first two colours as black and white
-			*/
-			unsigned int *entry = gif->global_colour_table;
-
-			entry[0] = 0x00000000;
-			/* Force Alpha channel to opaque */
-			((unsigned char *) entry)[3] = 0xff;
-
-			entry[1] = 0xffffffff;
-		}
-	}
-
-	/*	Repeatedly try to initialise frames
-	*/
-	while ((return_value = gif_initialise_frame(gif)) == GIF_WORKING);
-
-	/*	If there was a memory error tell the caller
-	*/
-	if ((return_value == GIF_INSUFFICIENT_MEMORY) ||
-			(return_value == GIF_DATA_ERROR))
-		return return_value;
-
-	/*	If we didn't have some frames then a GIF_INSUFFICIENT_DATA becomes a
-		GIF_INSUFFICIENT_FRAME_DATA
-	*/
-	if ((return_value == GIF_INSUFFICIENT_DATA) && (gif->frame_count_partial > 0))
-		return GIF_INSUFFICIENT_FRAME_DATA;
-
-	/*	Return how many we got
-	*/
-	return return_value;
+static gif_result gif_initialise_frame(gif_animation *gif)
+{
+        int frame;
+        gif_frame *temp_buf;
+
+        unsigned char *gif_data, *gif_end;
+        int gif_bytes;
+        unsigned int flags = 0;
+        unsigned int width, height, offset_x, offset_y;
+        unsigned int block_size, colour_table_size;
+        bool first_image = true;
+        gif_result return_value;
+
+        /* Get the frame to decode and our data position */
+        frame = gif->frame_count;
+
+        /* Get our buffer position etc. */
+        gif_data = (unsigned char *)(gif->gif_data + gif->buffer_position);
+        gif_end = (unsigned char *)(gif->gif_data + gif->buffer_size);
+        gif_bytes = (gif_end - gif_data);
+
+        /* Check if we've finished */
+        if ((gif_bytes > 0) && (gif_data[0] == GIF_TRAILER)) {
+                return GIF_OK;
+        }
+
+        /* Check if there is enough data remaining. The shortest block of data
+         * is a 4-byte comment extension + 1-byte block terminator + 1-byte gif
+         * trailer
+         */
+        if (gif_bytes < 6) {
+                return GIF_INSUFFICIENT_DATA;
+        }
+
+        /* We could theoretically get some junk data that gives us millions of
+         * frames, so we ensure that we don't have a silly number
+         */
+        if (frame > 4096) {
+                return GIF_FRAME_DATA_ERROR;
+        }
+
+        /* Get some memory to store our pointers in etc. */
+        if ((int)gif->frame_holders <= frame) {
+                /* Allocate more memory */
+                temp_buf = (gif_frame *)realloc(gif->frames, (frame + 1) * sizeof(gif_frame));
+                if (temp_buf == NULL) {
+                        return GIF_INSUFFICIENT_MEMORY;
+                }
+                gif->frames = temp_buf;
+                gif->frame_holders = frame + 1;
+        }
+
+        /* Store our frame pointer. We would do it when allocating except we
+         * start off with one frame allocated so we can always use realloc.
+         */
+        gif->frames[frame].frame_pointer = gif->buffer_position;
+        gif->frames[frame].display = false;
+        gif->frames[frame].virgin = true;
+        gif->frames[frame].disposal_method = 0;
+        gif->frames[frame].transparency = false;
+        gif->frames[frame].frame_delay = 100;
+        gif->frames[frame].redraw_required = false;
+
+        /* Invalidate any previous decoding we have of this frame */
+        if (gif->decoded_frame == frame) {
+                gif->decoded_frame = GIF_INVALID_FRAME;
+        }
+
+        /* We pretend to initialise the frames, but really we just skip over
+         * all the data contained within. This is all basically a cut down
+         * version of gif_decode_frame that doesn't have any of the LZW bits in
+         * it.
+         */
+
+        /* Initialise any extensions */
+        gif->buffer_position = gif_data - gif->gif_data;
+        return_value = gif_initialise_frame_extensions(gif, frame);
+        if (return_value != GIF_OK) {
+                return return_value;
+        }
+        gif_data = (gif->gif_data + gif->buffer_position);
+        gif_bytes = (gif_end - gif_data);
+
+        /* Check if we've finished */
+        if ((gif_bytes = (gif_end - gif_data)) < 1) {
+                return GIF_INSUFFICIENT_FRAME_DATA;
+        }
+
+        if (gif_data[0] == GIF_TRAILER) {
+                gif->buffer_position = (gif_data - gif->gif_data);
+                gif->frame_count = frame + 1;
+                return GIF_OK;
+        }
+
+        /* If we're not done, there should be an image descriptor */
+        if (gif_data[0] != GIF_IMAGE_SEPARATOR) {
+                return GIF_FRAME_DATA_ERROR;
+        }
+
+        /* Do some simple boundary checking */
+        if (gif_bytes < 10) {
+                return GIF_INSUFFICIENT_FRAME_DATA;
+        }
+        offset_x = gif_data[1] | (gif_data[2] << 8);
+        offset_y = gif_data[3] | (gif_data[4] << 8);
+        width = gif_data[5] | (gif_data[6] << 8);
+        height = gif_data[7] | (gif_data[8] << 8);
+
+        /* Set up the redraw characteristics. We have to check for extending
+         * the area due to multi-image frames.
+         */
+        if (!first_image) {
+                if (gif->frames[frame].redraw_x > offset_x) {
+                        gif->frames[frame].redraw_width += (gif->frames[frame].redraw_x - offset_x);
+                        gif->frames[frame].redraw_x = offset_x;
+                }
+
+                if (gif->frames[frame].redraw_y > offset_y) {
+                        gif->frames[frame].redraw_height += (gif->frames[frame].redraw_y - offset_y);
+                        gif->frames[frame].redraw_y = offset_y;
+                }
+
+                if ((offset_x + width) > (gif->frames[frame].redraw_x + gif->frames[frame].redraw_width)) {
+                        gif->frames[frame].redraw_width = (offset_x + width) - gif->frames[frame].redraw_x;
+                }
+
+                if ((offset_y + height) > (gif->frames[frame].redraw_y + gif->frames[frame].redraw_height)) {
+                        gif->frames[frame].redraw_height = (offset_y + height) - gif->frames[frame].redraw_y;
+                }
+        } else {
+                first_image = false;
+                gif->frames[frame].redraw_x = offset_x;
+                gif->frames[frame].redraw_y = offset_y;
+                gif->frames[frame].redraw_width = width;
+                gif->frames[frame].redraw_height = height;
+        }
+
+        /* if we are clearing the background then we need to redraw enough to
+         * cover the previous frame too
+         */
+        gif->frames[frame].redraw_required = ((gif->frames[frame].disposal_method == GIF_FRAME_CLEAR) ||
+                                                (gif->frames[frame].disposal_method == GIF_FRAME_RESTORE));
+
+        /* Boundary checking - shouldn't ever happen except with junk data */
+        if (gif_initialise_sprite(gif, (offset_x + width), (offset_y + height))) {
+                return GIF_INSUFFICIENT_MEMORY;
+        }
+
+        /* Decode the flags */
+        flags = gif_data[9];
+        colour_table_size = 2 << (flags & GIF_COLOUR_TABLE_SIZE_MASK);
+
+        /* Move our data onwards and remember we've got a bit of this frame */
+        gif_data += 10;
+        gif_bytes = (gif_end - gif_data);
+        gif->frame_count_partial = frame + 1;
+
+        /* Skip the local colour table */
+        if (flags & GIF_COLOUR_TABLE_MASK) {
+                gif_data += 3 * colour_table_size;
+                if ((gif_bytes = (gif_end - gif_data)) < 0) {
+                        return GIF_INSUFFICIENT_FRAME_DATA;
+                }
+        }
+
+        /* Ensure we have a correct code size */
+        if (gif_bytes < 1) {
+                return GIF_INSUFFICIENT_FRAME_DATA;
+        }
+        if (gif_data[0] > GIF_MAX_LZW) {
+                return GIF_DATA_ERROR;
+        }
+
+        /* Move our pointer to the actual image data */
+        gif_data++;
+        --gif_bytes;
+
+        /* Repeatedly skip blocks until we get a zero block or run out of data
+         * These blocks of image data are processed later by gif_decode_frame()
+         */
+        block_size = 0;
+        while (block_size != 1) {
+                if (gif_bytes < 1) return GIF_INSUFFICIENT_FRAME_DATA;
+                block_size = gif_data[0] + 1;
+                /* Check if the frame data runs off the end of the file	*/
+                if ((int)(gif_bytes - block_size) < 0) {
+                        /* Try to recover by signaling the end of the gif.
+                         * Once we get garbage data, there is no logical way to
+                         * determine where the next frame is.  It's probably
+                         * better to partially load the gif than not at all.
+                         */
+                        if (gif_bytes >= 2) {
+                                gif_data[0] = 0;
+                                gif_data[1] = GIF_TRAILER;
+                                gif_bytes = 1;
+                                ++gif_data;
+                                break;
+                        } else {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+                } else {
+                        gif_bytes -= block_size;
+                        gif_data += block_size;
+                }
+        }
+
+        /* Add the frame and set the display flag */
+        gif->buffer_position = gif_data - gif->gif_data;
+        gif->frame_count = frame + 1;
+        gif->frames[frame].display = true;
+
+        /* Check if we've finished */
+        if (gif_bytes < 1) {
+                return GIF_INSUFFICIENT_FRAME_DATA;
+        } else {
+                if (gif_data[0] == GIF_TRAILER) {
+                        return GIF_OK;
+                }
+        }
+        return GIF_WORKING;
 }
 
 
-/**	Updates the sprite memory size
-
-	@return GIF_INSUFFICIENT_MEMORY for a memory error
-		GIF_OK for success
-*/
-static gif_result gif_initialise_sprite(gif_animation *gif, unsigned int width, unsigned int height) {
-	unsigned int max_width;
-	unsigned int max_height;
-	struct bitmap *buffer;
-
-	/*	Check if we've changed
-	*/
-	if ((width <= gif->width) && (height <= gif->height))
-		return GIF_OK;
-
-	/*	Get our maximum values
-	*/
-	max_width = (width > gif->width) ? width : gif->width;
-	max_height = (height > gif->height) ? height : gif->height;
-
-	/*	Allocate some more memory
-	*/
-	assert(gif->bitmap_callbacks.bitmap_create);
-	if ((buffer = gif->bitmap_callbacks.bitmap_create(max_width, max_height)) == NULL)
-		return GIF_INSUFFICIENT_MEMORY;
-	assert(gif->bitmap_callbacks.bitmap_destroy);
-	gif->bitmap_callbacks.bitmap_destroy(gif->frame_image);
-	gif->frame_image = buffer;
-	gif->width = max_width;
-	gif->height = max_height;
-
-	/*	Invalidate our currently decoded image
-	*/
-	gif->decoded_frame = GIF_INVALID_FRAME;
-	return GIF_OK;
+/**
+ * Attempts to initialise the frame's extensions
+ *
+ * \param gif The animation context
+ * \param frame The frame number
+ * @return GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the
+ *         frame GIF_OK for successful initialisation.
+ */
+static gif_result
+gif_initialise_frame_extensions(gif_animation *gif, const int frame)
+{
+        unsigned char *gif_data, *gif_end;
+        int gif_bytes;
+        unsigned int block_size;
+
+        /* Get our buffer position etc.	*/
+        gif_data = (unsigned char *)(gif->gif_data + gif->buffer_position);
+        gif_end = (unsigned char *)(gif->gif_data + gif->buffer_size);
+
+        /* Initialise the extensions */
+        while (gif_data < gif_end && gif_data[0] == GIF_EXTENSION_INTRODUCER) {
+                ++gif_data;
+                if ((gif_bytes = (gif_end - gif_data)) < 1) {
+                        return GIF_INSUFFICIENT_FRAME_DATA;
+                }
+
+                /* Switch on extension label */
+                switch (gif_data[0]) {
+                case GIF_EXTENSION_GRAPHIC_CONTROL:
+                        /* 6-byte Graphic Control Extension is:
+                         *
+                         *	+0	CHAR	Graphic Control Label
+                         *	+1	CHAR	Block Size
+                         *	+2	CHAR	__Packed Fields__
+                         *			3BITS	Reserved
+                         *			3BITS	Disposal Method
+                         *			1BIT	User Input Flag
+                         *			1BIT	Transparent Color Flag
+                         *	+3	SHORT	Delay Time
+                         *	+5	CHAR	Transparent Color Index
+                         */
+                        if (gif_bytes < 6) {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+
+                        gif->frames[frame].frame_delay = gif_data[3] | (gif_data[4] << 8);
+                        if (gif_data[2] & GIF_TRANSPARENCY_MASK) {
+                                gif->frames[frame].transparency = true;
+                                gif->frames[frame].transparency_index = gif_data[5];
+                        }
+                        gif->frames[frame].disposal_method = ((gif_data[2] & GIF_DISPOSAL_MASK) >> 2);
+                        /* I have encountered documentation and GIFs in the
+                         * wild that use 0x04 to restore the previous frame,
+                         * rather than the officially documented 0x03.  I
+                         * believe some (older?)  software may even actually
+                         * export this way.  We handle this as a type of
+                         * "quirks" mode.
+                         */
+                        if (gif->frames[frame].disposal_method == GIF_FRAME_QUIRKS_RESTORE) {
+                                gif->frames[frame].disposal_method = GIF_FRAME_RESTORE;
+                        }
+                        gif_data += (2 + gif_data[1]);
+                        break;
+
+                case GIF_EXTENSION_APPLICATION:
+                        /* 14-byte+ Application Extension is:
+                         *
+                         *	+0    CHAR    Application Extension Label
+                         *	+1    CHAR    Block Size
+                         *	+2    8CHARS  Application Identifier
+                         *	+10   3CHARS  Appl. Authentication Code
+                         *	+13   1-256   Application Data (Data sub-blocks)
+                         */
+                        if (gif_bytes < 17) {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+                        if ((gif_data[1] == 0x0b) &&
+                            (strncmp((const char *) gif_data + 2,
+                                     "NETSCAPE2.0", 11) == 0) &&
+                            (gif_data[13] == 0x03) &&
+                            (gif_data[14] == 0x01)) {
+                                gif->loop_count = gif_data[15] | (gif_data[16] << 8);
+                        }
+                        gif_data += (2 + gif_data[1]);
+                        break;
+
+                case GIF_EXTENSION_COMMENT:
+                        /* Move the pointer to the first data sub-block Skip 1
+                         * byte for the extension label
+                         */
+                        ++gif_data;
+                        break;
+
+                default:
+                        /* Move the pointer to the first data sub-block Skip 2
+                         * bytes for the extension label and size fields Skip
+                         * the extension size itself
+                         */
+                        if (gif_bytes < 2) {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+                        gif_data += (2 + gif_data[1]);
+                }
+
+                /* Repeatedly skip blocks until we get a zero block or run out
+                 * of data This data is ignored by this gif decoder
+                 */
+                gif_bytes = (gif_end - gif_data);
+                block_size = 0;
+                while (gif_data < gif_end && gif_data[0] != GIF_BLOCK_TERMINATOR) {
+                        block_size = gif_data[0] + 1;
+                        if ((gif_bytes -= block_size) < 0) {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+                        gif_data += block_size;
+                }
+                ++gif_data;
+        }
+
+        /* Set buffer position and return */
+        gif->buffer_position = (gif_data - gif->gif_data);
+        return GIF_OK;
 }
 
 
-/**	Attempts to initialise the next frame
+/**
+ * Skips the frame's extensions (which have been previously initialised)
+ *
+ * \param gif The animation context
+ * \return GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the
+ *         frame GIF_OK for successful decoding
+ */
+static gif_result gif_skip_frame_extensions(gif_animation *gif)
+{
+        unsigned char *gif_data, *gif_end;
+        int gif_bytes;
+        unsigned int block_size;
+
+        /* Get our buffer position etc.	*/
+        gif_data = (unsigned char *)(gif->gif_data + gif->buffer_position);
+        gif_end = (unsigned char *)(gif->gif_data + gif->buffer_size);
+        gif_bytes = (gif_end - gif_data);
+
+        /* Skip the extensions */
+        while (gif_data < gif_end && gif_data[0] == GIF_EXTENSION_INTRODUCER) {
+                ++gif_data;
+                if (gif_data >= gif_end) {
+                        return GIF_INSUFFICIENT_FRAME_DATA;
+                }
+
+                /* Switch on extension label */
+                switch(gif_data[0]) {
+                case GIF_EXTENSION_COMMENT:
+                        /* Move the pointer to the first data sub-block
+                         * 1 byte for the extension label
+                         */
+                        ++gif_data;
+                        break;
+
+                default:
+                        /* Move the pointer to the first data sub-block 2 bytes
+                         * for the extension label and size fields Skip the
+                         * extension size itself
+                         */
+                        if (gif_data + 1 >= gif_end) {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+                        gif_data += (2 + gif_data[1]);
+                }
+
+                /* Repeatedly skip blocks until we get a zero block or run out
+                 * of data This data is ignored by this gif decoder
+                 */
+                gif_bytes = (gif_end - gif_data);
+                block_size = 0;
+                while (gif_data < gif_end && gif_data[0] != GIF_BLOCK_TERMINATOR) {
+                        block_size = gif_data[0] + 1;
+                        if ((gif_bytes -= block_size) < 0) {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+                        gif_data += block_size;
+                }
+                ++gif_data;
+        }
+
+        /* Set buffer position and return */
+        gif->buffer_position = (gif_data - gif->gif_data);
+        return GIF_OK;
+}
 
-	@return GIF_INSUFFICIENT_DATA for insufficient data to do anything
-		GIF_FRAME_DATA_ERROR for GIF frame data error
-		GIF_INSUFFICIENT_MEMORY for insufficient memory to process
-		GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the frame
-		GIF_DATA_ERROR for GIF error (invalid frame header)
-		GIF_OK for successful decoding
-		GIF_WORKING for successful decoding if more frames are expected
-*/
-static gif_result gif_initialise_frame(gif_animation *gif) {
-	int frame;
-	gif_frame *temp_buf;
-
-	unsigned char *gif_data, *gif_end;
-	int gif_bytes;
-	unsigned int flags = 0;
-	unsigned int width, height, offset_x, offset_y;
-	unsigned int block_size, colour_table_size;
-	bool first_image = true;
-	gif_result return_value;
-
-	/*	Get the frame to decode and our data position
-	*/
-	frame = gif->frame_count;
-
-	/*	Get our buffer position etc.
-	*/
-	gif_data = (unsigned char *)(gif->gif_data + gif->buffer_position);
-	gif_end = (unsigned char *)(gif->gif_data + gif->buffer_size);
-	gif_bytes = (gif_end - gif_data);
-
-	/*	Check if we've finished
-	*/
-	if ((gif_bytes > 0) && (gif_data[0] == GIF_TRAILER)) return GIF_OK;
-	
-	/*	Check if we have enough data
-	 *	The shortest block of data is a 4-byte comment extension + 1-byte block terminator + 1-byte gif trailer
-	*/
-	if (gif_bytes < 6) return GIF_INSUFFICIENT_DATA;
-
-	/*	We could theoretically get some junk data that gives us millions of frames, so
-		we ensure that we don't have a silly number
-	*/
-	if (frame > 4096) return GIF_FRAME_DATA_ERROR;
-
-	/*	Get some memory to store our pointers in etc.
-	*/
-	if ((int)gif->frame_holders <= frame) {
-		/*	Allocate more memory
-		*/
-		if ((temp_buf = (gif_frame *)realloc(gif->frames,
-					(frame + 1) * sizeof(gif_frame))) == NULL)
-			return GIF_INSUFFICIENT_MEMORY;
-		gif->frames = temp_buf;
-		gif->frame_holders = frame + 1;
-	}
-
-	/*	Store our frame pointer. We would do it when allocating except we
-		start off with one frame allocated so we can always use realloc.
-	*/
-	gif->frames[frame].frame_pointer = gif->buffer_position;
-	gif->frames[frame].display = false;
-	gif->frames[frame].virgin = true;
-	gif->frames[frame].disposal_method = 0;
-	gif->frames[frame].transparency = false;
-	gif->frames[frame].frame_delay = 100;
-	gif->frames[frame].redraw_required = false;
-
-	/*	Invalidate any previous decoding we have of this frame
-	*/
-	if (gif->decoded_frame == frame)
-		gif->decoded_frame = GIF_INVALID_FRAME;
-
-	/*	We pretend to initialise the frames, but really we just skip over all
-		the data contained within. This is all basically a cut down version of
-		gif_decode_frame that doesn't have any of the LZW bits in it.
-	*/
-
-	/*	Initialise any extensions
-	*/
-	gif->buffer_position = gif_data - gif->gif_data;
-	if ((return_value = gif_initialise_frame_extensions(gif, frame)) != GIF_OK)
-		return return_value;
-	gif_data = (gif->gif_data + gif->buffer_position);
-	gif_bytes = (gif_end - gif_data);
-
-	/*	Check if we've finished
-	*/
-	if ((gif_bytes = (gif_end - gif_data)) < 1)
-		return GIF_INSUFFICIENT_FRAME_DATA;
-	else if (gif_data[0] == GIF_TRAILER) {
-		gif->buffer_position = (gif_data - gif->gif_data);
-		gif->frame_count = frame + 1;
-		return GIF_OK;
-	}
-
-	/*	If we're not done, there should be an image descriptor
-	*/
-	if (gif_data[0] != GIF_IMAGE_SEPARATOR) return GIF_FRAME_DATA_ERROR;
-
-	/*	Do some simple boundary checking
-	*/
-	if (gif_bytes < 10) return GIF_INSUFFICIENT_FRAME_DATA;
-	offset_x = gif_data[1] | (gif_data[2] << 8);
-	offset_y = gif_data[3] | (gif_data[4] << 8);
-	width = gif_data[5] | (gif_data[6] << 8);
-	height = gif_data[7] | (gif_data[8] << 8);
-
-	/*	Set up the redraw characteristics. We have to check for extending the area
-		due to multi-image frames.
-	*/
-	if (!first_image) {
-		if (gif->frames[frame].redraw_x > offset_x) {
-			gif->frames[frame].redraw_width += (gif->frames[frame].redraw_x - offset_x);
-			gif->frames[frame].redraw_x = offset_x;
-		}
-		if (gif->frames[frame].redraw_y > offset_y) {
-			gif->frames[frame].redraw_height += (gif->frames[frame].redraw_y - offset_y);
-			gif->frames[frame].redraw_y = offset_y;
-		}
-		if ((offset_x + width) > (gif->frames[frame].redraw_x + gif->frames[frame].redraw_width))
-			gif->frames[frame].redraw_width = (offset_x + width) - gif->frames[frame].redraw_x;
-		if ((offset_y + height) > (gif->frames[frame].redraw_y + gif->frames[frame].redraw_height))
-			gif->frames[frame].redraw_height = (offset_y + height) - gif->frames[frame].redraw_y;
-	} else {
-		first_image = false;
-		gif->frames[frame].redraw_x = offset_x;
-		gif->frames[frame].redraw_y = offset_y;
-		gif->frames[frame].redraw_width = width;
-		gif->frames[frame].redraw_height = height;
-	}
-
-	/*	if we are clearing the background then we need to redraw enough to cover the previous
-		frame too
-	*/
-	gif->frames[frame].redraw_required = ((gif->frames[frame].disposal_method == GIF_FRAME_CLEAR) ||
-						(gif->frames[frame].disposal_method == GIF_FRAME_RESTORE));
-
-	/*	Boundary checking - shouldn't ever happen except with junk data
-	*/
-	if (gif_initialise_sprite(gif, (offset_x + width), (offset_y + height)))
-		return GIF_INSUFFICIENT_MEMORY;
-
-	/*	Decode the flags
-	*/
-	flags = gif_data[9];
-	colour_table_size = 2 << (flags & GIF_COLOUR_TABLE_SIZE_MASK);
-
-	/*	Move our data onwards and remember we've got a bit of this frame
-	*/
-	gif_data += 10;
-	gif_bytes = (gif_end - gif_data);
-	gif->frame_count_partial = frame + 1;
-
-	/*	Skip the local colour table
-	*/
-	if (flags & GIF_COLOUR_TABLE_MASK) {
-		gif_data += 3 * colour_table_size;
-		if ((gif_bytes = (gif_end - gif_data)) < 0)
-			return GIF_INSUFFICIENT_FRAME_DATA;
-	}
-
-	/*	Ensure we have a correct code size
-	*/
-	if (gif_bytes < 1)
-		return GIF_INSUFFICIENT_FRAME_DATA;
-	if (gif_data[0] > GIF_MAX_LZW)
-		return GIF_DATA_ERROR;
-
-	/*	Move our pointer to the actual image data
-	*/
-	gif_data++;
-	--gif_bytes;
-
-	/*	Repeatedly skip blocks until we get a zero block or run out of data
-	 *	These blocks of image data are processed later by gif_decode_frame()
-	*/
-	block_size = 0;
-	while (block_size != 1) {
-		if (gif_bytes < 1) return GIF_INSUFFICIENT_FRAME_DATA;
-		block_size = gif_data[0] + 1;
-		/*	Check if the frame data runs off the end of the file
-		*/
-		if ((int)(gif_bytes - block_size) < 0) {
-			/*	Try to recover by signaling the end of the gif.
-			 *	Once we get garbage data, there is no logical
-			 *	way to determine where the next frame is.
-			 *	It's probably better to partially load the gif
-			 *	than not at all.
-			*/
-			if (gif_bytes >= 2) {
-				gif_data[0] = 0;
-				gif_data[1] = GIF_TRAILER;
-				gif_bytes = 1;
-				++gif_data;
-				break;
-			} else
-				return GIF_INSUFFICIENT_FRAME_DATA;
-		} else {
-			gif_bytes -= block_size;
-			gif_data += block_size;
-		}
-	}
-
-	/*	Add the frame and set the display flag
-	*/
-	gif->buffer_position = gif_data - gif->gif_data;
-	gif->frame_count = frame + 1;
-	gif->frames[frame].display = true;
-
-	/*	Check if we've finished
-	*/
-	if (gif_bytes < 1)
-		return GIF_INSUFFICIENT_FRAME_DATA;
-	else
-		if (gif_data[0] == GIF_TRAILER) return GIF_OK;
-	return GIF_WORKING;
+static unsigned int gif_interlaced_line(int height, int y) {
+        if ((y << 3) < height) {
+                return (y << 3);
+        }
+        y -= ((height + 7) >> 3);
+        if ((y << 3) < (height - 4)) {
+                return (y << 3) + 4;
+        }
+        y -= ((height + 3) >> 3);
+        if ((y << 2) < (height - 2)) {
+                return (y << 2) + 2;
+        }
+        y -= ((height + 1) >> 2);
+        return (y << 1) + 1;
 }
 
-/**	Attempts to initialise the frame's extensions
 
-	@return GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the frame
-		GIF_OK for successful initialisation
-*/
-static gif_result gif_initialise_frame_extensions(gif_animation *gif, const int frame) {
-	unsigned char *gif_data, *gif_end;
-	int gif_bytes;
-	unsigned int block_size;
-
-	/*	Get our buffer position etc.
-	*/
-	gif_data = (unsigned char *)(gif->gif_data + gif->buffer_position);
-	gif_end = (unsigned char *)(gif->gif_data + gif->buffer_size);
-	
-	/*	Initialise the extensions
-	*/
-	while (gif_data < gif_end && gif_data[0] == GIF_EXTENSION_INTRODUCER) {
-		++gif_data;
-		if ((gif_bytes = (gif_end - gif_data)) < 1)
-			return GIF_INSUFFICIENT_FRAME_DATA;
-
-		/*	Switch on extension label
-		*/
-		switch(gif_data[0]) {
-			/* 6-byte Graphic Control Extension is:
-			 *
-			 *	+0	CHAR	Graphic Control Label
-			 *	+1	CHAR	Block Size
-			 *	+2	CHAR	__Packed Fields__
-			 *			3BITS	Reserved
-			 *			3BITS	Disposal Method
-			 *			1BIT	User Input Flag
-			 *			1BIT	Transparent Color Flag
-			 *	+3	SHORT	Delay Time
-			 *	+5	CHAR	Transparent Color Index
-			*/
-			case GIF_EXTENSION_GRAPHIC_CONTROL:
-				if (gif_bytes < 6) return GIF_INSUFFICIENT_FRAME_DATA;
-				gif->frames[frame].frame_delay = gif_data[3] | (gif_data[4] << 8);
-				if (gif_data[2] & GIF_TRANSPARENCY_MASK) {
-					gif->frames[frame].transparency = true;
-					gif->frames[frame].transparency_index = gif_data[5];
-				}
-				gif->frames[frame].disposal_method = ((gif_data[2] & GIF_DISPOSAL_MASK) >> 2);
-				/*	I have encountered documentation and GIFs in the wild that use
-				 *	0x04 to restore the previous frame, rather than the officially
-				 *	documented 0x03.  I believe some (older?) software may even actually
-				 *	export this way.  We handle this as a type of "quirks" mode.
-				*/
-				if (gif->frames[frame].disposal_method == GIF_FRAME_QUIRKS_RESTORE)
-					gif->frames[frame].disposal_method = GIF_FRAME_RESTORE;
-				gif_data += (2 + gif_data[1]);
-				break;
-
-			/* 14-byte+ Application Extension is:
-			 *
-			 *	+0	CHAR	Application Extension Label
-			 *	+1	CHAR	Block Size
-			 *	+2	8CHARS	Application Identifier
-			 *	+10	3CHARS	Appl. Authentication Code
-			 *	+13	1-256	Application Data (Data sub-blocks)
-			*/
-			case GIF_EXTENSION_APPLICATION:
-				if (gif_bytes < 17) return GIF_INSUFFICIENT_FRAME_DATA;
-				if ((gif_data[1] == 0x0b) &&
-					(strncmp((const char *) gif_data + 2,
-						"NETSCAPE2.0", 11) == 0) &&
-					(gif_data[13] == 0x03) &&
-					(gif_data[14] == 0x01)) {
-						gif->loop_count = gif_data[15] | (gif_data[16] << 8);
-				}
-				gif_data += (2 + gif_data[1]);
-				break;
-
-			/*	Move the pointer to the first data sub-block
-			 *	Skip 1 byte for the extension label
-			*/
-			case GIF_EXTENSION_COMMENT:
-				++gif_data;
-				break;
-
-			/*	Move the pointer to the first data sub-block
-			 *	Skip 2 bytes for the extension label and size fields
-			 *	Skip the extension size itself
-			*/
-			default:
-				if (gif_bytes < 2) return GIF_INSUFFICIENT_FRAME_DATA;
-				gif_data += (2 + gif_data[1]);
-		}
-
-		/*	Repeatedly skip blocks until we get a zero block or run out of data
-		 *	This data is ignored by this gif decoder
-		*/
-		gif_bytes = (gif_end - gif_data);
-		block_size = 0;
-		while (gif_data < gif_end && gif_data[0] != GIF_BLOCK_TERMINATOR) {
-			block_size = gif_data[0] + 1;
-			if ((gif_bytes -= block_size) < 0)
-				return GIF_INSUFFICIENT_FRAME_DATA;
-			gif_data += block_size;
-		}
-		++gif_data;
-	}
-
-	/*	Set buffer position and return
-	*/
-	gif->buffer_position = (gif_data - gif->gif_data);
-	return GIF_OK;
+/**
+ * Initialise LZW decoding
+ */
+void gif_init_LZW(gif_animation *gif)
+{
+        int i;
+
+        gif->current_error = 0;
+        if (clear_code >= (1 << GIF_MAX_LZW)) {
+                stack_pointer = stack;
+                gif->current_error = GIF_FRAME_DATA_ERROR;
+                return;
+        }
+
+        /* initialise our table */
+        memset(table, 0x00, (1 << GIF_MAX_LZW) * 8);
+        for (i = 0; i < clear_code; ++i) {
+                table[1][i] = i;
+        }
+
+        /* update our LZW parameters */
+        code_size = set_code_size + 1;
+        max_code_size = clear_code << 1;
+        max_code = clear_code + 2;
+        stack_pointer = stack;
+        do {
+                firstcode = oldcode = gif_next_code(gif, code_size);
+        } while (firstcode == clear_code);
+        *stack_pointer++ =firstcode;
 }
 
 
-/**	Decodes a GIF frame.
+static bool gif_next_LZW(gif_animation *gif)
+{
+        int code, incode;
+        int block_size;
+        int new_code;
+
+        code = gif_next_code(gif, code_size);
+        if (code < 0) {
+                gif->current_error = code;
+                return false;
+        } else if (code == clear_code) {
+                gif_init_LZW(gif);
+                return true;
+        } else if (code == end_code) {
+                /* skip to the end of our data so multi-image GIFs work */
+                if (zero_data_block) {
+                        gif->current_error = GIF_FRAME_DATA_ERROR;
+                        return false;
+                }
+                block_size = 0;
+                while (block_size != 1 &&
+                       gif->buffer_position < gif->buffer_size) {
+                        block_size = gif->gif_data[gif->buffer_position] + 1;
+                        gif->buffer_position += block_size;
+                }
+                gif->current_error = GIF_FRAME_DATA_ERROR;
+                return false;
+        }
+
+        incode = code;
+        if (code >= max_code) {
+                if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2)) {
+                        gif->current_error = GIF_FRAME_DATA_ERROR;
+                        return false;
+                }
+                *stack_pointer++ = firstcode;
+                code = oldcode;
+        }
+
+        /* The following loop is the most important in the GIF decoding cycle
+         * as every single pixel passes through it.
+         *
+         * Note: our stack is always big enough to hold a complete decompressed
+         * chunk.
+         */
+        while (code >= clear_code) {
+                if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2) ||
+                    code >= (1 << GIF_MAX_LZW)) {
+                        gif->current_error = GIF_FRAME_DATA_ERROR;
+                        return false;
+                }
+                *stack_pointer++ = table[1][code];
+                new_code = table[0][code];
+                if (new_code < clear_code) {
+                        code = new_code;
+                        break;
+                }
+
+                if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2) ||
+                    new_code >= (1 << GIF_MAX_LZW)) {
+                        gif->current_error = GIF_FRAME_DATA_ERROR;
+                        return false;
+                }
+                *stack_pointer++ = table[1][new_code];
+                code = table[0][new_code];
+                if (code == new_code) {
+                        gif->current_error = GIF_FRAME_DATA_ERROR;
+                        return false;
+                }
+        }
+
+        if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2)) {
+                gif->current_error = GIF_FRAME_DATA_ERROR;
+                return false;
+        }
+        *stack_pointer++ = firstcode = table[1][code];
+
+        if ((code = max_code) < (1 << GIF_MAX_LZW)) {
+                table[0][code] = oldcode;
+                table[1][code] = firstcode;
+                ++max_code;
+                if ((max_code >= max_code_size) &&
+                    (max_code_size < (1 << GIF_MAX_LZW))) {
+                        max_code_size = max_code_size << 1;
+                        ++code_size;
+                }
+        }
+        oldcode = incode;
+        return true;
+}
 
-	@return GIF_FRAME_DATA_ERROR for GIF frame data error
-		GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the frame
-		GIF_DATA_ERROR for GIF error (invalid frame header)
-		GIF_INSUFFICIENT_DATA for insufficient data to do anything
-		GIF_INSUFFICIENT_MEMORY for insufficient memory to process
-		GIF_OK for successful decoding
-		If a frame does not contain any image data, GIF_OK is returned and
-			gif->current_error is set to GIF_FRAME_NO_DISPLAY
-*/
-gif_result gif_decode_frame(gif_animation *gif, unsigned int frame) {
-	unsigned int index = 0;
-	unsigned char *gif_data, *gif_end;
-	int gif_bytes;
-	unsigned int width, height, offset_x, offset_y;
-	unsigned int flags, colour_table_size, interlace;
-	unsigned int *colour_table;
-	unsigned int *frame_data = 0;	// Set to 0 for no warnings
-	unsigned int *frame_scanline;
-	unsigned int save_buffer_position;
-	unsigned int return_value = 0;
-	unsigned int x, y, decode_y, burst_bytes;
-	int last_undisposed_frame = (frame - 1);
-	register unsigned char colour;
-
-	/*	Ensure this frame is supposed to be decoded
-	*/
-	if (gif->frames[frame].display == false) {
-		gif->current_error = GIF_FRAME_NO_DISPLAY;
-		return GIF_OK;
-	}
-
-	/*	Ensure we have a frame to decode
-	*/
-	if (frame > gif->frame_count_partial)
-		return GIF_INSUFFICIENT_DATA;
-	if ((!clear_image) && ((int)frame == gif->decoded_frame))
-		return GIF_OK;
-
-	/*	Get the start of our frame data and the end of the GIF data
-	*/
-	gif_data = gif->gif_data + gif->frames[frame].frame_pointer;
-	gif_end = gif->gif_data + gif->buffer_size;
-	gif_bytes = (gif_end - gif_data);
-
-	/*	Check if we have enough data
-	 *	The shortest block of data is a 10-byte image descriptor + 1-byte gif trailer
-	*/
-	if (gif_bytes < 12) return GIF_INSUFFICIENT_FRAME_DATA;
-
-	/*	Save the buffer position
-	*/
-	save_buffer_position = gif->buffer_position;
-	gif->buffer_position = gif_data - gif->gif_data;
-
-	/*	Skip any extensions because we all ready processed them
-	*/
-	if ((return_value = gif_skip_frame_extensions(gif)) != GIF_OK)
-		goto gif_decode_frame_exit;
-	gif_data = (gif->gif_data + gif->buffer_position);
-	gif_bytes = (gif_end - gif_data);
-
-	/*	Ensure we have enough data for the 10-byte image descriptor + 1-byte gif trailer
-	*/
-	if (gif_bytes < 12) {
-		return_value = GIF_INSUFFICIENT_FRAME_DATA;
-		goto gif_decode_frame_exit;
-	}
-
-	/* 10-byte Image Descriptor is:
-	 *
-	 *	+0	CHAR	Image Separator (0x2c)
-	 *	+1	SHORT	Image Left Position
-	 *	+3	SHORT	Image Top Position
-	 *	+5	SHORT	Width
-	 *	+7	SHORT	Height
-	 *	+9	CHAR	__Packed Fields__
-	 *			1BIT	Local Colour Table Flag
-	 *			1BIT	Interlace Flag
-	 *			1BIT	Sort Flag
-	 *			2BITS	Reserved
-	 *			3BITS	Size of Local Colour Table
-	*/
-	if (gif_data[0] != GIF_IMAGE_SEPARATOR) {
-		return_value = GIF_DATA_ERROR;
-		goto gif_decode_frame_exit;
-	}
-	offset_x = gif_data[1] | (gif_data[2] << 8);
-	offset_y = gif_data[3] | (gif_data[4] << 8);
-	width = gif_data[5] | (gif_data[6] << 8);
-	height = gif_data[7] | (gif_data[8] << 8);
-
-	/*	Boundary checking - shouldn't ever happen except unless the data has been
-		modified since initialisation.
-	*/
-	if ((offset_x + width > gif->width) || (offset_y + height > gif->height)) {
-		return_value = GIF_DATA_ERROR;
-		goto gif_decode_frame_exit;
-	}
-
-	/*	Decode the flags
-	*/
-	flags = gif_data[9];
-	colour_table_size = 2 << (flags & GIF_COLOUR_TABLE_SIZE_MASK);
-	interlace = flags & GIF_INTERLACE_MASK;
-
-	/*	Move our pointer to the colour table or image data (if no colour table is given)
-	*/
-	gif_data += 10;
-	gif_bytes = (gif_end - gif_data);
-
-	/*	Set up the colour table
-	*/
-	if (flags & GIF_COLOUR_TABLE_MASK) {
-		if (gif_bytes < (int)(3 * colour_table_size)) {
-			return_value = GIF_INSUFFICIENT_FRAME_DATA;
-			goto gif_decode_frame_exit;
-		}
-		colour_table = gif->local_colour_table;
-		if (!clear_image) {
-			for (index = 0; index < colour_table_size; index++) {
-				/* Gif colour map contents are r,g,b.
-				 *
-				 * We want to pack them bytewise into the 
-				 * colour table, such that the red component
-				 * is in byte 0 and the alpha component is in
-				 * byte 3.
-				 */
-				unsigned char *entry = 
-					(unsigned char *) &colour_table[index];
-
-				entry[0] = gif_data[0];	/* r */
-				entry[1] = gif_data[1];	/* g */
-				entry[2] = gif_data[2];	/* b */
-				entry[3] = 0xff;	/* a */
-
-				gif_data += 3;
-			}
-		} else {
-			gif_data += 3 * colour_table_size;
-		}
-		gif_bytes = (gif_end - gif_data);
-	} else {
-		colour_table = gif->global_colour_table;
-	}
-
-	/*	Check if we've finished
-	*/
-	if (gif_bytes < 1) {
-		return_value = GIF_INSUFFICIENT_FRAME_DATA;
-		goto gif_decode_frame_exit;
-	} else if (gif_data[0] == GIF_TRAILER) {
-		return_value = GIF_OK;
-		goto gif_decode_frame_exit;
-	}
-
-	/*	Get the frame data
-	*/
-	assert(gif->bitmap_callbacks.bitmap_get_buffer);
-	frame_data = (void *)gif->bitmap_callbacks.bitmap_get_buffer(gif->frame_image);
-	if (!frame_data)
-		return GIF_INSUFFICIENT_MEMORY;
-
-	/*	If we are clearing the image we just clear, if not decode
-	*/
-	if (!clear_image) {
-		/*	Ensure we have enough data for a 1-byte LZW code size + 1-byte gif trailer
-		*/
-		if (gif_bytes < 2) {
-			return_value = GIF_INSUFFICIENT_FRAME_DATA;
-			goto gif_decode_frame_exit;
-		/*	If we only have a 1-byte LZW code size + 1-byte gif trailer, we're finished
-		*/
-		} else if ((gif_bytes == 2) && (gif_data[1] == GIF_TRAILER)) {
-			return_value = GIF_OK;
-			goto gif_decode_frame_exit;
-		}
-
-		/*	If the previous frame's disposal method requires we restore the background
-		 *	colour or this is the first frame, clear the frame data
-		*/
-		if ((frame == 0) || (gif->decoded_frame == GIF_INVALID_FRAME)) {
-			memset((char*)frame_data, GIF_TRANSPARENT_COLOUR, gif->width * gif->height * sizeof(int));
-			gif->decoded_frame = frame;
-			/* The line below would fill the image with its background color, but because GIFs support
-			 * transparency we likely wouldn't want to do that. */
-			/* memset((char*)frame_data, colour_table[gif->background_index], gif->width * gif->height * sizeof(int)); */
-		} else if ((frame != 0) && (gif->frames[frame - 1].disposal_method == GIF_FRAME_CLEAR)) {
-			clear_image = true;
-			if ((return_value = gif_decode_frame(gif, (frame - 1))) != GIF_OK)
-				goto gif_decode_frame_exit;
-			clear_image = false;
-		/*	If the previous frame's disposal method requires we restore the previous
-		 *	image, find the last image set to "do not dispose" and get that frame data
-		*/
-		} else if ((frame != 0) && (gif->frames[frame - 1].disposal_method == GIF_FRAME_RESTORE)) {
-			while ((last_undisposed_frame != -1) && (gif->frames[--last_undisposed_frame].disposal_method == GIF_FRAME_RESTORE))
-				;
-
-			/*	If we don't find one, clear the frame data
-			 */
-			if (last_undisposed_frame == -1) {
-				/* see notes above on transparency vs. background color */
-				memset((char*)frame_data, GIF_TRANSPARENT_COLOUR, gif->width * gif->height * sizeof(int));
-			} else {
-				if ((return_value = gif_decode_frame(gif, last_undisposed_frame)) != GIF_OK)
-					goto gif_decode_frame_exit;
-				/*	Get this frame's data
-				*/
-				assert(gif->bitmap_callbacks.bitmap_get_buffer);
-				frame_data = (void *)gif->bitmap_callbacks.bitmap_get_buffer(gif->frame_image);
-				if (!frame_data)
-					return GIF_INSUFFICIENT_MEMORY;
-			}
-		}
-		gif->decoded_frame = frame;
-
-		/*	Initialise the LZW decoding
-		*/
-		set_code_size = gif_data[0];
-		gif->buffer_position = (gif_data - gif->gif_data) + 1;
-
-		/*	Set our code variables
-		*/
-		code_size = set_code_size + 1;
-		clear_code = (1 << set_code_size);
-		end_code = clear_code + 1;
-		max_code_size = clear_code << 1;
-		max_code = clear_code + 2;
-		curbit = lastbit = 0;
-		last_byte = 2;
-		get_done = false;
-		direct = buf;
-		gif_init_LZW(gif);
-
-		/*	Decompress the data
-		*/
-		for (y = 0; y < height; y++) {
-			if (interlace)
-				decode_y = gif_interlaced_line(height, y) + offset_y;
-			else
-				decode_y = y + offset_y;
-			frame_scanline = frame_data + offset_x + (decode_y * gif->width);
-
-			/*	Rather than decoding pixel by pixel, we try to burst out streams
-				of data to remove the need for end-of data checks every pixel.
-			*/
-			x = width;
-			while (x > 0) {
-				burst_bytes = (stack_pointer - stack);
-				if (burst_bytes > 0) {
-					if (burst_bytes > x)
-						burst_bytes = x;
-					x -= burst_bytes;
-					while (burst_bytes-- > 0) {
-						colour = *--stack_pointer;
-						if (((gif->frames[frame].transparency) &&
-							(colour != gif->frames[frame].transparency_index)) ||
-							(!gif->frames[frame].transparency))
-								*frame_scanline = colour_table[colour];
-						frame_scanline++;
-					}
-				} else {
-					if (!gif_next_LZW(gif)) {
-						/*	Unexpected end of frame, try to recover
-						*/
-						if (gif->current_error == GIF_END_OF_FRAME)
-							return_value = GIF_OK;
-						else
-							return_value = gif->current_error;
-						goto gif_decode_frame_exit;
-					}
-				}
-			}
-		}
-	} else {
-		/*	Clear our frame
-		*/
-		if (gif->frames[frame].disposal_method == GIF_FRAME_CLEAR) {
-			for (y = 0; y < height; y++) {
-				frame_scanline = frame_data + offset_x + ((offset_y + y) * gif->width);
-				if (gif->frames[frame].transparency)
-					memset(frame_scanline, GIF_TRANSPARENT_COLOUR, width * 4);
-				else
-					memset(frame_scanline, colour_table[gif->background_index], width * 4);
-			}
-		}
-	}
-gif_decode_frame_exit:
+static int gif_next_code(gif_animation *gif, int code_size)
+{
+        int i, j, end, count, ret;
+        unsigned char *b;
+
+        end = curbit + code_size;
+        if (end >= lastbit) {
+                if (get_done) {
+                        return GIF_END_OF_FRAME;
+                }
+                buf[0] = direct[last_byte - 2];
+                buf[1] = direct[last_byte - 1];
+
+                /* get the next block */
+                direct = gif->gif_data + gif->buffer_position;
+                if (gif->buffer_position >= gif->buffer_size) {
+                        return GIF_INSUFFICIENT_FRAME_DATA;
+                }
+                zero_data_block = ((count = direct[0]) == 0);
+                if ((gif->buffer_position + count) >= gif->buffer_size) {
+                        return GIF_INSUFFICIENT_FRAME_DATA;
+                }
+                if (count == 0) {
+                        get_done = true;
+                } else {
+                        if (gif->buffer_position + 3 >= gif->buffer_size) {
+                                return GIF_INSUFFICIENT_FRAME_DATA;
+                        }
+                        direct -= 1;
+                        buf[2] = direct[2];
+                        buf[3] = direct[3];
+                }
+                gif->buffer_position += count + 1;
+
+                /* update our variables */
+                last_byte = 2 + count;
+                curbit = (curbit - lastbit) + 16;
+                lastbit = (2 + count) << 3;
+                end = curbit + code_size;
+        }
+
+        i = curbit >> 3;
+        if (i < 2) {
+                b = buf;
+        } else {
+                b = direct;
+        }
+
+        ret = b[i];
+        j = (end >> 3) - 1;
+        if (i <= j) {
+                ret |= (b[i + 1] << 8);
+                if (i < j) {
+                        ret |= (b[i + 2] << 16);
+                }
+        }
+        ret = (ret >> (curbit % 8)) & maskTbl[code_size];
+        curbit += code_size;
+        return ret;
+}
 
-	/*	Check if we should test for optimisation
-	*/
-	if (gif->frames[frame].virgin) {
-		if (gif->bitmap_callbacks.bitmap_test_opaque)
-			gif->frames[frame].opaque = gif->bitmap_callbacks.bitmap_test_opaque(gif->frame_image);
-		else
-			gif->frames[frame].opaque = false;
-		gif->frames[frame].virgin = false;
-	}
-	if (gif->bitmap_callbacks.bitmap_set_opaque)
-		gif->bitmap_callbacks.bitmap_set_opaque(gif->frame_image, gif->frames[frame].opaque);
-	if (gif->bitmap_callbacks.bitmap_modified)
-		gif->bitmap_callbacks.bitmap_modified(gif->frame_image);
-
-	/*	Restore the buffer position
-	*/
-	gif->buffer_position = save_buffer_position;
-
-	/*	Success!
-	*/
-	return return_value;
 
+/* exported function documented in libnsgif.h */
+void gif_create(gif_animation *gif, gif_bitmap_callback_vt *bitmap_callbacks)
+{
+        memset(gif, 0, sizeof(gif_animation));
+        gif->bitmap_callbacks = *bitmap_callbacks;
+        gif->decoded_frame = GIF_INVALID_FRAME;
 }
 
-/**	Skips the frame's extensions (which have been previously initialised)
 
-	@return GIF_INSUFFICIENT_FRAME_DATA for insufficient data to complete the frame
-		GIF_OK for successful decoding
-*/
-static gif_result gif_skip_frame_extensions(gif_animation *gif) {
-	unsigned char *gif_data, *gif_end;
-	int gif_bytes;
-	unsigned int block_size;
-
-	/*	Get our buffer position etc.
-	*/
-	gif_data = (unsigned char *)(gif->gif_data + gif->buffer_position);
-	gif_end = (unsigned char *)(gif->gif_data + gif->buffer_size);
-	gif_bytes = (gif_end - gif_data);
-
-	/*	Skip the extensions
-	*/
-	while (gif_data < gif_end && gif_data[0] == GIF_EXTENSION_INTRODUCER) {
-		++gif_data;
-		if (gif_data >= gif_end)
-			return GIF_INSUFFICIENT_FRAME_DATA;
-
-		/*	Switch on extension label
-		*/
-		switch(gif_data[0]) {
-			/*	Move the pointer to the first data sub-block
-			 *	1 byte for the extension label
-			*/
-			case GIF_EXTENSION_COMMENT:
-				++gif_data;
-				break;
-
-			/*	Move the pointer to the first data sub-block
-			 *	2 bytes for the extension label and size fields
-			 *	Skip the extension size itself
-			*/
-			default:
-				if (gif_data + 1 >= gif_end)
-					return GIF_INSUFFICIENT_FRAME_DATA;
-				gif_data += (2 + gif_data[1]);
-		}
-
-		/*	Repeatedly skip blocks until we get a zero block or run out of data
-		 *	This data is ignored by this gif decoder
-		*/
-		gif_bytes = (gif_end - gif_data);
-		block_size = 0;
-		while (gif_data < gif_end && gif_data[0] != GIF_BLOCK_TERMINATOR) {
-			block_size = gif_data[0] + 1;
-			if ((gif_bytes -= block_size) < 0)
-				return GIF_INSUFFICIENT_FRAME_DATA;
-			gif_data += block_size;
-		}
-		++gif_data;
-	}
-
-	/*	Set buffer position and return
-	*/
-	gif->buffer_position = (gif_data - gif->gif_data);
-	return GIF_OK;
+/* exported function documented in libnsgif.h */
+gif_result gif_initialise(gif_animation *gif, size_t size, unsigned char *data)
+{
+        unsigned char *gif_data;
+        unsigned int index;
+        gif_result return_value;
+
+        /* Initialize values */
+        gif->buffer_size = size;
+        gif->gif_data = data;
+
+        /* Check for sufficient data to be a GIF (6-byte header + 7-byte
+         * logical screen descriptor)
+         */
+        if (gif->buffer_size < 13) {
+                return GIF_INSUFFICIENT_DATA;
+        }
+
+        /* Get our current processing position */
+        gif_data = gif->gif_data + gif->buffer_position;
+
+        /* See if we should initialise the GIF */
+        if (gif->buffer_position == 0) {
+                /* We want everything to be NULL before we start so we've no
+                 * chance of freeing bad pointers (paranoia)
+                 */
+                gif->frame_image = NULL;
+                gif->frames = NULL;
+                gif->local_colour_table = NULL;
+                gif->global_colour_table = NULL;
+
+                /* The caller may have been lazy and not reset any values */
+                gif->frame_count = 0;
+                gif->frame_count_partial = 0;
+                gif->decoded_frame = GIF_INVALID_FRAME;
+
+                /* 6-byte GIF file header is:
+                 *
+                 *	+0	3CHARS	Signature ('GIF')
+                 *	+3	3CHARS	Version ('87a' or '89a')
+                 */
+                if (strncmp((const char *) gif_data, "GIF", 3) != 0) {
+                        return GIF_DATA_ERROR;
+                }
+                gif_data += 3;
+
+                /* Ensure GIF reports version 87a or 89a */
+                /*
+                if ((strncmp(gif_data, "87a", 3) != 0) &&
+                    (strncmp(gif_data, "89a", 3) != 0))
+                               LOG(("Unknown GIF format - proceeding anyway"));
+                */
+                gif_data += 3;
+
+                /* 7-byte Logical Screen Descriptor is:
+                 *
+                 *	+0	SHORT	Logical Screen Width
+                 *	+2	SHORT	Logical Screen Height
+                 *	+4	CHAR	__Packed Fields__
+                 *                      1BIT	Global Colour Table Flag
+                 *                      3BITS	Colour Resolution
+                 *                      1BIT	Sort Flag
+                 *                      3BITS	Size of Global Colour Table
+                 *	+5	CHAR	Background Colour Index
+                 *	+6	CHAR	Pixel Aspect Ratio
+                 */
+                gif->width = gif_data[0] | (gif_data[1] << 8);
+                gif->height = gif_data[2] | (gif_data[3] << 8);
+                gif->global_colours = (gif_data[4] & GIF_COLOUR_TABLE_MASK);
+                gif->colour_table_size = (2 << (gif_data[4] & GIF_COLOUR_TABLE_SIZE_MASK));
+                gif->background_index = gif_data[5];
+                gif->aspect_ratio = gif_data[6];
+                gif->loop_count = 1;
+                gif_data += 7;
+
+                /* Some broken GIFs report the size as the screen size they
+                 * were created in. As such, we detect for the common cases and
+                 * set the sizes as 0 if they are found which results in the
+                 * GIF being the maximum size of the frames.
+                 */
+                if (((gif->width == 640) && (gif->height == 480)) ||
+                    ((gif->width == 640) && (gif->height == 512)) ||
+                    ((gif->width == 800) && (gif->height == 600)) ||
+                    ((gif->width == 1024) && (gif->height == 768)) ||
+                    ((gif->width == 1280) && (gif->height == 1024)) ||
+                    ((gif->width == 1600) && (gif->height == 1200)) ||
+                    ((gif->width == 0) || (gif->height == 0)) ||
+                    ((gif->width > 2048) || (gif->height > 2048))) {
+                        gif->width = 1;
+                        gif->height = 1;
+                }
+
+                /* Allocate some data irrespective of whether we've got any
+                 * colour tables. We always get the maximum size in case a GIF
+                 * is lying to us. It's far better to give the wrong colours
+                 * than to trample over some memory somewhere.
+                */
+                gif->global_colour_table = calloc(GIF_MAX_COLOURS, sizeof(unsigned int));
+                gif->local_colour_table = calloc(GIF_MAX_COLOURS, sizeof(unsigned int));
+                if ((gif->global_colour_table == NULL) ||
+                    (gif->local_colour_table == NULL)) {
+                        gif_finalise(gif);
+                        return GIF_INSUFFICIENT_MEMORY;
+                }
+
+                /* Set the first colour to a value that will never occur in
+                 * reality so we know if we've processed it
+                */
+                gif->global_colour_table[0] = GIF_PROCESS_COLOURS;
+
+                /* Check if the GIF has no frame data (13-byte header + 1-byte
+                 * termination block) Although generally useless, the GIF
+                 * specification does not expressly prohibit this
+                 */
+                if (gif->buffer_size == 14) {
+                        if (gif_data[0] == GIF_TRAILER) {
+                                return GIF_OK;
+                        } else {
+                                return GIF_INSUFFICIENT_DATA;
+                        }
+                }
+
+                /* Initialise enough workspace for 4 frames initially */
+                if ((gif->frames = (gif_frame *)malloc(sizeof(gif_frame))) == NULL) {
+                        gif_finalise(gif);
+                        return GIF_INSUFFICIENT_MEMORY;
+                }
+                gif->frame_holders = 1;
+
+                /* Initialise the bitmap header */
+                assert(gif->bitmap_callbacks.bitmap_create);
+                gif->frame_image = gif->bitmap_callbacks.bitmap_create(gif->width, gif->height);
+                if (gif->frame_image == NULL) {
+                        gif_finalise(gif);
+                        return GIF_INSUFFICIENT_MEMORY;
+                }
+
+                /* Remember we've done this now */
+                gif->buffer_position = gif_data - gif->gif_data;
+        }
+
+        /*  Do the colour map if we haven't already. As the top byte is always
+         *  0xff or 0x00 depending on the transparency we know if it's been
+         *  filled in.
+         */
+        if (gif->global_colour_table[0] == GIF_PROCESS_COLOURS) {
+                /* Check for a global colour map signified by bit 7 */
+                if (gif->global_colours) {
+                        if (gif->buffer_size < (gif->colour_table_size * 3 + 12)) {
+                                return GIF_INSUFFICIENT_DATA;
+                        }
+                        for (index = 0; index < gif->colour_table_size; index++) {
+                                /* Gif colour map contents are r,g,b.
+                                 *
+                                 * We want to pack them bytewise into the
+                                 * colour table, such that the red component
+                                 * is in byte 0 and the alpha component is in
+                                 * byte 3.
+                                 */
+                                unsigned char *entry = (unsigned char *) &gif->
+                                                       global_colour_table[index];
+
+                                entry[0] = gif_data[0];	/* r */
+                                entry[1] = gif_data[1];	/* g */
+                                entry[2] = gif_data[2];	/* b */
+                                entry[3] = 0xff;	/* a */
+
+                                gif_data += 3;
+                        }
+                        gif->buffer_position = (gif_data - gif->gif_data);
+                } else {
+                        /* Create a default colour table with the first two
+                         * colours as black and white
+                         */
+                        unsigned int *entry = gif->global_colour_table;
+
+                        entry[0] = 0x00000000;
+                        /* Force Alpha channel to opaque */
+                        ((unsigned char *) entry)[3] = 0xff;
+
+                        entry[1] = 0xffffffff;
+                }
+        }
+
+        /* Repeatedly try to initialise frames */
+        while ((return_value = gif_initialise_frame(gif)) == GIF_WORKING);
+
+        /* If there was a memory error tell the caller */
+        if ((return_value == GIF_INSUFFICIENT_MEMORY) ||
+            (return_value == GIF_DATA_ERROR)) {
+                return return_value;
+        }
+
+        /* If we didn't have some frames then a GIF_INSUFFICIENT_DATA becomes a
+         * GIF_INSUFFICIENT_FRAME_DATA
+         */
+        if ((return_value == GIF_INSUFFICIENT_DATA) &&
+            (gif->frame_count_partial > 0)) {
+                return GIF_INSUFFICIENT_FRAME_DATA;
+        }
+
+        /* Return how many we got */
+        return return_value;
 }
 
-static unsigned int gif_interlaced_line(int height, int y) {
-	if ((y << 3) < height) return (y << 3);
-	y -= ((height + 7) >> 3);
-	if ((y << 3) < (height - 4)) return (y << 3) + 4;
-	y -= ((height + 3) >> 3);
-	if ((y << 2) < (height - 2)) return (y << 2) + 2;
-	y -= ((height + 1) >> 2);
-	return (y << 1) + 1;
-}
 
-/*	Releases any workspace held by the animation
-*/
-void gif_finalise(gif_animation *gif) {
-	/*	Release all our memory blocks
-	*/
-	if (gif->frame_image) {
-		assert(gif->bitmap_callbacks.bitmap_destroy);
-		gif->bitmap_callbacks.bitmap_destroy(gif->frame_image);
-	}
-	gif->frame_image = NULL;
-	free(gif->frames);
-	gif->frames = NULL;
-	free(gif->local_colour_table);
-	gif->local_colour_table = NULL;
-	free(gif->global_colour_table);
-	gif->global_colour_table = NULL;
-}
+/* exported function documented in libnsgif.h */
+gif_result gif_decode_frame(gif_animation *gif, unsigned int frame)
+{
+        unsigned int index = 0;
+        unsigned char *gif_data, *gif_end;
+        int gif_bytes;
+        unsigned int width, height, offset_x, offset_y;
+        unsigned int flags, colour_table_size, interlace;
+        unsigned int *colour_table;
+        unsigned int *frame_data = 0;	// Set to 0 for no warnings
+        unsigned int *frame_scanline;
+        unsigned int save_buffer_position;
+        unsigned int return_value = 0;
+        unsigned int x, y, decode_y, burst_bytes;
+        int last_undisposed_frame = (frame - 1);
+        register unsigned char colour;
+
+        /* Ensure this frame is supposed to be decoded */
+        if (gif->frames[frame].display == false) {
+                gif->current_error = GIF_FRAME_NO_DISPLAY;
+                return GIF_OK;
+        }
+
+        /* Ensure the frame is in range to decode */
+        if (frame > gif->frame_count_partial) {
+                return GIF_INSUFFICIENT_DATA;
+        }
+
+        /* done if frame is already decoded */
+        if ((!clear_image) &&
+            ((int)frame == gif->decoded_frame)) {
+                return GIF_OK;
+        }
+
+        /* Get the start of our frame data and the end of the GIF data */
+        gif_data = gif->gif_data + gif->frames[frame].frame_pointer;
+        gif_end = gif->gif_data + gif->buffer_size;
+        gif_bytes = (gif_end - gif_data);
+
+        /*
+         * Ensure there is a minimal amount of data to proceed.  The shortest
+         * block of data is a 10-byte image descriptor + 1-byte gif trailer
+         */
+        if (gif_bytes < 12) {
+                return GIF_INSUFFICIENT_FRAME_DATA;
+        }
+
+        /* Save the buffer position */
+        save_buffer_position = gif->buffer_position;
+        gif->buffer_position = gif_data - gif->gif_data;
+
+        /* Skip any extensions because they have allready been processed */
+        if ((return_value = gif_skip_frame_extensions(gif)) != GIF_OK) {
+                goto gif_decode_frame_exit;
+        }
+        gif_data = (gif->gif_data + gif->buffer_position);
+        gif_bytes = (gif_end - gif_data);
+
+        /* Ensure we have enough data for the 10-byte image descriptor + 1-byte
+         * gif trailer
+         */
+        if (gif_bytes < 12) {
+                return_value = GIF_INSUFFICIENT_FRAME_DATA;
+                goto gif_decode_frame_exit;
+        }
+
+        /* 10-byte Image Descriptor is:
+         *
+         *	+0	CHAR	Image Separator (0x2c)
+         *	+1	SHORT	Image Left Position
+         *	+3	SHORT	Image Top Position
+         *	+5	SHORT	Width
+         *	+7	SHORT	Height
+         *	+9	CHAR	__Packed Fields__
+         *			1BIT	Local Colour Table Flag
+         *			1BIT	Interlace Flag
+         *			1BIT	Sort Flag
+         *			2BITS	Reserved
+         *			3BITS	Size of Local Colour Table
+         */
+        if (gif_data[0] != GIF_IMAGE_SEPARATOR) {
+                return_value = GIF_DATA_ERROR;
+                goto gif_decode_frame_exit;
+        }
+        offset_x = gif_data[1] | (gif_data[2] << 8);
+        offset_y = gif_data[3] | (gif_data[4] << 8);
+        width = gif_data[5] | (gif_data[6] << 8);
+        height = gif_data[7] | (gif_data[8] << 8);
+
+        /* Boundary checking - shouldn't ever happen except unless the data has
+         * been modified since initialisation.
+         */
+        if ((offset_x + width > gif->width) ||
+            (offset_y + height > gif->height)) {
+                return_value = GIF_DATA_ERROR;
+                goto gif_decode_frame_exit;
+        }
+
+        /* Decode the flags */
+        flags = gif_data[9];
+        colour_table_size = 2 << (flags & GIF_COLOUR_TABLE_SIZE_MASK);
+        interlace = flags & GIF_INTERLACE_MASK;
+
+        /* Advance data pointer to next block either colour table or image
+         * data.
+         */
+        gif_data += 10;
+        gif_bytes = (gif_end - gif_data);
+
+        /* Set up the colour table */
+        if (flags & GIF_COLOUR_TABLE_MASK) {
+                if (gif_bytes < (int)(3 * colour_table_size)) {
+                        return_value = GIF_INSUFFICIENT_FRAME_DATA;
+                        goto gif_decode_frame_exit;
+                }
+                colour_table = gif->local_colour_table;
+                if (!clear_image) {
+                        for (index = 0; index < colour_table_size; index++) {
+                                /* Gif colour map contents are r,g,b.
+                                 *
+                                 * We want to pack them bytewise into the
+                                 * colour table, such that the red component
+                                 * is in byte 0 and the alpha component is in
+                                 * byte 3.
+                                 */
+                                unsigned char *entry =
+                                        (unsigned char *) &colour_table[index];
+
+                                entry[0] = gif_data[0];	/* r */
+                                entry[1] = gif_data[1];	/* g */
+                                entry[2] = gif_data[2];	/* b */
+                                entry[3] = 0xff;	/* a */
+
+                                gif_data += 3;
+                        }
+                } else {
+                        gif_data += 3 * colour_table_size;
+                }
+                gif_bytes = (gif_end - gif_data);
+        } else {
+                colour_table = gif->global_colour_table;
+        }
+
+        /* Ensure sufficient data remains */
+        if (gif_bytes < 1) {
+                return_value = GIF_INSUFFICIENT_FRAME_DATA;
+                goto gif_decode_frame_exit;
+        }
+
+        /* check for an end marker */
+        if (gif_data[0] == GIF_TRAILER) {
+                return_value = GIF_OK;
+                goto gif_decode_frame_exit;
+        }
+
+        /* Get the frame data */
+        assert(gif->bitmap_callbacks.bitmap_get_buffer);
+        frame_data = (void *)gif->bitmap_callbacks.bitmap_get_buffer(gif->frame_image);
+        if (!frame_data) {
+                return GIF_INSUFFICIENT_MEMORY;
+        }
+
+        /* If we are clearing the image we just clear, if not decode */
+        if (!clear_image) {
+                /* Ensure we have enough data for a 1-byte LZW code size +
+                 * 1-byte gif trailer
+                 */
+                if (gif_bytes < 2) {
+                        return_value = GIF_INSUFFICIENT_FRAME_DATA;
+                        goto gif_decode_frame_exit;
+                }
+
+                /* If we only have a 1-byte LZW code size + 1-byte gif trailer,
+                 * we're finished
+                 */
+                if ((gif_bytes == 2) && (gif_data[1] == GIF_TRAILER)) {
+                        return_value = GIF_OK;
+                        goto gif_decode_frame_exit;
+                }
+
+                /* If the previous frame's disposal method requires we restore
+                 * the background colour or this is the first frame, clear
+                 * the frame data
+                 */
+                if ((frame == 0) || (gif->decoded_frame == GIF_INVALID_FRAME)) {
+                        memset((char*)frame_data,
+                               GIF_TRANSPARENT_COLOUR,
+                               gif->width * gif->height * sizeof(int));
+                        gif->decoded_frame = frame;
+                        /* The line below would fill the image with its
+                         * background color, but because GIFs support
+                         * transparency we likely wouldn't want to do that. */
+                        /* memset((char*)frame_data, colour_table[gif->background_index], gif->width * gif->height * sizeof(int)); */
+                } else if ((frame != 0) &&
+                           (gif->frames[frame - 1].disposal_method == GIF_FRAME_CLEAR)) {
+                        clear_image = true;
+                        if ((return_value = gif_decode_frame(gif, (frame - 1))) != GIF_OK) {
+                                goto gif_decode_frame_exit;
+                        }
+                        clear_image = false;
+                        /*
+                         * If the previous frame's disposal method requires we
+                         * restore the previous image, find the last image set
+                         * to "do not dispose" and get that frame data
+                         */
+                } else if ((frame != 0) &&
+                           (gif->frames[frame - 1].disposal_method == GIF_FRAME_RESTORE)) {
+                        while ((last_undisposed_frame != -1) &&
+                               (gif->frames[--last_undisposed_frame].disposal_method == GIF_FRAME_RESTORE));
+
+                        /* If we don't find one, clear the frame data */
+                        if (last_undisposed_frame == -1) {
+                                /* see notes above on transparency
+                                 * vs. background color */
+                                memset((char*)frame_data,
+                                       GIF_TRANSPARENT_COLOUR,
+                                       gif->width * gif->height * sizeof(int));
+                        } else {
+                                if ((return_value = gif_decode_frame(gif, last_undisposed_frame)) != GIF_OK) {
+                                        goto gif_decode_frame_exit;
+                                }
+                                /* Get this frame's data */
+                                assert(gif->bitmap_callbacks.bitmap_get_buffer);
+                                frame_data = (void *)gif->bitmap_callbacks.bitmap_get_buffer(gif->frame_image);
+                                if (!frame_data) {
+                                        return GIF_INSUFFICIENT_MEMORY;
+                                }
+                        }
+                }
+                gif->decoded_frame = frame;
+
+                /* Initialise the LZW decoding */
+                set_code_size = gif_data[0];
+                gif->buffer_position = (gif_data - gif->gif_data) + 1;
+
+                /* Set our code variables */
+                code_size = set_code_size + 1;
+                clear_code = (1 << set_code_size);
+                end_code = clear_code + 1;
+                max_code_size = clear_code << 1;
+                max_code = clear_code + 2;
+                curbit = lastbit = 0;
+                last_byte = 2;
+                get_done = false;
+                direct = buf;
+                gif_init_LZW(gif);
+
+                /* Decompress the data */
+                for (y = 0; y < height; y++) {
+                        if (interlace) {
+                                decode_y = gif_interlaced_line(height, y) + offset_y;
+                        } else {
+                                decode_y = y + offset_y;
+                        }
+                        frame_scanline = frame_data + offset_x + (decode_y * gif->width);
+
+                        /* Rather than decoding pixel by pixel, we try to burst
+                         * out streams of data to remove the need for end-of
+                         * data checks every pixel.
+                         */
+                        x = width;
+                        while (x > 0) {
+                                burst_bytes = (stack_pointer - stack);
+                                if (burst_bytes > 0) {
+                                        if (burst_bytes > x) {
+                                                burst_bytes = x;
+                                        }
+                                        x -= burst_bytes;
+                                        while (burst_bytes-- > 0) {
+                                                colour = *--stack_pointer;
+                                                if (((gif->frames[frame].transparency) &&
+                                                     (colour != gif->frames[frame].transparency_index)) ||
+                                                    (!gif->frames[frame].transparency)) {
+                                                        *frame_scanline = colour_table[colour];
+                                                }
+                                                frame_scanline++;
+                                        }
+                                } else {
+                                        if (!gif_next_LZW(gif)) {
+                                                /* Unexpected end of frame, try to recover */
+                                                if (gif->current_error == GIF_END_OF_FRAME) {
+                                                        return_value = GIF_OK;
+                                                } else {
+                                                        return_value = gif->current_error;
+                                                }
+                                                goto gif_decode_frame_exit;
+                                        }
+                                }
+                        }
+                }
+        } else {
+                /* Clear our frame */
+                if (gif->frames[frame].disposal_method == GIF_FRAME_CLEAR) {
+                        for (y = 0; y < height; y++) {
+                                frame_scanline = frame_data + offset_x + ((offset_y + y) * gif->width);
+                                if (gif->frames[frame].transparency) {
+                                        memset(frame_scanline,
+                                               GIF_TRANSPARENT_COLOUR,
+                                               width * 4);
+                                } else {
+                                        memset(frame_scanline,
+                                               colour_table[gif->background_index],
+                                               width * 4);
+                                }
+                        }
+                }
+        }
+gif_decode_frame_exit:
 
-/**
- * Initialise LZW decoding
- */
-void gif_init_LZW(gif_animation *gif) {
-	int i;
-
-	gif->current_error = 0;
-	if (clear_code >= (1 << GIF_MAX_LZW)) {
-		stack_pointer = stack;
-		gif->current_error = GIF_FRAME_DATA_ERROR;
-		return;
-	}
-
-	/* initialise our table */
-	memset(table, 0x00, (1 << GIF_MAX_LZW) * 8);
-	for (i = 0; i < clear_code; ++i)
-		table[1][i] = i;
-
-	/* update our LZW parameters */
-	code_size = set_code_size + 1;
-	max_code_size = clear_code << 1;
-	max_code = clear_code + 2;
-	stack_pointer = stack;
-	do {
-		firstcode = oldcode = gif_next_code(gif, code_size);
-	} while (firstcode == clear_code);
-	*stack_pointer++ =firstcode;
-}
+        /* Check if we should test for optimisation */
+        if (gif->frames[frame].virgin) {
+                if (gif->bitmap_callbacks.bitmap_test_opaque) {
+                        gif->frames[frame].opaque = gif->bitmap_callbacks.bitmap_test_opaque(gif->frame_image);
+                } else {
+                        gif->frames[frame].opaque = false;
+                }
+                gif->frames[frame].virgin = false;
+        }
+
+        if (gif->bitmap_callbacks.bitmap_set_opaque) {
+                gif->bitmap_callbacks.bitmap_set_opaque(gif->frame_image, gif->frames[frame].opaque);
+        }
 
+        if (gif->bitmap_callbacks.bitmap_modified) {
+                gif->bitmap_callbacks.bitmap_modified(gif->frame_image);
+        }
 
-static bool gif_next_LZW(gif_animation *gif) {
-	int code, incode;
-	int block_size;
-	int new_code;
-
-	code = gif_next_code(gif, code_size);
-	if (code < 0) {
-	  	gif->current_error = code;
-		return false;
-	} else if (code == clear_code) {
-		gif_init_LZW(gif);
-		return true;
-	} else if (code == end_code) {
-		/* skip to the end of our data so multi-image GIFs work */
-		if (zero_data_block) {
-			gif->current_error = GIF_FRAME_DATA_ERROR;
-			return false;
-		}
-		block_size = 0;
-		while (block_size != 1 &&
-				gif->buffer_position < gif->buffer_size) {
-			block_size = gif->gif_data[gif->buffer_position] + 1;
-			gif->buffer_position += block_size;
-		}
-		gif->current_error = GIF_FRAME_DATA_ERROR;
-		return false;
-	}
-
-	incode = code;
-	if (code >= max_code) {
-		if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2)) {
-			gif->current_error = GIF_FRAME_DATA_ERROR;
-			return false;
-		}
-		*stack_pointer++ = firstcode;
-		code = oldcode;
-	}
-
-	/* The following loop is the most important in the GIF decoding cycle as every
-	 * single pixel passes through it.
-	 *
-	 * Note: our stack is always big enough to hold a complete decompressed chunk. */
-	while (code >= clear_code) {
-		if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2) ||
-				code >= (1 << GIF_MAX_LZW)) {
-			gif->current_error = GIF_FRAME_DATA_ERROR;
-			return false;
-		}
-		*stack_pointer++ = table[1][code];
-		new_code = table[0][code];
-		if (new_code < clear_code) {
-			code = new_code;
-			break;
-		}
-
-		if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2) ||
-				new_code >= (1 << GIF_MAX_LZW)) {
-			gif->current_error = GIF_FRAME_DATA_ERROR;
-			return false;
-		}
-		*stack_pointer++ = table[1][new_code];
-		code = table[0][new_code];
-		if (code == new_code) {
-		  	gif->current_error = GIF_FRAME_DATA_ERROR;
-			return false;
-		}
-	}
-
-	if (stack_pointer >= stack + ((1 << GIF_MAX_LZW) * 2)) {
-		gif->current_error = GIF_FRAME_DATA_ERROR;
-		return false;
-	}
-	*stack_pointer++ = firstcode = table[1][code];
-
-	if ((code = max_code) < (1 << GIF_MAX_LZW)) {
-		table[0][code] = oldcode;
-		table[1][code] = firstcode;
-		++max_code;
-		if ((max_code >= max_code_size) && (max_code_size < (1 << GIF_MAX_LZW))) {
-			max_code_size = max_code_size << 1;
-			++code_size;
-		}
-	}
-	oldcode = incode;
-	return true;
+        /* Restore the buffer position */
+        gif->buffer_position = save_buffer_position;
+
+        return return_value;
 }
 
-static int gif_next_code(gif_animation *gif, int code_size) {
-	int i, j, end, count, ret;
-	unsigned char *b;
-
-	end = curbit + code_size;
-	if (end >= lastbit) {
-		if (get_done)
-			return GIF_END_OF_FRAME;
-		buf[0] = direct[last_byte - 2];
-		buf[1] = direct[last_byte - 1];
-
-		/* get the next block */
-		direct = gif->gif_data + gif->buffer_position;
-		if (gif->buffer_position >= gif->buffer_size)
-			return GIF_INSUFFICIENT_FRAME_DATA;
-		zero_data_block = ((count = direct[0]) == 0);
-		if ((gif->buffer_position + count) >= gif->buffer_size)
-			return GIF_INSUFFICIENT_FRAME_DATA;
-		if (count == 0)
-			get_done = true;
-		else {
-			if (gif->buffer_position + 3 >= gif->buffer_size)
-				return GIF_INSUFFICIENT_FRAME_DATA;
-			direct -= 1;
-			buf[2] = direct[2];
-			buf[3] = direct[3];
-		}
-		gif->buffer_position += count + 1;
-
-		/* update our variables */
-		last_byte = 2 + count;
-		curbit = (curbit - lastbit) + 16;
-		lastbit = (2 + count) << 3;
-		end = curbit + code_size;
-	}
-
-	i = curbit >> 3;
-	if (i < 2)
-		b = buf;
-	else
-		b = direct;
-
-	ret = b[i];
-	j = (end >> 3) - 1;
-	if (i <= j) {
-		ret |= (b[i + 1] << 8);
-		if (i < j)
-			ret |= (b[i + 2] << 16);
-	}
-	ret = (ret >> (curbit % 8)) & maskTbl[code_size];
-	curbit += code_size;
-	return ret;
+
+/* exported function documented in libnsgif.h */
+void gif_finalise(gif_animation *gif)
+{
+        /* Release all our memory blocks */
+        if (gif->frame_image) {
+                assert(gif->bitmap_callbacks.bitmap_destroy);
+                gif->bitmap_callbacks.bitmap_destroy(gif->frame_image);
+        }
+
+        gif->frame_image = NULL;
+        free(gif->frames);
+        gif->frames = NULL;
+        free(gif->local_colour_table);
+        gif->local_colour_table = NULL;
+        free(gif->global_colour_table);
+        gif->global_colour_table = NULL;
 }