Units: Add support for length unit conversion to libcss.

Currently only used for unit conversion.

5 files changed, 677 insertions, 97 deletions

diff --git a/src/select/Makefile b/src/select/Makefile
index 8b47673..f5ddb18 100644
--- a/src/select/Makefile
+++ b/src/select/Makefile
@@ -2,6 +2,6 @@
 select_generator:
 	python3 src/select/select_generator.py
 
-DIR_SOURCES := arena.c computed.c dispatch.c hash.c select.c font_face.c format_list_style.c
+DIR_SOURCES := arena.c computed.c dispatch.c hash.c select.c font_face.c format_list_style.c unit.c
 
 include $(NSBUILD)/Makefile.subdir
diff --git a/src/select/hash.c b/src/select/hash.c
index 4b11702..4dedec9 100644
--- a/src/select/hash.c
+++ b/src/select/hash.c
@@ -8,6 +8,8 @@
 #include <stdio.h>
 #include <string.h>
 
+#include <libcss/hint.h>
+
 #include "stylesheet.h"
 #include "select/hash.h"
 #include "select/mq.h"
diff --git a/src/select/mq.h b/src/select/mq.h
index 6f98387..080a6ba 100644
--- a/src/select/mq.h
+++ b/src/select/mq.h
@@ -10,96 +10,7 @@
 #define css_select_mq_h_
 
 #include "select/helpers.h"
-
-static inline css_fixed css_len2px(
-		css_fixed length,
-		css_unit unit,
-		const css_media *media)
-{
-	css_fixed px_per_unit;
-
-	switch (unit) {
-	case CSS_UNIT_VI:
-		/* TODO: Assumes writing mode. */
-		unit = CSS_UNIT_VW;
-		break;
-	case CSS_UNIT_VB:
-		/* TODO: Assumes writing mode. */
-		unit = CSS_UNIT_VH;
-		break;
-	case CSS_UNIT_VMIN:
-		unit = (media->height < media->width) ?
-				CSS_UNIT_VH : CSS_UNIT_VW;
-		break;
-	case CSS_UNIT_VMAX:
-		unit = (media->width > media->height) ?
-				CSS_UNIT_VH : CSS_UNIT_VW;
-		break;
-	default:
-		break;
-	}
-
-	switch (unit) {
-	case CSS_UNIT_EM:
-	case CSS_UNIT_EX:
-	case CSS_UNIT_CH:
-	{
-		px_per_unit = FDIV(FMUL(media->client_font_size, F_96), F_72);
-
-		/* TODO: Handling these as fixed ratios of CSS_UNIT_EM. */
-		switch (unit) {
-		case CSS_UNIT_EX:
-			px_per_unit = FMUL(px_per_unit, FLTTOFIX(0.6));
-			break;
-		case CSS_UNIT_CH:
-			px_per_unit = FMUL(px_per_unit, FLTTOFIX(0.4));
-			break;
-		default:
-			break;
-		}
-	}
-		break;
-	case CSS_UNIT_PX:
-		return length;
-	case CSS_UNIT_IN:
-		px_per_unit = F_96;
-		break;
-	case CSS_UNIT_CM:
-		px_per_unit = FDIV(F_96, FLTTOFIX(2.54));
-		break;
-	case CSS_UNIT_MM:
-		px_per_unit = FDIV(F_96, FLTTOFIX(25.4));
-		break;
-	case CSS_UNIT_Q:
-		px_per_unit = FDIV(F_96, FLTTOFIX(101.6));
-		break;
-	case CSS_UNIT_PT:
-		px_per_unit = FDIV(F_96, F_72);
-		break;
-	case CSS_UNIT_PC:
-		px_per_unit = FDIV(F_96, INTTOFIX(6));
-		break;
-	case CSS_UNIT_REM:
-		px_per_unit = FDIV(FMUL(media->client_font_size, F_96), F_72);
-		break;
-	case CSS_UNIT_VH:
-		px_per_unit = FDIV(media->height, F_100);
-		break;
-	case CSS_UNIT_VW:
-		px_per_unit = FDIV(media->width, F_100);
-		break;
-	default:
-		px_per_unit = 0;
-		break;
-	}
-
-	/* Ensure we round px_per_unit to the nearest whole number of pixels:
-	 * the use of FIXTOINT() below will truncate. */
-	px_per_unit += F_0_5;
-
-	/* Calculate total number of pixels */
-	return FMUL(length, TRUNCATEFIX(px_per_unit));
-}
+#include "select/unit.h"
 
 static inline bool mq_match_feature_range_length_op1(
 		css_mq_feature_op op,
@@ -114,9 +25,9 @@ static inline bool mq_match_feature_range_length_op1(
 	}
 
 	if (value->data.dim.unit != UNIT_PX) {
-		v = css_len2px(value->data.dim.len,
-				css__to_css_unit(value->data.dim.unit),
-				media);
+		v = css_unit_len2px_mq(media,
+				value->data.dim.len,
+				css__to_css_unit(value->data.dim.unit));
 	} else {
 		v = value->data.dim.len;
 	}
@@ -149,9 +60,9 @@ static inline bool mq_match_feature_range_length_op2(
 	}
 
 	if (value->data.dim.unit != UNIT_PX) {
-		v = css_len2px(value->data.dim.len,
-				css__to_css_unit(value->data.dim.unit),
-				media);
+		v = css_unit_len2px_mq(media,
+				value->data.dim.len,
+				css__to_css_unit(value->data.dim.unit));
 	} else {
 		v = value->data.dim.len;
 	}
diff --git a/src/select/unit.c b/src/select/unit.c
new file mode 100644
index 0000000..f9ecf83
--- /dev/null
+++ b/src/select/unit.c
@@ -0,0 +1,498 @@
+/*
+ * This file is part of LibCSS
+ * Licensed under the MIT License,
+ *                http://www.opensource.org/licenses/mit-license.php
+ *
+ * Copyright 2021 Michael Drake <tlsa@netsurf-browser.org>
+ */
+
+#include <libcss/stylesheet.h>
+
+#include "utils/utils.h"
+
+#include "propget.h"
+#include "unit.h"
+
+/**
+ * Map viewport-relative length units to either vh or vw.
+ *
+ * Non-viewport-relative units are unchanged.
+ *
+ * \param[in] style            Reference style.
+ * \param[in] viewport_height  Viewport height in px.
+ * \param[in] viewport_width   Viewport width in px.
+ * \param[in] unit             Unit to map.
+ * \return the mapped unit.
+ */
+static inline css_unit css_unit__map_viewport_units(
+		const css_computed_style *style,
+		const css_fixed viewport_height,
+		const css_fixed viewport_width,
+		const css_unit unit)
+{
+	switch (unit) {
+	case CSS_UNIT_VI:
+		return (style != NULL && get_writing_mode(style) !=
+				CSS_WRITING_MODE_HORIZONTAL_TB) ?
+				CSS_UNIT_VH : CSS_UNIT_VW;
+
+	case CSS_UNIT_VB:
+		return (style != NULL && get_writing_mode(style) !=
+				CSS_WRITING_MODE_HORIZONTAL_TB) ?
+				CSS_UNIT_VW : CSS_UNIT_VH;
+
+	case CSS_UNIT_VMIN:
+		return (viewport_height < viewport_width) ?
+				CSS_UNIT_VH : CSS_UNIT_VW;
+
+	case CSS_UNIT_VMAX:
+		return (viewport_height > viewport_width) ?
+				CSS_UNIT_VH : CSS_UNIT_VW;
+
+	default:
+		return unit;
+	}
+}
+
+/**
+ * Convert an absolute length to points (pt).
+ *
+ * \param[in] style            Style to get font-size from, or NULL.
+ * \param[in] viewport_height  Client viewport height.
+ * \param[in] viewport_width   Client viewport width.
+ * \param[in] length           Length to convert.
+ * \param[in] unit             Current unit of length.
+ * \return length in points (pt).
+ */
+static inline css_fixed css_unit__absolute_len2pt(
+		const css_computed_style *style,
+		const css_fixed viewport_height,
+		const css_fixed viewport_width,
+		const css_fixed length,
+		const css_unit unit)
+{
+	/* Length must not be relative */
+	assert(unit != CSS_UNIT_EM &&
+	       unit != CSS_UNIT_EX &&
+	       unit != CSS_UNIT_CH &&
+	       unit != CSS_UNIT_REM);
+
+	switch (css_unit__map_viewport_units(style,
+			viewport_height,
+			viewport_width,
+			unit)) {
+	case CSS_UNIT_PX:
+		return FDIV(FMUL(length, F_72), F_96);
+
+	case CSS_UNIT_IN:
+		return FMUL(length, F_72);
+
+	case CSS_UNIT_CM:
+		return FMUL(length, FDIV(F_72, FLTTOFIX(2.54)));
+
+	case CSS_UNIT_MM:
+		return FMUL(length, FDIV(F_72, FLTTOFIX(25.4)));
+
+	case CSS_UNIT_Q:
+		return FMUL(length, FDIV(F_72, FLTTOFIX(101.6)));
+
+	case CSS_UNIT_PT:
+		return length;
+
+	case CSS_UNIT_PC:
+		return FMUL(length, INTTOFIX(12));
+
+	case CSS_UNIT_VH:
+		return FDIV(FMUL(FDIV(FMUL(length, viewport_height), F_100),
+				F_72), F_96);
+
+	case CSS_UNIT_VW:
+		return FDIV(FMUL(FDIV(FMUL(length, viewport_width), F_100),
+				F_72), F_96);
+
+	default:
+		return 0;
+	}
+}
+
+/* Exported function, documented in unit.h. */
+css_fixed css_unit_font_size_len2pt(
+		const css_unit_len_ctx *ctx,
+		const css_fixed length,
+		const css_unit unit)
+{
+	return css_unit__absolute_len2pt(
+			ctx->ref_style,
+			ctx->viewport_height,
+			ctx->viewport_width,
+			length,
+			unit);
+}
+
+/**
+ * Get font size from a style in CSS pixels.
+ *
+ * The style should have font size in absolute units.
+ *
+ * \param[in] style              Style to get font-size from, or NULL.
+ * \param[in] font_size_default  Client font size for NULL style.
+ * \param[in] font_size_minimum  Client minimum font size clamp.
+ * \param[in] viewport_height    Client viewport height.
+ * \param[in] viewport_width     Client viewport width.
+ * \return font-size in CSS pixels.
+ */
+static inline css_fixed css_unit__font_size_px(
+		const css_computed_style *style,
+		const css_fixed font_size_default,
+		const css_fixed font_size_minimum,
+		const css_fixed viewport_height,
+		const css_fixed viewport_width)
+{
+	css_fixed font_length = 0;
+	css_unit font_unit = CSS_UNIT_PT;
+
+	if (style == NULL) {
+		return font_size_default;
+	}
+
+	get_font_size(style, &font_length, &font_unit);
+
+	if (font_unit != CSS_UNIT_PX) {
+		font_length = css_unit__absolute_len2pt(style,
+				viewport_height,
+				viewport_width,
+				font_length,
+				font_unit);
+
+		/* Convert from pt to CSS pixels.*/
+		font_length = FDIV(FMUL(font_length, F_96), F_72);
+	}
+
+	/* Clamp to configured minimum */
+	if (font_length < font_size_minimum) {
+		font_length = font_size_minimum;
+	}
+
+	return font_length;
+}
+
+/**
+ * Get the number of CSS pixels for a given unit.
+ *
+ * \param[in] measure            Client callback for font measuring.
+ * \param[in] ref_style          Reference style.  (Element or parent, or NULL).
+ * \param[in] root_style         Root element style or NULL.
+ * \param[in] font_size_default  Client default font size in CSS pixels.
+ * \param[in] font_size_minimum  Client minimum font size in CSS pixels.
+ * \param[in] viewport_height    Viewport height in CSS pixels.
+ * \param[in] viewport_width     Viewport width in CSS pixels.
+ * \param[in] unit               The unit to convert from.
+ * \param[in] pw                 Client private word for measure callback.
+ * \return Number of CSS pixels equivalent to the given unit.
+ */
+static inline css_fixed css_unit__px_per_unit(
+		const css_unit_len_measure measure,
+		const css_computed_style *ref_style,
+		const css_computed_style *root_style,
+		const css_fixed font_size_default,
+		const css_fixed font_size_minimum,
+		const css_fixed viewport_height,
+		const css_fixed viewport_width,
+		const css_unit unit,
+		void *pw)
+{
+	switch (css_unit__map_viewport_units(
+			ref_style,
+			viewport_height,
+			viewport_width,
+			unit)) {
+	case CSS_UNIT_EM:
+		return css_unit__font_size_px(
+				ref_style,
+				font_size_default,
+				font_size_minimum,
+				viewport_height,
+				viewport_width);
+
+	case CSS_UNIT_EX:
+		if (measure != NULL) {
+			return measure(pw, ref_style, CSS_UNIT_EX);
+		}
+		return FMUL(css_unit__font_size_px(
+				ref_style,
+				font_size_default,
+				font_size_minimum,
+				viewport_height,
+				viewport_width), FLTTOFIX(0.6));
+
+	case CSS_UNIT_CH:
+		if (measure != NULL) {
+			return measure(pw, ref_style, CSS_UNIT_CH);
+		}
+		return FMUL(css_unit__font_size_px(
+				ref_style,
+				font_size_default,
+				font_size_minimum,
+				viewport_height,
+				viewport_width), FLTTOFIX(0.4));
+
+	case CSS_UNIT_PX:
+		return F_1;
+
+	case CSS_UNIT_IN:
+		return F_96;
+
+	case CSS_UNIT_CM:
+		return FDIV(F_96, FLTTOFIX(2.54));
+
+	case CSS_UNIT_MM:
+		return FDIV(F_96, FLTTOFIX(25.4));
+
+	case CSS_UNIT_Q:
+		return FDIV(F_96, FLTTOFIX(101.6));
+
+	case CSS_UNIT_PT:
+		return FDIV(F_96, F_72);
+
+	case CSS_UNIT_PC:
+		return FDIV(F_96, INTTOFIX(6));
+
+	case CSS_UNIT_REM:
+		return css_unit__font_size_px(
+				root_style,
+				font_size_default,
+				font_size_minimum,
+				viewport_height,
+				viewport_width);
+
+	case CSS_UNIT_VH:
+		return FDIV(viewport_width, F_100);
+
+	case CSS_UNIT_VW:
+		return FDIV(viewport_height, F_100);
+
+	default:
+		return 0;
+	}
+}
+
+/* Exported function, documented in unit.h. */
+css_fixed css_unit_len2px_mq(
+		const css_media *media,
+		const css_fixed length,
+		const css_unit unit)
+{
+	/* In the media query context there is no reference or root element
+	 * style, so these are hardcoded to NULL. */
+	css_fixed px_per_unit = css_unit__px_per_unit(
+			NULL,
+			NULL,
+			NULL,
+			media->client_font_size,
+			INTTOFIX(0),
+			media->height,
+			media->width,
+			unit,
+			NULL);
+
+	/* Ensure we round px_per_unit to the nearest whole number of pixels:
+	 * the use of FIXTOINT() below will truncate. */
+	px_per_unit += F_0_5;
+
+	/* Calculate total number of pixels */
+	return FMUL(length, TRUNCATEFIX(px_per_unit));
+}
+
+/* Exported function, documented in unit.h. */
+css_fixed css_unit_len2css_px(
+		const css_unit_len_ctx *ctx,
+		const css_fixed length,
+		const css_unit unit)
+{
+	css_fixed px_per_unit = css_unit__px_per_unit(
+			ctx->measure,
+			ctx->ref_style,
+			ctx->root_style,
+			ctx->font_size_default,
+			ctx->font_size_minimum,
+			ctx->viewport_height,
+			ctx->viewport_width,
+			unit,
+			ctx->pw);
+
+	/* Ensure we round px_per_unit to the nearest whole number of pixels:
+	 * the use of FIXTOINT() below will truncate. */
+	px_per_unit += F_0_5;
+
+	/* Calculate total number of pixels */
+	return FMUL(length, TRUNCATEFIX(px_per_unit));
+}
+
+/* Exported function, documented in unit.h. */
+css_fixed css_unit_len2device_px(
+		const css_unit_len_ctx *ctx,
+		const css_fixed length,
+		const css_unit unit)
+{
+	css_fixed px_per_unit = css_unit__px_per_unit(
+			ctx->measure,
+			ctx->ref_style,
+			ctx->root_style,
+			ctx->font_size_default,
+			ctx->font_size_minimum,
+			ctx->viewport_height,
+			ctx->viewport_width,
+			unit,
+			ctx->pw);
+
+	px_per_unit = css_unit_css2device_px(px_per_unit, ctx->device_dpi);
+
+	/* Ensure we round px_per_unit to the nearest whole number of pixels:
+	 * the use of FIXTOINT() below will truncate. */
+	px_per_unit += F_0_5;
+
+	/* Calculate total number of pixels */
+	return FMUL(length, TRUNCATEFIX(px_per_unit));
+}
+
+/**
+ * Get font size from a computed style.
+ *
+ * The computed style will have font-size with an absolute unit.
+ * If no computed style is given, the client default font-size will be returned.
+ *
+ * \param[in]  ctx    Length unit conversion context.
+ * \param[in]  style  The style to get the font-size for, or NULL.
+ * \return The font size in absolute units.
+ */
+static inline css_hint_length css_unit__get_font_size(
+		const css_unit_len_ctx *ctx,
+		const css_computed_style *style)
+{
+	css_hint_length size;
+
+	if (style == NULL) {
+		size.value = ctx->font_size_default;
+		size.unit = CSS_UNIT_PX;
+	} else {
+		enum css_font_size_e status = get_font_size(
+				style, &size.value, &size.unit);
+
+		UNUSED(status);
+
+		assert(status == CSS_FONT_SIZE_DIMENSION);
+		assert(size.unit != CSS_UNIT_EM);
+		assert(size.unit != CSS_UNIT_EX);
+		assert(size.unit != CSS_UNIT_PCT);
+	}
+
+	return size;
+}
+
+/* Exported function, documented in unit.h. */
+css_error css_unit_compute_absolute_font_size(
+		const css_unit_len_ctx *ctx,
+		css_hint *size)
+{
+	css_hint_length ref_len;
+
+	assert(size->status != CSS_FONT_SIZE_INHERIT);
+
+	switch (size->status) {
+	case CSS_FONT_SIZE_XX_SMALL: /* Fall-through. */
+	case CSS_FONT_SIZE_X_SMALL:  /* Fall-through. */
+	case CSS_FONT_SIZE_SMALL:    /* Fall-through. */
+	case CSS_FONT_SIZE_MEDIUM:   /* Fall-through. */
+	case CSS_FONT_SIZE_LARGE:    /* Fall-through. */
+	case CSS_FONT_SIZE_X_LARGE:  /* Fall-through. */
+	case CSS_FONT_SIZE_XX_LARGE:
+	{
+		static const css_fixed factors[CSS_FONT_SIZE_XX_LARGE] = {
+			[CSS_FONT_SIZE_XX_SMALL - 1] = FLTTOFIX(0.5625),
+			[CSS_FONT_SIZE_X_SMALL  - 1] = FLTTOFIX(0.6250),
+			[CSS_FONT_SIZE_SMALL    - 1] = FLTTOFIX(0.8125),
+			[CSS_FONT_SIZE_MEDIUM   - 1] = FLTTOFIX(1.0000),
+			[CSS_FONT_SIZE_LARGE    - 1] = FLTTOFIX(1.1250),
+			[CSS_FONT_SIZE_X_LARGE  - 1] = FLTTOFIX(1.5000),
+			[CSS_FONT_SIZE_XX_LARGE - 1] = FLTTOFIX(2.0000),
+		};
+		assert(CSS_FONT_SIZE_INHERIT  == 0);
+		assert(CSS_FONT_SIZE_XX_SMALL == 1);
+
+		size->data.length.value = FMUL(factors[size->status - 1],
+				ctx->font_size_default);
+		size->data.length.unit = CSS_UNIT_PX;
+		size->status = CSS_FONT_SIZE_DIMENSION;
+		break;
+	}
+	case CSS_FONT_SIZE_LARGER:
+		ref_len = css_unit__get_font_size(ctx, ctx->ref_style);
+		size->data.length.value = FMUL(ref_len.value, FLTTOFIX(1.2));
+		size->data.length.unit = ref_len.unit;
+		size->status = CSS_FONT_SIZE_DIMENSION;
+		break;
+
+	case CSS_FONT_SIZE_SMALLER:
+		ref_len = css_unit__get_font_size(ctx, ctx->ref_style);
+		size->data.length.value = FDIV(ref_len.value, FLTTOFIX(1.2));
+		size->data.length.unit = ref_len.unit;
+		size->status = CSS_FONT_SIZE_DIMENSION;
+		break;
+
+	case CSS_FONT_SIZE_DIMENSION:
+		/* Convert any relative units to absolute. */
+		switch (size->data.length.unit) {
+		case CSS_UNIT_PCT:
+			ref_len = css_unit__get_font_size(ctx, ctx->ref_style);
+			size->data.length.value = FDIV(
+					FMUL(size->data.length.value,
+					ref_len.value), INTTOFIX(100));
+			size->data.length.unit = ref_len.unit;
+			break;
+
+		case CSS_UNIT_EM: /* Fall-through */
+		case CSS_UNIT_EX: /* Fall-through */
+		case CSS_UNIT_CH:
+			/* Parent relative units. */
+			ref_len = css_unit__get_font_size(ctx, ctx->ref_style);
+
+			size->data.length.unit = ref_len.unit;
+			size->data.length.value = FMUL(size->data.length.value,
+						ref_len.value);
+
+			switch (size->data.length.unit) {
+			case CSS_UNIT_EX:
+				size->data.length.value = FMUL(
+						size->data.length.value,
+						FLTTOFIX(0.6));
+				break;
+
+			case CSS_UNIT_CH:
+				size->data.length.value = FMUL(
+						size->data.length.value,
+						FLTTOFIX(0.4));
+				break;
+
+			default:
+				break;
+			}
+			break;
+
+		case CSS_UNIT_REM:
+			/* Root element relative units. */
+			ref_len = css_unit__get_font_size(ctx, ctx->root_style);
+
+			size->data.length.unit = ref_len.unit;
+			size->data.length.value = FMUL(size->data.length.value,
+						ref_len.value);
+			break;
+
+		default:
+			break;
+		}
+	default:
+		break;
+	}
+
+	return CSS_OK;
+}
diff --git a/src/select/unit.h b/src/select/unit.h
new file mode 100644
index 0000000..738c181
--- /dev/null
+++ b/src/select/unit.h
@@ -0,0 +1,169 @@
+/*
+ * This file is part of LibCSS
+ * Licensed under the MIT License,
+ *                http://www.opensource.org/licenses/mit-license.php
+ *
+ * Copyright 2021 Michael Drake <tlsa@netsurf-browser.org>
+ */
+
+#ifndef css_select_unit_h_
+#define css_select_unit_h_
+
+/**
+ * Client callback for font measuring.
+ *
+ * \param[in]  pw     Client data.
+ * \param[in]  style  Style to measure font for, or NULL.
+ * \param[in]  unit   Either CSS_UNIT_EX, or CSS_UNIT_CH.
+ * \return length in CSS pixels.
+ */
+typedef css_fixed (*css_unit_len_measure)(
+		void *pw,
+		const css_computed_style *style,
+		const css_unit unit);
+
+/**
+ * LibCSS unit conversion context.
+ *
+ * The client callback is optional.  It is used for measuring "ch"
+ * (glyph '0' advance) and "ex" (height of the letter 'x') units.
+ * If a NULL pointer is given, LibCSS will use a fixed scaling of
+ * the "em" unit.
+ */
+typedef struct css_unit_len_ctx {
+	/**
+	 * Viewport width in CSS pixels.
+	 * Used if unit is vh, vw, vi, vb, vmin, or vmax.
+	 */
+	css_fixed viewport_width;
+	/**
+	 * Viewport height in CSS pixels.
+	 * Used if unit is vh, vw, vi, vb, vmin, or vmax.
+	 */
+	css_fixed viewport_height;
+	/**
+	 * Client default font size in CSS pixels.
+	 */
+	css_fixed font_size_default;
+	/**
+	 * Client minimum font size in CSS pixels.  May be zero.
+	 */
+	css_fixed font_size_minimum;
+	/**
+	 * DPI of the device the style is selected for.
+	 */
+	css_fixed device_dpi;
+	/**
+	 * Reference style.  Most of the time, this is the element's style.
+	 * When converting a length for a typographical property, such as
+	 * font-size, then this should be the parent node.  If the node has
+	 * no parent then this may be NULL.
+	 */
+	const css_computed_style *ref_style;
+	/**
+	 * Computed style for the document root element.
+	 * May be NULL if unit is not rem.
+	 */
+	const css_computed_style *root_style;
+	/**
+	 * Client private word for measure callback.
+	 */
+	void *pw;
+	/**
+	 * Client callback for font measuring.
+	 */
+	const css_unit_len_measure measure;
+} css_unit_len_ctx;
+
+/**
+ * Convert css pixels to physical pixels.
+ *
+ * \param[in] css_pixels  Length in css pixels.
+ * \param[in] device_dpi  Device dots per inch.
+ * \return Length in device pixels.
+ */
+static inline css_fixed css_unit_css2device_px(
+		const css_fixed css_pixels,
+		const css_fixed device_dpi)
+{
+	return FDIV(FMUL(css_pixels, device_dpi), F_96);
+}
+
+/**
+ * Convert device pixels to css pixels.
+ *
+ * \param[in] device_pixels  Length in physical pixels.
+ * \param[in] device_dpi     Device dots per inch.
+ * \return Length in css pixels.
+ */
+static inline css_fixed css_unit_device2css_px(
+		const css_fixed device_pixels,
+		const css_fixed device_dpi)
+{
+	return FDIV(FMUL(device_pixels, F_96), device_dpi);
+}
+
+/**
+ * Convert a length to points (pt).
+ *
+ * \param[in]  ctx     Length unit conversion context.
+ * \param[in]  length  Length to convert.
+ * \param[in]  unit    Current unit of length.
+ * \return A length in points.
+ */
+css_fixed css_unit_font_size_len2pt(
+		const css_unit_len_ctx *ctx,
+		const css_fixed length,
+		const css_unit unit);
+
+/**
+ * Convert a length to CSS pixels.
+ *
+ * \param[in]  ctx     Length unit conversion context.
+ * \param[in]  length  Length to convert.
+ * \param[in]  unit    Current unit of length.
+ * \return A length in CSS pixels.
+ */
+css_fixed css_unit_len2css_px(
+		const css_unit_len_ctx *ctx,
+		const css_fixed length,
+		const css_unit unit);
+
+/**
+ * Convert a length to device pixels.
+ *
+ * \param[in]  ctx     Length unit conversion context.
+ * \param[in]  length  Length to convert.
+ * \param[in]  unit    Current unit of length.
+ * \return A length in device pixels.
+ */
+css_fixed css_unit_len2device_px(
+		const css_unit_len_ctx *ctx,
+		const css_fixed length,
+		const css_unit unit);
+
+/**
+ * Convert a length to CSS pixels for a media query context.
+ *
+ * \param[in]  media   Client media specification.
+ * \param[in]  length  Length to convert.
+ * \param[in]  unit    Current unit of length.
+ * \return A length in CSS pixels.
+ */
+css_fixed css_unit_len2px_mq(
+		const css_media *media,
+		const css_fixed length,
+		const css_unit unit);
+
+/**
+ * Convert relative font size units to absolute units.
+ *
+ * \param[in]     ctx   Length unit conversion context.
+ * \param[in,out] size  The length to convert.
+ * \return CSS_OK on success, or appropriate error otherwise.
+ */
+css_error css_unit_compute_absolute_font_size(
+		const css_unit_len_ctx *ctx,
+		css_hint *size);
+
+#endif