layout: Implement word-spacing per CSS 2.1 § 16.4.

This assumes that there are no ligatures that span across multiple
words. Since we have a per-word shape cache, this is a safe assumption
as of now. I have left comments to ensure that, if and when this is
revisted, we make sure to handle it properly.

components/gfx/font.rs

@@ -115,6 +115,8 @@ pub struct ShapingOptions {
     /// Spacing to add between each letter. Corresponds to the CSS 2.1 `letter-spacing` property.
     /// NB: You will probably want to set the `IGNORE_LIGATURES_SHAPING_FLAG` if this is non-null.
     pub letter_spacing: Option<Au>,
+    /// Spacing to add between each word. Corresponds to the CSS 2.1 `word-spacing` property.
+    pub word_spacing: Au,
     /// Various flags.
     pub flags: ShapingFlags,
 }

components/gfx/text/shaping/harfbuzz.rs

@@ -42,8 +42,8 @@ use harfbuzz::{hb_shape, hb_buffer_get_glyph_infos};
 use libc::{c_uint, c_int, c_void, c_char};
 use servo_util::geometry::Au;
 use servo_util::range::Range;
-use std::mem;
 use std::char;
+use std::mem;
 use std::cmp;
 use std::ptr;
 
@@ -436,8 +436,12 @@ impl Shaper {
                 // for now, just pretend that every character is a cluster start.
                 // (i.e., pretend there are no combining character sequences).
                 // 1-to-1 mapping of character to glyph also treated as ligature start.
+                //
+                // NB: When we acquire the ability to handle ligatures that cross word boundaries,
+                // we'll need to do something special to handle `word-spacing` properly.
                 let shape = glyph_data.get_entry_for_glyph(glyph_span.begin(), &mut y_pos);
-                let advance = self.advance_for_shaped_glyph(shape.advance, options);
+                let character = text.char_at(char_byte_span.begin() as uint);
+                let advance = self.advance_for_shaped_glyph(shape.advance, character, options);
                 let data = GlyphData::new(shape.codepoint,
                                           advance,
                                           shape.offset,
@@ -488,11 +492,22 @@ impl Shaper {
         glyphs.finalize_changes();
     }
 
-    fn advance_for_shaped_glyph(&self, advance: Au, options: &ShapingOptions) -> Au {
+    fn advance_for_shaped_glyph(&self, mut advance: Au, character: char, options: &ShapingOptions)
+                                -> Au {
         match options.letter_spacing {
-            None => advance,
-            Some(spacing) => advance + spacing,
+            None => {}
+            Some(letter_spacing) => advance = advance + letter_spacing,
+        };
+
+        // CSS 2.1 § 16.4 states that "word spacing affects each space (U+0020) and non-breaking
+        // space (U+00A0) left in the text after the white space processing rules have been
+        // applied. The effect of the property on other word-separator characters is undefined."
+        // We elect to only space the two required code points.
+        if character == ' ' || character == '\u00a0' {
+            advance = advance + options.word_spacing
         }
+
+        advance
     }
 }
 

components/layout/text.rs

@@ -107,16 +107,19 @@ impl TextRunScanner {
             let compression;
             let text_transform;
             let letter_spacing;
+            let word_spacing;
             {
                 let in_fragment = self.clump.front().unwrap();
                 let font_style = in_fragment.style().get_font_arc();
+                let inherited_text_style = in_fragment.style().get_inheritedtext();
                 fontgroup = font_context.get_layout_font_group_for_style(font_style);
                 compression = match in_fragment.white_space() {
                     white_space::normal | white_space::nowrap => CompressWhitespaceNewline,
                     white_space::pre => CompressNone,
                 };
-                text_transform = in_fragment.style().get_inheritedtext().text_transform;
-                letter_spacing = in_fragment.style().get_inheritedtext().letter_spacing;
+                text_transform = inherited_text_style.text_transform;
+                letter_spacing = inherited_text_style.letter_spacing;
+                word_spacing = inherited_text_style.word_spacing.unwrap_or(Au(0));
             }
 
             // First, transform/compress text of all the nodes.
@@ -160,6 +163,7 @@ impl TextRunScanner {
             // `fi n a l l y`.
             let options = ShapingOptions {
                 letter_spacing: letter_spacing,
+                word_spacing: word_spacing,
                 flags: match letter_spacing {
                     Some(Au(0)) | None => ShapingFlags::empty(),
                     Some(_) => IGNORE_LIGATURES_SHAPING_FLAG,

components/style/properties/mod.rs.mako

@@ -1111,6 +1111,29 @@ pub mod longhands {
         }
     </%self:single_component_value>
 
+    <%self:single_component_value name="word-spacing">
+        pub type SpecifiedValue = Option<specified::Length>;
+        pub mod computed_value {
+            use super::super::Au;
+            pub type T = Option<Au>;
+        }
+        #[inline]
+        pub fn get_initial_value() -> computed_value::T {
+            None
+        }
+        #[inline]
+        pub fn to_computed_value(value: SpecifiedValue, context: &computed::Context)
+                                 -> computed_value::T {
+            value.map(|length| computed::compute_Au(length, context))
+        }
+        pub fn from_component_value(input: &ComponentValue, _: &Url) -> Result<SpecifiedValue,()> {
+            match input {
+                &Ident(ref value) if value.eq_ignore_ascii_case("normal") => Ok(None),
+                _ => specified::Length::parse_non_negative(input).map(|length| Some(length)),
+            }
+        }
+    </%self:single_component_value>
+
     ${predefined_type("text-indent", "LengthOrPercentage", "computed::LP_Length(Au(0))")}
 
     ${new_style_struct("Text", is_inherited=False)}

tests/ref/basic.list

@@ -198,3 +198,4 @@ fragment=top != ../html/acid2.html acid2_ref.html
 != border_black_ridge.html border_black_solid.html
 != border_black_ridge.html border_black_groove.html
 == text_indent_a.html text_indent_ref.html
+== word_spacing_a.html word_spacing_ref.html

tests/ref/word_spacing_a.html

@@ -0,0 +1,16 @@
+<html>
+<head>
+<!-- Tests that `word-spacing` works. -->
+<link rel="stylesheet" type="text/css" href="css/ahem.css">
+<style>
+* {
+    word-spacing: 100px;
+    color: chartreuse;
+}
+body {
+    margin: 0;
+}
+</style>
+<body>X XX</body>
+</head>
+

tests/ref/word_spacing_ref.html

@@ -0,0 +1,26 @@
+<html>
+<head>
+<!-- Tests that `word-spacing` works. -->
+<style>
+section, nav, main {
+    background-color: chartreuse;
+    width: 100px;
+    height: 100px;
+    position: absolute;
+    top: 0;
+}
+section {
+    left: 0;
+}
+nav {
+    left: 300px;
+}
+main {
+    left: 400px;
+}
+</style>
+<body><section></section><nav></nav><main></main></body>
+</head>
+
+
+