Lazy load fonts in a FontGroup

This is a step towards fixing #17267. To fix that, we need to be able to
try various different fallback fonts in turn, which would become
unweildy with the prior eager-loading strategy.

Prior to this change, FontGroup loaded up all Font instances, including
the fallback font, before any of them were checked for the presence of
the glyphs we're trying to render.

So for the following CSS:

    font-family: Helvetica, Arial;

The FontGroup would contain a Font instance for Helvetica, and a Font
instance for Arial, and a Font instance for the fallback font.

It may be that Helvetica contains glyphs for every character in the
document, and therefore Arial and the fallback font are not needed at
all.

This change makes the strategy lazy, so that we'll only create a Font
for Arial if we cannot find a glyph within Helvetica. I've also
substantially refactored the existing code in the process and added
some documentation along the way.

1 files changed, 37 insertions, 25 deletions

diff --git a/components/layout/text.rs b/components/layout/text.rs
index 6541b27f3ac..1bfb1305ecb 100644
--- a/components/layout/text.rs
+++ b/components/layout/text.rs
@@ -9,7 +9,7 @@
 use app_units::Au;
 use fragment::{Fragment, ScannedTextFlags};
 use fragment::{ScannedTextFragmentInfo, SpecificFragmentInfo, UnscannedTextFragmentInfo};
-use gfx::font::{FontMetrics, RunMetrics, ShapingFlags, ShapingOptions};
+use gfx::font::{FontRef, FontMetrics, RunMetrics, ShapingFlags, ShapingOptions};
 use gfx::font_context::FontContext;
 use gfx::text::glyph::ByteIndex;
 use gfx::text::text_run::TextRun;
@@ -18,6 +18,7 @@ use inline::{InlineFragmentNodeFlags, InlineFragments};
 use linked_list::split_off_head;
 use ordered_float::NotNaN;
 use range::Range;
+use servo_atoms::Atom;
 use std::borrow::ToOwned;
 use std::collections::LinkedList;
 use std::mem;
@@ -28,7 +29,7 @@ use style::computed_values::white_space::T as WhiteSpace;
 use style::computed_values::word_break::T as WordBreak;
 use style::logical_geometry::{LogicalSize, WritingMode};
 use style::properties::ComputedValues;
-use style::properties::style_structs;
+use style::properties::style_structs::Font as FontStyleStruct;
 use style::values::generics::text::LineHeight;
 use unicode_bidi as bidi;
 use unicode_script::{Script, get_script};
@@ -136,7 +137,7 @@ impl TextRunScanner {
     /// for correct painting order. Since we compress several leaf fragments here, the mapping must
     /// be adjusted.
     fn flush_clump_to_list(&mut self,
-                           font_context: &mut FontContext,
+                           mut font_context: &mut FontContext,
                            out_fragments: &mut Vec<Fragment>,
                            paragraph_bytes_processed: &mut usize,
                            bidi_levels: Option<&[bidi::Level]>,
@@ -159,7 +160,7 @@ impl TextRunScanner {
         // Concatenate all of the transformed strings together, saving the new character indices.
         let mut mappings: Vec<RunMapping> = Vec::new();
         let runs = {
-            let fontgroup;
+            let font_group;
             let compression;
             let text_transform;
             let letter_spacing;
@@ -170,7 +171,7 @@ impl TextRunScanner {
                 let in_fragment = self.clump.front().unwrap();
                 let font_style = in_fragment.style().clone_font();
                 let inherited_text_style = in_fragment.style().get_inheritedtext();
-                fontgroup = font_context.layout_font_group_for_style(font_style);
+                font_group = font_context.font_group(font_style);
                 compression = match in_fragment.white_space() {
                     WhiteSpace::Normal |
                     WhiteSpace::Nowrap => CompressionMode::CompressWhitespaceNewline,
@@ -214,14 +215,7 @@ impl TextRunScanner {
 
                 let (mut start_position, mut end_position) = (0, 0);
                 for (byte_index, character) in text.char_indices() {
-                    // Search for the first font in this font group that contains a glyph for this
-                    // character.
-                    let font_index = fontgroup.fonts.iter().position(|font| {
-                        font.borrow().glyph_index(character).is_some()
-                    }).unwrap_or(0);
-
-                    // The following code panics one way or another if this condition isn't met.
-                    assert!(fontgroup.fonts.len() > 0);
+                    let font = font_group.borrow_mut().find_by_codepoint(&mut font_context, character);
 
                     let bidi_level = match bidi_levels {
                         Some(levels) => levels[*paragraph_bytes_processed],
@@ -245,7 +239,7 @@ impl TextRunScanner {
                     };
 
                     // Now, if necessary, flush the mapping we were building up.
-                    let flush_run = run_info.font_index != font_index ||
+                    let flush_run = !run_info.has_font(&font) ||
                                     run_info.bidi_level != bidi_level ||
                                     !compatible_script;
                     let new_mapping_needed = flush_run || mapping.selected != selected;
@@ -272,7 +266,7 @@ impl TextRunScanner {
                             mapping = RunMapping::new(&run_info_list[..],
                                                       fragment_index);
                         }
-                        run_info.font_index = font_index;
+                        run_info.font = font;
                         run_info.bidi_level = bidi_level;
                         run_info.script = script;
                         mapping.selected = selected;
@@ -328,9 +322,14 @@ impl TextRunScanner {
                 if run_info.bidi_level.is_rtl() {
                     options.flags.insert(ShapingFlags::RTL_FLAG);
                 }
-                let mut font = fontgroup.fonts.get(run_info.font_index).unwrap().borrow_mut();
 
-                let (run, break_at_zero) = TextRun::new(&mut *font,
+                // If no font is found (including fallbacks), there's no way we can render.
+                let font =
+                    run_info.font
+                        .or_else(|| font_group.borrow_mut().first(&mut font_context))
+                        .expect("No font found for text run!");
+
+                let (run, break_at_zero) = TextRun::new(&mut *font.borrow_mut(),
                                                         run_info.text,
                                                         &options,
                                                         run_info.bidi_level,
@@ -456,15 +455,20 @@ fn bounding_box_for_run_metrics(metrics: &RunMetrics, writing_mode: WritingMode)
         metrics.bounding_box.size.height)
 }
 
-/// Returns the metrics of the font represented by the given `style_structs::Font`, respectively.
+/// Returns the metrics of the font represented by the given `FontStyleStruct`.
 ///
 /// `#[inline]` because often the caller only needs a few fields from the font metrics.
+///
+/// # Panics
+///
+/// Panics if no font can be found for the given font style.
 #[inline]
-pub fn font_metrics_for_style(font_context: &mut FontContext, font_style: ::ServoArc<style_structs::Font>)
+pub fn font_metrics_for_style(mut font_context: &mut FontContext, style: ::ServoArc<FontStyleStruct>)
                               -> FontMetrics {
-    let fontgroup = font_context.layout_font_group_for_style(font_style);
-    // FIXME(https://github.com/rust-lang/rust/issues/23338)
-    let font = fontgroup.fonts[0].borrow();
+    let font_group = font_context.font_group(style);
+    let font = font_group.borrow_mut().first(&mut font_context);
+    let font = font.as_ref().unwrap().borrow();
+
     font.metrics.clone()
 }
 
@@ -546,8 +550,8 @@ struct RunInfo {
     text: String,
     /// The insertion point in this text run, if applicable.
     insertion_point: Option<ByteIndex>,
-    /// The index of the applicable font in the font group.
-    font_index: usize,
+    /// The font that the text should be rendered with.
+    font: Option<FontRef>,
     /// The bidirection embedding level of this text run.
     bidi_level: bidi::Level,
     /// The Unicode script property of this text run.
@@ -559,7 +563,7 @@ impl RunInfo {
         RunInfo {
             text: String::new(),
             insertion_point: None,
-            font_index: 0,
+            font: None,
             bidi_level: bidi::Level::ltr(),
             script: Script::Common,
         }
@@ -584,6 +588,14 @@ impl RunInfo {
         }
         list.push(self);
     }
+
+    fn has_font(&self, font: &Option<FontRef>) -> bool {
+        fn identifier(font: &Option<FontRef>) -> Option<Atom> {
+            font.as_ref().map(|f| f.borrow().identifier())
+        }
+
+        identifier(&self.font) == identifier(font)
+    }
 }
 
 /// A mapping from a portion of an unscanned text fragment to the text run we're going to create