Add a fast path for shaping ASCII text

1 files changed, 51 insertions, 1 deletions

diff --git a/components/gfx/text/shaping/harfbuzz.rs b/components/gfx/text/shaping/harfbuzz.rs
index a1457e4ffd0..79d36f9b6e0 100644
--- a/components/gfx/text/shaping/harfbuzz.rs
+++ b/components/gfx/text/shaping/harfbuzz.rs
@@ -4,7 +4,7 @@
 
 use app_units::Au;
 use euclid::Point2D;
-use font::{DISABLE_KERNING_SHAPING_FLAG, Font, FontTableMethods, FontTableTag};
+use font::{DISABLE_KERNING_SHAPING_FLAG, Font, FontHandleMethods, FontTableMethods, FontTableTag};
 use font::{IGNORE_LIGATURES_SHAPING_FLAG, KERN, RTL_FLAG, ShapingOptions};
 use harfbuzz::{HB_DIRECTION_LTR, HB_DIRECTION_RTL, HB_MEMORY_MODE_READONLY};
 use harfbuzz::{hb_blob_create, hb_face_create_for_tables};
@@ -34,10 +34,12 @@ use harfbuzz::{hb_glyph_position_t};
 use harfbuzz::{hb_position_t, hb_tag_t};
 use libc::{c_char, c_int, c_uint, c_void};
 use platform::font::FontTable;
+use std::ascii::AsciiExt;
 use std::{char, cmp, ptr};
 use text::glyph::{ByteIndex, GlyphData, GlyphId, GlyphStore};
 use text::shaping::ShaperMethods;
 use text::util::{fixed_to_float, float_to_fixed, is_bidi_control};
+use unicode_script::Script;
 
 const NO_GLYPH: i32 = -1;
 const LIGA: u32 = ot_tag!('l', 'i', 'g', 'a');
@@ -199,7 +201,14 @@ impl ShaperMethods for Shaper {
     /// Calculate the layout metrics associated with the given text when painted in a specific
     /// font.
     fn shape_text(&self, text: &str, options: &ShapingOptions, glyphs: &mut GlyphStore) {
+        if self.can_use_fast_path(text, options) {
+            debug!("shape_text: Using ASCII fast path.");
+            self.shape_text_fast(text, options, glyphs);
+            return;
+        }
         unsafe {
+            debug!("shape_text: Using Harfbuzz.");
+
             let hb_buffer: *mut hb_buffer_t = hb_buffer_create();
             hb_buffer_set_direction(hb_buffer, if options.flags.contains(RTL_FLAG) {
                 HB_DIRECTION_RTL
@@ -241,6 +250,47 @@ impl ShaperMethods for Shaper {
 }
 
 impl Shaper {
+    fn can_use_fast_path(&self, text: &str, options: &ShapingOptions) -> bool {
+        let font = self.font_and_shaping_options.font;
+
+        options.script == Script::Latin &&
+            !options.flags.contains(RTL_FLAG) &&
+            unsafe { (*font).handle.can_do_fast_shaping() } &&
+            text.is_ascii()
+    }
+
+    /// Fast path for ASCII text that only needs simple horizontal LTR kerning.
+    fn shape_text_fast(&self, text: &str, options: &ShapingOptions, glyphs: &mut GlyphStore) {
+        let font = unsafe { &mut *self.font_and_shaping_options.font };
+
+        let mut prev_glyph_id = None;
+        for (i, byte) in text.bytes().enumerate() {
+            let character = byte as char;
+            let glyph_id = match font.glyph_index(character) {
+                Some(id) => id,
+                None => continue,
+            };
+
+            let mut advance = Au::from_f64_px(font.glyph_h_advance(glyph_id));
+            if character == ' ' {
+                advance += options.word_spacing;
+            }
+            if let Some(letter_spacing) = options.letter_spacing {
+                advance += letter_spacing;
+            }
+            let offset = prev_glyph_id.map(|prev| {
+                let h_kerning = Au::from_f64_px(font.glyph_h_kerning(prev, glyph_id));
+                advance += h_kerning;
+                Point2D::new(h_kerning, Au(0))
+            });
+
+            let glyph = GlyphData::new(glyph_id, advance, offset, true, true);
+            glyphs.add_glyph_for_byte_index(ByteIndex(i as isize), character, &glyph);
+            prev_glyph_id = Some(glyph_id);
+        }
+        glyphs.finalize_changes();
+    }
+
     fn save_glyph_results(&self,
                           text: &str,
                           options: &ShapingOptions,