Add Windows font fallback

4 files changed, 253 insertions, 101 deletions

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 5e39a660..8011be88 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -16,6 +16,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Vi mode for copying text and opening links
 - `CopySelection` action which copies into selection buffer on Linux/BSD
 - Option `cursor.thickness` to set terminal cursor thickness
+- Font fallback on Windows
 
 ### Changed
 
diff --git a/Cargo.lock b/Cargo.lock
index bb9bb521..b91f66bd 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -465,7 +465,7 @@ source = "registry+https://github.com/rust-lang/crates.io-index"
 
 [[package]]
 name = "dwrote"
-version = "0.9.0"
+version = "0.11.0"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 dependencies = [
  "lazy_static 1.4.0 (registry+https://github.com/rust-lang/crates.io-index)",
@@ -473,6 +473,7 @@ dependencies = [
  "serde 1.0.104 (registry+https://github.com/rust-lang/crates.io-index)",
  "serde_derive 1.0.104 (registry+https://github.com/rust-lang/crates.io-index)",
  "winapi 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
+ "wio 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -548,13 +549,14 @@ dependencies = [
  "core-foundation-sys 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)",
  "core-graphics 0.17.3 (registry+https://github.com/rust-lang/crates.io-index)",
  "core-text 13.3.2 (registry+https://github.com/rust-lang/crates.io-index)",
- "dwrote 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "dwrote 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "euclid 0.20.7 (registry+https://github.com/rust-lang/crates.io-index)",
  "foreign-types 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "freetype-rs 0.24.0 (registry+https://github.com/rust-lang/crates.io-index)",
  "libc 0.2.67 (registry+https://github.com/rust-lang/crates.io-index)",
  "log 0.4.8 (registry+https://github.com/rust-lang/crates.io-index)",
  "servo-fontconfig 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)",
+ "winapi 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
 ]
 
 [[package]]
@@ -2124,6 +2126,14 @@ dependencies = [
 ]
 
 [[package]]
+name = "wio"
+version = "0.2.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+dependencies = [
+ "winapi 0.3.8 (registry+https://github.com/rust-lang/crates.io-index)",
+]
+
+[[package]]
 name = "ws2_32-sys"
 version = "0.2.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
@@ -2239,7 +2249,7 @@ dependencies = [
 "checksum dlib 0.4.1 (registry+https://github.com/rust-lang/crates.io-index)" = "77e51249a9d823a4cb79e3eca6dcd756153e8ed0157b6c04775d04bf1b13b76a"
 "checksum downcast-rs 1.1.1 (registry+https://github.com/rust-lang/crates.io-index)" = "52ba6eb47c2131e784a38b726eb54c1e1484904f013e576a25354d0124161af6"
 "checksum dtoa 0.4.5 (registry+https://github.com/rust-lang/crates.io-index)" = "4358a9e11b9a09cf52383b451b49a169e8d797b68aa02301ff586d70d9661ea3"
-"checksum dwrote 0.9.0 (registry+https://github.com/rust-lang/crates.io-index)" = "0bd1369e02db5e9b842a9b67bce8a2fcc043beafb2ae8a799dd482d46ea1ff0d"
+"checksum dwrote 0.11.0 (registry+https://github.com/rust-lang/crates.io-index)" = "439a1c2ba5611ad3ed731280541d36d2e9c4ac5e7fb818a27b604bdc5a6aa65b"
 "checksum embed-resource 1.3.1 (registry+https://github.com/rust-lang/crates.io-index)" = "bbaba4684ab0af1cbb3ef0b1f540ddc4b57b31940c920ea594efe09ab86e2a6c"
 "checksum env_logger 0.7.1 (registry+https://github.com/rust-lang/crates.io-index)" = "44533bbbb3bb3c1fa17d9f2e4e38bbbaf8396ba82193c4cb1b6445d711445d36"
 "checksum euclid 0.20.7 (registry+https://github.com/rust-lang/crates.io-index)" = "3f852d320142e1cceb15dccef32ed72a9970b83109d8a4e24b1ab04d579f485d"
@@ -2431,6 +2441,7 @@ dependencies = [
 "checksum winpty 0.2.0 (registry+https://github.com/rust-lang/crates.io-index)" = "92c5a39bb2408a307dd5ab774039ec3f2c68d052970ae4dacc346101abe202b2"
 "checksum winpty-sys 0.5.0 (registry+https://github.com/rust-lang/crates.io-index)" = "ed8a179a59760dc51d30b5e6eaf1bd6da88461f72f2616e962ddebef7e413210"
 "checksum winreg 0.6.2 (registry+https://github.com/rust-lang/crates.io-index)" = "b2986deb581c4fe11b621998a5e53361efe6b48a151178d0cd9eeffa4dc6acc9"
+"checksum wio 0.2.2 (registry+https://github.com/rust-lang/crates.io-index)" = "5d129932f4644ac2396cb456385cbf9e63b5b30c6e8dc4820bdca4eb082037a5"
 "checksum ws2_32-sys 0.2.1 (registry+https://github.com/rust-lang/crates.io-index)" = "d59cefebd0c892fa2dd6de581e937301d8552cb44489cdff035c6187cb63fa5e"
 "checksum x11-clipboard 0.5.1 (registry+https://github.com/rust-lang/crates.io-index)" = "e5e937afd03b64b7be4f959cc044e09260a47241b71e56933f37db097bf7859d"
 "checksum x11-dl 2.18.5 (registry+https://github.com/rust-lang/crates.io-index)" = "2bf981e3a5b3301209754218f962052d4d9ee97e478f4d26d4a6eced34c1fef8"
diff --git a/font/Cargo.toml b/font/Cargo.toml
index f647c275..866b14cf 100644
--- a/font/Cargo.toml
+++ b/font/Cargo.toml
@@ -23,4 +23,5 @@ core-graphics = "0.17"
 core-foundation-sys = "0.6"
 
 [target.'cfg(windows)'.dependencies]
-dwrote = { version = "0.9.0" }
+dwrote = { version = "0.11" }
+winapi = { version = "0.3", features = ["impl-default"] }
diff --git a/font/src/directwrite/mod.rs b/font/src/directwrite/mod.rs
index 5f3bdfc1..87130a37 100644
--- a/font/src/directwrite/mod.rs
+++ b/font/src/directwrite/mod.rs
@@ -13,30 +13,164 @@
 // limitations under the License.
 //
 //! Rasterization powered by DirectWrite
-use dwrote::{FontCollection, FontStretch, FontStyle, FontWeight, GlyphOffset, GlyphRunAnalysis};
+use std::borrow::Cow;
+use std::collections::HashMap;
+use std::ffi::OsString;
+use std::fmt::{self, Display, Formatter};
+use std::os::windows::ffi::OsStringExt;
+
+use dwrote::{
+    FontCollection, FontFace, FontFallback, FontStretch, FontStyle, FontWeight, GlyphOffset,
+    GlyphRunAnalysis, TextAnalysisSource, TextAnalysisSourceMethods, DWRITE_GLYPH_RUN,
+};
+
+use winapi::shared::ntdef::{HRESULT, LOCALE_NAME_MAX_LENGTH};
+use winapi::um::dwrite;
+use winapi::um::winnls::GetUserDefaultLocaleName;
 
 use super::{
     BitmapBuffer, FontDesc, FontKey, GlyphKey, Metrics, RasterizedGlyph, Size, Slant, Style, Weight,
 };
 
+/// Cached DirectWrite font.
+struct Font {
+    face: FontFace,
+    family_name: String,
+    weight: FontWeight,
+    style: FontStyle,
+    stretch: FontStretch,
+}
+
 pub struct DirectWriteRasterizer {
-    fonts: Vec<dwrote::FontFace>,
+    fonts: HashMap<FontKey, Font>,
+    keys: HashMap<FontDesc, FontKey>,
     device_pixel_ratio: f32,
+    available_fonts: FontCollection,
+    fallback_sequence: Option<FontFallback>,
+}
+
+impl DirectWriteRasterizer {
+    fn rasterize_glyph(
+        &self,
+        face: &FontFace,
+        size: Size,
+        c: char,
+    ) -> Result<RasterizedGlyph, Error> {
+        let glyph_index = self.get_glyph_index(face, c)?;
+
+        let em_size = em_size(size);
+
+        let glyph_run = DWRITE_GLYPH_RUN {
+            fontFace: unsafe { face.as_ptr() },
+            fontEmSize: em_size,
+            glyphCount: 1,
+            glyphIndices: &glyph_index,
+            glyphAdvances: &0.0,
+            glyphOffsets: &GlyphOffset::default(),
+            isSideways: 0,
+            bidiLevel: 0,
+        };
+
+        let rendering_mode = face.get_recommended_rendering_mode_default_params(
+            em_size,
+            self.device_pixel_ratio,
+            dwrote::DWRITE_MEASURING_MODE_NATURAL,
+        );
+
+        let glyph_analysis = GlyphRunAnalysis::create(
+            &glyph_run,
+            self.device_pixel_ratio,
+            None,
+            rendering_mode,
+            dwrote::DWRITE_MEASURING_MODE_NATURAL,
+            0.0,
+            0.0,
+        )
+        .map_err(Error::DirectWriteError)?;
+
+        let bounds = glyph_analysis
+            .get_alpha_texture_bounds(dwrote::DWRITE_TEXTURE_CLEARTYPE_3x1)
+            .map_err(Error::DirectWriteError)?;
+
+        let buf = glyph_analysis
+            .create_alpha_texture(dwrote::DWRITE_TEXTURE_CLEARTYPE_3x1, bounds)
+            .map_err(Error::DirectWriteError)?;
+
+        Ok(RasterizedGlyph {
+            c,
+            width: (bounds.right - bounds.left) as i32,
+            height: (bounds.bottom - bounds.top) as i32,
+            top: -bounds.top,
+            left: bounds.left,
+            buf: BitmapBuffer::RGB(buf),
+        })
+    }
+
+    fn get_loaded_font(&self, font_key: FontKey) -> Result<&Font, Error> {
+        self.fonts.get(&font_key).ok_or(Error::FontNotLoaded)
+    }
+
+    fn get_glyph_index(&self, face: &FontFace, c: char) -> Result<u16, Error> {
+        let idx = *face
+            .get_glyph_indices(&[c as u32])
+            .first()
+            // DirectWrite returns 0 if the glyph does not exist in the font
+            .filter(|glyph_index| **glyph_index != 0)
+            .ok_or_else(|| Error::MissingGlyph(c))?;
+
+        Ok(idx)
+    }
+
+    fn get_fallback_font(&self, loaded_font: &Font, c: char) -> Option<dwrote::Font> {
+        let fallback = self.fallback_sequence.as_ref()?;
+
+        let mut buf = [0u16; 2];
+        c.encode_utf16(&mut buf);
+
+        let length = c.len_utf16() as u32;
+        let utf16_codepoints = &buf[..length as usize];
+
+        let locale = get_current_locale();
+
+        let text_analysis_source_data = TextAnalysisSourceData { locale: &locale, length };
+        let text_analysis_source = TextAnalysisSource::from_text(
+            Box::new(text_analysis_source_data),
+            Cow::Borrowed(utf16_codepoints),
+        );
+
+        let fallback_result = fallback.map_characters(
+            &text_analysis_source,
+            0,
+            length,
+            &self.available_fonts,
+            Some(&loaded_font.family_name),
+            loaded_font.weight,
+            loaded_font.style,
+            loaded_font.stretch,
+        );
+
+        fallback_result.mapped_font
+    }
 }
 
 impl crate::Rasterize for DirectWriteRasterizer {
     type Err = Error;
 
     fn new(device_pixel_ratio: f32, _: bool) -> Result<DirectWriteRasterizer, Error> {
-        Ok(DirectWriteRasterizer { fonts: Vec::new(), device_pixel_ratio })
+        Ok(DirectWriteRasterizer {
+            fonts: HashMap::new(),
+            keys: HashMap::new(),
+            device_pixel_ratio,
+            available_fonts: FontCollection::system(),
+            fallback_sequence: FontFallback::get_system_fallback(),
+        })
     }
 
     fn metrics(&self, key: FontKey, size: Size) -> Result<Metrics, Error> {
-        let font = self.fonts.get(key.token as usize).ok_or(Error::FontNotLoaded)?;
+        let face = &self.get_loaded_font(key)?.face;
+        let vmetrics = face.metrics().metrics0();
 
-        let vmetrics = font.metrics();
-        let scale = (size.as_f32_pts() * self.device_pixel_ratio * (96.0 / 72.0))
-            / f32::from(vmetrics.designUnitsPerEm);
+        let scale = em_size(size) * self.device_pixel_ratio / f32::from(vmetrics.designUnitsPerEm);
 
         let underline_position = f32::from(vmetrics.underlinePosition) * scale;
         let underline_thickness = f32::from(vmetrics.underlineThickness) * scale;
@@ -50,10 +184,12 @@ impl crate::Rasterize for DirectWriteRasterizer {
 
         let line_height = f64::from(ascent - descent + line_gap);
 
-        // We assume that all monospace characters have the same width
-        // Because of this we take '!', the first drawable character, for measurements
-        let glyph_metrics = font.get_design_glyph_metrics(&[33], false);
-        let hmetrics = glyph_metrics.first().ok_or(Error::MissingGlyph('!'))?;
+        // Since all monospace characters have the same width, we use `!` for horizontal metrics
+        let c = '!';
+        let glyph_index = self.get_glyph_index(face, c)?;
+
+        let glyph_metrics = face.get_design_glyph_metrics(&[glyph_index], false);
+        let hmetrics = glyph_metrics.first().ok_or_else(|| Error::MissingGlyph(c))?;
 
         let average_advance = f64::from(hmetrics.advanceWidth) * f64::from(scale);
 
@@ -69,27 +205,22 @@ impl crate::Rasterize for DirectWriteRasterizer {
     }
 
     fn load_font(&mut self, desc: &FontDesc, _size: Size) -> Result<FontKey, Error> {
-        let system_fc = FontCollection::system();
+        // Fast path if face is already loaded
+        if let Some(key) = self.keys.get(desc) {
+            return Ok(*key);
+        }
 
-        let family = system_fc
+        let family = self
+            .available_fonts
             .get_font_family_by_name(&desc.name)
             .ok_or_else(|| Error::MissingFont(desc.clone()))?;
 
         let font = match desc.style {
             Style::Description { weight, slant } => {
-                let weight =
-                    if weight == Weight::Bold { FontWeight::Bold } else { FontWeight::Regular };
-
-                let style = match slant {
-                    Slant::Normal => FontStyle::Normal,
-                    Slant::Oblique => FontStyle::Oblique,
-                    Slant::Italic => FontStyle::Italic,
-                };
-
                 // This searches for the "best" font - should mean we don't have to worry about
                 // fallbacks if our exact desired weight/style isn't available
-                Ok(family.get_first_matching_font(weight, FontStretch::Normal, style))
-            },
+                Ok(family.get_first_matching_font(weight.into(), FontStretch::Normal, slant.into()))
+            }
             Style::Specific(ref style) => {
                 let mut idx = 0;
                 let count = family.get_font_count();
@@ -107,73 +238,26 @@ impl crate::Rasterize for DirectWriteRasterizer {
 
                     idx += 1;
                 }
-            },
+            }
         }?;
 
-        let face = font.create_font_face();
-        self.fonts.push(face);
+        let key = FontKey::next();
+        self.keys.insert(desc.clone(), key);
+        self.fonts.insert(key, font.into());
 
-        Ok(FontKey { token: (self.fonts.len() - 1) as u32 })
+        Ok(key)
     }
 
     fn get_glyph(&mut self, glyph: GlyphKey) -> Result<RasterizedGlyph, Error> {
-        let font = self.fonts.get(glyph.font_key.token as usize).ok_or(Error::FontNotLoaded)?;
-
-        let offset = GlyphOffset { advanceOffset: 0.0, ascenderOffset: 0.0 };
+        let loaded_font = self.get_loaded_font(glyph.font_key)?;
 
-        let glyph_index = *font
-            .get_glyph_indices(&[glyph.c as u32])
-            .first()
-            .ok_or_else(|| Error::MissingGlyph(glyph.c))?;
-        if glyph_index == 0 {
-            // The DirectWrite documentation states that we should get 0 returned if the glyph
-            // does not exist in the font
-            return Err(Error::MissingGlyph(glyph.c));
+        match self.rasterize_glyph(&loaded_font.face, glyph.size, glyph.c) {
+            Err(err @ Error::MissingGlyph(_)) => {
+                let fallback_font = self.get_fallback_font(&loaded_font, glyph.c).ok_or(err)?;
+                self.rasterize_glyph(&fallback_font.create_font_face(), glyph.size, glyph.c)
+            },
+            result => result,
         }
-
-        let glyph_run = dwrote::DWRITE_GLYPH_RUN {
-            fontFace: unsafe { font.as_ptr() },
-            fontEmSize: glyph.size.as_f32_pts(),
-            glyphCount: 1,
-            glyphIndices: &(glyph_index),
-            glyphAdvances: &(0.0),
-            glyphOffsets: &(offset),
-            isSideways: 0,
-            bidiLevel: 0,
-        };
-
-        let rendering_mode = font.get_recommended_rendering_mode_default_params(
-            glyph.size.as_f32_pts(),
-            self.device_pixel_ratio * (96.0 / 72.0),
-            dwrote::DWRITE_MEASURING_MODE_NATURAL,
-        );
-
-        let glyph_analysis = GlyphRunAnalysis::create(
-            &glyph_run,
-            self.device_pixel_ratio * (96.0 / 72.0),
-            None,
-            rendering_mode,
-            dwrote::DWRITE_MEASURING_MODE_NATURAL,
-            0.0,
-            0.0,
-        )
-        .or_else(|_| Err(Error::MissingGlyph(glyph.c)))?;
-
-        let bounds = glyph_analysis
-            .get_alpha_texture_bounds(dwrote::DWRITE_TEXTURE_CLEARTYPE_3x1)
-            .or_else(|_| Err(Error::MissingGlyph(glyph.c)))?;
-        let buf = glyph_analysis
-            .create_alpha_texture(dwrote::DWRITE_TEXTURE_CLEARTYPE_3x1, bounds)
-            .or_else(|_| Err(Error::MissingGlyph(glyph.c)))?;
-
-        Ok(RasterizedGlyph {
-            c: glyph.c,
-            width: (bounds.right - bounds.left) as i32,
-            height: (bounds.bottom - bounds.top) as i32,
-            top: -bounds.top,
-            left: bounds.left,
-            buf: BitmapBuffer::RGB(buf),
-        })
     }
 
     fn update_dpr(&mut self, device_pixel_ratio: f32) {
@@ -186,29 +270,84 @@ pub enum Error {
     MissingFont(FontDesc),
     MissingGlyph(char),
     FontNotLoaded,
+    DirectWriteError(HRESULT),
 }
 
-impl ::std::error::Error for Error {
-    fn description(&self) -> &str {
-        match *self {
-            Error::MissingFont(ref _desc) => "Couldn't find the requested font",
-            Error::MissingGlyph(ref _c) => "Couldn't find the requested glyph",
-            Error::FontNotLoaded => "Tried to operate on font that hasn't been loaded",
-        }
-    }
-}
+impl std::error::Error for Error {}
 
-impl ::std::fmt::Display for Error {
-    fn fmt(&self, f: &mut ::std::fmt::Formatter) -> ::std::fmt::Result {
-        match *self {
-            Error::MissingGlyph(ref c) => write!(f, "Glyph not found for char {:?}", c),
-            Error::MissingFont(ref desc) => write!(
+impl Display for Error {
+    fn fmt(&self, f: &mut Formatter) -> fmt::Result {
+        match self {
+            Error::MissingGlyph(c) => write!(f, "Glyph not found for char {:?}", c),
+            Error::MissingFont(desc) => write!(
                 f,
                 "Couldn't find a font with {}\n\tPlease check the font config in your \
                  alacritty.yml.",
                 desc
             ),
             Error::FontNotLoaded => f.write_str("Tried to use a font that hasn't been loaded"),
+            Error::DirectWriteError(hresult) => {
+                write!(f, "A DirectWrite rendering error occurred: {:#X}", hresult)
+            }
+        }
+    }
+}
+
+fn em_size(size: Size) -> f32 {
+    size.as_f32_pts() * (96.0 / 72.0)
+}
+
+impl From<dwrote::Font> for Font {
+    fn from(font: dwrote::Font) -> Font {
+        Font {
+            face: font.create_font_face(),
+            family_name: font.family_name(),
+            weight: font.weight(),
+            style: font.style(),
+            stretch: font.stretch(),
+        }
+    }
+}
+
+impl From<Weight> for FontWeight {
+    fn from(weight: Weight) -> FontWeight {
+        match weight {
+            Weight::Bold => FontWeight::Bold,
+            Weight::Normal => FontWeight::Regular,
+        }
+    }
+}
+
+impl From<Slant> for FontStyle {
+    fn from(slant: Slant) -> FontStyle {
+        match slant {
+            Slant::Oblique => FontStyle::Oblique,
+            Slant::Italic => FontStyle::Italic,
+            Slant::Normal => FontStyle::Normal,
         }
     }
 }
+
+fn get_current_locale() -> String {
+    let mut buf = vec![0u16; LOCALE_NAME_MAX_LENGTH];
+    let len = unsafe { GetUserDefaultLocaleName(buf.as_mut_ptr(), buf.len() as i32) as usize };
+
+    // `len` includes null byte, which we don't need in Rust
+    OsString::from_wide(&buf[..len - 1]).into_string().expect("Locale not valid unicode")
+}
+
+/// Font fallback information for dwrote's TextAnalysisSource.
+struct TextAnalysisSourceData<'a> {
+    locale: &'a str,
+    length: u32,
+}
+
+impl TextAnalysisSourceMethods for TextAnalysisSourceData<'_> {
+    fn get_locale_name(&self, _text_position: u32) -> (Cow<str>, u32) {
+        (Cow::Borrowed(self.locale), self.length)
+    }
+
+    fn get_paragraph_reading_direction(&self) -> dwrite::DWRITE_READING_DIRECTION {
+        dwrite::DWRITE_READING_DIRECTION_LEFT_TO_RIGHT
+    }
+}