Implement fallback priorities and han unification on Unix

examples/text/src/font/fallback/macos.rs

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+use unicode_script::Script;
+
+// Fallbacks to use after any script specific fallbacks
+pub fn common_fallback() -> &'static [&'static str] {
+    &[]
+}
+
+// Fallbacks to use per script
+pub fn script_fallback(script: &Script, locale: &str) -> &'static [&'static str] {
+    &[]
+}

examples/text/src/font/fallback/mod.rs

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+use unicode_script::Script;
+
+use super::Font;
+
+#[cfg(not(any(macos, unix, windows)))]
+use self::other::*;
+#[cfg(not(any(macos, unix, windows)))]
+mod other;
+
+#[cfg(macos)]
+use self::macos::*;
+#[cfg(macos)]
+mod macos;
+
+#[cfg(unix)]
+use self::unix::*;
+#[cfg(unix)]
+mod unix;
+
+#[cfg(windows)]
+use self::windows::*;
+#[cfg(windows)]
+mod windows;
+
+pub struct FontFallbackIter<'a> {
+    fonts: &'a [Font<'a>],
+    locale: &'a str,
+    scripts: Vec<Script>,
+    script_i: (usize, usize),
+    common_i: usize,
+    other_i: usize,
+}
+
+impl<'a> FontFallbackIter<'a> {
+    pub fn new(fonts: &'a [Font<'a>], scripts: Vec<Script>, locale: &'a str) -> Self {
+        Self {
+            fonts,
+            locale,
+            scripts,
+            script_i: (0, 0),
+            common_i: 0,
+            other_i: 0,
+        }
+    }
+}
+
+impl<'a> Iterator for FontFallbackIter<'a> {
+    type Item = &'a Font<'a>;
+    fn next(&mut self) -> Option<Self::Item> {
+        while self.script_i.0 < self.scripts.len() {
+            let script_families = script_fallback(&self.scripts[self.script_i.0], self.locale);
+            while self.script_i.1 < script_families.len() {
+                let script_family = script_families[self.script_i.1];
+                self.script_i.1 += 1;
+                for font in self.fonts.iter() {
+                    if font.info.family == script_family {
+                        return Some(font);
+                    }
+                }
+            }
+            self.script_i.0 += 1;
+            self.script_i.1 = 0;
+        }
+
+        let common_families = common_fallback();
+        while self.common_i < common_families.len() {
+            let common_family = common_families[self.common_i];
+            self.common_i += 1;
+            for font in self.fonts.iter() {
+                if font.info.family == common_family {
+                    return Some(font);
+                }
+            }
+        }
+
+        //TODO: do we need to do this?
+        //TODO: do not evaluate fonts more than once!
+        while self.other_i < self.fonts.len() {
+            let font = &self.fonts[self.other_i];
+            self.other_i += 1;
+            return Some(font);
+        }
+
+        None
+    }
+}

examples/text/src/font/fallback/other.rs

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+use unicode_script::Script;
+
+// Fallbacks to use after any script specific fallbacks
+pub fn common_fallback() -> &'static [&'static str] {
+    &[]
+}
+
+// Fallbacks to use per script
+pub fn script_fallback(script: &Script, locale: &str) -> &'static [&'static str] {
+    &[]
+}

examples/text/src/font/fallback/unix.rs

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+use unicode_script::Script;
+
+// Fallbacks to use after any script specific fallbacks
+pub fn common_fallback() -> &'static [&'static str] {
+    //TODO: abstract style (sans/serif/monospaced)
+    &[
+        "Fira Sans",
+        "DejaVu Sans",
+        "DejaVu Serif",
+        "Noto Color Emoji",
+    ]
+}
+
+// Fallbacks to use per script
+pub fn script_fallback(script: &Script, locale: &str) -> &'static [&'static str] {
+    //TODO: abstract style (sans/serif/monospaced)
+    match script {
+        Script::Han => match locale {
+            // Japan
+            "ja" => &["Noto Sans CJK JA"],
+            // Korea
+            "ko" => &["Noto Sans CJK KR"],
+            // China
+            "zh-CN" => &["Noto Sans CJK SC"],
+            // Hong Kong
+            "zh-HK" => &["Noto Sans CJK HK"],
+            // Taiwan
+            "zh-TW" => &["Noto Sans CJK TC"],
+            // Simplified Chinese is the default
+            _ => &["Noto Sans CJK SC"],
+        }
+        _ => &[],
+    }
+}

examples/text/src/font/fallback/windows.rs

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+use unicode_script::Script;
+
+// Fallbacks to use after any script specific fallbacks
+pub fn common_fallback() -> &'static [&'static str] {
+    &[]
+}
+
+// Fallbacks to use per script
+pub fn script_fallback(script: &Script, locale: &str) -> &'static [&'static str] {
+    &[]
+}