font/shaper: add Cache

src/font/shape.zig

@@ -4,6 +4,7 @@ pub const harfbuzz = @import("shaper/harfbuzz.zig");
 pub const coretext = @import("shaper/coretext.zig");
 pub const web_canvas = @import("shaper/web_canvas.zig");
 pub usingnamespace @import("shaper/run.zig");
+pub const Cache = @import("shaper/Cache.zig");
 
 /// Shaper implementation for our compile options.
 pub const Shaper = switch (options.backend) {
@@ -56,3 +57,8 @@ pub const Options = struct {
     /// support applying features globally.
     features: []const []const u8 = &.{},
 };
+
+test {
+    _ = Cache;
+    _ = Shaper;
+}

src/font/shaper/Cache.zig

@@ -0,0 +1,77 @@
+//! This structure caches the shaped cells for a given text run.
+//!
+//! At one point, shaping was the most expensive part of rendering text
+//! (accounting for 96% of frame time on my machine). To speed it up, this
+//! was introduced so that shaping results can be cached depending on the
+//! run.
+//!
+//! The cache key is the text run. The text run builds its own hash value
+//! based on the font, style, codepoint, etc. This just utilizes the hash that
+//! the text run provides.
+pub const Cache = @This();
+
+const std = @import("std");
+const assert = std.debug.assert;
+const Allocator = std.mem.Allocator;
+const font = @import("../main.zig");
+const lru = @import("../../lru.zig");
+
+/// Our LRU is the run hash to the shaped cells.
+const LRU = lru.AutoHashMap(u64, []font.shape.Cell);
+
+/// The cache of shaped cells.
+map: LRU,
+
+pub fn init() Cache {
+    // Note: this is very arbitrary. Increasing this number will increase
+    // the cache hit rate, but also increase the memory usage. We should do
+    // some more empirical testing to see what the best value is.
+    const capacity = 1024;
+
+    return .{ .map = LRU.init(capacity) };
+}
+
+pub fn deinit(self: *Cache, alloc: Allocator) void {
+    var it = self.map.map.iterator();
+    while (it.next()) |entry| alloc.free(entry.value_ptr.*.data.value);
+    self.map.deinit(alloc);
+}
+
+/// Get the shaped cells for the given text run or null if they are not
+/// in the cache.
+pub fn get(self: *const Cache, run: font.shape.TextRun) ?[]const font.shape.Cell {
+    return self.map.get(run.hash);
+}
+
+/// Insert the shaped cells for the given text run into the cache. The
+/// cells will be duplicated.
+pub fn put(
+    self: *Cache,
+    alloc: Allocator,
+    run: font.shape.TextRun,
+    cells: []const font.shape.Cell,
+) Allocator.Error!void {
+    const copy = try alloc.dupe(font.shape.Cell, cells);
+    const gop = try self.map.getOrPut(alloc, run.hash);
+    if (gop.evicted) |evicted| alloc.free(evicted.value);
+    gop.value_ptr.* = copy;
+}
+
+test Cache {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    var c = Cache.init();
+    defer c.deinit(alloc);
+
+    var run: font.shape.TextRun = undefined;
+    run.hash = 1;
+    try testing.expect(c.get(run) == null);
+    try c.put(alloc, run, &.{
+        .{ .x = 0, .glyph_index = 0 },
+        .{ .x = 1, .glyph_index = 1 },
+    });
+
+    const actual = c.get(run).?;
+    try testing.expect(actual.len == 2);
+}

src/lru.zig

@@ -150,7 +150,7 @@ pub fn HashMap(
         }
 
         /// Get a value for a key.
-        pub fn get(self: *Self, key: K) ?V {
+        pub fn get(self: *const Self, key: K) ?V {
             if (@sizeOf(Context) != 0) {
                 @compileError("getContext must be used.");
             }
@@ -158,7 +158,7 @@ pub fn HashMap(
         }
 
         /// See get
-        pub fn getContext(self: *Self, key: K, ctx: Context) ?V {
+        pub fn getContext(self: *const Self, key: K, ctx: Context) ?V {
             const node = self.map.getContext(key, ctx) orelse return null;
             return node.data.value;
         }