font: shapers init with allocator since web canvas needs to init buffer

src/font/shaper/harfbuzz.zig

@@ -26,7 +26,10 @@ pub const Shaper = struct {
 
     /// The cell_buf argument is the buffer to use for storing shaped results.
     /// This should be at least the number of columns in the terminal.
-    pub fn init(cell_buf: []font.shape.Cell) !Shaper {
+    pub fn init(alloc: Allocator, cell_buf: []font.shape.Cell) !Shaper {
+        // Allocator is not used because harfbuzz uses libc
+        _ = alloc;
+
         return Shaper{
             .hb_buf = try harfbuzz.Buffer.create(),
             .cell_buf = cell_buf,
@@ -547,7 +550,7 @@ fn testShaper(alloc: Allocator) !TestShaper {
     var cell_buf = try alloc.alloc(font.shape.Cell, 80);
     errdefer alloc.free(cell_buf);
 
-    var shaper = try Shaper.init(cell_buf);
+    var shaper = try Shaper.init(alloc, cell_buf);
     errdefer shaper.deinit();
 
     return TestShaper{

src/font/shaper/web_canvas.zig

@@ -2,24 +2,62 @@ const std = @import("std");
 const assert = std.debug.assert;
 const Allocator = std.mem.Allocator;
 const font = @import("../main.zig");
+const terminal = @import("../../terminal/main.zig");
 
 const log = std.log.scoped(.font_shaper);
 
 pub const Shaper = struct {
+    const RunBuf = std.ArrayList(u32);
+
     /// The shared memory used for shaping results.
     cell_buf: []font.shape.Cell,
 
+    /// The shared memory used for storing information about a run.
+    run_buf: RunBuf,
+
     /// The cell_buf argument is the buffer to use for storing shaped results.
     /// This should be at least the number of columns in the terminal.
-    pub fn init(cell_buf: []font.shape.Cell) !Shaper {
+    pub fn init(alloc: Allocator, cell_buf: []font.shape.Cell) !Shaper {
         return Shaper{
             .cell_buf = cell_buf,
+            .run_buf = try RunBuf.initCapacity(alloc, cell_buf.len),
         };
     }
 
     pub fn deinit(self: *Shaper) void {
-        _ = self;
+        self.run_buf.deinit();
+        self.* = undefined;
     }
+
+    /// Returns an iterator that returns one text run at a time for the
+    /// given terminal row. Note that text runs are are only valid one at a time
+    /// for a Shaper struct since they share state.
+    pub fn runIterator(
+        self: *Shaper,
+        group: *font.GroupCache,
+        row: terminal.Screen.Row,
+    ) font.shape.RunIterator {
+        return .{ .hooks = .{ .shaper = self }, .group = group, .row = row };
+    }
+
+    /// The hooks for RunIterator.
+    pub const RunIteratorHook = struct {
+        shaper: *Shaper,
+
+        pub fn prepare(self: RunIteratorHook) !void {
+            // Reset the buffer for our current run
+            self.shaper.run_buf.clearRetainingCapacity();
+        }
+
+        pub fn addCodepoint(self: RunIteratorHook, cp: u32, cluster: u32) !void {
+            _ = cluster;
+            try self.shaper.append(cp);
+        }
+
+        pub fn finalize(self: RunIteratorHook) !void {
+            _ = self;
+        }
+    };
 };
 
 /// The wasm-compatible API.

src/renderer/Metal.zig

@@ -180,7 +180,7 @@ pub fn init(alloc: Allocator, options: renderer.Options) !Metal {
     // avoid allocations later.
     var shape_buf = try alloc.alloc(font.shape.Cell, 160);
     errdefer alloc.free(shape_buf);
-    var font_shaper = try font.Shaper.init(shape_buf);
+    var font_shaper = try font.Shaper.init(alloc, shape_buf);
     errdefer font_shaper.deinit();
 
     // Initialize our Metal buffers

src/renderer/OpenGL.zig

@@ -162,7 +162,7 @@ pub fn init(alloc: Allocator, options: renderer.Options) !OpenGL {
     // Create the initial font shaper
     var shape_buf = try alloc.alloc(font.shape.Cell, 1);
     errdefer alloc.free(shape_buf);
-    var shaper = try font.Shaper.init(shape_buf);
+    var shaper = try font.Shaper.init(alloc, shape_buf);
     errdefer shaper.deinit();
 
     // Create our shader