From dc52c248e73386fef496c3bbe8643d6276b7fbfc Mon Sep 17 00:00:00 2001
From: Mitchell Hashimoto <m@mitchellh.com>
Date: Fri, 31 Jul 2026 19:59:34 -0700
Subject: [PATCH] benchmark: terminal-resize

---
 src/benchmark/TerminalResize.zig | 357 +++++++++++++++++++++++++++++++
 src/benchmark/cli.zig            |   2 +
 src/benchmark/main.zig           |   1 +
 3 files changed, 360 insertions(+)
 create mode 100644 src/benchmark/TerminalResize.zig

diff --git a/src/benchmark/TerminalResize.zig b/src/benchmark/TerminalResize.zig
new file mode 100644
index 000000000..61b908c83
--- /dev/null
+++ b/src/benchmark/TerminalResize.zig
@@ -0,0 +1,357 @@
+//! This benchmark tests the performance of Terminal.resize, with a
+//! primary focus on column resizes that reflow soft-wrapped text.
+//! Resize happens on the IO thread while holding the terminal lock,
+//! so a slow resize directly translates into dropped input and a
+//! frozen-feeling UI while the user drags the window edge (which
+//! produces a rapid stream of resizes).
+//!
+//! The terminal is populated once during setup (synthetic fill and/or
+//! a data file replayed through the VT stream) and then each step
+//! ping-pongs the terminal between two sizes. A full cycle returns the
+//! terminal to its original dimensions so the state reaches a steady
+//! state after the first cycle and every iteration performs
+//! equivalent work.
+const TerminalResize = @This();
+
+const std = @import("std");
+const assert = std.debug.assert;
+const Allocator = std.mem.Allocator;
+const terminalpkg = @import("../terminal/main.zig");
+const Benchmark = @import("Benchmark.zig");
+const options = @import("options.zig");
+const Terminal = terminalpkg.Terminal;
+const global = @import("../global.zig");
+
+const log = std.log.scoped(.@"terminal-resize-bench");
+
+opts: Options,
+alloc: Allocator,
+terminal: Terminal,
+
+pub const Options = struct {
+    /// The resize pattern to benchmark. See Mode.
+    mode: Mode = .cols,
+
+    /// Multiplier on the number of resize cycles each step runs. This
+    /// is useful to make a benchmark run long enough for profiling.
+    loops: u32 = 1,
+
+    /// The initial size of the terminal. This is also the size that
+    /// every resize cycle returns to.
+    @"terminal-rows": u16 = 80,
+    @"terminal-cols": u16 = 120,
+
+    /// The dimensions to resize to for the cols/rows/both modes. If
+    /// unset, they default to half of the respective terminal
+    /// dimension, which forces every soft-wrapped line to rewrap.
+    @"resize-cols": ?u16 = null,
+    @"resize-rows": ?u16 = null,
+
+    /// The number of synthetic lines written to the terminal during
+    /// setup. The content is deterministic: a mix of short lines,
+    /// soft-wrapped long lines, blank lines, styled cells, and wide
+    /// characters, since all of these hit different reflow paths.
+    /// Set to 0 to only use `data`.
+    @"fill-lines": u32 = 10_000,
+
+    /// The maximum scrollback size in bytes. Defaults to the Ghostty
+    /// application default. Reflow cost scales with the amount of
+    /// scrollback, not just the visible screen.
+    @"scrollback-bytes": usize = 50_000_000,
+
+    /// The data to read as a filepath. If this is "-" then
+    /// we will read stdin. If this is unset, only the synthetic fill
+    /// is used. The data is streamed into the terminal during setup
+    /// (not part of the benchmark) to build the screen contents that
+    /// get resized.
+    data: ?[]const u8 = null,
+};
+
+pub const Mode = enum {
+    /// Resize to the current dimensions. This measures the early-exit
+    /// path (mode updates, pixel geometry) and acts as a baseline.
+    noop,
+
+    /// Alternate the column count between `terminal-cols` and
+    /// `resize-cols`. With wraparound enabled (the default), this
+    /// reflows text in both directions: shrinking wraps long lines
+    /// and growing unwraps them. This is the primary reflow benchmark.
+    cols,
+
+    /// Like `cols`, but with wraparound mode disabled so the resize
+    /// does not reflow. Useful as a baseline to isolate the cost of
+    /// reflow itself from the rest of the resize.
+    @"cols-no-reflow",
+
+    /// Alternate the row count between `terminal-rows` and
+    /// `resize-rows`. Column count is unchanged so no text reflows;
+    /// this measures growing/trimming rows against scrollback.
+    rows,
+
+    /// Alternate both dimensions at once, like a diagonal window drag.
+    both,
+};
+
+pub fn create(
+    alloc: Allocator,
+    opts: Options,
+) !*TerminalResize {
+    const ptr = try alloc.create(TerminalResize);
+    errdefer alloc.destroy(ptr);
+
+    ptr.* = .{
+        .opts = opts,
+        .alloc = alloc,
+        .terminal = try .init(global.io(), alloc, .{
+            .rows = opts.@"terminal-rows",
+            .cols = opts.@"terminal-cols",
+            .max_scrollback_bytes = opts.@"scrollback-bytes",
+        }),
+    };
+
+    return ptr;
+}
+
+pub fn destroy(self: *TerminalResize, alloc: Allocator) void {
+    self.terminal.deinit(alloc);
+    alloc.destroy(self);
+}
+
+pub fn benchmark(self: *TerminalResize) Benchmark {
+    return .init(self, .{
+        .stepFn = switch (self.opts.mode) {
+            .noop => stepNoop,
+            .cols, .@"cols-no-reflow" => stepCols,
+            .rows => stepRows,
+            .both => stepBoth,
+        },
+        .setupFn = setup,
+    });
+}
+
+/// The column count used by the cols/both modes.
+fn targetCols(self: *const TerminalResize) u16 {
+    return self.opts.@"resize-cols" orelse
+        @max(1, self.opts.@"terminal-cols" / 2);
+}
+
+/// The row count used by the rows/both modes.
+fn targetRows(self: *const TerminalResize) u16 {
+    return self.opts.@"resize-rows" orelse
+        @max(1, self.opts.@"terminal-rows" / 2);
+}
+
+fn setup(ptr: *anyopaque) Benchmark.Error!void {
+    const self: *TerminalResize = @ptrCast(@alignCast(ptr));
+
+    // Always reset our terminal state. Note this doesn't resize, but
+    // create initializes (and steps return) the terminal to the
+    // requested dimensions so we're always at terminal-rows/cols here.
+    self.terminal.fullReset();
+    assert(self.terminal.cols == self.opts.@"terminal-cols");
+    assert(self.terminal.rows == self.opts.@"terminal-rows");
+
+    // Fill with synthetic content first, then replay the data file on
+    // top if given. Both go through the VT stream so soft wraps,
+    // styles, etc. are all set exactly as they would be in a real
+    // session.
+    self.fill();
+    try self.replayData();
+
+    // Reflow only happens when wraparound mode is set (it is by
+    // default). We only disable it after filling so the fill itself
+    // still soft-wraps identically in every mode.
+    if (self.opts.mode == .@"cols-no-reflow") {
+        self.terminal.modes.set(.wraparound, false);
+    }
+}
+
+/// Write deterministic synthetic content to the terminal. The goal is
+/// content that is representative of a real session so that reflow
+/// touches its interesting paths: soft-wrapped lines (must rewrap),
+/// short lines (copied as-is), blank lines, styled cells (styles must
+/// be moved across pages), and wide characters (can't be split at the
+/// wrap column).
+fn fill(self: *TerminalResize) void {
+    if (self.opts.@"fill-lines" == 0) return;
+
+    var s = self.terminal.vtStream();
+    defer s.deinit();
+
+    var prng: std.Random.DefaultPrng = .init(0xB3);
+    const rand = prng.random();
+
+    const cols: usize = self.terminal.cols;
+    var line_buf: [8192]u8 = undefined;
+
+    for (0..self.opts.@"fill-lines") |i| {
+        // Periodically toggle a background style so reflow has to
+        // carry styled cells into new pages.
+        if (i % 64 == 0) s.nextSlice("\x1b[48;2;20;40;60m");
+        if (i % 64 == 32) s.nextSlice("\x1b[m");
+
+        // A small portion of lines are blank.
+        if (i % 16 == 15) {
+            s.nextSlice("\r\n");
+            continue;
+        }
+
+        // Line lengths between ~25% and ~250% of the terminal width
+        // so we get a mix of short lines and soft-wrapped lines.
+        const min = @max(1, cols / 4);
+        const max = @min(line_buf.len, cols * 5 / 2);
+        const len = min + rand.uintLessThan(usize, max - min);
+
+        var j: usize = 0;
+        while (j < len) {
+            // Sprinkle wide characters into every 8th line.
+            if (i % 8 == 7 and j % 16 == 8 and j + 3 <= len) {
+                line_buf[j..][0..3].* = "漢".*;
+                j += 3;
+                continue;
+            }
+
+            // Words of ASCII separated by spaces.
+            line_buf[j] = if (j % 8 == 7)
+                ' '
+            else
+                rand.intRangeAtMost(u8, 'a', 'z');
+            j += 1;
+        }
+
+        s.nextSlice(line_buf[0..len]);
+        s.nextSlice("\r\n");
+    }
+}
+
+/// Stream the data file (if any) into the terminal.
+fn replayData(self: *TerminalResize) Benchmark.Error!void {
+    const data_f: std.Io.File = (options.dataFile(
+        self.opts.data,
+    ) catch |err| {
+        log.warn("error opening data file err={}", .{err});
+        return error.BenchmarkFailed;
+    }) orelse return;
+    defer data_f.close(global.io());
+
+    var stream = self.terminal.vtStream();
+    defer stream.deinit();
+
+    var read_buf: [4096]u8 align(std.atomic.cache_line) = undefined;
+    var f_reader = data_f.reader(global.io(), &read_buf);
+    const r = &f_reader.interface;
+
+    var buf: [4096]u8 = undefined;
+    while (true) {
+        const n = r.readSliceShort(&buf) catch {
+            log.warn("error reading data file err={?}", .{f_reader.err});
+            return error.BenchmarkFailed;
+        };
+        if (n == 0) break; // EOF reached
+        stream.nextSlice(buf[0..n]);
+    }
+}
+
+fn resizeTerminal(
+    self: *TerminalResize,
+    cols: u16,
+    rows: u16,
+) Benchmark.Error!void {
+    self.terminal.resize(self.alloc, .{
+        .cols = cols,
+        .rows = rows,
+        // Realistic cell pixel geometry: real apprt resizes always
+        // carry it, and it exercises the pixel dimension updates.
+        .cell_size_px = .{ .width = 10, .height = 20 },
+    }) catch |err| {
+        log.warn("error resizing terminal err={}", .{err});
+        return error.BenchmarkFailed;
+    };
+    std.mem.doNotOptimizeAway(&self.terminal);
+}
+
+fn stepNoop(ptr: *anyopaque) Benchmark.Error!void {
+    const self: *TerminalResize = @ptrCast(@alignCast(ptr));
+
+    const cols = self.opts.@"terminal-cols";
+    const rows = self.opts.@"terminal-rows";
+
+    // We loop because it's so fast (a few ns) that a single resize
+    // doesn't properly capture our speeds.
+    for (0..50_000_000 * @as(u64, self.opts.loops)) |_| {
+        try self.resizeTerminal(cols, rows);
+    }
+}
+
+fn stepCols(ptr: *anyopaque) Benchmark.Error!void {
+    const self: *TerminalResize = @ptrCast(@alignCast(ptr));
+
+    const cols = self.opts.@"terminal-cols";
+    const rows = self.opts.@"terminal-rows";
+    const target = self.targetCols();
+
+    // Per-cycle cost differs by orders of magnitude between the two
+    // modes (a reflow resize walks the entire scrollback; a non-reflow
+    // resize doesn't), so pick a cycle count that makes each run long
+    // enough to measure well above process startup and setup cost.
+    const cycles: u64 = switch (self.opts.mode) {
+        .cols => 25,
+        .@"cols-no-reflow" => 1_500,
+        else => unreachable,
+    };
+
+    // Each cycle shrinks (rewrapping long lines) and grows back
+    // (unwrapping them), ending at the original size.
+    for (0..cycles * @as(u64, self.opts.loops)) |_| {
+        try self.resizeTerminal(target, rows);
+        try self.resizeTerminal(cols, rows);
+    }
+}
+
+fn stepRows(ptr: *anyopaque) Benchmark.Error!void {
+    const self: *TerminalResize = @ptrCast(@alignCast(ptr));
+
+    const cols = self.opts.@"terminal-cols";
+    const rows = self.opts.@"terminal-rows";
+    const target = self.targetRows();
+
+    // Row-only resizes don't reflow so they're much cheaper (tens of
+    // ns); loop a lot more so the measurement isn't dominated by
+    // process overhead.
+    for (0..2_500_000 * @as(u64, self.opts.loops)) |_| {
+        try self.resizeTerminal(cols, target);
+        try self.resizeTerminal(cols, rows);
+    }
+}
+
+fn stepBoth(ptr: *anyopaque) Benchmark.Error!void {
+    const self: *TerminalResize = @ptrCast(@alignCast(ptr));
+
+    const cols = self.opts.@"terminal-cols";
+    const rows = self.opts.@"terminal-rows";
+    const target_cols = self.targetCols();
+    const target_rows = self.targetRows();
+
+    for (0..25 * @as(u64, self.opts.loops)) |_| {
+        try self.resizeTerminal(target_cols, target_rows);
+        try self.resizeTerminal(cols, rows);
+    }
+}
+
+test TerminalResize {
+    const testing = std.testing;
+    const alloc = testing.allocator;
+
+    // Small dimensions and fill so this is fast in debug builds while
+    // still exercising real reflow in both directions.
+    const impl: *TerminalResize = try .create(alloc, .{
+        .mode = .cols,
+        .@"terminal-rows" = 10,
+        .@"terminal-cols" = 20,
+        .@"fill-lines" = 50,
+    });
+    defer impl.destroy(alloc);
+
+    const bench = impl.benchmark();
+    _ = try bench.run(.once);
+}
diff --git a/src/benchmark/cli.zig b/src/benchmark/cli.zig
index 19db37a3d..1dc089b71 100644
--- a/src/benchmark/cli.zig
+++ b/src/benchmark/cli.zig
@@ -15,6 +15,7 @@ pub const Action = enum {
     @"scrollback-compression",
     @"screen-clone",
     @"terminal-parser",
+    @"terminal-resize",
     @"terminal-stream",
     @"is-symbol",
     @"osc-parser",
@@ -38,6 +39,7 @@ pub const Action = enum {
             .@"codepoint-width" => @import("CodepointWidth.zig"),
             .@"grapheme-break" => @import("GraphemeBreak.zig"),
             .@"terminal-parser" => @import("TerminalParser.zig"),
+            .@"terminal-resize" => @import("TerminalResize.zig"),
             .@"is-symbol" => @import("IsSymbol.zig"),
             .@"osc-parser" => @import("OscParser.zig"),
         };
diff --git a/src/benchmark/main.zig b/src/benchmark/main.zig
index f22891c71..fb038c0a3 100644
--- a/src/benchmark/main.zig
+++ b/src/benchmark/main.zig
@@ -7,6 +7,7 @@ pub const GraphemeBreak = @import("GraphemeBreak.zig");
 pub const HyperlinkMap = @import("HyperlinkMap.zig");
 pub const ScreenClone = @import("ScreenClone.zig");
 pub const TerminalParser = @import("TerminalParser.zig");
+pub const TerminalResize = @import("TerminalResize.zig");
 pub const IsSymbol = @import("IsSymbol.zig");
 pub const PageCompression = @import("PageCompression.zig");
 pub const ScrollbackCompression = @import("ScrollbackCompression.zig");