lib-vt: add color scheme report encoder

Add a shared encoder for CSI ? 997 ; Ps n color scheme reports and use 
it for both CSI ? 996 n replies and unsolicited Termio reports. Export the 
same encoder through the libghostty-vt C API with docs and an example.

This is a really light API, arguably easy for consumers to hardcode,
but it didn't match the rest of our style in the libghostty API so we 
should expose it.

Example: GHOSTTY_COLOR_SCHEME_DARK encodes to ESC [ ? 997 ; 1 n,
while GHOSTTY_COLOR_SCHEME_LIGHT encodes to ESC [ ? 997 ; 2 n.

src/lib_vt.zig

@@ -191,6 +191,7 @@ comptime {
         @export(&c.osc_end, .{ .name = "ghostty_osc_end" });
         @export(&c.osc_command_type, .{ .name = "ghostty_osc_command_type" });
         @export(&c.osc_command_data, .{ .name = "ghostty_osc_command_data" });
+        @export(&c.color_scheme_report_encode, .{ .name = "ghostty_color_scheme_report_encode" });
         @export(&c.focus_encode, .{ .name = "ghostty_focus_encode" });
         @export(&c.mode_report_encode, .{ .name = "ghostty_mode_report_encode" });
         @export(&c.paste_is_safe, .{ .name = "ghostty_paste_is_safe" });

src/terminal/c/color_scheme.zig

@@ -0,0 +1,63 @@
+const std = @import("std");
+const lib = @import("../lib.zig");
+const device_status = @import("../device_status.zig");
+const Result = @import("result.zig").Result;
+
+pub fn report_encode(
+    scheme: device_status.ColorScheme,
+    out_: ?[*]u8,
+    out_len: usize,
+    out_written: *usize,
+) callconv(lib.calling_conv) Result {
+    var writer: std.Io.Writer = .fixed(if (out_) |out| out[0..out_len] else &.{});
+    device_status.encodeColorSchemeReport(&writer, scheme) catch |err| switch (err) {
+        error.WriteFailed => {
+            var discarding: std.Io.Writer.Discarding = .init(&.{});
+            device_status.encodeColorSchemeReport(&discarding.writer, scheme) catch unreachable;
+            out_written.* = @intCast(discarding.count);
+            return .out_of_space;
+        },
+    };
+
+    out_written.* = writer.end;
+    return .success;
+}
+
+test "encode color scheme report dark" {
+    var buf: [device_status.max_color_scheme_report_encode_size]u8 = undefined;
+    var written: usize = 0;
+    const result = report_encode(.dark, &buf, buf.len, &written);
+    try std.testing.expectEqual(.success, result);
+    try std.testing.expectEqualStrings("\x1B[?997;1n", buf[0..written]);
+}
+
+test "encode color scheme report light" {
+    var buf: [device_status.max_color_scheme_report_encode_size]u8 = undefined;
+    var written: usize = 0;
+    const result = report_encode(.light, &buf, buf.len, &written);
+    try std.testing.expectEqual(.success, result);
+    try std.testing.expectEqualStrings("\x1B[?997;2n", buf[0..written]);
+}
+
+test "encode color scheme report with null buffer" {
+    var written: usize = 0;
+    const result = report_encode(.dark, null, 0, &written);
+    try std.testing.expectEqual(.out_of_space, result);
+    try std.testing.expectEqual(@as(usize, 9), written);
+}
+
+test "encode color scheme report with insufficient buffer" {
+    var buf: [3]u8 = undefined;
+    var written: usize = 0;
+    const result = report_encode(.light, &buf, buf.len, &written);
+    try std.testing.expectEqual(.out_of_space, result);
+    try std.testing.expectEqual(@as(usize, 9), written);
+}
+
+test "encode color scheme report with exact buffer" {
+    var buf: [9]u8 = undefined;
+    var written: usize = 0;
+    const result = report_encode(.dark, &buf, buf.len, &written);
+    try std.testing.expectEqual(.success, result);
+    try std.testing.expectEqual(@as(usize, 9), written);
+}

src/terminal/c/main.zig

@@ -5,6 +5,7 @@ const buildpkg = @import("build_info.zig");
 pub const allocator = @import("allocator.zig");
 pub const cell = @import("cell.zig");
 pub const color = @import("color.zig");
+pub const color_scheme = @import("color_scheme.zig");
 pub const focus = @import("focus.zig");
 pub const formatter = @import("formatter.zig");
 pub const grid_ref = @import("grid_ref.zig");
@@ -58,6 +59,8 @@ pub const osc_command_data = osc.commandData;
 
 pub const color_rgb_get = color.rgb_get;
 
+pub const color_scheme_report_encode = color_scheme.report_encode;
+
 pub const focus_encode = focus.encode;
 
 pub const mode_report_encode = modes.report_encode;
@@ -218,6 +221,7 @@ test {
     _ = buildpkg;
     _ = cell;
     _ = color;
+    _ = color_scheme;
     _ = grid_ref;
     _ = grid_ref_tracked;
     _ = kitty_graphics;

src/terminal/device_status.zig

@@ -7,6 +7,32 @@ pub const ColorScheme = lib.Enum(lib.target, &.{
     "dark",
 });
 
+/// Maximum number of bytes that `encodeColorSchemeReport` will write.
+pub const max_color_scheme_report_encode_size = max: {
+    var result: usize = 0;
+    for (@typeInfo(ColorScheme).@"enum".fields) |field| {
+        var discarding: std.Io.Writer.Discarding = .init(&.{});
+        encodeColorSchemeReport(
+            &discarding.writer,
+            @enumFromInt(field.value),
+        ) catch unreachable;
+        result = @max(result, @as(usize, @intCast(discarding.count)));
+    }
+
+    break :max result;
+};
+
+/// Encode a color scheme report response for CSI ? 996 n queries.
+pub fn encodeColorSchemeReport(
+    writer: *std.Io.Writer,
+    scheme: ColorScheme,
+) std.Io.Writer.Error!void {
+    try writer.writeAll(switch (scheme) {
+        .dark => "\x1B[?997;1n",
+        .light => "\x1B[?997;2n",
+    });
+}
+
 /// An enum(u16) of the available device status requests.
 pub const Request = dsr_enum: {
     const EnumField = std.builtin.Type.EnumField;
@@ -72,3 +98,19 @@ const entries: []const Entry = &.{
     .{ .name = "cursor_position", .value = 6 },
     .{ .name = "color_scheme", .value = 996, .question = true },
 };
+
+test "encode color scheme report dark" {
+    try std.testing.expectEqual(@as(usize, 9), max_color_scheme_report_encode_size);
+
+    var buf: [max_color_scheme_report_encode_size]u8 = undefined;
+    var writer: std.Io.Writer = .fixed(&buf);
+    try encodeColorSchemeReport(&writer, .dark);
+    try std.testing.expectEqualStrings("\x1B[?997;1n", writer.buffered());
+}
+
+test "encode color scheme report light" {
+    var buf: [max_color_scheme_report_encode_size]u8 = undefined;
+    var writer: std.Io.Writer = .fixed(&buf);
+    try encodeColorSchemeReport(&writer, .light);
+    try std.testing.expectEqualStrings("\x1B[?997;2n", writer.buffered());
+}

src/terminal/stream_terminal.zig

@@ -348,10 +348,11 @@ pub const Handler = struct {
             .color_scheme => {
                 const func = self.effects.color_scheme orelse return;
                 const scheme = func(self) orelse return;
-                self.writePty(switch (scheme) {
-                    .dark => "\x1B[?997;1n",
-                    .light => "\x1B[?997;2n",
-                });
+                var buf: [device_status.max_color_scheme_report_encode_size + 1]u8 = undefined;
+                var writer: std.Io.Writer = .fixed(buf[0..device_status.max_color_scheme_report_encode_size]);
+                device_status.encodeColorSchemeReport(&writer, scheme) catch return;
+                buf[writer.end] = 0;
+                self.writePty(buf[0..writer.end :0]);
             },
         }
     }

src/termio/Termio.zig

@@ -712,11 +712,15 @@ pub fn colorSchemeReportLocked(self: *Termio, td: *ThreadData, force: bool) !voi
     if (!force and !self.renderer_state.terminal.modes.get(.report_color_scheme)) {
         return;
     }
-    const output = switch (self.config.conditional_state.theme) {
-        .light => "\x1B[?997;2n",
-        .dark => "\x1B[?997;1n",
+    const scheme: terminalpkg.device_status.ColorScheme = switch (self.config.conditional_state.theme) {
+        .light => .light,
+        .dark => .dark,
     };
-    try self.queueWrite(td, output, false);
+
+    var buf: [terminalpkg.device_status.max_color_scheme_report_encode_size]u8 = undefined;
+    var writer: std.Io.Writer = .fixed(&buf);
+    try terminalpkg.device_status.encodeColorSchemeReport(&writer, scheme);
+    try self.queueWrite(td, writer.buffered(), false);
 }
 
 /// ThreadData is the data created and stored in the termio thread