feat(tui): diff viewer (#9260)

Signed-off-by: Douwe Osinga <douwe@squareup.com>
Co-authored-by: Douwe Osinga <douwe@squareup.com>

ui/text/src/slashCommands.tsx

@@ -0,0 +1,93 @@
+import { spawnSync } from "node:child_process";
+
+export interface SlashCommandContext {
+  cwd: string;
+}
+
+export type SlashCommandResult =
+  | { handled: true; message?: string }
+  | { handled: true; overlay: "diff"; content: string; truncated: boolean }
+  | { handled: false };
+
+export interface SlashCommand {
+  name: string;
+  description: string;
+  run: (ctx: SlashCommandContext) => SlashCommandResult;
+}
+
+function isGitRepo(cwd: string): boolean {
+  const result = spawnSync("git", ["rev-parse", "--is-inside-work-tree"], {
+    cwd,
+    stdio: ["ignore", "ignore", "ignore"],
+  });
+  return result.status === 0;
+}
+
+const MAX_DIFF_BYTES = 2_000_000;
+
+function readDiff(cwd: string): { text: string; truncated: boolean } | null {
+  const result = spawnSync(
+    "git",
+    ["--no-pager", "diff", "--no-color"],
+    {
+      cwd,
+      encoding: "utf8",
+      maxBuffer: 32 * 1024 * 1024,
+    },
+  );
+  if (result.status !== 0 && result.status !== null) return null;
+  const stdout = result.stdout ?? "";
+  if (stdout.length > MAX_DIFF_BYTES) {
+    return { text: stdout.slice(0, MAX_DIFF_BYTES), truncated: true };
+  }
+  return { text: stdout, truncated: false };
+}
+
+const diffCommand: SlashCommand = {
+  name: "diff",
+  description: "show unstaged changes",
+  run: (ctx) => {
+    if (!isGitRepo(ctx.cwd)) {
+      return {
+        handled: true,
+        message: `not a git repository: ${ctx.cwd}`,
+      };
+    }
+
+    const diff = readDiff(ctx.cwd);
+    if (diff === null) {
+      return { handled: true, message: "failed to run `git diff`" };
+    }
+
+    if (diff.text.trim().length === 0) {
+      return { handled: true, message: "no unstaged changes" };
+    }
+
+    return {
+      handled: true,
+      overlay: "diff",
+      content: diff.text,
+      truncated: diff.truncated,
+    };
+  },
+};
+
+const COMMANDS: Record<string, SlashCommand> = {
+  diff: diffCommand,
+};
+
+export function tryRunSlashCommand(
+  input: string,
+  ctx: SlashCommandContext,
+): SlashCommandResult {
+  const trimmed = input.trim();
+  if (!trimmed.startsWith("/")) return { handled: false };
+  const name = trimmed.slice(1).split(/\s+/)[0]?.toLowerCase() ?? "";
+  const cmd = COMMANDS[name];
+  if (!cmd) return { handled: false };
+  return cmd.run(ctx);
+}
+
+export function listSlashCommands(): SlashCommand[] {
+  return Object.values(COMMANDS);
+}