feat(cli): snapshot path + diff helpers [FR-004]

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: dmealing

server/typescript/packages/cli/src/lib/snapshot.ts

@@ -0,0 +1,50 @@
+// Path + diff helpers for `meta prompt-snapshot`. Pure (no I/O, no metadata) so
+// they unit-test trivially; the command does the filesystem work.
+
+import { join } from "node:path";
+
+export interface SnapshotPaths {
+  /** The per-template snapshot directory: <cwd>/.metaobjects/snapshots/<name>. */
+  dir: string;
+  /** The committed fixture payload (author-owned input). */
+  payloadPath: string;
+  /** The golden rendered output (tool-managed, byte-exact). */
+  snapPath: string;
+}
+
+export function snapshotPaths(cwd: string, templateName: string): SnapshotPaths {
+  const dir = join(cwd, ".metaobjects", "snapshots", templateName);
+  return {
+    dir,
+    payloadPath: join(dir, "payload.json"),
+    snapPath: join(dir, "output.snap"),
+  };
+}
+
+// A compact line diff: trim the common leading/trailing lines, then show the
+// differing middle as `- <expected>` followed by `+ <actual>`. Enough to make
+// drift reviewable in CI output without pulling in a diff dependency.
+export function unifiedDiff(expected: string, actual: string): string {
+  const e = expected.split("\n");
+  const a = actual.split("\n");
+
+  let pre = 0;
+  while (pre < e.length && pre < a.length && e[pre] === a[pre]) pre++;
+
+  let suf = 0;
+  while (
+    suf < e.length - pre &&
+    suf < a.length - pre &&
+    e[e.length - 1 - suf] === a[a.length - 1 - suf]
+  ) {
+    suf++;
+  }
+
+  const eMid = e.slice(pre, e.length - suf);
+  const aMid = a.slice(pre, a.length - suf);
+
+  const out: string[] = [`@@ line ${pre + 1} @@`];
+  for (const line of eMid) out.push(`- ${line}`);
+  for (const line of aMid) out.push(`+ ${line}`);
+  return out.join("\n");
+}

server/typescript/packages/cli/test/unit/snapshot.test.ts

@@ -0,0 +1,28 @@
+import { describe, test, expect } from "bun:test";
+import { join } from "node:path";
+import { snapshotPaths, unifiedDiff } from "../../src/lib/snapshot.js";
+
+describe("snapshotPaths", () => {
+  test("resolves the per-template dir + payload + golden under .metaobjects/snapshots", () => {
+    const p = snapshotPaths("/proj", "Greeting");
+    expect(p.dir).toBe(join("/proj", ".metaobjects", "snapshots", "Greeting"));
+    expect(p.payloadPath).toBe(join("/proj", ".metaobjects", "snapshots", "Greeting", "payload.json"));
+    expect(p.snapPath).toBe(join("/proj", ".metaobjects", "snapshots", "Greeting", "output.snap"));
+  });
+});
+
+describe("unifiedDiff", () => {
+  test("identical strings produce no -/+ lines", () => {
+    const d = unifiedDiff("a\nb\nc", "a\nb\nc");
+    expect(d).not.toContain("\n- ");
+    expect(d).not.toContain("\n+ ");
+  });
+  test("shows the differing region with - (expected) then + (actual)", () => {
+    const d = unifiedDiff("a\nOLD\nc", "a\nNEW\nc");
+    expect(d).toContain("- OLD");
+    expect(d).toContain("+ NEW");
+    // common leading/trailing lines are trimmed from the diff body
+    expect(d).not.toContain("- a");
+    expect(d).not.toContain("+ c");
+  });
+});