fix: trim diff output context and normalize +/- indentation

Author: thymikee

src/__tests__/cli-diff.test.ts

@@ -77,8 +77,8 @@ test('diff snapshot renders human-readable unified diff text', async () => {
   assert.equal(result.code, null);
   assert.equal(result.calls.length, 1);
   assert.match(result.stdout, /^@e2 \[window\]/m);
-  assert.match(result.stdout, /^-  @e3 \[text\] "67"$/m);
-  assert.match(result.stdout, /^\+  @e3 \[text\] "134"$/m);
+  assert.match(result.stdout, /^- @e3 \[text\] "67"$/m);
+  assert.match(result.stdout, /^\+ @e3 \[text\] "134"$/m);
   assert.match(result.stdout, /1 additions, 1 removals, 1 unchanged/);
   assert.equal(result.stderr, '');
 });

src/utils/__tests__/output.test.ts

@@ -5,21 +5,26 @@ import { formatSnapshotDiffText } from '../output.ts';
 test('formatSnapshotDiffText renders unified diff lines with summary', () => {
   const text = formatSnapshotDiffText({
     baselineInitialized: false,
-    summary: { additions: 2, removals: 2, unchanged: 2 },
+    summary: { additions: 2, removals: 2, unchanged: 4 },
     lines: [
+      { kind: 'unchanged', text: '@e0 [application]' },
       { kind: 'unchanged', text: '@e2 [window]' },
       { kind: 'removed', text: '  @e3 [other] "67"' },
       { kind: 'removed', text: '  @e4 [text] "67"' },
       { kind: 'added', text: '  @e3 [other] "134"' },
       { kind: 'added', text: '  @e4 [text] "134"' },
       { kind: 'unchanged', text: '  @e5 [button] "Increment"' },
+      { kind: 'unchanged', text: '  @e6 [text] "Footer"' },
     ],
   });
 
+  assert.doesNotMatch(text, /^@e0 \[application\]$/m);
   assert.match(text, /^@e2 \[window\]/m);
-  assert.match(text, /^-  @e3 \[other\] "67"$/m);
-  assert.match(text, /^\+  @e3 \[other\] "134"$/m);
-  assert.match(text, /2 additions, 2 removals, 2 unchanged/);
+  assert.match(text, /^- @e3 \[other\] "67"$/m);
+  assert.match(text, /^\+ @e3 \[other\] "134"$/m);
+  assert.match(text, /^  @e5 \[button\] "Increment"$/m);
+  assert.doesNotMatch(text, /^  @e6 \[text\] "Footer"$/m);
+  assert.match(text, /2 additions, 2 removals, 4 unchanged/);
 });
 
 test('formatSnapshotDiffText renders baseline initialization text', () => {

src/utils/output.ts

@@ -84,10 +84,11 @@ export function formatSnapshotDiffText(data: Record<string, unknown>): string {
     return `Baseline initialized (${unchanged} lines).\n`;
   }
   const rawLines = Array.isArray(data.lines) ? (data.lines as SnapshotDiffLine[]) : [];
-  const lines = rawLines.map((line) => {
+  const contextLines = applyContextWindow(rawLines, 1);
+  const lines = contextLines.map((line) => {
     const text = typeof line.text === 'string' ? line.text : '';
-    if (line.kind === 'added') return `+${text}`;
-    if (line.kind === 'removed') return `-${text}`;
+    if (line.kind === 'added') return `+ ${text.trimStart()}`;
+    if (line.kind === 'removed') return `- ${text.trimStart()}`;
     return text;
   });
   const body = lines.length > 0 ? `${lines.join('\n')}\n` : '';
@@ -97,3 +98,22 @@ export function formatSnapshotDiffText(data: Record<string, unknown>): string {
 function toNumber(value: unknown): number {
   return typeof value === 'number' && Number.isFinite(value) ? value : 0;
 }
+
+function applyContextWindow(lines: SnapshotDiffLine[], contextWindow: number): SnapshotDiffLine[] {
+  if (lines.length === 0) return lines;
+  const changedIndices = lines
+    .map((line, index) => ({ index, kind: line.kind }))
+    .filter((entry) => entry.kind === 'added' || entry.kind === 'removed')
+    .map((entry) => entry.index);
+  if (changedIndices.length === 0) return lines;
+
+  const keep = new Array<boolean>(lines.length).fill(false);
+  for (const index of changedIndices) {
+    const start = Math.max(0, index - contextWindow);
+    const end = Math.min(lines.length - 1, index + contextWindow);
+    for (let i = start; i <= end; i += 1) {
+      keep[i] = true;
+    }
+  }
+  return lines.filter((_, index) => keep[index]);
+}