Add compact JSON formatter with format-on-paste (§03A)

src/core/json-format.test.ts

@@ -0,0 +1,45 @@
+import { describe, expect, it } from 'vitest';
+import { formatJson } from './json-format';
+
+describe('formatJson', () => {
+  it('pretty-prints a valid object with the default indent', () => {
+    expect(formatJson('{"a":1,"b":2}')).toBe('{"a": 1, "b": 2}');
+  });
+
+  it('keeps short arrays compact (Vega house style)', () => {
+    expect(formatJson('{"values":[1,2,3]}')).toBe('{"values": [1, 2, 3]}');
+  });
+
+  it('wraps content that exceeds the line-length budget', () => {
+    expect(formatJson('{"a":1,"b":2,"c":3}', { maxLength: 5 })).toBe(
+      '{\n  "a": 1,\n  "b": 2,\n  "c": 3\n}',
+    );
+  });
+
+  it('respects a custom indent width', () => {
+    const out = formatJson('{"a":1,"b":2}', { indent: 4, maxLength: 5 });
+    expect(out).toBe('{\n    "a": 1,\n    "b": 2\n}');
+  });
+
+  it('returns null for invalid JSON so callers can skip', () => {
+    expect(formatJson('{"a":')).toBeNull();
+    expect(formatJson('not json')).toBeNull();
+  });
+
+  it('returns null for empty or whitespace-only input', () => {
+    expect(formatJson('')).toBeNull();
+    expect(formatJson('   \n  ')).toBeNull();
+  });
+
+  it('is idempotent — formatting formatted text changes nothing', () => {
+    const once = formatJson('{"a":1,"b":[1,2,3],"c":{"d":4}}');
+    expect(once).not.toBeNull();
+    expect(formatJson(once!)).toBe(once);
+  });
+
+  it('handles top-level arrays and primitives', () => {
+    expect(formatJson('[1,2,3]')).toBe('[1, 2, 3]');
+    expect(formatJson('42')).toBe('42');
+    expect(formatJson('true')).toBe('true');
+  });
+});

src/core/json-format.ts

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+/**
+ * JSON pretty-printer for Vega-Lite specs (spec §03A; docs/architecture/08 →
+ * "compact formatter").
+ *
+ * Uses `json-stringify-pretty-compact` for Vega's house style — arrays/objects
+ * stay on one line until they exceed a width budget, then wrap — which reads far
+ * better for VL specs than `JSON.stringify(…, null, 2)`. Pure and portable: no
+ * Monaco, no browser. The Monaco formatting adapter
+ * (`infrastructure/monaco-format`) is a thin wrapper over this.
+ */
+
+import stringify from 'json-stringify-pretty-compact';
+
+/** Width budget before an array/object wraps onto multiple lines. */
+export const DEFAULT_MAX_LINE = 80;
+
+/** Default indent (spaces) — matches the editor's default tab size (spec §07). */
+const DEFAULT_INDENT = 2;
+
+/**
+ * Reformat `text` as consistently-indented JSON, or return `null` when it is not
+ * valid JSON. Returning `null` (rather than throwing) lets callers skip a no-op
+ * rather than mangle an in-progress edit, mirroring auto-save's "skip when
+ * unparseable" rule (spec §03B). The result is idempotent: formatting
+ * already-formatted text returns it unchanged.
+ */
+export function formatJson(
+  text: string,
+  opts: { indent?: number; maxLength?: number } = {},
+): string | null {
+  let parsed: unknown;
+  try {
+    parsed = JSON.parse(text);
+  } catch {
+    return null;
+  }
+  return stringify(parsed, {
+    indent: opts.indent ?? DEFAULT_INDENT,
+    maxLength: opts.maxLength ?? DEFAULT_MAX_LINE,
+  });
+}