feat(rule): test detection rules (closes #7, #8, #9)

__tests__/tests/test-improvement.test.js

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+const rule = require('../../src/rules/tests/test-improvement');
+
+describe('Test Improvement Rule', () => {
+  it('labels when both source and tests change', () => {
+    const files = [{ filename: 'src/app.ts' }, { filename: '__tests__/app.test.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toContain('test-improvement');
+  });
+
+  it('labels for various source and test combinations', () => {
+    const files = [{ filename: 'lib/util.js' }, { filename: 'test/util.spec.js' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toContain('test-improvement');
+  });
+
+  it('does not label when only tests change', () => {
+    const files = [{ filename: '__tests__/app.test.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('does not label when only source changes', () => {
+    const files = [{ filename: 'src/app.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('does not label when only docs change', () => {
+    const files = [{ filename: 'README.md' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('handles empty files array', () => {
+    const labels = rule({ files: [], pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('handles debug mode', () => {
+    const files = [{ filename: 'src/app.js' }, { filename: '__tests__/app.test.js' }];
+    expect(() => rule({ files, pr: {}, enableDebug: true })).not.toThrow();
+  });
+});

__tests__/tests/test-missing.test.js

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+const rule = require('../../src/rules/tests/test-missing');
+
+describe('Test Missing Rule', () => {
+  it('labels when source changes without tests', () => {
+    const files = [{ filename: 'src/app.ts' }, { filename: 'src/util.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toContain('test-missing');
+  });
+
+  it('labels for various source file types', () => {
+    const files = [{ filename: 'lib/helper.js' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toContain('test-missing');
+  });
+
+  it('does not label when tests are present', () => {
+    const files = [{ filename: 'src/app.ts' }, { filename: '__tests__/app.test.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('does not label when only tests change', () => {
+    const files = [{ filename: '__tests__/app.test.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('does not label when only docs change', () => {
+    const files = [{ filename: 'README.md' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('handles empty files array', () => {
+    const labels = rule({ files: [], pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('handles debug mode', () => {
+    const files = [{ filename: 'src/app.js' }];
+    expect(() => rule({ files, pr: {}, enableDebug: true })).not.toThrow();
+  });
+});

__tests__/tests/test-only-change.test.js

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+const rule = require('../../src/rules/tests/test-only-change');
+
+describe('Test Only Change Rule', () => {
+  it('labels when only tests are changed', () => {
+    const files = [{ filename: '__tests__/a.test.js' }, { filename: 'tests/utils.spec.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toContain('test-only-change');
+  });
+
+  it('labels when test/ directory files change', () => {
+    const files = [{ filename: 'test/unit.js' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toContain('test-only-change');
+  });
+
+  it('does not label when source files also change', () => {
+    const files = [{ filename: '__tests__/a.test.js' }, { filename: 'src/app.ts' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('does not label when only source files change', () => {
+    const files = [{ filename: 'src/app.js' }];
+    const labels = rule({ files, pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('handles empty files array', () => {
+    const labels = rule({ files: [], pr: {}, enableDebug: false });
+    expect(labels).toEqual([]);
+  });
+
+  it('handles debug mode', () => {
+    const files = [{ filename: '__tests__/test.js' }];
+    expect(() => rule({ files, pr: {}, enableDebug: true })).not.toThrow();
+  });
+});

src/rules/tests/test-improvement.js

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+/**
+ * Test Improvement Detection Rule
+ *
+ * Adds `test-improvement` when both source and test files changed.
+ */
+
+module.exports = function testImprovementRule({ files, pr, enableDebug }) {
+  const labels = [];
+
+  const isTestFile = (name) => /(^|\/).__tests__\//i.test(name) || /\.(test|spec)\.[a-z0-9]+$/i.test(name) || /(^|\/)tests?\//i.test(name);
+  const isSourceFile = (name) => /\.(js|jsx|ts|tsx|java|go|py|rb|rs|php|cpp|c|cs|scala|kt|swift|vue|svelte)$/i.test(name) && !isTestFile(name);
+
+  const names = (files || []).map(f => (f && f.filename ? String(f.filename).toLowerCase() : ''));
+  const hasSource = names.some(n => n && isSourceFile(n));
+  const hasTests = names.some(n => n && isTestFile(n));
+
+  if (hasSource && hasTests) {
+    labels.push('test-improvement');
+  }
+
+  if (enableDebug) {
+    console.log(`[Test Improvement Rule] hasSource=${hasSource} hasTests=${hasTests} → ${labels.join(', ') || 'none'}`);
+  }
+
+  return labels;
+};
+
+module.exports.metadata = {
+  name: 'Test Improvement Detection',
+  description: 'Detects PRs that modify tests alongside code changes',
+  labels: [
+    { name: 'test-improvement', color: '0E8A16', description: 'Tests improved/added with code changes' }
+  ],
+  author: 'pr-auto-labeler',
+  version: '1.0.0',
+  category: 'tests'
+};
+
+

src/rules/tests/test-missing.js

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+/**
+ * Source Changed Without Tests Detection Rule
+ *
+ * Adds `test-missing` when source files changed but no test files changed.
+ */
+
+module.exports = function testMissingRule({ files, pr, enableDebug }) {
+  const labels = [];
+
+  const isTestFile = (name) => /(^|\/)__tests__\//i.test(name) || /\.(test|spec)\.[a-z0-9]+$/i.test(name) || /(^|\/)tests?\//i.test(name);
+  const isSourceFile = (name) => /\.(js|jsx|ts|tsx|java|go|py|rb|rs|php|cpp|c|cs|scala|kt|swift|vue|svelte)$/i.test(name);
+
+  const names = (files || []).map(f => (f && f.filename ? String(f.filename).toLowerCase() : ''));
+  const hasSource = names.some(n => n && isSourceFile(n));
+  const hasTests = names.some(n => n && isTestFile(n));
+
+  if (hasSource && !hasTests) {
+    labels.push('test-missing');
+  }
+
+  if (enableDebug) {
+    console.log(`[Test Missing Rule] hasSource=${hasSource} hasTests=${hasTests} → ${labels.join(', ') || 'none'}`);
+  }
+
+  return labels;
+};
+
+module.exports.metadata = {
+  name: 'Source Changed Without Tests',
+  description: 'Detects code changes without accompanying tests',
+  labels: [
+    { name: 'test-missing', color: 'D93F0B', description: 'Code changes without tests' }
+  ],
+  author: 'pr-auto-labeler',
+  version: '1.0.0',
+  category: 'tests'
+};
+
+

src/rules/tests/test-only-change.js

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+/**
+ * Test-Only Change Detection Rule
+ *
+ * Adds `test-only-change` when only test files are changed.
+ */
+
+module.exports = function testOnlyChangeRule({ files, pr, enableDebug }) {
+  const labels = [];
+
+  const isTestFile = (name) => /(^|\/)__tests__\//i.test(name) || /\.(test|spec)\.[a-z0-9]+$/i.test(name) || /(^|\/)tests?\//i.test(name);
+
+  const names = (files || []).map(f => (f && f.filename ? String(f.filename).toLowerCase() : ''));
+  const hasAny = names.length > 0;
+  const allAreTests = hasAny && names.every(n => n && isTestFile(n));
+
+  if (allAreTests) {
+    labels.push('test-only-change');
+  }
+
+  if (enableDebug) {
+    console.log(`[Test Only Change Rule] allAreTests=${allAreTests} → ${labels.join(', ') || 'none'}`);
+  }
+
+  return labels;
+};
+
+module.exports.metadata = {
+  name: 'Test-only Change Detection',
+  description: 'Detects PRs that modify only test files',
+  labels: [
+    { name: 'test-only-change', color: '0E8A16', description: 'Only test files changed' }
+  ],
+  author: 'pr-auto-labeler',
+  version: '1.0.0',
+  category: 'tests'
+};
+
+