fix(instrumenter): switch case mutant placer (#2518)

Add a `MutantPlacer` for `SwitchCase` nodes. It places the mutants in the `consequence` part of the switch case. This can dramatically improve the performance of instrumenting large switch-case statements.

This:

```js
switch(foo){
  case 'bar':
    console.log('bar');
    break;
  case 'baz':
    console.log('baz');
    break;

}
```

Is now instrumented as:

```js
switch (foo) {
  case stryMutAct_9fa48(1) ? \\"\\" : (stryCov_9fa48(1), 'bar'):
    if (stryMutAct_9fa48(0)) {} else {
      stryCov_9fa48(0);
      console.log(stryMutAct_9fa48(2) ? \\"\\" : (stryCov_9fa48(2), 'bar'));
      break;
    }

  case stryMutAct_9fa48(4) ? \\"\\" : (stryCov_9fa48(4), 'baz'):
    if (stryMutAct_9fa48(3)) {} else {
      stryCov_9fa48(3);
      console.log(stryMutAct_9fa48(5) ? \\"\\" : (stryCov_9fa48(5), 'baz'));
      break;
    }
}
```

While previously the entire `SwitchCaseExpression` was duplicated for each empty switch case mutation.

packages/instrumenter/src/mutant-placers/index.ts

@@ -8,8 +8,9 @@ import { Mutant } from '../mutant';
 import { MutantPlacer } from './mutant-placer';
 import { statementMutantPlacer } from './statement-mutant-placer';
 import { expressionMutantPlacer } from './expression-mutant-placer';
+import { switchCaseMutantPlacer } from './switch-case-mutant-placer';
 
-export const MUTANT_PLACERS = Object.freeze([expressionMutantPlacer, statementMutantPlacer]);
+export const MUTANT_PLACERS = Object.freeze([expressionMutantPlacer, statementMutantPlacer, switchCaseMutantPlacer]);
 
 /**
  * Represents a mutant placer, tries to place a mutant in the AST with corresponding mutation switch and mutant covering expression

packages/instrumenter/src/mutant-placers/switch-case-mutant-placer.ts

@@ -0,0 +1,44 @@
+import { types } from '@babel/core';
+
+import { createMutatedAst, mutantTestExpression, mutationCoverageSequenceExpression } from '../util';
+
+import { MutantPlacer } from './mutant-placer';
+
+/**
+ * Places the mutants with consequent of a SwitchCase node. Uses an if-statement to do so.
+ * @example
+ *  case 'foo':
+ *    if (stryMutAct_9fa48(0)) {} else {
+ *      stryCov_9fa48(0);
+ *      console.log('bar');
+ *      break;
+ *   }
+ */
+const switchCaseMutantPlacer: MutantPlacer = (path, mutants): boolean => {
+  if (path.isSwitchCase()) {
+    // First transform the mutated ast before we start to apply mutants.
+    const appliedMutants = mutants.map((mutant) => {
+      const ast = createMutatedAst(path, mutant);
+      if (!types.isSwitchCase(ast)) {
+        throw new Error(`${switchCaseMutantPlacer.name} can only place SwitchCase syntax`);
+      }
+      return {
+        ast,
+        mutant,
+      };
+    });
+
+    const instrumentedConsequent = appliedMutants.reduce(
+      // Add if statements per mutant
+      (prev: types.Statement, { ast, mutant }) => types.ifStatement(mutantTestExpression(mutant.id), types.blockStatement(ast.consequent), prev),
+      types.blockStatement([types.expressionStatement(mutationCoverageSequenceExpression(mutants)), ...path.node.consequent])
+    );
+    path.replaceWith(types.switchCase(path.node.test, [instrumentedConsequent]));
+    return true;
+  }
+
+  return false;
+};
+
+// Export it after initializing so `fn.name` is properly set
+export { switchCaseMutantPlacer };

packages/instrumenter/test/integration/instrumenter.it.spec.ts

@@ -49,6 +49,9 @@ describe('instrumenter integration', () => {
   it('should be able to instrument js files with a shebang in them', async () => {
     await arrangeAndActAssert('shebang.js');
   });
+  it('should be able to instrument switch case statements (using the switchCaseMutantPlacer)', async () => {
+    await arrangeAndActAssert('switch-case.js');
+  });
 
   describe('type declarations', () => {
     it('should not produce mutants for TS type definitions', async () => {

packages/instrumenter/test/unit/mutant-placers/switch-case-mutant-placer.spec.ts

@@ -0,0 +1,103 @@
+import { NodePath, types } from '@babel/core';
+import generate from '@babel/generator';
+import { normalizeWhitespaces } from '@stryker-mutator/util';
+import { expect } from 'chai';
+
+import { switchCaseMutantPlacer as sut } from '../../../src/mutant-placers/switch-case-mutant-placer';
+import { createMutant } from '../../helpers/factories';
+import { findNodePath, parseJS } from '../../helpers/syntax-test-helpers';
+
+describe(sut.name, () => {
+  it('should have the correct name', () => {
+    expect(sut.name).eq('switchCaseMutantPlacer');
+  });
+
+  it('should not place mutants on non-switch-case nodes', () => {
+    [
+      findNodePath(parseJS('foo + bar'), (p) => p.isBinaryExpression()),
+      findNodePath(parseJS('switch(foo) { }'), (p) => p.isSwitchStatement()),
+    ].forEach((node) => {
+      expect(sut(node, [])).false;
+    });
+  });
+
+  it('should only place SwitchCase nodes', () => {
+    const switchCase = findNodePath(parseJS('switch(foo) { case "bar": console.log("bar"); break; }'), (p) => p.isSwitchCase());
+    const mutant = createMutant({ original: switchCase.node, replacement: types.stringLiteral('foo') });
+    expect(() => sut(switchCase, [mutant])).throws('switchCaseMutantPlacer can only place SwitchCase syntax');
+  });
+
+  describe('given a SwitchCase node', () => {
+    let ast: types.File;
+    let switchCase: NodePath;
+
+    beforeEach(() => {
+      ast = parseJS('switch(foo) { case "bar": console.log("bar"); break; }');
+      switchCase = findNodePath(ast, (p) => p.isSwitchCase());
+    });
+
+    it('should place a mutant in the "consequent" part of a switch-case', () => {
+      // Arrange
+      const mutant = createMutant({ id: 42, original: switchCase.node, replacement: types.switchCase(types.stringLiteral('bar'), []) });
+
+      // Act
+      const actual = sut(switchCase, [mutant]);
+      const actualCode = normalizeWhitespaces(generate(ast).code);
+
+      // Assert
+      expect(actual).true;
+      expect(actualCode).contains(normalizeWhitespaces('switch (foo) { case "bar": if (stryMutAct_9fa48(42))'));
+    });
+
+    it('should place the original code as alternative (inside `else`)', () => {
+      // Arrange
+      const mutant = createMutant({ id: 42, original: switchCase.node, replacement: types.switchCase(types.stringLiteral('bar'), []) });
+
+      // Act
+      const actual = sut(switchCase, [mutant]);
+      const actualCode = normalizeWhitespaces(generate(ast).code);
+
+      // Assert
+      expect(actual).true;
+      expect(actualCode).matches(/else {.* console\.log\("bar"\); break; }/);
+    });
+
+    it('should add mutant coverage syntax', () => {
+      // Arrange
+      const mutant = createMutant({ id: 42, original: switchCase.node, replacement: types.switchCase(types.stringLiteral('bar'), []) });
+
+      // Act
+      const actual = sut(switchCase, [mutant]);
+      const actualCode = normalizeWhitespaces(generate(ast).code);
+
+      // Assert
+      expect(actual).true;
+      expect(actualCode).matches(/else\s*{\s*stryCov_9fa48\(42\)/);
+    });
+
+    it('should be able to place multiple mutants', () => {
+      // Arrange
+      const mutants = [
+        createMutant({ id: 42, original: switchCase.node, replacement: types.switchCase(types.stringLiteral('bar'), []) }),
+        createMutant({
+          id: 156,
+          original: switchCase.node,
+          replacement: types.switchCase(types.stringLiteral('bar'), [types.expressionStatement(types.callExpression(types.identifier('foo'), []))]),
+        }),
+      ];
+
+      // Act
+      sut(switchCase, mutants);
+      const actualCode = normalizeWhitespaces(generate(ast).code);
+
+      // Assert
+      expect(actualCode).contains(
+        normalizeWhitespaces(`if (stryMutAct_9fa48(156)) {
+          foo();
+          } else if (stryMutAct_9fa48(42)) {} 
+          else {
+            stryCov_9fa48(42, 156)`)
+      );
+    });
+  });
+});

packages/instrumenter/testResources/instrumenter/switch-case.js

@@ -0,0 +1,9 @@
+switch(foo){
+  case 'bar':
+    console.log('bar');
+    break;
+  case 'baz': 
+    console.log('baz');
+    break;
+
+}

packages/instrumenter/testResources/instrumenter/switch-case.js.out.snap

@@ -0,0 +1,74 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`instrumenter integration should be able to instrument switch case statements (using the switchCaseMutantPlacer) 1`] = `
+"function stryNS_9fa48() {
+  var g = new Function(\\"return this\\")();
+  var ns = g.__stryker__ || (g.__stryker__ = {});
+
+  if (ns.activeMutant === undefined && g.process && g.process.env && g.process.env.__STRYKER_ACTIVE_MUTANT__) {
+    ns.activeMutant = Number(g.process.env.__STRYKER_ACTIVE_MUTANT__);
+  }
+
+  function retrieveNS() {
+    return ns;
+  }
+
+  stryNS_9fa48 = retrieveNS;
+  return retrieveNS();
+}
+
+stryNS_9fa48();
+
+function stryCov_9fa48() {
+  var ns = stryNS_9fa48();
+  var cov = ns.mutantCoverage || (ns.mutantCoverage = {
+    static: {},
+    perTest: {}
+  });
+
+  function cover() {
+    var c = cov.static;
+
+    if (ns.currentTestId) {
+      c = cov.perTest[ns.currentTestId] = cov.perTest[ns.currentTestId] || {};
+    }
+
+    var a = arguments;
+
+    for (var i = 0; i < a.length; i++) {
+      c[a[i]] = (c[a[i]] || 0) + 1;
+    }
+  }
+
+  stryCov_9fa48 = cover;
+  cover.apply(null, arguments);
+}
+
+function stryMutAct_9fa48(id) {
+  var ns = stryNS_9fa48();
+
+  function isActive(id) {
+    return ns.activeMutant === id;
+  }
+
+  stryMutAct_9fa48 = isActive;
+  return isActive(id);
+}
+
+switch (foo) {
+  case stryMutAct_9fa48(1) ? \\"\\" : (stryCov_9fa48(1), 'bar'):
+    if (stryMutAct_9fa48(0)) {} else {
+      stryCov_9fa48(0);
+      console.log(stryMutAct_9fa48(2) ? \\"\\" : (stryCov_9fa48(2), 'bar'));
+      break;
+    }
+
+  case stryMutAct_9fa48(4) ? \\"\\" : (stryCov_9fa48(4), 'baz'):
+    if (stryMutAct_9fa48(3)) {} else {
+      stryCov_9fa48(3);
+      console.log(stryMutAct_9fa48(5) ? \\"\\" : (stryCov_9fa48(5), 'baz'));
+      break;
+    }
+
+}"
+`;