Add UnbiasedProperty class to force non-biased values

src/check/property/UnbiasedProperty.ts

@@ -0,0 +1,12 @@
+import Random from '../../random/generator/Random';
+import Arbitrary from '../arbitrary/definition/Arbitrary';
+import Shrinkable from '../arbitrary/definition/Shrinkable';
+import IProperty from './IProperty';
+
+/** @hidden */
+export class UnbiasedProperty<Ts> implements IProperty<Ts> {
+  constructor(readonly property: IProperty<Ts>) {}
+  isAsync = () => this.property.isAsync();
+  generate = (mrng: Random, freq?: number) => this.property.generate(mrng);
+  run = (v: Ts) => this.property.run(v);
+}

test/unit/check/property/UnbiasedProperty.spec.ts

@@ -0,0 +1,51 @@
+import * as assert from 'assert';
+
+import Arbitrary from '../../../../src/check/arbitrary/definition/Arbitrary';
+import Shrinkable from '../../../../src/check/arbitrary/definition/Shrinkable';
+import IProperty from '../../../../src/check/property/IProperty';
+import { UnbiasedProperty } from '../../../../src/check/property/UnbiasedProperty';
+
+import * as stubRng from '../../stubs/generators';
+
+describe('UnbiasedProperty', () => {
+  it('Should forward parameters correctly (asynchronous property)', () => {
+    let calledWithFreq: number | undefined = undefined;
+    const pAsync = new class implements IProperty<number> {
+      isAsync = () => true;
+      generate = (mrng: any, freq?: number) => {
+        calledWithFreq = freq;
+        return new Shrinkable(42);
+      };
+      run = (v: number) => 'pAsync:' + v;
+    }();
+
+    const unbiasedAsyncProp = new UnbiasedProperty(pAsync);
+
+    assert.equal(unbiasedAsyncProp.isAsync(), true);
+    assert.equal(unbiasedAsyncProp.generate(stubRng.mutable.nocall()).value, 42);
+    assert.strictEqual(calledWithFreq, undefined);
+    assert.equal(unbiasedAsyncProp.generate(stubRng.mutable.nocall(), 52).value, 42);
+    assert.strictEqual(calledWithFreq, undefined);
+    assert.equal(unbiasedAsyncProp.run(47), 'pAsync:47');
+  });
+  it('Should forward parameters correctly (synchronous property)', () => {
+    let calledWithFreq: number | undefined = undefined;
+    const pSync = new class implements IProperty<number> {
+      isAsync = () => false;
+      generate = (mrng: any, freq?: number) => {
+        calledWithFreq = freq;
+        return new Shrinkable(48);
+      };
+      run = (v: number) => 'pSync:' + v;
+    }();
+
+    const unbiasedSyncProp = new UnbiasedProperty(pSync);
+
+    assert.equal(unbiasedSyncProp.isAsync(), false);
+    assert.equal(unbiasedSyncProp.generate(stubRng.mutable.nocall()).value, 48);
+    assert.strictEqual(calledWithFreq, undefined);
+    assert.equal(unbiasedSyncProp.generate(stubRng.mutable.nocall(), 52).value, 48);
+    assert.strictEqual(calledWithFreq, undefined);
+    assert.equal(unbiasedSyncProp.run(29), 'pSync:29');
+  });
+});