stress.{sample, sampleSize}

src/random/number.ts

@@ -1,18 +1,5 @@
 export const defaultRng = Math.random;
 
-function sample<T>(from: ReadonlyArray<T>, count: number, rng = defaultRng): T[] {
-  const sampled: T[] = from.slice(0, count); // 'reservoir'
-
-  for (let i = count; i < from.length; i++) {
-    const j = Math.floor(1 + Math.random() * i);
-    if (j < count) {
-      sampled[j] = from[i];
-    }
-  }
-
-  return sampled;
-}
-
 export function binomialRandom(n: number, p: number, rng = defaultRng): number {
   let s = 0;
   for (let sample = 0; sample < n; ++sample) {

src/stress/__snapshots__/sample.spec.ts.snap

@@ -0,0 +1,10 @@
+// Jest Snapshot v1, https://goo.gl/fbAQLP
+
+exports[`sample picks random value: sample([1,2,3]) 1`] = `1`;
+
+exports[`sampleSize picks specified size: sampleSize([1,2,3], 2) 1`] = `
+[
+  1,
+  2,
+]
+`;

src/stress/sample.spec.ts

@@ -0,0 +1,23 @@
+import { sample, sampleSize } from './sample';
+
+function fakeRng() {
+  return 0.3;
+}
+
+describe('sample', () => {
+  it('picks random value', () => {
+    expect(sample([1, 2, 3], fakeRng)).toMatchSnapshot('sample([1,2,3])');
+  });
+  it('returns undefined on empty array', () => {
+    expect(sample([])).toBeUndefined();
+  });
+});
+
+describe('sampleSize', () => {
+  it('picks specified size', () => {
+    expect(sampleSize([1, 2, 3], 2, fakeRng)).toMatchSnapshot('sampleSize([1,2,3], 2)');
+  });
+  it('returns undefined on empty array', () => {
+    expect(sampleSize([2, 3], 3, fakeRng)).toHaveLength(2);
+  });
+});

src/stress/sample.ts

@@ -0,0 +1,19 @@
+import { defaultRng } from '../random/number';
+
+export function sample<T>(input: readonly T[], rng = defaultRng): T {
+  const index = Math.floor(input.length * rng());
+  return input[index];
+}
+
+export function sampleSize<T>(from: T[], count: number, rng = defaultRng): T[] {
+  const reservoir: T[] = from.slice(0, Math.min(count, from.length));
+
+  for (let i = count; i < from.length; i++) {
+    const j = Math.floor(1 + Math.random() * i);
+    if (j < count) {
+      reservoir[j] = from[i];
+    }
+  }
+
+  return reservoir;
+}