spec.js

const sample = require('.');

const samples = (n, l = 10) => new Array(l).fill(0).map(() => sample(n));

describe('sample-size', () => {
	it('Should never pass sample rate 0', () => {
		const results = samples(0);
		expect(results).not.to.contain(true);
	});
	it('Should always pass sample rate 1', () => {
		const results = samples(1);
		expect(results).not.to.contain(false);
	});
	it('Should pass only some', () => {
		const results = samples(.5);
		expect(results).to.contain(false).and.to.contain(true);
	});
	[
		.05,
		.2,
		.5,
		.8,
		.95,
	].forEach(rate => {
		const margin = .025;
		const min = rate - margin;
		const max = rate + margin;

		const rangeDescription = `(between ${Math.round(min * 100)}% and ${Math.round(max * 100)}%)`;

		it(
			`Should sample about ${100 * rate}% for rate ${rate} ${rangeDescription}`,
			function() {
				this.retries(2);

				const size = 1000;
				const results = samples(rate, size).filter(Boolean);
				expect(results)
					.to.have.lengthOf.at.least(size * min).and
					.to.have.lengthOf.at.most(size * max);
			},
		);
	});
	it('Should sample an array', function() {
		this.retries(2);

		const filtered = new Array(100).fill(0).filter(() => sample(.2));
		expect(filtered)
			.to.have.lengthOf.at.least(15).and
			.to.have.lengthOf.at.most(25);
	});
});