Check for random sampling

rndsampling.js

const assert = require('assert');

const checks = {
/*	original_post: (val, range, maxNum) => {
		return val + range - (val % range) < maxNum;
	},*/
	traditional: (val, range, maxNum) => {
		return val < Math.floor(maxNum / range) * range;
	},
	correct: (val, range, maxNum) => {
		return val < maxNum - maxNum % range;
	},
};

const maxNum = 256;
for (let range = 1;range < maxNum;range++) {
	const all_counts = {};
	for (const check_name in checks) {
		const check = checks[check_name];

		const counts = new Array(range).fill(0);
		for (let val = 0;val < maxNum;val++) {
			if (check(val, range, maxNum)) {
				const res = val % range;
				counts[res]++;
			}
		}

		all_counts[check_name] = counts;
		for (c of counts) {
			if (c !== counts[0]) {
				throw new Error(
					'[' + check_name + '] Biased for range ' + range +
					': ' + JSON.stringify(counts));
			}
		}
	}

	const ref_counts = all_counts.traditional;
	for (const check_name in all_counts) {
		const this_counts = all_counts[check_name];
		assert.deepStrictEqual(
			this_counts, ref_counts,
			'Counts of ' + check_name + ' incorrect for range = ' + range + ': ' + JSON.stringify(this_counts));
	}
}
console.log('All tests passed');