diff --git a/Maths/Prime.ts b/Maths/Prime.ts new file mode 100644 index 00000000..5474fade --- /dev/null +++ b/Maths/Prime.ts @@ -0,0 +1,57 @@ +const assert = (condition: boolean, message: string) => { + if (!condition) throw Error(message); +}; + +/** + * https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes + * @param limit An integer _n_ > 1 + * @returns All prime numbers from 2 through {@linkcode limit} + */ +export function sieveOfEratosthenes(limit: number): number[] { + assert(limit > 1, "limit should be an integer greater than 1"); + assert(Number.isInteger(limit), "limit should be an integer greater than 1"); + + const maybePrime: boolean[] = new Array(limit + 1).fill(true); + for (let i = 2; i * i <= limit; i++) { + if (!maybePrime[i]) continue; + for (let j = i * i; j <= limit; j += i) { + maybePrime[j] = false; + } + } + + return maybePrime + .reduce( + (primes, isPrime, number) => (isPrime ? [...primes, number] : primes), + [] as number[] + ) + .slice(2); +} + +/** + * Generator that yields primes. + * + * Inspired by https://gist.github.com/e-nikolov/cd94db0de2a6b70da144124ae93a6458 + */ +export function* primeGenerator() { + type NumberGen = Generator; + + function* filter(input: NumberGen, prime: number): NumberGen { + while (true) { + const { done, value } = input.next(); + if (done) break; + if (value % prime !== 0) yield value; + } + } + + let chain: NumberGen = (function* () { + let i = 2; + while (true) yield i++; + })(); + + while (true) { + const { done, value } = chain.next(); + if (done) break; + yield value; + chain = filter(chain, value); + } +} diff --git a/Maths/test/Prime.test.ts b/Maths/test/Prime.test.ts new file mode 100644 index 00000000..b55ace80 --- /dev/null +++ b/Maths/test/Prime.test.ts @@ -0,0 +1,35 @@ +import { primeGenerator, sieveOfEratosthenes } from "../Prime"; + +describe(sieveOfEratosthenes, () => { + test.each([-1, 0, 1, 2.123, 1337.80085])( + "should throw an error when given an invalid limit=%d", + (invalidLimit) => { + expect(() => sieveOfEratosthenes(invalidLimit)).toThrow(); + } + ); + test.each([ + [2, [2]], + [3, [2, 3]], + [4, [2, 3]], + [5, [2, 3, 5]], + [6, [2, 3, 5]], + [7, [2, 3, 5, 7]], + [8, [2, 3, 5, 7]], + [9, [2, 3, 5, 7]], + ])( + "should return the expected list of primes for limit=%i", + (limit, expected) => { + expect(sieveOfEratosthenes(limit)).toEqual(expected); + } + ); +}); + +describe(primeGenerator, () => { + it("should generate prime numbers", () => { + const expectedPrimes = sieveOfEratosthenes(123); + const primeGen = primeGenerator(); + for (const prime of expectedPrimes) { + expect(primeGen.next().value).toBe(prime); + } + }); +});