Seed Generators

FuzzSeed is a deterministic pseudo-random source. The same seed produces the same sequence, which is what makes failures reproducible. You receive one inside .generate() and call its methods to build each input.

.generate((seed: FuzzSeed, run: (n: i32, s: string) => bool): void => {
  run(seed.i32({ min: 0, max: 255 }), seed.string({ charset: "alpha", max: 16 }));
});

Generators

Method	Returns	Options
boolean() / bool()	bool	—
pick<T>(values)	a random element of values	—
i8 / u8 / i16 / u16 / i32 / u32 / i64 / u64	integer	{ min, max, exclude }
f32 / f64	float (default range [0, 1))	{ min, max, exclude }
bytes(options?)	Uint8Array	{ min, max, include, exclude }
buffer(options?)	ArrayBuffer	same as bytes
string(options?)	string	{ charset, min, max, prefix, suffix, include, exclude }
array<T>(item, options?)	Array<T>	{ min, max }
reseed(seed)	resets the sequence	—

Integer and float exclude lists drop specific values from the range; bytes include/exclude and string include/exclude operate on byte values and codepoints respectively.

String charsets

seed.string({ charset }) selects from a named character set:

Charset	Characters
ascii (default)	printable ASCII
alpha	A–Z, a–z
digit	0–9
hex	0–9, a–f
alnum	letters and digits
base64	base64 alphabet
identifier	valid identifier characters
whitespace	whitespace characters

seed.string({ charset: "hex", min: 8, max: 8 });        // 8 hex chars
seed.string({ charset: "alnum", prefix: "id_", max: 12 });

Composing generators

array takes a per-item generator, so you can build structured inputs:

.generate((seed: FuzzSeed, run: (xs: i32[]) => bool): void => {
  run(seed.array<i32>((s: FuzzSeed): i32 => s.i32({ min: 0, max: 9 }), { min: 1, max: 16 }));
});

Example

import { expect, fuzz, FuzzSeed } from "as-test";

fuzz("sorting is idempotent", (xs: i32[]): bool => {
  const once = sort(xs);
  const twice = sort(once);
  expect(twice).toEqual(once);
  return true;
}).generate((seed: FuzzSeed, run: (xs: i32[]) => bool): void => {
  run(seed.array<i32>((s: FuzzSeed): i32 => s.i32(), { min: 0, max: 32 }));
});