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import Prando from "prando";
type Result = 'OK' | 'Falsified' | 'Exception';
type ShrinkResult<A> = { counterexample: A, shrinks: number };
type TestResult<A> = {
result: Result,
counterexample: ShrinkResult<A> | null
};
type Predicate<A> = (a: A) => boolean;
export type Gen<A> = (rng: Prando) => ((size: number) => A);
type Shrinker<A> = (a: A) => A[];
type Property<A> = (rng: Prando,
size: number) => TestResult<A>;
const MAX_SUCCESS = 100;
function findMinimalCounterExample<A>(x: A,
predicate: Predicate<A>,
shrinker: Shrinker<A>,
depth: number = 0): ShrinkResult<A> {
let xs = shrinker(x);
let counterexample = x;
let shrinks = depth;
for (let y of xs) {
if (!predicate(y)) {
let shrink = findMinimalCounterExample(y, predicate, shrinker, depth + 1);
if (shrink.shrinks > depth) {
counterexample = shrink.counterexample;
shrinks = shrink.shrinks;
}
}
}
return { counterexample, shrinks };
}
export function generate<A>(gen: Gen<A>): A {
let rng = new Prando(Math.random() * 1000);
return gen(rng)(100);
}
export function property<A>(
predicate: Predicate<A>,
generator: Gen<A>,
shrinker: Shrinker<A>): Property<A> {
return (rng: Prando,
size: number) => {
let gen = generator(rng);
let i = 0;
for (; i < MAX_SUCCESS; i++) {
let x = gen(size);
if (!predicate(x)) {
let counterexample = findMinimalCounterExample(x, predicate, shrinker);
return { result: 'Falsified', tests: i+1, counterexample };
}
size++;
}
return { result: 'OK', tests: i, counterexample: null };
};
}
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