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import Prando from 'prando';
type Result = 'OK' | 'Falsified' | 'Exception';
type ShrinkResult<A> = {counterexample: A; shrinks: number};
type TestResult<A> = {
seed: number | null;
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> = (seed: number, 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>(
generator: Gen<A>,
predicate: Predicate<A>,
shrinker?: Shrinker<A>,
): Property<A> {
return (seed: number, size: number) => {
let rng = new Prando(seed);
let gen = generator(rng);
let i = 0;
for (; i < MAX_SUCCESS; i++) {
let x = gen(size);
if (!predicate(x)) {
let counterexample = {counterexample: x, shrinks: 0};
if (shrinker !== undefined) {
counterexample = findMinimalCounterExample(x, predicate, shrinker);
}
return {seed, result: 'Falsified', tests: i + 1, counterexample};
}
size++;
}
return {seed, result: 'OK', tests: i, counterexample: null};
};
}
// basic generators
export function listOf<A>(gen: Gen<A>): Gen<A[]> {
return (rng: Prando) => {
let g = gen(rng);
return (size: number) => {
let result = [];
for (let i = 0; i < size; i++) {
result.push(g(size));
}
return result;
};
};
}
/// generates a map with exactly size elements
export function mapOf<K, V>(genK: Gen<K>, genV: Gen<V>): Gen<Map<K, V>> {
return (rng: Prando) => {
let gk = genK(rng);
let gv = genV(rng);
return (size: number) => {
let result = new Map<K, V>();
for (let i = 0; i < size; i++) {
result.set(gk(size), gv(size));
}
return result;
};
};
}
/// generates a map with at least size+1 elements. As size is expected
/// to be positive or null this guarantees at least one element hence
/// the name
export function mapOf1<K, V>(genK: Gen<K>, genV: Gen<V>): Gen<Map<K, V>> {
return (rng: Prando) => {
let gk = genK(rng);
let gv = genV(rng);
return (size: number) => {
let result = new Map<K, V>();
for (let i = 0; i <= size; i++) {
result.set(gk(size), gv(size));
}
return result;
};
};
}
export function elements<A>(arr: A[]): Gen<A> {
return (rng: Prando) => (_size: number) => {
let index = rng.nextInt(0, arr.length - 1);
return arr[index];
};
}
export function keyValueOf<K, V>(map: Map<K, V>): Gen<[K, V]> {
let keys = Array.from(map.keys());
return (rng: Prando) => {
let gkey = elements(keys)(rng);
return (size: number) => {
let key = gkey(size);
let value = map.get(key)!;
return [key, value];
};
};
}
export function choose(min: number, max: number): Gen<number> {
return (rng: Prando) => (size: number) => rng.nextInt(min, max);
}
export let someInt: Gen<number> = (rng: Prando) => (size: number) =>
rng.nextInt(-size, size);
// basic shrinkers
export function shrinklist<A>(arr: A[]): A[][] {
let result = [];
let tail = arr.slice(1);
let keep1 = arr.slice();
keep1.splice(1, 1);
result.push(tail, keep1);
if (arr.length > 3) {
let half = Math.floor(arr.length / 2);
let copy = arr.slice();
let secondHalf = copy.slice(half);
result.push(secondHalf);
}
return result;
}
export const Property = {
run<A>(prop: Property<A>): TestResult<A> {
let seed = Math.floor(Math.random() * 1000000);
return prop(seed, 0);
},
};
// helpers
export function forall<A, B>(
gen: Gen<A>,
k: (a: A) => Property<B>,
): Property<B> {
return (seed: number, size: number) => {
let rng = new Prando(seed);
let a = gen(rng)(size);
return k(a)(seed, size);
};
}
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