3 files changed, 86 insertions, 3 deletions

diff --git a/pbt/ts/src/bank.ts b/pbt/ts/src/bank.ts
index bee9dfc..c5adc05 100644
--- a/pbt/ts/src/bank.ts
+++ b/pbt/ts/src/bank.ts
@@ -5,6 +5,16 @@ export type Account = string;
 
 export interface Bank {
   balance: (account: Account) => Option<number>;
+
+  /// Deposit amount into account
+  ///
+  /// This function increases the balance of the given account by the
+  /// specified amount. It's a `void` function hence implicitly
+  /// stateful: It's expected to change some internal state of the
+  /// implementing class.
+  ///
+  /// @param account The account to deposit into
+  /// @param amt The amount to deposit
   deposit: (account: Account, amt: number) => void;
   withdraw: (account: Account, amt: number) => void;
 }
diff --git a/pbt/ts/src/property.test.ts b/pbt/ts/src/property.test.ts
new file mode 100644
index 0000000..c4e089d
--- /dev/null
+++ b/pbt/ts/src/property.test.ts
@@ -0,0 +1,34 @@
+import {
+  pairOf,
+  positiveInt,
+  property,
+  shrinkPair,
+  shrinkPositive,
+  someInt,
+} from './property';
+
+// from https://github.com/jlink/shrinking-challenge/blob/main/challenges/difference.md
+const predicate1: (pair: [number, number]) => boolean = ([x, y]) =>
+  x < 10 || x != y;
+
+describe('Find minimal counterexample', () => {
+  test('do not shrink example if no shrinker given', () => {
+    const result = property(pairOf(positiveInt), predicate1)(42, 1);
+    expect(result.result).toBe('Falsified');
+    expect(result.counterexample).toBeNull;
+  });
+
+  test('shrink example if shrinker given', () => {
+    const result = property(
+      pairOf(positiveInt),
+      predicate1,
+      shrinkPair(shrinkPositive, shrinkPositive),
+    )(42, 1);
+    expect(result.result).toBe('Falsified');
+    if (result.counterexample !== null) {
+      expect(result.counterexample.counterexample).toStrictEqual([10, 10]);
+    } else {
+      fail('no counterexample');
+    }
+  });
+});
diff --git a/pbt/ts/src/property.ts b/pbt/ts/src/property.ts
index 7b3ee59..500c423 100644
--- a/pbt/ts/src/property.ts
+++ b/pbt/ts/src/property.ts
@@ -20,7 +20,7 @@ type Property<A> = (seed: number, size: number) => TestResult<A>;
 
 const MAX_SUCCESS = 100;
 
-function findMinimalCounterExample<A>(
+export function findMinimalCounterExample<A>(
   x: A,
   predicate: Predicate<A>,
   shrinker: Shrinker<A>,
@@ -117,6 +117,7 @@ export function mapOf1<K, V>(genK: Gen<K>, genV: Gen<V>): Gen<Map<K, V>> {
   };
 }
 
+/// Select randomly one element from given list
 export function elements<A>(arr: A[]): Gen<A> {
   return (rng: Prando) => (_size: number) => {
     let index = rng.nextInt(0, arr.length - 1);
@@ -124,6 +125,7 @@ export function elements<A>(arr: A[]): Gen<A> {
   };
 }
 
+/// Select one key-value pair from given map
 export function keyValueOf<K, V>(map: Map<K, V>): Gen<[K, V]> {
   let keys = Array.from(map.keys());
   return (rng: Prando) => {
@@ -136,12 +138,27 @@ export function keyValueOf<K, V>(map: Map<K, V>): Gen<[K, V]> {
   };
 }
 
+/// Choose number between some bounds.
 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);
+/// Choose some random integer.
+export let someInt: Gen<number> = (rng: Prando) => (size: number) => {
+  const bound = Math.pow(2, size);
+  return rng.nextInt(-bound, bound);
+};
+
+/// Choose some random positive integer.
+export let positiveInt: Gen<number> = (rng: Prando) => (size: number) => {
+  const bound = Math.pow(2, Math.round(Math.sqrt(size)));
+  return rng.nextInt(0, bound);
+};
+
+/// Generate a pair of generated values
+export function pairOf<A>(gen: Gen<A>): Gen<[A, A]> {
+  return (rng: Prando) => (size: number) => [gen(rng)(size), gen(rng)(size)];
+}
 
 // basic shrinkers
 export function shrinklist<A>(arr: A[]): A[][] {
@@ -162,6 +179,28 @@ export function shrinklist<A>(arr: A[]): A[][] {
   return result;
 }
 
+export function shrinkPositive(i: number): number[] {
+  if (i == 0) return [];
+
+  return [i - 1, Math.round(Math.sqrt(i))];
+}
+
+// shrink pair
+export function shrinkPair<A, B>(
+  shrinkA: Shrinker<A>,
+  shrinkB: Shrinker<B>,
+): Shrinker<[A, B]> {
+  return ([x, y]) => {
+    const shrinkx = shrinkA(x);
+    const shrinky = shrinkB(y);
+    // assumes x >= y
+    const resulta: [A, B][] = shrinkx.map(e => [e, y]);
+    const resultb: [A, B][] = shrinky.map(e => [x, e]);
+    const resultboth: [A, B][] = shrinkx.map((e, i) => [e, shrinky[i]]);
+    return resulta.concat(resultb, resultboth);
+  };
+}
+
 export const Property = {
   run<A>(prop: Property<A>): TestResult<A> {
     let seed = Math.floor(Math.random() * 1000000);