more checks on shrinking

1 files changed, 34 insertions, 0 deletions

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');
+    }
+  });
+});