1 files changed, 53 insertions, 0 deletions
diff --git a/pbt/ts/src/property.ts b/pbt/ts/src/property.ts
new file mode 100644
index 0000000..e8d3a21
--- /dev/null
+++ b/pbt/ts/src/property.ts
@@ -0,0 +1,53 @@
+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];
+
+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 property<A>(rng: Prando,
+  size: number,
+  predicate: Predicate<A>,
+  generator: Gen<A>,
+  shrinker: Shrinker<A>): TestResult<A> {
+  let gen = generator(rng);
+  for (let i = 0; i < MAX_SUCCESS; i++) {
+    let x = gen(size);
+    if (!predicate(x)) {
+      let counterexample = findMinimalCounterExample(x, predicate, shrinker);
+      return { result: 'Falsified', counterexample };
+    }
+  }
+  return { result: 'OK', counterexample: null };
+}