1 files changed, 27 insertions, 0 deletions
diff --git a/pbt/ts/src/property.ts b/pbt/ts/src/property.ts
index 500c423..04f2a62 100644
--- a/pbt/ts/src/property.ts
+++ b/pbt/ts/src/property.ts
@@ -1,3 +1,5 @@
+import {Option, getOrElse, isSome, none, some} from 'fp-ts/lib/Option';
+import {LazyArg, pipe} from 'fp-ts/lib/function';
 import Prando from 'prando';
 
 type Result = 'OK' | 'Falsified' | 'Exception';
@@ -125,6 +127,31 @@ export function elements<A>(arr: A[]): Gen<A> {
   };
 }
 
+export function frequency<A>(distribution: [number, Gen<A>][]): Gen<A> {
+  const total: number = distribution.reduce((acc, [freq, _]) => acc + freq, 0);
+
+  return (rng: Prando) => (size: number) => {
+    const idx = rng.next(0, total);
+    let selectedGen: Option<Gen<A>> = none;
+    selectedGen = distribution.reduce((found, [freq, gen]) => {
+      if (isSome(found)) {
+        return found;
+      } else if (idx <= freq) {
+        return some(gen);
+      } else {
+        return none;
+      }
+    }, selectedGen);
+    const generator: Gen<A> = pipe(
+      selectedGen,
+      getOrElse((() => {
+        throw new Error(`invalid frequency distribution ${distribution}`);
+      }) as LazyArg<Gen<A>>),
+    );
+    return generator(rng)(size);
+  };
+}
+
 /// 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());