1 files changed, 17 insertions, 12 deletions
diff --git a/pbt/ts/src/property.ts b/pbt/ts/src/property.ts
index 04f2a62..da05328 100644
--- a/pbt/ts/src/property.ts
+++ b/pbt/ts/src/property.ts
@@ -131,21 +131,26 @@ 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 idx = Math.floor(rng.next(0, total));
+    const [selectedGen, _] = distribution.reduce(
+      ([found, sum], [freq, gen]) => {
+        const current = sum + freq;
+        if (isSome(found)) {
+          return [found, current];
+        } else if (idx <= current) {
+          return [some(gen), current];
+        } else {
+          return [none, current];
+        }
+      },
+      [none as Option<Gen<A>>, 0],
+    );
     const generator: Gen<A> = pipe(
       selectedGen,
       getOrElse((() => {
-        throw new Error(`invalid frequency distribution ${distribution}`);
+        throw new Error(
+          `invalid frequency distribution ${distribution}, idx=${idx}`,
+        );
       }) as LazyArg<Gen<A>>),
     );
     return generator(rng)(size);