Adds set.choose

src/array/array.ts

@@ -30,6 +30,14 @@ export function concat<T>(...elems: Wrapped<T>[]): Thunk<T[]> {
   };
 }
 
+/*
+ * Given a list of fuzzers, and a density function, retrieve a subset of elements
+ *
+ * @param elems A list of fuzzers
+ * @param density A function that takes a min and max, and returns a BigInt
+ *
+ * @returns A thunk that returns a subset of the umwrapped fuzzers
+ */
 export function choose<T>(elems: Wrapped<T[]>, density: DensityBigInt): Thunk<T[]> {
   return () => {
     const concreteElems = unwrap(elems);

src/set/set.test.ts

@@ -13,3 +13,52 @@ Deno.test({
     }
   },
 });
+
+
+Deno.test({
+  name: "Set.choose | returns a subset of the provided collection",
+  fn() {
+    const integers = Peach.Set.from(
+      Peach.Number.uniform(0, 100),
+      Peach.Number.uniform(0, 100));
+
+    for (let idx = 0; idx < 1_000; idx++) {
+      const sample = integers();
+
+      const result = Peach.Set.choose(sample, Peach.BigInt.uniform);
+      if (result.length > 100) {
+        throw new Error(`out of range 0...100: ${result}`);
+      }
+
+      for (const elem of sample) {
+        if (elem < 0 || elem > 100) {
+          throw new Error(`out of range 0...100: ${elem}`);
+        }
+      }
+    }
+  }
+});
+
+Deno.test({
+  name: "Set.choose | preserves elements",
+  fn() {
+    const integers = Peach.Set.from(
+      0,
+      Peach.Number.uniform(0, 100));
+
+    for (let idx = 0; idx < 1_000; idx++) {
+      const sample = integers();
+
+      const result = Peach.Set.choose(sample, Peach.BigInt.uniform);
+      if (result.length > 100) {
+        throw new Error(`out of range 0...100: ${result}`);
+      }
+
+      for (const elem of sample) {
+        if (elem !== 0) {
+          throw new Error(`elem was not zero: ${elem}`);
+        }
+      }
+    }
+  }
+})

src/set/set.ts

@@ -1,4 +1,4 @@
-import type { Thunk, Wrapped } from "../types.ts";
+import type { Thunk, Wrapped, DensityBigInt } from "../types.ts";
 import { unwrap } from "../types.ts";
 
 /**
@@ -26,3 +26,36 @@ export function from<T>(
     return elems;
   };
 }
+
+
+/*
+ * Given a list of fuzzers, and a density function, retrieve a subset of elements
+ *
+ * @param elems A list of fuzzers
+ * @param density A function that takes a min and max, and returns a BigInt
+ *
+ * @returns A thunk that returns a subset of the umwrapped fuzzers
+ */
+export function choose<T>(elems: Wrapped<Set<T>>, density: DensityBigInt): Thunk<T[]> {
+  return () => {
+    const concreteElems = Array.from(unwrap(elems));
+    const subsetCount = BigInt(2) ^ BigInt(concreteElems.length);
+    const index = unwrap(density(BigInt(0), subsetCount));
+
+    // bits correspond to a include-or-don't for each element
+    const bits = index
+      .toString(2)
+      .padStart(concreteElems.length, "0")
+      .split("");
+
+    const output: T[] = [];
+
+    for (let idx = 0; idx < bits.length; idx++) {
+      if (bits[idx] === "1") {
+        output.push(concreteElems[idx]);
+      }
+    }
+
+    return output;
+  }
+}