Fix randomBetween overflow and rounding issues

random/between.ts

@@ -39,5 +39,7 @@ export function randomBetween(
     throw new RangeError("max must be greater than or equal to min");
   }
 
-  return (options?.prng ?? Math.random)() * (max - min) + min;
+  const x = (options?.prng ?? Math.random)();
+  const y = min * (1 - x) + max * x;
+  return y >= min && y < max ? y : min;
 }

random/between_test.ts

@@ -2,10 +2,12 @@
 import { randomBetween } from "./between.ts";
 import { randomSeeded } from "./seeded.ts";
 import {
+  assert,
   assertAlmostEquals,
   assertEquals,
   assertGreaterOrEqual,
   assertLessOrEqual,
+  assertNotEquals,
   assertThrows,
 } from "@std/assert";
 
@@ -86,3 +88,21 @@ Deno.test("randomBetween() allows min and max to be the same, in which case it r
   const results = Array.from({ length: 3 }, () => randomBetween(9.99, 9.99));
   assertEquals(results, [9.99, 9.99, 9.99]);
 });
+
+Deno.test("randomBetween() never returns max, even if the prng returns its max value", () => {
+  const prng = () => 0.9999999999999999;
+  const result = randomBetween(1, 2, { prng });
+  assertNotEquals(result, 2);
+});
+
+Deno.test("randomBetween() doesn't overflow even for min = -Number.MAX_VALUE; max = Number.MAX_VALUE", () => {
+  for (const val of [0, 0.5, 0.9999999999999999]) {
+    const result = randomBetween(
+      -Number.MAX_VALUE,
+      Number.MAX_VALUE,
+      { prng: () => val! },
+    );
+    console.log(result)
+    assert(Number.isFinite(result));
+  }
+});