Add weightedSample snippet

snippets/weightedSample.md

@@ -0,0 +1,31 @@
+---
+title: weightedSample
+tags: array,random,advanced
+---
+
+Returns a random element from an array, using the provided `weights` as the probabilities for each element.
+
+Use `Array.prototype.reduce()` to create an array of partial sums for each value in `weights`.
+Use `Math.random()` to generate a random number and `Array.prototype.findIndex()` to find the correct index based on the array previously produced.
+Finally, return the element of `arr` with the produced index.
+
+
+```js
+const weightedSample = (arr, weights) => {
+  let roll = Math.random();
+  return arr[
+    weights
+      .reduce((acc, w, i) =>
+        i === 0 ? [w] : [...acc, acc[acc.length - 1] + w],
+        []
+      )
+      .findIndex((v, i, s) =>
+        roll >= (i === 0 ? 0 : s[i - 1]) && roll < v
+      )
+  ];
+}
+```
+
+```js
+weightedSample([3, 7, 9, 11], [0.1, 0.2, 0.6, 0.1]); // 9
+```