Add the "Weighted Random" algorithm (#792)

* Add the link to the Weighted Random algorithm to the main README.

* Add Weighted Random implementation and tests.

* Add Weighted Random README.

* Add Weighted Random README.

* Add Weighted Random README.

src/algorithms/statistics/weighted-random/weightedRandom.js

@@ -0,0 +1,52 @@
+/**
+ * Picks the random item based on its weight.
+ * The items with higher weight will be picked more often (with a higher probability).
+ *
+ * For example:
+ * - items = ['banana', 'orange', 'apple']
+ * - weights = [0, 0.2, 0.8]
+ * - weightedRandom(items, weights) in 80% of cases will return 'apple', in 20% of cases will return
+ * 'orange' and it will never return 'banana' (because probability of picking the banana is 0%)
+ *
+ * @param {any[]} items
+ * @param {number[]} weights
+ * @returns {{item: any, index: number}}
+ */
+/* eslint-disable consistent-return */
+export default function weightedRandom(items, weights) {
+  if (items.length !== weights.length) {
+    throw new Error('Items and weights must be of the same size');
+  }
+
+  if (!items.length) {
+    throw new Error('Items must not be empty');
+  }
+
+  // Preparing the cumulative weights array.
+  // For example:
+  // - weights = [1, 4, 3]
+  // - cumulativeWeights = [1, 5, 8]
+  const cumulativeWeights = [];
+  for (let i = 0; i < weights.length; i += 1) {
+    cumulativeWeights[i] = weights[i] + (cumulativeWeights[i - 1] || 0);
+  }
+
+  // Getting the random number in a range of [0...sum(weights)]
+  // For example:
+  // - weights = [1, 4, 3]
+  // - maxCumulativeWeight = 8
+  // - range for the random number is [0...8]
+  const maxCumulativeWeight = cumulativeWeights[cumulativeWeights.length - 1];
+  const randomNumber = maxCumulativeWeight * Math.random();
+
+  // Picking the random item based on its weight.
+  // The items with higher weight will be picked more often.
+  for (let itemIndex = 0; itemIndex < items.length; itemIndex += 1) {
+    if (cumulativeWeights[itemIndex] >= randomNumber) {
+      return {
+        item: items[itemIndex],
+        index: itemIndex,
+      };
+    }
+  }
+}