Entropy Library and making existing operations use it

src/core/lib/Entropy.mjs

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+/**
+ * @author n1073645 [n1073645@gmail.com]
+ * @copyright Crown Copyright 2019
+ * @license Apache-2.0
+ */
+
+/**
+ * Calculates the frequency of bytes in the input.
+ *
+ * @param {Uint8Array} input
+ * @returns {number}
+ */
+export function freqDist(input) {
+    const prob = [],
+        occurrences = new Array(256).fill(0);
+
+    // Count occurrences of each byte in the input
+    let i;
+    for (i = 0; i < input.length; i++) {
+        occurrences[input[i]]++;
+    }
+
+    // Store probability list
+    for (i = 0; i < occurrences.length; i++) {
+        if (occurrences[i] > 0) {
+            prob.push(occurrences[i] / input.length);
+        }
+    }
+    return prob;
+}
+
+/**
+ * Calculates the entropy of the input.
+ *
+ * @param {Uint8Array} input
+ * @returns {number}
+ */
+export function calculateShannonEntropy(input) {
+
+    const prob = freqDist(input);
+
+    // Calculate Shannon entropy
+    let entropy = 0,
+        p;
+
+    for (let i = 0; i < prob.length; i++) {
+        p = prob[i];
+        entropy += p * Math.log(p) / Math.log(2);
+    }
+
+    return -entropy;
+}
+
+/**
+ * Calculates the entropy of the input from the passed probability.
+ *
+ * @param {Uint8Array} prob
+ * @returns {number}
+ */
+export function calculateShannonEntropyFromProb(prob) {
+
+    // Calculate Shannon entropy
+    let entropy = 0,
+        p;
+
+    for (let i = 0; i < prob.length; i++) {
+        p = prob[i];
+        entropy += p * Math.log(p) / Math.log(2);
+    }
+
+    return -entropy;
+}
+
+/**
+ * Calculates the scanning entropy of the input.
+ *
+ * @param {Uint8Array} inputBytes
+ * @param {number} binWidth
+ * @returns {Object}
+ */
+export function calculateScanningEntropy(inputBytes, binWidth) {
+    const entropyData = [];
+
+    for (let bytePos = 0; bytePos < inputBytes.length; bytePos += binWidth) {
+        const block = inputBytes.slice(bytePos, bytePos+binWidth);
+        entropyData.push(calculateShannonEntropy(block));
+    }
+
+    return { entropyData, binWidth };
+}