Converted Affine/Atbash operations to mjs & added tests

src/core/lib/Ciphers.mjs

@@ -0,0 +1,41 @@
+/**
+ * Cipher functions.
+ *
+ * @author Matt C [matt@artemisbot.uk]
+ * @copyright Crown Copyright 2018
+ * @license Apache-2.0
+ *
+ */
+
+/**
+ * Affine Cipher Encode operation.
+ *
+ * @author Matt C [matt@artemisbot.uk]
+ * @param {string} input
+ * @param {Object[]} args
+ * @returns {string}
+ */
+export function affineEncode(input, args) {
+    const alphabet = "abcdefghijklmnopqrstuvwxyz",
+        a = args[0],
+        b = args[1];
+    let output = "";
+
+    if (!/^\+?(0|[1-9]\d*)$/.test(a) || !/^\+?(0|[1-9]\d*)$/.test(b)) {
+        return "The values of a and b can only be integers.";
+    }
+
+    for (let i = 0; i < input.length; i++) {
+        if (alphabet.indexOf(input[i]) >= 0) {
+            // Uses the affine function ax+b % m = y (where m is length of the alphabet)
+            output += alphabet[((a * alphabet.indexOf(input[i])) + b) % 26];
+        } else if (alphabet.indexOf(input[i].toLowerCase()) >= 0) {
+            // Same as above, accounting for uppercase
+            output += alphabet[((a * alphabet.indexOf(input[i].toLowerCase())) + b) % 26].toUpperCase();
+        } else {
+            // Non-alphabetic characters
+            output += input[i];
+        }
+    }
+    return output;
+}