Add Length Value Decoder Operatoin

src/core/lib/LengthValue.mjs

@@ -0,0 +1,71 @@
+/**
+ * @author gchq77703 []
+ * @copyright Crown Copyright 2018
+ * @license Apache-2.0
+ */
+
+const defaults = {
+    location: 0,
+    bytesInLength: 1,
+    basicEncodingRules: false
+};
+
+/**
+ * Length Value library
+ */
+export default class LengthValue {
+
+    /**
+     * LengthValue constructor
+     */
+    constructor(input, options) {
+        this.input = input;
+        Object.assign(this, defaults, options);
+    }
+
+    /**
+     * @returns {Number}
+     */
+    getLength() {
+        if (this.basicEncodingRules) {
+            const bit = this.input[this.location];
+            if (bit & 0x80) {
+                this.bytesInLength = bit & ~0x80;
+            } else {
+                this.location++;
+                return bit & ~0x80;
+            }
+        }
+
+        let length = 0;
+
+        for (let i = 0; i < this.bytesInLength; i++) {
+            length += this.input[this.location] * Math.pow(Math.pow(2, 8), i);
+            this.location++;
+        }
+
+        return length;
+    }
+
+    /**
+     * @param {Number} length
+     * @returns {Number[]}
+     */
+    getValue(length) {
+        const value = [];
+
+        for (let i = 0; i < length; i++) {
+            value.push(this.input[this.location]);
+            this.location++;
+        }
+
+        return value;
+    }
+
+    /**
+     * @returns {Boolean}
+     */
+    atEnd() {
+        return this.input.length <= this.location;
+    }
+}