Add operation

Added 'Take nth bytes' operation.

src/core/config/Categories.json

@@ -238,7 +238,8 @@
             "Escape string",
             "Unescape string",
             "Pseudo-Random Number Generator",
-            "Sleep"
+            "Sleep",
+            "Take nth bytes"
         ]
     },
     {

src/core/operations/TakeNthBytes.mjs

@@ -0,0 +1,78 @@
+/**
+ * @author Oshawk [oshawk@protonmail.com]
+ * @copyright Crown Copyright 2019
+ * @license Apache-2.0
+ */
+
+import Operation from "../Operation.mjs";
+import OperationError from "../errors/OperationError.mjs";
+
+/**
+ * Take nth bytes operation
+ */
+class TakeNthBytes extends Operation {
+
+    /**
+     * TakeNthBytes constructor
+     */
+    constructor() {
+        super();
+
+        this.name = "Take nth bytes";
+        this.module = "Default";
+        this.description = "Takes every nth byte starting with a given byte.";
+        this.infoURL = "";
+        this.inputType = "byteArray";
+        this.outputType = "byteArray";
+        this.args = [
+            {
+                name: "Take every",
+                type: "number",
+                value: 4
+            },
+            {
+                name: "Starting at",
+                type: "number",
+                value: 0
+            },
+            {
+                name: "Apply to each line",
+                type: "boolean",
+                value: false
+            }
+        ];
+    }
+
+    /**
+     * @param {byteArray} input
+     * @param {Object[]} args
+     * @returns {byteArray}
+     */
+    run(input, args) {
+        let n = args[0];
+        let start = args[1];
+        let eachLine = args[2];
+
+        if (parseInt(n) !== n || n <= 0) {
+            throw new OperationError("'Take every' must be a positive integer.")
+        }
+        if (parseInt(start) !== start || start < 0) {
+            throw new OperationError("'Starting at' must be a positive or zero integer.")
+        }
+        
+        let offset = 0;
+        let output = [];
+        for (let i = 0; i < input.length; i++) {
+            if (eachLine && input[i] == 0x0a) {
+                offset = i + 1;
+            } else if (i - offset >= start && (i - (start + offset)) % n == 0) {
+                output.push(input[i]);
+            }
+        }
+
+        return output;
+    }
+
+}
+
+export default TakeNthBytes;