From 857af059a7da37ced3150c6088df4eb48f85051d Mon Sep 17 00:00:00 2001
From: Matthew Grove <me@mgrove.uk>
Date: Sun, 30 Jan 2022 12:50:57 +0000
Subject: [PATCH] Add universal Turing machine (Node.JS)

---
 universal-turing-machine.js | 86 +++++++++++++++++++++++++++++++++++++
 1 file changed, 86 insertions(+)
 create mode 100644 universal-turing-machine.js

diff --git a/universal-turing-machine.js b/universal-turing-machine.js
new file mode 100644
index 0000000..315f69a
--- /dev/null
+++ b/universal-turing-machine.js
@@ -0,0 +1,86 @@
+// © Matthew Grove 2022
+
+String.prototype.replaceAt = function(index, replacement) {
+    return this.substr(0, index) + replacement + this.substr(index + replacement.length);
+}
+const delay = ms => new Promise(resolve => setTimeout(resolve, ms));
+
+let turingMachine = class {
+    /**
+     * Constructs a turing machine class object.
+     * @param {tape} String A string containing the contents of the tape.
+     * @param {transitions} String A string containing the transition functions for the machine.
+     *                          States are separated by / and transition functions are separated by ,
+     *                          Each transition function must contain four characters: input character, output character, direction to move (l or r), and state to transition into
+     *                          States are numbered automatically by index, and accepting states must be defined with no transition functions
+     * @param {startPosition} Integer The starting index of the pointer on the tape.
+     * @param {startState} Integer The index of the machine's starting state.
+     * @param {delay} Integer The time period (in ms) to wait before each transition.
+     */
+    constructor(tape, transitions, startPosition, startState, delay) {
+        this.position = startPosition;
+        this.tape = tape;
+        let exportedTransitions = {};
+        transitions.split("/").map((state, stateIndex) => {
+            if (state.length > 0) {
+                exportedTransitions[stateIndex] = {
+                    accepting: false,
+                };
+                state.split(",").map(transition => {
+                    exportedTransitions[stateIndex][transition.charAt(0)] = {
+                        write: transition.charAt(1),
+                        move: transition.charAt(2),
+                        newState: transition.charAt(3),
+                    };
+                }, this);
+            } else {
+                exportedTransitions[stateIndex] = {
+                    accepting: true,
+                };
+            }
+        }, this);
+        this.transitions = exportedTransitions;
+        this.state = startState;
+        this.delay = delay;
+    }
+
+    run = async () => {
+        console.log(`Machine is now running. Tape is currently:
+${this.tape}
+${" ".repeat(this.position)}^`);
+        while (!this.transitions[this.state].accepting) {
+            await delay(this.delay);
+            let transitionData = this.transitions[this.state][this.tape.charAt(this.position)]; // storing write, move, newState
+            let originalChar = this.tape.charAt(this.position);
+            this.tape = this.tape.replaceAt(this.position, transitionData.write);
+            // shift position
+            if (transitionData.move.toLowerCase() === "l") {
+                if (this.position === 0) {
+                    this.tape = ` ${this.tape}`;
+                } else {
+                    this.position--;
+                }
+            } else {
+                this.position++;
+                if (this.position == this.tape.length) {
+                    this.tape = `${this.tape} `;
+                }
+            }
+            console.log(`Read ${originalChar} at position ${this.position} in state ${this.state}, and changed it to ${transitionData.write}. Machine has moved to state ${transitionData.newState} and tape is now:
+${this.tape}
+${" ".repeat(this.position)}^`)
+            this.state = transitionData.newState;
+        }
+
+        if (this.transitions[this.state].accepting) {
+            console.log(`Machine is accepting in state ${this.state}. Tape is now:
+${this.tape}
+${" ".repeat(this.position)}^`)
+        } else {
+            console.log("error...")
+        }
+    }
+}
+
+let machineOne = new turingMachine("01#    ", "00r0,11r0,##r0, 0l2/00r1,11r1,##r1, 1l2/00l2,11l2,##l2,x0r3,y1r3/0xr0,1yr1,##r4/",0,3, 1000);
+machineOne.run();
\ No newline at end of file