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Task/Universal-Turing-machine/D/universal-turing-machine.d
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190
Task/Universal-Turing-machine/D/universal-turing-machine.d
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import std.stdio, std.algorithm, std.string, std.conv, std.array,
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std.exception;
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struct UTM(bool doShow=true) {
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alias Symbol = uint;
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alias State = char; // Typedef?
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enum Direction { right, left, stay }
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private TapeHead head;
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private const TuringMachine tm;
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static struct Rule {
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Symbol toWrite;
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Direction direction;
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State nextState;
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}
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static struct TuringMachine {
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Symbol[] symbols;
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Symbol blank;
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State initialState;
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State[] haltStates, runningStates;
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Rule[Symbol][State] rules;
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Symbol[] input;
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}
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static struct TapeHead {
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immutable Symbol blank;
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Symbol[] tape;
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size_t position;
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this(in ref TuringMachine t) pure /*nothrow*/ {
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this.blank = t.blank;
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this.tape = t.input.dup; // Not nothrow.
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if (this.tape.empty)
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this.tape = [this.blank];
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this.position = 0;
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}
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pure nothrow invariant() {
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assert(this.position < this.tape.length);
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}
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Symbol read() const pure nothrow {
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return this.tape[this.position];
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}
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void show() const {
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.write(this);
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}
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void write(in Symbol symbol) {
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static if (doShow)
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show();
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this.tape[this.position] = symbol;
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}
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void right() pure nothrow {
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this.position++;
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if (this.position == this.tape.length)
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this.tape ~= this.blank;
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}
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void left() pure nothrow {
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if (this.position == 0)
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this.tape = this.blank ~ this.tape;
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else
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this.position--;
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}
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void move(in Direction dir) {
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final switch (dir) {
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case Direction.left: left(); break;
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case Direction.right: right(); break;
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case Direction.stay: /*Do nothing.*/ break;
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}
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}
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string toString() const {
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return format("...%(%)...", this.tape)
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~ '\n'
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~ format("%" ~ text(this.position + 4) ~ "s", "^")
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~ '\n';
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}
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}
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void show() const {
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head.show();
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}
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this(const ref TuringMachine tm_) {
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immutable errMsg = "Invalid input.";
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enforce(!tm_.runningStates.empty, errMsg);
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enforce(!tm_.haltStates.empty, errMsg);
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enforce(!tm_.symbols.empty, errMsg);
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enforce(tm_.rules.length, errMsg);
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enforce(tm_.runningStates.canFind(tm_.initialState), errMsg);
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enforce(tm_.symbols.canFind(tm_.blank), errMsg);
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const allStates = tm_.runningStates ~ tm_.haltStates;
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foreach (const s; tm_.rules.keys.to!(dchar[])().sort())
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enforce(tm_.runningStates.canFind(s), errMsg);
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foreach (const aa; tm_.rules.byValue)
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foreach (/*const*/ s, const rule; aa) {
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enforce(tm_.symbols.canFind(s), errMsg);
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enforce(tm_.symbols.canFind(rule.toWrite), errMsg);
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enforce(allStates.canFind(rule.nextState), errMsg);
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}
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this.tm = tm_;
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this.head = TapeHead(this.tm);
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State state = this.tm.initialState;
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while (true) {
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if (tm.haltStates.canFind(state))
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break;
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if (!tm.runningStates.canFind(state))
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throw new Exception("Unknown state.");
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immutable symbol = this.head.read();
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immutable rule = this.tm.rules[state][symbol];
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this.head.write(rule.toWrite);
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this.head.move(rule.direction);
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state = rule.nextState;
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}
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static if (doShow)
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show();
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}
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}
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void main() {
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with (UTM!()) {
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alias R = Rule;
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writeln("Incrementer:");
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TuringMachine tm1;
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tm1.symbols = [0, 1];
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tm1.blank = 0;
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tm1.initialState = 'A';
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tm1.haltStates = ['H'];
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tm1.runningStates = ['A'];
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with (Direction)
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tm1.rules = ['A': [0: R(1, left, 'H'),
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1: R(1, right, 'A')]];
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tm1.input = [1, 1, 1];
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UTM!()(tm1);
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// http://en.wikipedia.org/wiki/Busy_beaver
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writeln("\nBusy beaver machine (3-state, 2-symbol):");
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TuringMachine tm2;
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tm2.symbols = [0, 1];
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tm2.blank = 0;
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tm2.initialState = 'A';
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tm2.haltStates = ['H'];
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tm2.runningStates = ['A', 'B', 'C'];
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with (Direction)
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tm2.rules = ['A': [0: R(1, right, 'B'),
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1: R(1, left, 'C')],
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'B': [0: R(1, left, 'A'),
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1: R(1, right, 'B')],
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'C': [0: R(1, left, 'B'),
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1: R(1, stay, 'H')]];
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UTM!()(tm2);
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}
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with (UTM!false) {
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writeln("\nSorting stress test (12212212121212):");
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alias R = Rule;
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TuringMachine tm3;
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tm3.symbols = [0, 1, 2, 3];
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tm3.blank = 0;
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tm3.initialState = 'A';
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tm3.haltStates = ['H'];
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tm3.runningStates = ['A', 'B', 'C', 'D', 'E'];
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with (Direction)
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tm3.rules = ['A': [1: R(1, right, 'A'),
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2: R(3, right, 'B'),
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0: R(0, left, 'E')],
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'B': [1: R(1, right, 'B'),
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2: R(2, right, 'B'),
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0: R(0, left, 'C')],
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'C': [1: R(2, left, 'D'),
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2: R(2, left, 'C'),
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3: R(2, left, 'E')],
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'D': [1: R(1, left, 'D'),
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2: R(2, left, 'D'),
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3: R(1, right, 'A')],
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'E': [1: R(1, left, 'E'),
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0: R(0, right, 'H')]];
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tm3.input = [1, 2, 2, 1, 2, 2, 1, 2, 1, 2, 1, 2, 1, 2];
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UTM!false(tm3).show();
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}
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}
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