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Universal Turing machine: Difference between revisions
→{{header|Mercury}}
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111111 STOP
^</pre>
=={{header|Erlang}}==
The following code is an Escript which can be placed into a file and run as <code>escript filename</code> or simply marked as executable and run directly via the provided shebang header. <code>-type</code> and <code>-spec</code> declarations have not been used; using the <code>typer</code> utility can get a head start on this process should a more robust solution be desired.
In this universal Turing machine simulator, a machine is defined by giving it a configuration function that returns the initial state, the halting states and the blank symbol, as well as a function for the rules. These are passed in to the public interface <code>turing/3</code> as funs, together with the initial tape setup.
<lang erlang>#!/usr/bin/env escript
-module(turing).
-mode(compile).
-export([main/1]).
% Incrementer definition:
% States: a | halt
% Initial state: a
% Halting states: halt
% Symbols: b | '1'
% Blank symbol: b
incrementer_config() -> {a, [halt], b}.
incrementer(a, '1') -> {'1', right, a};
incrementer(a, b) -> {'1', stay, halt}.
% Busy beaver definition:
% States: a | b | c | halt
% Initial state: a
% Halting states: halt
% Symbols: '0' | '1'
% Blank symbol: '0'
busy_beaver_config() -> {a, [halt], '0'}.
busy_beaver(a, '0') -> {'1', right, b};
busy_beaver(a, '1') -> {'1', left, c};
busy_beaver(b, '0') -> {'1', left, a};
busy_beaver(b, '1') -> {'1', right, b};
busy_beaver(c, '0') -> {'1', left, b};
busy_beaver(c, '1') -> {'1', stay, halt}.
% Mainline code.
main([]) ->
io:format("==============================~n"),
io:format("Turing machine simulator test.~n"),
io:format("==============================~n"),
Tape2 = turing(fun incrementer_config/0, fun incrementer/2, ['1','1','1']),
io:format("~w~n", [Tape2]).
Tape1 = turing(fun busy_beaver_config/0, fun busy_beaver/2, []),
io:format("~w~n", [Tape1]),
% Universal Turing machine simulator.
turing(Config, Rules, Input) ->
{Start, _, _} = Config(),
{Left, Right} = perform(Config, Rules, Start, {[], Input}),
lists:reverse(Left) ++ Right.
perform(Config, Rules, State, Input = {LeftInput, RightInput}) ->
{_, Halts, Blank} = Config(),
case lists:member(State, Halts) of
true -> Input;
false ->
{NewRight, Symbol} = symbol(RightInput, Blank),
{NewSymbol, Action, NewState} = Rules(State, Symbol),
NewInput = action(Action, Blank, {LeftInput, [NewSymbol| NewRight]}),
perform(Config, Rules, NewState, NewInput)
end.
symbol([], Blank) -> {[], Blank};
symbol([S|R], _) -> {R, S}.
action(left, Blank, {[], Right}) -> {[], [Blank|Right]};
action(left, _, {[L|Ls], Right}) -> {Ls, [L|Right]};
action(stay, _, Tape) -> Tape;
action(right, Blank, {Left, []}) -> {[Blank|Left], []};
action(right, _, {Left, [R|Rs]}) -> {[R|Left], Rs}.</lang>
=={{header|Mercury}}==
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