Task: Universal Turing machine





Java: import java.util.HashMap;
import java.util.HashSet;
import java.util.LinkedList;
import java.util.ListIterator;
import java.util.List;
import java.util.Set;
import java.util.Map;

public class UTM {
    private List<String> tape;
    private String blankSymbol;
    private ListIterator<String> head;
    private Map<StateTapeSymbolPair, Transition> transitions = new HashMap<StateTapeSymbolPair, Transition>();
    private Set<String> terminalStates;
    private String initialState;
    
    public UTM(Set<Transition> transitions, Set<String> terminalStates, String initialState, String blankSymbol) {
        this.blankSymbol = blankSymbol;
        for (Transition t : transitions) {
            this.transitions.put(t.from, t);
        }
        this.terminalStates = terminalStates;
        this.initialState = initialState;
    }
    
    public static class StateTapeSymbolPair {
        private String state;
        private String tapeSymbol;

        public StateTapeSymbolPair(String state, String tapeSymbol) {
            this.state = state;
            this.tapeSymbol = tapeSymbol;
        }

        
        @Override
        public int hashCode() {
            final int prime = 31;
            int result = 1;
            result = prime * result
                    + ((state == null) ? 0 : state.hashCode());
            result = prime
                    * result
                    + ((tapeSymbol == null) ? 0 : tapeSymbol
                            .hashCode());
            return result;
        }

        
        @Override
        public boolean equals(Object obj) {
            if (this == obj)
                return true;
            if (obj == null)
                return false;
            if (getClass() != obj.getClass())
                return false;
            StateTapeSymbolPair other = (StateTapeSymbolPair) obj;
            if (state == null) {
                if (other.state != null)
                    return false;
            } else if (!state.equals(other.state))
                return false;
            if (tapeSymbol == null) {
                if (other.tapeSymbol != null)
                    return false;
            } else if (!tapeSymbol.equals(other.tapeSymbol))
                return false;
            return true;
        }

        @Override
        public String toString() {
            return "(" + state + "," + tapeSymbol + ")";
        }
    }
    
    public static class Transition {
        private StateTapeSymbolPair from;
        private StateTapeSymbolPair to;
        private int direction; 

        public Transition(StateTapeSymbolPair from, StateTapeSymbolPair to, int direction) {
             this.from = from;
            this.to = to;
            this.direction = direction;
        }

        @Override
        public String toString() {
            return from + "=>" + to + "/" + direction;
        }
    }
    
    public void initializeTape(List<String> input) { 
        tape = input;
    }
    
    public void initializeTape(String input) { 
        tape = new LinkedList<String>();
        for (int i = 0; i < input.length(); i++) {
            tape.add(input.charAt(i) + "");
        }
    }
    
    public List<String> runTM() { 
        if (tape.size() == 0) {
            tape.add(blankSymbol);
        }
        
        head = tape.listIterator();
        head.next();
        head.previous();
        
        StateTapeSymbolPair tsp = new StateTapeSymbolPair(initialState, tape.get(0));
        
        while (transitions.containsKey(tsp)) { 
            System.out.println(this + " --- " + transitions.get(tsp));
            Transition trans = transitions.get(tsp);
            head.set(trans.to.tapeSymbol); 
            tsp.state = trans.to.state; 
            if (trans.direction == -1) { 
                if (!head.hasPrevious()) {
                    head.add(blankSymbol); 
                }
                tsp.tapeSymbol = head.previous(); 
            } else if (trans.direction == 1) { 
                head.next();
                if (!head.hasNext()) {
                    head.add(blankSymbol); 
                    head.previous();
                }
                tsp.tapeSymbol = head.next(); 
                head.previous();
            } else {
                tsp.tapeSymbol = trans.to.tapeSymbol;
            }
        }
        
        System.out.println(this + " --- " + tsp);
        
        if (terminalStates.contains(tsp.state)) {
            return tape;
        } else {
            return null;
        }
    }

    @Override
    public String toString() {
        try {
        	int headPos = head.previousIndex();
            String s = "[ ";
            
            for (int i = 0; i <= headPos; i++) {
                s += tape.get(i) + " ";
            }

            s += "[H] ";
            
            for (int i = headPos + 1; i < tape.size(); i++) {
                s += tape.get(i) + " ";
            }

            return s + "]";
        } catch (Exception e) {
            return "";
        }
    }
    
    public static void main(String[] args) {
        
        String init = "q0";
        String blank = "b";
        
        Set<String> term = new HashSet<String>();
        term.add("qf");
        
        Set<Transition> trans = new HashSet<Transition>();
        
        trans.add(new Transition(new StateTapeSymbolPair("q0", "1"), new StateTapeSymbolPair("q0", "1"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("q0", "b"), new StateTapeSymbolPair("qf", "1"), 0));
        
        UTM machine = new UTM(trans, term, init, blank);
        machine.initializeTape("111");
        System.out.println("Output (si): " + machine.runTM() + "\n");
        
        
        init = "a";
        
        term.clear();
        term.add("halt");
        
        blank = "0";
        
        trans.clear();

        
        trans.add(new Transition(new StateTapeSymbolPair("a", "0"), new StateTapeSymbolPair("b", "1"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("a", "1"), new StateTapeSymbolPair("c", "1"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("b", "0"), new StateTapeSymbolPair("a", "1"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("b", "1"), new StateTapeSymbolPair("b", "1"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("c", "0"), new StateTapeSymbolPair("b", "1"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("c", "1"), new StateTapeSymbolPair("halt", "1"), 0));
        
        machine = new UTM(trans, term, init, blank);
        machine.initializeTape("");
        System.out.println("Output (bb): " + machine.runTM());

        
        init = "s0";
        blank = "*";
        
        term = new HashSet<String>();
        term.add("see");
        
        trans = new HashSet<Transition>();
        
        trans.add(new Transition(new StateTapeSymbolPair("s0", "a"), new StateTapeSymbolPair("s0", "a"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("s0", "b"), new StateTapeSymbolPair("s1", "B"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("s0", "*"), new StateTapeSymbolPair("se", "*"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("s1", "a"), new StateTapeSymbolPair("s1", "a"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("s1", "b"), new StateTapeSymbolPair("s1", "b"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("s1", "*"), new StateTapeSymbolPair("s2", "*"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("s2", "a"), new StateTapeSymbolPair("s3", "b"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("s2", "b"), new StateTapeSymbolPair("s2", "b"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("s2", "B"), new StateTapeSymbolPair("se", "b"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("s3", "a"), new StateTapeSymbolPair("s3", "a"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("s3", "b"), new StateTapeSymbolPair("s3", "b"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("s3", "B"), new StateTapeSymbolPair("s0", "a"), 1));
        trans.add(new Transition(new StateTapeSymbolPair("se", "a"), new StateTapeSymbolPair("se", "a"), -1));
        trans.add(new Transition(new StateTapeSymbolPair("se", "*"), new StateTapeSymbolPair("see", "*"), 1));

        machine = new UTM(trans, term, init, blank);
        machine.initializeTape("babbababaa");
        System.out.println("Output (sort): " + machine.runTM() + "\n");
    }
}




VB: Option Base 1
Public Enum sett
    name_ = 1
    initState
    endState
    blank
    rules
End Enum
Public incrementer As Variant, threeStateBB As Variant, fiveStateBB As Variant

Private Sub init()
    incrementer = Array("Simple incrementer", _
        "q0", _
        "qf", _
        "B", _
         Array( _
         Array("q0", "1", "1", "right", "q0"), _
         Array("q0", "B", "1", "stay", "qf")))
    threeStateBB = Array("Three-state busy beaver", _
       "a", _
       "halt", _
       "0", _
        Array( _
        Array("a", "0", "1", "right", "b"), _
        Array("a", "1", "1", "left", "c"), _
        Array("b", "0", "1", "left", "a"), _
        Array("b", "1", "1", "right", "b"), _
        Array("c", "0", "1", "left", "b"), _
        Array("c", "1", "1", "stay", "halt")))
    fiveStateBB = Array("Five-state busy beaver", _
        "A", _
        "H", _
        "0", _
         Array( _
         Array("A", "0", "1", "right", "B"), _
         Array("A", "1", "1", "left", "C"), _
         Array("B", "0", "1", "right", "C"), _
         Array("B", "1", "1", "right", "B"), _
         Array("C", "0", "1", "right", "D"), _
         Array("C", "1", "0", "left", "E"), _
         Array("D", "0", "1", "left", "A"), _
         Array("D", "1", "1", "left", "D"), _
         Array("E", "0", "1", "stay", "H"), _
         Array("E", "1", "0", "left", "A")))
End Sub

Private Sub show(state As String, headpos As Long, tape As Collection)
    Debug.Print " "; state; String$(7 - Len(state), " "); "| ";
    For p = 1 To tape.Count
        Debug.Print IIf(p = headpos, "[" & tape(p) & "]", " " & tape(p) & " ");
    Next p
    Debug.Print
End Sub
 

Private Sub UTM(machine As Variant, tape As Collection, Optional countOnly As Long = 0)
    Dim state As String: state = machine(initState)
    Dim headpos As Long: headpos = 1
    Dim counter As Long, rule As Variant
    Debug.Print machine(name_); vbCrLf; String$(Len(machine(name_)), "=")
    If Not countOnly Then Debug.Print " State  | Tape [head]" & vbCrLf & "---------------------"
    Do While True
        If headpos > tape.Count Then
            tape.Add machine(blank)
        Else
            If headpos < 1 Then
                tape.Add machine(blank), Before:=1
                headpos = 1
            End If
        End If
        If Not countOnly Then show state, headpos, tape
        For i = LBound(machine(rules)) To UBound(machine(rules))
            rule = machine(rules)(i)
            If rule(1) = state And rule(2) = tape(headpos) Then
                tape.Remove headpos
                If headpos > tape.Count Then
                    tape.Add rule(3)
                Else
                    tape.Add rule(3), Before:=headpos
                End If
                If rule(4) = "left" Then headpos = headpos - 1
                If rule(4) = "right" Then headpos = headpos + 1
                state = rule(5)
                Exit For
            End If
        Next i
        counter = counter + 1
        If counter Mod 100000 = 0 Then
            Debug.Print counter
            DoEvents
            DoEvents
        End If
        If state = machine(endState) Then Exit Do
    Loop
    DoEvents
    If countOnly Then
        Debug.Print "Steps taken: ", counter
    Else
        show state, headpos, tape
        Debug.Print
    End If
End Sub
 
Public Sub main()
    init
    Dim tap As New Collection
    tap.Add "1": tap.Add "1": tap.Add "1"
    UTM incrementer, tap
    Set tap = New Collection
    UTM threeStateBB, tap
    Set tap = New Collection
    UTM fiveStateBB, tap, countOnly:=-1
End Sub