// Do one step: change state, write letter, move.
public void DoOneStep()
{
if (Halted)
{
// If machine has halted, do nothing at all.
return;
}
int previousLetter = Cursor.Value;
int previousState = State;
// Update machine state.
State = definition.TransitionMatrix(previousState, 3 * previousLetter);
// Write new letter.
Cursor.Value = definition.TransitionMatrix(previousState, 3 * previousLetter + 1);
// Move.
int direction = definition.TransitionMatrix(previousState, 3 * previousLetter + 2);
switch (direction)
{
case 0:
// We don't move.
break;
case 1:
{
// We will move to the right, so future position is next node in linked list.
LinkedListNode <int> futurePosition = Cursor.Next;
// If we are on the left end of the tape, and we have written a blank
// on current position, then "cut" this position from the tape.
// (to save RAM)
if ((Cursor.Previous == null) && (Cursor.Value == definition.Blank))
{
Tape.RemoveFirst();
}
// Now check if the current position is not the right end of the tape,
// if it is, we must add a piece of tape to the right.
if (futurePosition == null)
{
Tape.AddLast(definition.Blank);
futurePosition = Tape.Last;
}
// Now move.
Cursor = futurePosition;
}
break;
case -1:
{
// We will move to the left, so future position is previous node in linked list.
LinkedListNode <int> futurePosition = Cursor.Previous;
// If we are on the right end of the tape, and we have written a blank
// on current position, then "cut" this position from the tape.
// (to save RAM)
if ((Cursor.Next == null) && (Cursor.Value == definition.Blank))
{
Tape.RemoveLast();
}
// Now check if the current position is not the left end of the tape,
// if it is, we must add a piece of tape to the left.
if (futurePosition == null)
{
Tape.AddFirst(definition.Blank);
futurePosition = Tape.First;
}
// Now move.
Cursor = futurePosition;
}
break;
default:
throw new FiniteStateMachineException("Internal error, direction = " + direction);
}
}