using System;

namespace TuringMachine.Core
{
    public class Machine
    {
        public Tape Tape { get; }
        public TransitionTable Transitions { get; }
        
        public int HeadPosition { get; private set; }
        public string CurrentState { get; private set; }
        public bool IsHalted { get; private set; }
        public string InitialState { get; set; } = "q0";

        public event Action? OnStep;
        public event Action? OnHalt;

        public Machine()
        {
            Tape = new Tape();
            Transitions = new TransitionTable();
            CurrentState = InitialState;
        }

        public void ForceHeadPosition(int position)
        {
            HeadPosition = position;
        }

        public void Reset()
        {
            Tape.Clear();
            HeadPosition = 0;
            CurrentState = InitialState;
            IsHalted = false;
            OnStep?.Invoke();
        }

        public void LoadProgram(string initialState, IEnumerable<(string state, char read, string newState, char write, MoveDirection move, string comment)> transitions)
        {
            InitialState = initialState;
            Transitions.Clear();
            foreach (var t in transitions)
            {
                Transitions.Add(t.state, t.read, t.newState, t.write, t.move, t.comment);
            }
            Reset();
        }

        public bool Step()
        {
            if (IsHalted) return false;

            char readSymbol = Tape[HeadPosition];
            
            if (Transitions.TryGetTransition(CurrentState, readSymbol, out var result))
            {
                if (result == null)
                {
                     // Should not simplify happen due to TryGetTransition semantics, but for safety
                     IsHalted = true;
                     OnHalt?.Invoke();
                     return false;
                }

                Tape[HeadPosition] = result.WriteSymbol;
                CurrentState = result.NewState;
                
                switch (result.MoveDirection)
                {
                    case MoveDirection.Left:
                        HeadPosition--;
                        break;
                    case MoveDirection.Right:
                        HeadPosition++;
                        break;
                }
                
                OnStep?.Invoke();
                return true;
            }
            else
            {
                // No transition defined = Halt
                IsHalted = true;
                OnHalt?.Invoke();
                return false;
            }
        }
    }
}