using System.Collections.Generic;
using UnityEngine;

// This class inherits from MonoBehaviour so it can be attached to a GameObject.
public class StateMachine : MonoBehaviour 
{
    // A Stack is used to manage states. The top of the stack is the current state.
    // This allows for easily returning to previous states by "popping" the current one.
    public Stack<State> States { get; set; }

    private void Awake()
    {
        // Initialize the stack when the object is created.
        States = new Stack<State>();
    }

    private void Update()
    {
        // Each frame, execute the logic of the current state (if one exists).
        State currentState = GetCurrentState();
        if (currentState != null)
        {
            currentState.Execute();
        }
    }

    // Pushes a new state onto the stack, making it the current state.
    public void PushState(System.Action active, System.Action onEnter, System.Action onExit)
    {
        // Run the OnExit method of the old state before adding the new one.
        if (GetCurrentState() != null)
        {
            GetCurrentState().OnExit();
        }

        // Create the new state and push it to the stack.
        State state = new State(active, onEnter, onExit);
        States.Push(state);
        
        // Run the OnEnter method for the new current state.
        GetCurrentState().OnEnter();
    }

    // Removes the current state from the stack, returning to the one below it.
    public void PopState()
    {
        if (GetCurrentState() != null)
        {
            GetCurrentState().OnExit();
            States.Pop();
            // Run the OnEnter of the newly revealed state.
            if (GetCurrentState() != null)
            {
                GetCurrentState().OnEnter();
            }
        }
    }

    // A helper method to safely get the state at the top of the stack.
    private State GetCurrentState()
    {
        // Uses a ternary operator to return the top state or null if the stack is empty.
        return States.Count > 0 ? States.Peek() : null;
    }
}