package snake

/**
 * m04 state (optional, Exercise 4.7): the general pattern behind Random. A State[S, A] is a function
 * s => (A, nextS): it reads a state, produces a value and hands on a new state. Random[A] is State[Long, A].
 * Nothing in the game uses this file; it is here to show that Random was one example of a bigger idea.
 */
final case class State[S, A](run: S => (A, S)):

  def map[B](f: A => B): State[S, B] =
    State { s =>
      val (a, next) = run(s)
      (f(a), next)
    }

  def flatMap[B](f: A => State[S, B]): State[S, B] =
    State { s =>
      val (a, next) = run(s)
      f(a).run(next)
    }

object State:

  def pure[S, A](a: A): State[S, A] = State(s => (a, s))

  /** The current state as the value; the state is left untouched. */
  def get[S]: State[S, S] = State(s => (s, s))

  /** Replace the state with `s`; there is no interesting value, so it is `()`. */
  def put[S](s: S): State[S, Unit] = State(_ => ((), s))

  def modify[S](f: S => S): State[S, Unit] = State(s => ((), f(s)))