Monadic-ScalaFX-Demo/src/main/scala/nova/monadic_sfx/util/History.scala

package nova.monadic_sfx.util
import com.softwaremill.quicklens._

final class MutHistory[T](initValue: T) {
  private var _values = Vector(initValue)
  def values = _values
  private var _sp = 0
  def sp = _sp

  // def current = if (ints.isEmpty) None else Some(ints(sp))
  def current = _values(_sp)

  def push(v: T) = {
    if (_sp < _values.length - 1) {
      _values = _values.splitAt(_sp)._1
      _values = _values :+ v
    } else {
      _values = _values :+ v
      _sp = _values.length - 1
    }
  }

  def forward() = {
    if (!_values.isEmpty && _sp < _values.length - 1) _sp += 1
  }

  def backward() = {
    if (_sp > 0) {
      _sp -= 1
    }
  }
}

final class History[T] private (val state: History.State[T]) {
  def current = state.values(state.sp)
  def push(v: T) = {
    val nextState =
      //   state.copy(state.values.splitAt(state.sp)._1 :+ v, state.sp + 1)
      if (state.sp < state.values.length - 1) {
        state.modify(_.values).using(_.splitAt(state.sp)._1 :+ v)
      } else {
        val s1 = state
          .modify(_.values)
          .using(_ :+ v)
        s1.modify(_.sp)
          .setTo(s1.values.length - 1)
      }
    new History(nextState)
  }
  def forward = {
    // val nextState =
    if (!state.values.isEmpty && state.sp < state.values.length - 1)
      new History(state.modify(_.sp).using(_ + 1))
    else this
    // new History(nextState)
  }
  def backward = {
    // val nextState =
    if (state.sp > 0) new History(state.modify(_.sp).using(_ - 1)) else this
    // new History(nextState)
  }
}

// final class HistoryImpl()

object History {
  case class State[T](values: Vector[T], sp: Int)

  def apply[T](intialValue: T) =
    new History(State(Vector(intialValue), 0))

//   val history = new History(History.State(Vector.empty, 0))

  implicit class HistoryOps[T](private val h: History[T]) extends AnyVal {
    def :+(v: T) = h.push(v)
  }

}
// History.history.push(1).forward