'use strict'

function History () {
  this.index = 0
  this.a = []

  this.clear = function () {
    this.a = []
    this.index = 0
  }

  this.push = function (data) {
    if (this.index < this.a.length - 1) {
      this.fork()
    }
    this.index = this.a.length
    this.a = this.a.slice(0, this.index)
    this.a.push(copy(data))

    if (this.a.length > 20) {
      this.a.shift()
    }
  }

  this.fork = function () {
    this.a = this.a.slice(0, this.index + 1)
  }

  this.pop = function () {
    return this.a.pop()
  }

  this.prev = function () {
    this.index = clamp(this.index - 1, 0, this.a.length - 1)
    return copy(this.a[this.index])
  }

  this.next = function () {
    this.index = clamp(this.index + 1, 0, this.a.length - 1)
    return copy(this.a[this.index])
  }

  function copy (data) { return data ? JSON.parse(JSON.stringify(data)) : [] }
  function clamp (v, min, max) { return v < min ? min : v > max ? max : v }
}