class DoublyLinkedList {
  static from(array) {
    const dll = new DoublyLinkedList()
    for (const val of array) {
      dll.append(val)
    }
    return dll
  }

  constructor() {
    this.head = null
    this.tail = null
    this.size = 0
  }

  append(value) {
    this.size++

    if (this.tail == null) {
      this.head = this.tail = { value, prev: null, next: null }
    } else {
      const oldTail = this.tail
      const node = { value, prev: oldTail, next: null }
      oldTail.next = node
      this.tail = node
    }

    return this
  }

  prepend(value) {
    this.size++

    if (this.tail == null) {
      this.head = this.tail = { value, prev: null, next: null }
    } else {
      const oldHead = this.head
      const node = { value, prev: null, next: oldHead }
      oldHead.prev = node
      this.head = node
    }

    return this
  }

  insert(at, value) {
    if (at === 0) {
      return this.prepend(value)
    }

    if (at === this.size) {
      return this.append(value)
    }

    if (at > this.size) {
      throw new TypeError('insert index out of bounds')
    }

    this.walk((node, i) => {
      if (i === at - 1) {
        const oldNext = node.next
        node.next = oldNext.prev = { value, next: oldNext, prev: node }
      }
    })

    return this
  }

  removeAt(at) {
    this.walk((node, i) => {
      if (i === at) {
        node.prev.next = node.next
        node.next.prev = node.prev
      }
    })

    return this
  }

  reverse() {
    let current = this.tail
    while (current != null) {
      [current.next, current.prev] = [current.prev, current.next]
      current = current.next
    }

    [this.head, this.tail] = [this.tail, this.head]

    return this
  }

  equals(other) {
    if (this.size !== other.size) { // assuming we can rely on this...
      return false
    }

    if (this === other) {
      return true
    }

    let thisCurrent = this.head,
      otherCurrent = other.head
    while (thisCurrent != null && otherCurrent != null) {
      if (thisCurrent.value !== otherCurrent.value) {
        return false
      }

      thisCurrent = thisCurrent.next
      otherCurrent = otherCurrent.next
    }

    return true
  }

  walk(callback) {
    let current = this.head,
      i = 0
    while (current != null) {
      callback(current, i++)
      current = current.next
    }
    return this
  }

  prettyPrint() {
    let output = ''
    this.walk(node => {
      if (node.next != null) {
        output += `${node.value} <-> `
      } else {
        output += `${node.value}`
      }
    })
    return output
  }
}

console.log(
  DoublyLinkedList
    .from([1, 2, 3, 4])
    .equals(
      DoublyLinkedList
        .from([4, 3, 2, 1])
        .reverse()
    )
)

console.log(
  DoublyLinkedList
    .from(['🌑', '🌒', '🌓', '🌔', '🌕'])
    .reverse()
    .prettyPrint()
)

console.log(
  DoublyLinkedList
    .from(['😄', '😄', '😭', '😄', '😄', '😄'])
    .removeAt(2)
    .prettyPrint()
)

for (let i = 0; i < 5; i++) {
  let dll = DoublyLinkedList
    .from(['🥶', '🥶', '🥶', '🥶'])
    .insert(i, '🥵')
  console.log(dll.prettyPrint())
}