/**
 * 写一个函数，传入一个数组，
 * 要求返回数组中所有的元素组合的数组
 *
 * [1, 2] => [
 *   12,
 *   21,
 * ]
 * [1, 2, 3] => [
 *   123, 132,
 *   213, 231,
 *   312, 321,
 * ]
 * [1, 2, 3, 4] => [
 *   1234, 1243, 1324, 1342, 1423, 1432,
 *   2134, 2143, 2314, 2341, 2413, 2431,
 *   3124, 3142, 3214, 3241, 3412, 3421,
 *   4123, 4132, 4213, 4231, 4312, 4321,
 * ]
 *
 */

const DEBUG = false

const combineArray = (data, arr, comb) => {
  arr = arr || data
  comb = comb || ''

  return arr.map((value, index, currentArr) => {
    const _comb = comb ? `${comb},${value}` : `${value}`

    const l = index - 1
    const r = index + 1

    const p = currentArr.slice(0, Math.max(0, l + 1))
    const n = currentArr.slice(Math.min(r, currentArr.length))

    const _arr = p.concat(n)

    if (DEBUG) {
      console.group('flag')

      console.log(`%cdata:`, 'color: purple', `[${data}]`)

      console.log(`@:`, index)
      console.log(`currentArr  => ${currentArr}`)
      console.log(`comb  => ${comb}`)
      console.log(`arr   => ${arr}`)
      console.log(`-----------------------`)

      console.log(`_comb => ${_comb}`)
      console.log(`_arr  => ${_arr}`)
      console.log(`=======================`)

      console.groupEnd()
    }

    if (_arr.length) {
      return combineArray(data, _arr, _comb)
    } else {
      if (DEBUG) {
        console.log(
          `%c● %c${_comb}`,
          'color: red; font-size: 14px',
          'padding: .2em .42em; font-weight: 700; background-color: #ddd; border-radius: 4px'
        )
      }

      return _comb
    }

  })
}

const formatArray = (data, fn, result) => {
  fn = fn || factorial(data.length)
  result = result || []

  for (let o of data) {
    if (Array.isArray(o)) {
      formatArray(o, fn, result)
    } else {
      result.push(o)
    }
  }

  if (result.length === fn) {
    return result
  }
}

const factorial = (num) => {
  if (Number.isInteger(num)) {
    if (num === 0) {
      return 1
    } else {
      return num * factorial(num - 1)
    }
  }
}

const a = [1, 2]
const b = [1, 2, 3]
const c = [1, 2, 3, 4]


// console.log( combineArray(a) )
// console.log( combineArray(b) )
console.log( combineArray(c) )
console.log( formatArray(combineArray(c)) )

// console.log( combineArray( ['A'] ) )
// console.log( combineArray( ['A', 'B'] ) )
console.log( combineArray( ['A', 'B', 'C'] ) )

console.log( formatArray(combineArray( ['A', 'B', 'C'] )) )