interface Vec2 { x: number; y: number }

export type CompareVectorFn = (v1: Vec2, v2: Vec2) => number

const comparePointCache: Map<string, CompareVectorFn> = new Map()

const ALLKEY = ['x', 'X', 'y', 'Y', 'z', 'Z']
const KEY = ['x', 'y', 'z']

/**
 * 构建返回一个用来排序的函数.根据key创建排序规则.
 *
 * 当key = "xyz" 时,点集按 x从小到大,y从小到大 z从小到大
 *   key = "X"   时,点集按 x从大到小
 * 以此类推.
 *
 * 例子:
 * let pts:Vector3[] =...;
 * pts.sort(comparePoint("x")); //x从小到大排序
 * pts.sort(comparePoint("zX")); //z从小到大 x从大到小
 *
 * @export
 * @param {string} sortKey
 */
export function ComparePoint(sortKey: string): CompareVectorFn
{
  if (comparePointCache.has(sortKey))
    return comparePointCache.get(sortKey)

  let sortIndex = []

  for (let char of sortKey)
  {
    let index = ALLKEY.indexOf(char)

    let i2 = index / 2
    let ci = Math.floor(i2)
    sortIndex.push([KEY[ci], i2 > ci ? 1 : -1])
  }

  let compareFunction = (v1: Vec2, v2: Vec2): number =>
  {
    if (!v1)
      return -1
    if (!v2)
      return 1
    for (let s of sortIndex)
    {
      let vv1 = v1[s[0]]
      let vv2 = v2[s[0]]
      if (vv1 === vv2)
        continue
      if (vv2 > vv1)
        return s[1]
      else return -s[1]
    }
    return 0
  }

  comparePointCache.set(sortKey, compareFunction)
  return compareFunction
}