// 转码表
const table = [
  'A',
  'B',
  'C',
  'D',
  'E',
  'F',
  'G',
  'H',
  'I',
  'J',
  'K',
  'L',
  'M',
  'N',
  'O',
  'P',
  'Q',
  'R',
  'S',
  'T',
  'U',
  'V',
  'W',
  'X',
  'Y',
  'Z',
  'a',
  'b',
  'c',
  'd',
  'e',
  'f',
  'g',
  'h',
  'i',
  'j',
  'k',
  'l',
  'm',
  'n',
  'o',
  'p',
  'q',
  'r',
  's',
  't',
  'u',
  'v',
  'w',
  'x',
  'y',
  'z',
  '0',
  '1',
  '2',
  '3',
  '4',
  '5',
  '6',
  '7',
  '8',
  '9',
  '+',
  '/',
]

export function UTF16ToUTF8(str) {
  let res = [],
    len = str.length
  for (let i = 0; i < len; i++) {
    let code = str.charCodeAt(i)
    if (code > 0x0000 && code <= 0x007f) {
      // 单字节，这里并不考虑0x0000，因为它是空字节
      // U+00000000 – U+0000007F  0xxxxxxx
      res.push(str.charAt(i))
    } else if (code >= 0x0080 && code <= 0x07ff) {
      // 双字节
      // U+00000080 – U+000007FF  110xxxxx 10xxxxxx
      // 110xxxxx
      let byte1 = 0xc0 | ((code >> 6) & 0x1f)
      // 10xxxxxx
      let byte2 = 0x80 | (code & 0x3f)
      res.push(String.fromCharCode(byte1), String.fromCharCode(byte2))
    } else if (code >= 0x0800 && code <= 0xffff) {
      // 三字节
      // U+00000800 – U+0000FFFF  1110xxxx 10xxxxxx 10xxxxxx
      // 1110xxxx
      let byte1 = 0xe0 | ((code >> 12) & 0x0f)
      // 10xxxxxx
      let byte2 = 0x80 | ((code >> 6) & 0x3f)
      // 10xxxxxx
      let byte3 = 0x80 | (code & 0x3f)
      res.push(
        String.fromCharCode(byte1),
        String.fromCharCode(byte2),
        String.fromCharCode(byte3)
      )
    } else if (code >= 0x00010000 && code <= 0x001fffff) {
      // 四字节
      // U+00010000 – U+001FFFFF  11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
    } else if (code >= 0x00200000 && code <= 0x03ffffff) {
      // 五字节
      // U+00200000 – U+03FFFFFF  111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
    } /** if (code >= 0x04000000 && code <= 0x7FFFFFFF)*/ else {
      // 六字节
      // U+04000000 – U+7FFFFFFF  1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
    }
  }

  return res.join('')
}

export function UTF8ToUTF16(str) {
  let res = [],
    len = str.length
  for (let i = 0; i < len; i++) {
    let code = str.charCodeAt(i)
    // 对第一个字节进行判断
    if (((code >> 7) & 0xff) === 0x0) {
      // 单字节
      // 0xxxxxxx
      res.push(str.charAt(i))
    } else if (((code >> 5) & 0xff) === 0x6) {
      // 双字节
      // 110xxxxx 10xxxxxx
      let code2 = str.charCodeAt(++i)
      let byte1 = (code & 0x1f) << 6
      let byte2 = code2 & 0x3f
      let utf16 = byte1 | byte2
      res.push(String.fromCharCode(utf16))
    } else if (((code >> 4) & 0xff) === 0xe) {
      // 三字节
      // 1110xxxx 10xxxxxx 10xxxxxx
      let code2 = str.charCodeAt(++i)
      let code3 = str.charCodeAt(++i)
      let byte1 = (code << 4) | ((code2 >> 2) & 0x0f)
      let byte2 = ((code2 & 0x03) << 6) | (code3 & 0x3f)
      let utf16 = ((byte1 & 0x00ff) << 8) | byte2
      res.push(String.fromCharCode(utf16))
    } else if (((code >> 3) & 0xff) === 0x1e) {
      // 四字节
      // 11110xxx 10xxxxxx 10xxxxxx 10xxxxxx
    } else if (((code >> 2) & 0xff) === 0x3e) {
      // 五字节
      // 111110xx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
    } /** if (((code >> 1) & 0xFF) == 0x7E)*/ else {
      // 六字节
      // 1111110x 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx 10xxxxxx
    }
  }

  return res.join('')
}

export function code(str) {
  let index = 0
  for (let i = 0; i < table.length; i++) {
    if (str === table[i]) {
      index = i
      break
    }
  }
  return index
}

export function encode(str) {
  if (!str) return ''

  let utf8 = UTF16ToUTF8(str) // 转成UTF8
  let i = 0 // 遍历索引
  let len = utf8.length
  let res = []
  while (i < len) {
    let c1 = utf8.charCodeAt(i++) & 0xff
    res.push(table[c1 >> 2])
    // 需要补2个=
    if (i === len) {
      res.push(table[(c1 & 0x3) << 4])
      res.push('==')
      break
    }
    let c2 = utf8.charCodeAt(i++)
    // 需要补1个=
    if (i === len) {
      res.push(table[((c1 & 0x3) << 4) | ((c2 >> 4) & 0x0f)])
      res.push(table[(c2 & 0x0f) << 2])
      res.push('=')
      break
    }
    let c3 = utf8.charCodeAt(i++)
    res.push(table[((c1 & 0x3) << 4) | ((c2 >> 4) & 0x0f)])
    res.push(table[((c2 & 0x0f) << 2) | ((c3 & 0xc0) >> 6)])
    res.push(table[c3 & 0x3f])
  }

  return res.join('')
}

export function decode(str) {
  if (!str) return ''

  let len = str.length
  let i = 0
  let res = []

  while (i < len) {
    /*
    //火狐能用，IE8不能用
    alert("下标："+code(str.charAt(i++)));
    code1 = table.indexOf(str.charAt(i++));
    code2 = table.indexOf(str.charAt(i++));
    code3 = table.indexOf(str.charAt(i++));
    code4 = table.indexOf(str.charAt(i++));
    */
    //IE、火狐都能用
    let code1 = code(str.charAt(i++))
    let code2 = code(str.charAt(i++))
    let code3 = code(str.charAt(i++))
    let code4 = code(str.charAt(i++))
    let c1 = (code1 << 2) | (code2 >> 4)
    let c2 = ((code2 & 0xf) << 4) | (code3 >> 2)
    let c3 = ((code3 & 0x3) << 6) | code4
    res.push(String.fromCharCode(c1))
    if (code3 !== 64) res.push(String.fromCharCode(c2))

    if (code4 !== 64) res.push(String.fromCharCode(c3))
  }
  return UTF8ToUTF16(res.join(''))
}