base64字符串互转

// base64 、 字符串 互转 
var _PADCHAR = "=",
  _ALPHA = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/",
  _VERSION = "1.1";//Mr. Ruan fix to 1.1 to support asian char(utf8) 
function _getbyte64(s, i) {
  var idx = _ALPHA.indexOf(s.charAt(i));
  if (idx === -1) {
    throw "Cannot decode base64";
  }
  return idx;
}

function _decode_chars(y, x) {
  while (y.length > 0) {
    var ch = y[0];
    if (ch < 0x80) {
      y.shift();
      x.push(String.fromCharCode(ch));
    } else if ((ch & 0x80) == 0xc0) {
      if (y.length < 2) break;
      ch = y.shift();
      var ch1 = y.shift();
      x.push(String.fromCharCode(((ch & 0x1f) << 6) + (ch1 & 0x3f)));
    } else {
      if (y.length < 3) break;
      ch = y.shift();
      var ch1 = y.shift();
      var ch2 = y.shift();
      x.push(String.fromCharCode(((ch & 0x0f) << 12) + ((ch1 & 0x3f) << 6) + (ch2 & 0x3f)));
    }
  }
}

export function decode(s) {
  var pads = 0,
    i,
    b10,
    imax = s.length,
    x = [],
    y = [];

  s = String(s);

  if (imax === 0) {
    return s;
  }

  if (imax % 4 !== 0) {
    throw "Cannot decode base64";
  }

  if (s.charAt(imax - 1) === _PADCHAR) {
    pads = 1;

    if (s.charAt(imax - 2) === _PADCHAR) {
      pads = 2;
    }

    // either way, we want to ignore this last block
    imax -= 4;
  }

  for (i = 0; i < imax; i += 4) {
    var ch1 = _getbyte64(s, i);
    var ch2 = _getbyte64(s, i + 1);
    var ch3 = _getbyte64(s, i + 2);
    var ch4 = _getbyte64(s, i + 3);

    b10 = (_getbyte64(s, i) << 18) | (_getbyte64(s, i + 1) << 12) | (_getbyte64(s, i + 2) << 6) | _getbyte64(s, i + 3);
    y.push(b10 >> 16);
    y.push((b10 >> 8) & 0xff);
    y.push(b10 & 0xff);
    _decode_chars(y, x);
  }
  switch (pads) {
    case 1:
      b10 = (_getbyte64(s, i) << 18) | (_getbyte64(s, i + 1) << 12) | (_getbyte64(s, i + 2) << 6);
      y.push(b10 >> 16);
      y.push((b10 >> 8) & 0xff);
      break;

    case 2:
      b10 = (_getbyte64(s, i) << 18) | (_getbyte64(s, i + 1) << 12);
      y.push(b10 >> 16);
      break;
  }
  _decode_chars(y, x);
  if (y.length > 0) throw "Cannot decode base64";
  return x.join("");
}
function _get_chars(ch, y) {
  if (ch < 0x80) y.push(ch);
  else if (ch < 0x800) {
    y.push(0xc0 + ((ch >> 6) & 0x1f));
    y.push(0x80 + (ch & 0x3f));
  } else {
    y.push(0xe0 + ((ch >> 12) & 0xf));
    y.push(0x80 + ((ch >> 6) & 0x3f));
    y.push(0x80 + (ch & 0x3f));
  }
}

export function encode(s) {
  if (arguments.length !== 1) {
    throw "SyntaxError: exactly one argument required";
  }

  s = String(s);
  if (s.length === 0) {
    return s;
  }

  //s = _encode_utf8(s);
  var i,
    b10,
    y = [],
    x = [],
    len = s.length;
  i = 0;
  while (i < len) {
    _get_chars(s.charCodeAt(i), y);
    while (y.length >= 3) {
      var ch1 = y.shift();
      var ch2 = y.shift();
      var ch3 = y.shift();
      b10 = (ch1 << 16) | (ch2 << 8) | ch3;
      x.push(_ALPHA.charAt(b10 >> 18));
      x.push(_ALPHA.charAt((b10 >> 12) & 0x3F));
      x.push(_ALPHA.charAt((b10 >> 6) & 0x3f));
      x.push(_ALPHA.charAt(b10 & 0x3f));
    }
    i++;
  }


  switch (y.length) {
    case 1:
      var ch = y.shift();
      b10 = ch << 16;
      x.push(_ALPHA.charAt(b10 >> 18) + _ALPHA.charAt((b10 >> 12) & 0x3F) + _PADCHAR + _PADCHAR);
      break;

    case 2:
      var ch1 = y.shift();
      var ch2 = y.shift();
      b10 = (ch1 << 16) | (ch2 << 8);
      x.push(_ALPHA.charAt(b10 >> 18) + _ALPHA.charAt((b10 >> 12) & 0x3F) + _ALPHA.charAt((b10 >> 6) & 0x3f) + _PADCHAR);
      break;
  }

  return x.join("");
}