import sha1 from "js-sha1";

function getEtag(buffer, callback) {
  // sha1算法
  var shA1 = sha1.digest;

  // 以4M为单位分割
  var blockSize = 4 * 1024 * 1024;
  var sha1String = [];
  var prefix = 0x16;
  var blockCount = 0;

  var bufferSize = buffer.size || buffer.length || buffer.byteLength;
  blockCount = Math.ceil(bufferSize / blockSize);

  for (var i = 0; i < blockCount; i++) {
    sha1String.push(shA1(buffer.slice(i * blockSize, (i + 1) * blockSize)));
  }
  function concatArr2Uint8(s) {//Array 2 Uint8Array
    var tmp = [];
    for (var i of s) tmp = tmp.concat(i);
    return new Uint8Array(tmp);
  }
  function Uint8ToBase64(u8Arr, urisafe) {//Uint8Array 2 Base64
    var CHUNK_SIZE = 0x8000; //arbitrary number
    var index = 0;
    var length = u8Arr.length;
    var result = '';
    var slice;
    while (index < length) {
      slice = u8Arr.subarray(index, Math.min(index + CHUNK_SIZE, length));
      result += String.fromCharCode.apply(null, slice);
      index += CHUNK_SIZE;
    }
    return urisafe ? btoa(result).replace(/\//g, '_').replace(/\+/g, '-') : btoa(result);
  }
  function calcEtag() {
    if (!sha1String.length) return 'Fto5o-5ea0sNMlW_75VgGJCv2AcJ';
    var sha1Buffer = concatArr2Uint8(sha1String);
    // 如果大于4M，则对各个块的sha1结果再次sha1
    if (blockCount > 1) {
      prefix = 0x96;
      sha1Buffer = shA1(sha1Buffer.buffer);
    } else {
      sha1Buffer = Array.apply([], sha1Buffer);
    }
    sha1Buffer = concatArr2Uint8([[prefix], sha1Buffer]);
    return Uint8ToBase64(sha1Buffer, true);
  }
  return (calcEtag());
}

export { getEtag }