import sha1JS from 'js-sha1';

export const getEtag = (buffer) => {
  return new Promise((resolve, reject) => {
    // sha1算法
    const sha1 = sha1JS.digest;

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

    const fileReader = new FileReader();
    fileReader.onerror = (e) => {
      reject(e);
    };

    const concatArr2Uint8 = (s) => {
      // Array 2 Uint8Array
      let tmp = [];
      s.forEach((e) => {
        tmp = tmp.concat(e);
      });
      return new Uint8Array(tmp);
    };

    const Uint8ToBase64 = (u8Arr, urisafe) => {
      // Uint8Array 2 Base64
      const CHUNK_SIZE = 0x8000; // arbitrary number
      let index = 0;
      const { length } = u8Arr;
      let result = '';
      let 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);
    };

    const calcEtag = () => {
      if (!sha1String.length) {
        return '';
      }
      let sha1Buffer = concatArr2Uint8(sha1String);
      let sha1Array = [] as number[];

      // 如果大于4M，则对各个块的sha1结果再次sha1
      if (blockCount > 1) {
        prefix = 0x96;
        sha1Array = sha1(sha1Buffer.buffer);
      } else {
        sha1Array = Array.from(sha1Buffer);
      }
      sha1Buffer = concatArr2Uint8([[prefix], sha1Array]);

      return Uint8ToBase64(sha1Buffer, true);
    };

    const processBlock = (block_start: number) => {
      const block_end = Math.min(buffer.size, block_start + blockSize);
      fileReader.readAsArrayBuffer(buffer.slice(block_start, block_end));
    };

    fileReader.onload = (e: any) => {
      sha1String.push(sha1(e.target.result));
      blockCount += 1;
      if (blockCount * blockSize < buffer.size) {
        processBlock(blockCount * blockSize);
      } else {
        resolve(calcEtag());
      }
    };
    processBlock(0);
  });
};