content: package org.weixin4j.misc;
  
  import java.io.OutputStream;
  import java.io.PushbackInputStream;
  
  public class BASE64Decoder extends CharacterDecoder {
  
      @Override
      protected int bytesPerAtom() {
          return (4);
      }
  
      @Override
      protected int bytesPerLine() {
          return (72);
      }
  
      private final static char pem_array[] = {
          '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', '+', '/'
      };
  
      private final static byte pem_convert_array[] = new byte[256];
  
      static {
          for (int i = 0; i < 255; i++) {
              pem_convert_array[i] = -1;
          }
          for (int i = 0; i < pem_array.length; i++) {
              pem_convert_array[pem_array[i]] = (byte) i;
          }
      }
      byte decode_buffer[] = new byte[4];
  
      @Override
      protected void decodeAtom(PushbackInputStream inStream, OutputStream outStream, int rem)
              throws java.io.IOException {
          int i;
          byte a = -1, b = -1, c = -1, d = -1;
  
          if (rem < 2) {
              throw new CEFormatException("BASE64Decoder: Not enough bytes for an atom.");
          }
          do {
              i = inStream.read();
              if (i == -1) {
                  throw new CEStreamExhausted();
              }
          } while (i == '\n' || i == '\r');
          decode_buffer[0] = (byte) i;
  
          i = readFully(inStream, decode_buffer, 1, rem - 1);
          if (i == -1) {
              throw new CEStreamExhausted();
          }
  
          if (rem > 3 && decode_buffer[3] == '=') {
              rem = 3;
          }
          if (rem > 2 && decode_buffer[2] == '=') {
              rem = 2;
          }
          switch (rem) {
              case 4:
                  d = pem_convert_array[decode_buffer[3] & 0xff];
              // NOBREAK
              case 3:
                  c = pem_convert_array[decode_buffer[2] & 0xff];
              // NOBREAK
              case 2:
                  b = pem_convert_array[decode_buffer[1] & 0xff];
                  a = pem_convert_array[decode_buffer[0] & 0xff];
                  break;
          }
  
          switch (rem) {
              case 2:
                  outStream.write((byte) (((a << 2) & 0xfc) | ((b >>> 4) & 3)));
                  break;
              case 3:
                  outStream.write((byte) (((a << 2) & 0xfc) | ((b >>> 4) & 3)));
                  outStream.write((byte) (((b << 4) & 0xf0) | ((c >>> 2) & 0xf)));
                  break;
              case 4:
                  outStream.write((byte) (((a << 2) & 0xfc) | ((b >>> 4) & 3)));
                  outStream.write((byte) (((b << 4) & 0xf0) | ((c >>> 2) & 0xf)));
                  outStream.write((byte) (((c << 6) & 0xc0) | (d & 0x3f)));
                  break;
          }
      }
  }
  