content: package com.liyi.sutils.utils.encrypt;
  
  import android.text.TextUtils;
  
  import java.io.ByteArrayInputStream;
  import java.io.ByteArrayOutputStream;
  import java.io.FilterInputStream;
  import java.io.IOException;
  import java.io.InputStream;
  
  /**
   * base64 解密
   */
  public class Base64Decoder extends FilterInputStream {
  
      private static final char[] chars = {'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', '+', '/'};
  
      // A mapping between char values and six-bit integers
      private static final int[] ints = new int[128];
  
      static {
          for (int i = 0; i < 64; i++) {
              ints[chars[i]] = i;
          }
      }
  
      private int charCount;
      private int carryOver;
  
      /***
       * Constructs a new Base64 decoder that reads input from the given
       * InputStream.
       *
       * @param in
       *            the input stream
       */
      private Base64Decoder(InputStream in) {
          super(in);
      }
  
      /***
       * Returns the next decoded character from the stream, or -1 if end of
       * stream was reached.
       *
       * @return the decoded character, or -1 if the end of the input stream is
       *         reached
       * @exception IOException
       *                if an I/O error occurs
       */
      public int read() throws IOException {
          // Read the next non-whitespace character
          int x;
          do {
              x = in.read();
              if (x == -1) {
                  return -1;
              }
          } while (Character.isWhitespace((char) x));
          charCount++;
  
          // The '=' sign is just padding
          if (x == '=') {
              return -1; // effective end of stream
          }
  
          // Convert from raw form to 6-bit form
          x = ints[x];
  
          // Calculate which character we're decoding now
          int mode = (charCount - 1) % 4;
  
          // First char save all six bits, go for another
          if (mode == 0) {
              carryOver = x & 63;
              return read();
          }
          // Second char use previous six bits and first two new bits,
          // save last four bits
          else if (mode == 1) {
              int decoded = ((carryOver << 2) + (x >> 4)) & 255;
              carryOver = x & 15;
              return decoded;
          }
          // Third char use previous four bits and first four new bits,
          // save last two bits
          else if (mode == 2) {
              int decoded = ((carryOver << 4) + (x >> 2)) & 255;
              carryOver = x & 3;
              return decoded;
          }
          // Fourth char use previous two bits and all six new bits
          else if (mode == 3) {
              int decoded = ((carryOver << 6) + x) & 255;
              return decoded;
          }
          return -1; // can't actually reach this line
      }
  
      /***
       * Reads decoded data into an array of bytes and returns the actual number
       * of bytes read, or -1 if end of stream was reached.
       *
       * @param buf
       *            the buffer into which the data is read
       * @param off
       *            the start offset of the data
       * @param len
       *            the maximum number of bytes to read
       * @return the actual number of bytes read, or -1 if the end of the input
       *         stream is reached
       * @exception IOException
       *                if an I/O error occurs
       */
      public int read(byte[] buf, int off, int len) throws IOException {
          if (buf.length < (len + off - 1)) {
              throw new IOException("The input buffer is too small: " + len + " bytes requested starting at offset " + off + " while the buffer " + " is only " + buf.length + " bytes long.");
          }
  
          // This could of course be optimized
          int i;
          for (i = 0; i < len; i++) {
              int x = read();
              if (x == -1 && i == 0) { // an immediate -1 returns -1
                  return -1;
              } else if (x == -1) { // a later -1 returns the chars read so far
                  break;
              }
              buf[off + i] = (byte) x;
          }
          return i;
      }
  
      /***
       * Returns the decoded form of the given encoded string, as a String. Note
       * that not all binary data can be represented as a String, so this method
       * should only be used for encoded String data. Use decodeToBytes()
       * otherwise.
       *
       * @param encoded
       *            the string to decode
       * @return the decoded form of the encoded string
       */
      public static String decode(String encoded) {
          if (TextUtils.isEmpty(encoded)) {
              return "";
          }
          return new String(decodeToBytes(encoded));
      }
  
      /***
       * Returns the decoded form of the given encoded string, as bytes.
       *
       * @param encoded
       *            the string to decode
       * @return the decoded form of the encoded string
       */
      public static byte[] decodeToBytes(String encoded) {
          byte[] bytes = encoded.getBytes();
          Base64Decoder in = new Base64Decoder(new ByteArrayInputStream(bytes));
          ByteArrayOutputStream out = new ByteArrayOutputStream((int) (bytes.length * 0.75));
          try {
              byte[] buf = new byte[4 * 1024]; // 4K buffer
              int bytesRead;
              while ((bytesRead = in.read(buf)) != -1) {
                  out.write(buf, 0, bytesRead);
              }
              return out.toByteArray();
          } catch (IOException e) {
              throw new RuntimeException(e);
          } finally {
              try {
                  in.close();
              } catch (IOException e) {
                  throw new RuntimeException(e);
              }
              try {
                  out.close();
              } catch (IOException e) {
                  throw new RuntimeException(e);
              }
          }
      }
  }