content: /*
   * Copyright (C) 2010 The Android Open Source Project
   * Licensed under the Apache License, Version 2.0 (the "License");
   * you may not use this file except in compliance with the License.
   * You may obtain a copy of the License at
   *      http://www.apache.org/licenses/LICENSE-2.0
   * Unless required by applicable law or agreed to in writing, software
   * distributed under the License is distributed on an "AS IS" BASIS,
   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
   * See the License for the specific language governing permissions and
   * limitations under the License.
   */
  package dev.utils.common.cipher;
  
  import java.io.UnsupportedEncodingException;
  
  /**
   * detail: Base64 工具类
   * @author Android
   */
  public final class Base64 {
      /**
       * Default values for encoder/decoder flags.
       */
      public static final int DEFAULT = 0;
  
      /**
       * Encoder flag bit to omit the padding '=' characters at the end
       * of the output (if any).
       */
      public static final int NO_PADDING = 1;
  
      /**
       * Encoder flag bit to omit all line terminators (i.e., the output
       * will be on one long line).
       */
      public static final int NO_WRAP = 2;
  
      /**
       * Encoder flag bit to indicate lines should be terminated with a
       * CRLF pair instead of just an LF.  Has no effect if {@code
       * NO_WRAP} is specified as well.
       */
      public static final int CRLF = 4;
  
      /**
       * Encoder/decoder flag bit to indicate using the "URL and
       * filename safe" variant of Base64 (see RFC 3548 section 4) where
       * {@code -} and {@code _} are used in place of {@code +} and
       * {@code /}.
       */
      public static final int URL_SAFE = 8;
  
      /**
       * Flag to pass to Base64OutputStream to indicate that it
       * should not close the output stream it is wrapping when it
       * itself is closed.
       */
      public static final int NO_CLOSE = 16;
  
      //  --------------------------------------------------------
      //  shared code
      //  --------------------------------------------------------
  
      /* package */ static abstract class Coder {
          public byte[] output;
          public int    op;
  
          /**
           * Encode/decode another block of input data.  this.output is
           * provided by the caller, and must be big enough to hold all
           * the coded data.  On exit, this.opwill be set to the length
           * of the coded data.
           * @param finish true if this is the final call to process for
           *               this object.  Will finalize the coder state and
           *               include any final bytes in the output.
           * @return true if the input so far is good; false if some
           * error has been detected in the input stream..
           */
          public abstract boolean process(
                  byte[] input,
                  int offset,
                  int len,
                  boolean finish
          );
  
          /**
           * @return the maximum number of bytes a call to process()
           * could produce for the given number of input bytes.  This may
           * be an overestimate.
           */
          public abstract int maxOutputSize(int len);
      }
  
      //  --------------------------------------------------------
      //  decoding
      //  --------------------------------------------------------
  
      /**
       * Decode the Base64-encoded data in input and return the data in
       * a new byte array.
       * <p>
       * The padding '=' characters at the end are considered optional, but
       * if any are present, there must be the correct number of them.
       * @param str   the input String to decode, which is converted to
       *              bytes using the default charset
       * @param flags controls certain features of the decoded output.
       *              Pass {@code DEFAULT} to decode standard Base64.
       * @throws IllegalArgumentException if the input contains
       *                                  incorrect padding
       */
      public static byte[] decode(
              String str,
              int flags
      ) {
          return decode(str.getBytes(), flags);
      }
  
      /**
       * Decode the Base64-encoded data in input and return the data in
       * a new byte array.
       * <p>
       * The padding '=' characters at the end are considered optional, but
       * if any are present, there must be the correct number of them.
       * @param input the input array to decode
       * @param flags controls certain features of the decoded output.
       *              Pass {@code DEFAULT} to decode standard Base64.
       * @throws IllegalArgumentException if the input contains
       *                                  incorrect padding
       */
      public static byte[] decode(
              byte[] input,
              int flags
      ) {
          return decode(input, 0, input.length, flags);
      }
  
      /**
       * Decode the Base64-encoded data in input and return the data in
       * a new byte array.
       * <p>
       * The padding '=' characters at the end are considered optional, but
       * if any are present, there must be the correct number of them.
       * @param input  the data to decode
       * @param offset the position within the input array at which to start
       * @param len    the number of bytes of input to decode
       * @param flags  controls certain features of the decoded output.
       *               Pass {@code DEFAULT} to decode standard Base64.
       * @throws IllegalArgumentException if the input contains
       *                                  incorrect padding
       */
      public static byte[] decode(
              byte[] input,
              int offset,
              int len,
              int flags
      ) {
          // Allocate space for the most data the input could represent.
          // (It could contain less if it contains whitespace, etc.)
          Decoder decoder = new Decoder(flags, new byte[len * 3 / 4]);
  
          if (!decoder.process(input, offset, len, true)) {
              throw new IllegalArgumentException("bad base-64");
          }
  
          // Maybe we got lucky and allocated exactly enough output space.
          if (decoder.op == decoder.output.length) {
              return decoder.output;
          }
  
          // Need to shorten the array, so allocate a new one of the
          // right size and copy.
          byte[] temp = new byte[decoder.op];
          System.arraycopy(decoder.output, 0, temp, 0, decoder.op);
          return temp;
      }
  
      /* package */ static class Decoder
              extends Coder {
          /**
           * Lookup table for turning bytes into their position in the
           * Base64 alphabet.
           */
          private static final int[] DECODE = {
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1, -1, 63,
                  52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -2, -1, -1,
                  -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
                  15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, -1,
                  -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
                  41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
          };
  
          /**
           * Decode lookup table for the "web safe" variant (RFC 3548
           * sec. 4) where - and _ replace + and /.
           */
          private static final int[] DECODE_WEBSAFE = {
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 62, -1, -1,
                  52, 53, 54, 55, 56, 57, 58, 59, 60, 61, -1, -1, -1, -2, -1, -1,
                  -1, 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14,
                  15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, -1, -1, -1, -1, 63,
                  -1, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40,
                  41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
                  -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
          };
  
          /**
           * Non-data values in the DECODE arrays.
           */
          private static final int SKIP   = -1;
          private static final int EQUALS = -2;
  
          /**
           * States 0-3 are reading through the next input tuple.
           * State 4 is having read one '=' and expecting exactly
           * one more.
           * State 5 is expecting no more data or padding characters
           * in the input.
           * State 6 is the error state; an error has been detected
           * in the input and no future input can "fix" it.
           */
          private int state;   // state number (0 to 6)
          private int value;
  
          final private int[] alphabet;
  
          public Decoder(
                  int flags,
                  byte[] output
          ) {
              this.output = output;
  
              alphabet = ((flags & URL_SAFE) == 0) ? DECODE : DECODE_WEBSAFE;
              state    = 0;
              value    = 0;
          }
  
          /**
           * @return an overestimate for the number of bytes {@code
           * len} bytes could decode to.
           */
          public int maxOutputSize(int len) {
              return len * 3 / 4 + 10;
          }
  
          /**
           * Decode another block of input data.
           * @return true if the state machine is still healthy.  false if
           * bad base-64 data has been detected in the input stream.
           */
          public boolean process(
                  byte[] input,
                  int offset,
                  int len,
                  boolean finish
          ) {
              if (this.state == 6) return false;
  
              int p = offset;
              len += offset;
  
              // Using local variables makes the decoder about 12%
              // faster than if we manipulate the member variables in
              // the loop.  (Even alphabet makes a measurable
              // difference, which is somewhat surprising to me since
              // the member variable is final.)
              int          state    = this.state;
              int          value    = this.value;
              int          op       = 0;
              final byte[] output   = this.output;
              final int[]  alphabet = this.alphabet;
  
              while (p < len) {
                  // Try the fast path:  we're starting a new tuple and the
                  // next four bytes of the input stream are all data
                  // bytes.  This corresponds to going through states
                  // 0-1-2-3-0.  We expect to use this method for most of
                  // the data.
                  //
                  // If any of the next four bytes of input are non-data
                  // (whitespace, etc.), value will end up negative.  (All
                  // the non-data values in decode are small negative
                  // numbers, so shifting any of them up and or'ing them
                  // together will result in a value with its top bit set.)
                  //
                  // You can remove this whole block and the output should
                  // be the same, just slower.
                  if (state == 0) {
                      while (p + 4 <= len &&
                              (value = ((alphabet[input[p] & 0xff] << 18) |
                                      (alphabet[input[p + 1] & 0xff] << 12) |
                                      (alphabet[input[p + 2] & 0xff] << 6) |
                                      (alphabet[input[p + 3] & 0xff]))) >= 0) {
                          output[op + 2] = (byte) value;
                          output[op + 1] = (byte) (value >> 8);
                          output[op]     = (byte) (value >> 16);
                          op += 3;
                          p += 4;
                      }
                      if (p >= len) break;
                  }
  
                  // The fast path isn't available either we've read a
                  // partial tuple, or the next four input bytes aren't all
                  // data, or whatever.  Fall back to the slower state
                  // machine implementation.
  
                  int d = alphabet[input[p++] & 0xff];
  
                  switch (state) {
                      case 0:
                          if (d >= 0) {
                              value = d;
                              ++state;
                          } else if (d != SKIP) {
                              this.state = 6;
                              return false;
                          }
                          break;
  
                      case 1:
                          if (d >= 0) {
                              value = (value << 6) | d;
                              ++state;
                          } else if (d != SKIP) {
                              this.state = 6;
                              return false;
                          }
                          break;
  
                      case 2:
                          if (d >= 0) {
                              value = (value << 6) | d;
                              ++state;
                          } else if (d == EQUALS) {
                              // Emit the last (partial) output tuple;
                              // expect exactly one more padding character.
                              output[op++] = (byte) (value >> 4);
                              state        = 4;
                          } else if (d != SKIP) {
                              this.state = 6;
                              return false;
                          }
                          break;
  
                      case 3:
                          if (d >= 0) {
                              // Emit the output triple and return to state 0.
                              value          = (value << 6) | d;
                              output[op + 2] = (byte) value;
                              output[op + 1] = (byte) (value >> 8);
                              output[op]     = (byte) (value >> 16);
                              op += 3;
                              state          = 0;
                          } else if (d == EQUALS) {
                              // Emit the last (partial) output tuple;
                              // expect no further data or padding characters.
                              output[op + 1] = (byte) (value >> 2);
                              output[op]     = (byte) (value >> 10);
                              op += 2;
                              state          = 5;
                          } else if (d != SKIP) {
                              this.state = 6;
                              return false;
                          }
                          break;
  
                      case 4:
                          if (d == EQUALS) {
                              ++state;
                          } else if (d != SKIP) {
                              this.state = 6;
                              return false;
                          }
                          break;
  
                      case 5:
                          if (d != SKIP) {
                              this.state = 6;
                              return false;
                          }
                          break;
                  }
              }
  
              if (!finish) {
                  // We're out of input, but a future call could provide
                  // more.
                  this.state = state;
                  this.value = value;
                  this.op    = op;
                  return true;
              }
  
              // Done reading input.  Now figure out where we are left in
              // the state machine and finish up.
  
              switch (state) {
                  case 0:
                      // Output length is a multiple of three.  Fine.
                      break;
                  case 1:
                      // Read one extra input byte, which isn't enough to
                      // make another output byte.  Illegal.
                      this.state = 6;
                      return false;
                  case 2:
                      // Read two extra input bytes, enough to emit 1 more
                      // output byte.  Fine.
                      output[op++] = (byte) (value >> 4);
                      break;
                  case 3:
                      // Read three extra input bytes, enough to emit 2 more
                      // output bytes.  Fine.
                      output[op++] = (byte) (value >> 10);
                      output[op++] = (byte) (value >> 2);
                      break;
                  case 4:
                      // Read one padding '=' when we expected 2.  Illegal.
                      this.state = 6;
                      return false;
                  case 5:
                      // Read all the padding '='s we expected and no more.
                      // Fine.
                      break;
              }
  
              this.state = state;
              this.op    = op;
              return true;
          }
      }
  
      //  --------------------------------------------------------
      //  encoding
      //  --------------------------------------------------------
  
      /**
       * Base64-encode the given data and return a newly allocated
       * String with the result.
       * @param input the data to encode
       * @param flags controls certain features of the encoded output.
       *              Passing {@code DEFAULT} results in output that
       *              adheres to RFC 2045.
       */
      public static String encodeToString(
              byte[] input,
              int flags
      ) {
          try {
              return new String(encode(input, flags), "US-ASCII");
          } catch (UnsupportedEncodingException e) {
              // US-ASCII is guaranteed to be available.
              throw new AssertionError(e);
          }
      }
  
      /**
       * Base64-encode the given data and return a newly allocated
       * String with the result.
       * @param input  the data to encode
       * @param offset the position within the input array at which to
       *               start
       * @param len    the number of bytes of input to encode
       * @param flags  controls certain features of the encoded output.
       *               Passing {@code DEFAULT} results in output that
       *               adheres to RFC 2045.
       */
      public static String encodeToString(
              byte[] input,
              int offset,
              int len,
              int flags
      ) {
          try {
              return new String(encode(input, offset, len, flags), "US-ASCII");
          } catch (UnsupportedEncodingException e) {
              // US-ASCII is guaranteed to be available.
              throw new AssertionError(e);
          }
      }
  
      /**
       * Base64-encode the given data and return a newly allocated
       * byte[] with the result.
       * @param input the data to encode
       * @param flags controls certain features of the encoded output.
       *              Passing {@code DEFAULT} results in output that
       *              adheres to RFC 2045.
       */
      public static byte[] encode(
              byte[] input,
              int flags
      ) {
          return encode(input, 0, input.length, flags);
      }
  
      /**
       * Base64-encode the given data and return a newly allocated
       * byte[] with the result.
       * @param input  the data to encode
       * @param offset the position within the input array at which to
       *               start
       * @param len    the number of bytes of input to encode
       * @param flags  controls certain features of the encoded output.
       *               Passing {@code DEFAULT} results in output that
       *               adheres to RFC 2045.
       */
      public static byte[] encode(
              byte[] input,
              int offset,
              int len,
              int flags
      ) {
          Encoder encoder = new Encoder(flags, null);
  
          // Compute the exact length of the array we will produce.
          int output_len = len / 3 * 4;
  
          // Account for the tail of the data and the padding bytes, if any.
          if (encoder.do_padding) {
              if (len % 3 > 0) {
                  output_len += 4;
              }
          } else {
              switch (len % 3) {
                  case 0:
                      break;
                  case 1:
                      output_len += 2;
                      break;
                  case 2:
                      output_len += 3;
                      break;
              }
          }
  
          // Account for the newlines, if any.
          if (encoder.do_newline && len > 0) {
              output_len += (((len - 1) / (3 * Encoder.LINE_GROUPS)) + 1) *
                      (encoder.do_cr ? 2 : 1);
          }
  
          encoder.output = new byte[output_len];
          encoder.process(input, offset, len, true);
  
          assert encoder.op == output_len;
  
          return encoder.output;
      }
  
      /* package */ static class Encoder
              extends Coder {
          /**
           * Emit a new line every this many output tuples.  Corresponds to
           * a 76-character line length (the maximum allowable according to
           * <a href="http://www.ietf.org/rfc/rfc2045.txt">RFC 2045</a>).
           */
          public static final int LINE_GROUPS = 19;
  
          /**
           * Lookup table for turning Base64 alphabet positions (6 bits)
           * into output bytes.
           */
          private static final byte[] ENCODE = {
                  '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', '+', '/',
          };
  
          /**
           * Lookup table for turning Base64 alphabet positions (6 bits)
           * into output bytes.
           */
          private static final byte[] ENCODE_WEBSAFE = {
                  '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', '-', '_',
          };
  
          final private byte[] tail;
          /* package */ int tailLen;
          private int count;
  
          final public  boolean do_padding;
          final public  boolean do_newline;
          final public  boolean do_cr;
          final private byte[]  alphabet;
  
          public Encoder(
                  int flags,
                  byte[] output
          ) {
              this.output = output;
  
              do_padding = (flags & NO_PADDING) == 0;
              do_newline = (flags & NO_WRAP) == 0;
              do_cr      = (flags & CRLF) != 0;
              alphabet   = ((flags & URL_SAFE) == 0) ? ENCODE : ENCODE_WEBSAFE;
  
              tail    = new byte[2];
              tailLen = 0;
  
              count = do_newline ? LINE_GROUPS : -1;
          }
  
          /**
           * @return an overestimate for the number of bytes {@code
           * len} bytes could encode to.
           */
          public int maxOutputSize(int len) {
              return len * 8 / 5 + 10;
          }
  
          public boolean process(
                  byte[] input,
                  int offset,
                  int len,
                  boolean finish
          ) {
              // Using local variables makes the encoder about 9% faster.
              final byte[] alphabet = this.alphabet;
              final byte[] output   = this.output;
              int          op       = 0;
              int          count    = this.count;
  
              int p = offset;
              len += offset;
              int v = -1;
  
              // First we need to concatenate the tail of the previous call
              // with any input bytes available now and see if we can empty
              // the tail.
  
              switch (tailLen) {
                  case 0:
                      // There was no tail.
                      break;
  
                  case 1:
                      if (p + 2 <= len) {
                          // A 1-byte tail with at least 2 bytes of
                          // input available now.
                          v       = ((tail[0] & 0xff) << 16) |
                                  ((input[p++] & 0xff) << 8) |
                                  (input[p++] & 0xff);
                          tailLen = 0;
                      }
                      ;
                      break;
  
                  case 2:
                      if (p + 1 <= len) {
                          // A 2-byte tail with at least 1 byte of input.
                          v       = ((tail[0] & 0xff) << 16) |
                                  ((tail[1] & 0xff) << 8) |
                                  (input[p++] & 0xff);
                          tailLen = 0;
                      }
                      break;
              }
  
              if (v != -1) {
                  output[op++] = alphabet[(v >> 18) & 0x3f];
                  output[op++] = alphabet[(v >> 12) & 0x3f];
                  output[op++] = alphabet[(v >> 6) & 0x3f];
                  output[op++] = alphabet[v & 0x3f];
                  if (--count == 0) {
                      if (do_cr) output[op++] = '\r';
                      output[op++] = '\n';
                      count        = LINE_GROUPS;
                  }
              }
  
              // At this point either there is no tail, or there are fewer
              // than 3 bytes of input available.
  
              // The main loop, turning 3 input bytes into 4 output bytes on
              // each iteration.
              while (p + 3 <= len) {
                  v              = ((input[p] & 0xff) << 16) |
                          ((input[p + 1] & 0xff) << 8) |
                          (input[p + 2] & 0xff);
                  output[op]     = alphabet[(v >> 18) & 0x3f];
                  output[op + 1] = alphabet[(v >> 12) & 0x3f];
                  output[op + 2] = alphabet[(v >> 6) & 0x3f];
                  output[op + 3] = alphabet[v & 0x3f];
                  p += 3;
                  op += 4;
                  if (--count == 0) {
                      if (do_cr) output[op++] = '\r';
                      output[op++] = '\n';
                      count        = LINE_GROUPS;
                  }
              }
  
              if (finish) {
                  // Finish up the tail of the input.  Note that we need to
                  // consume any bytes in tail before any bytes
                  // remaining in input; there should be at most two bytes
                  // total.
  
                  if (p - tailLen == len - 1) {
                      int t = 0;
                      v            = ((tailLen > 0 ? tail[t++] : input[p++]) & 0xff) << 4;
                      tailLen -= t;
                      output[op++] = alphabet[(v >> 6) & 0x3f];
                      output[op++] = alphabet[v & 0x3f];
                      if (do_padding) {
                          output[op++] = '=';
                          output[op++] = '=';
                      }
                      if (do_newline) {
                          if (do_cr) output[op++] = '\r';
                          output[op++] = '\n';
                      }
                  } else if (p - tailLen == len - 2) {
                      int t = 0;
                      v            = (((tailLen > 1 ? tail[t++] : input[p++]) & 0xff) << 10) |
                              (((tailLen > 0 ? tail[t++] : input[p++]) & 0xff) << 2);
                      tailLen -= t;
                      output[op++] = alphabet[(v >> 12) & 0x3f];
                      output[op++] = alphabet[(v >> 6) & 0x3f];
                      output[op++] = alphabet[v & 0x3f];
                      if (do_padding) {
                          output[op++] = '=';
                      }
                      if (do_newline) {
                          if (do_cr) output[op++] = '\r';
                          output[op++] = '\n';
                      }
                  } else if (do_newline && op > 0 && count != LINE_GROUPS) {
                      if (do_cr) output[op++] = '\r';
                      output[op++] = '\n';
                  }
  
                  assert tailLen == 0;
                  assert p == len;
              } else {
                  // Save the leftovers in tail to be consumed on the next
                  // call to encodeInternal.
  
                  if (p == len - 1) {
                      tail[tailLen++] = input[p];
                  } else if (p == len - 2) {
                      tail[tailLen++] = input[p];
                      tail[tailLen++] = input[p + 1];
                  }
              }
  
              this.op    = op;
              this.count = count;
  
              return true;
          }
      }
  
      private Base64() {
      }   // don't instantiate
  }