Android Open Source - bitfynd-wallet-android Base43

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The source code is released under:

GNU General Public License

Java Source Code

/*
 * Copyright 2011 Google Inc./*from  w w  w .  j a  v a 2s.co m*/
 *
 * 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 de.schildbach.wallet.util;

import javax.annotation.Nonnull;

import com.google.common.base.Charsets;

/**
 * Base43, derived from bitcoinj Base58
 * 
 * @author Andreas Schildbach
 */
public class Base43
{
  private static final char[] ALPHABET = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ$*+-./:".toCharArray();

  private static final int[] INDEXES = new int[128];
  static
  {
    for (int i = 0; i < INDEXES.length; i++)
      INDEXES[i] = -1;

    for (int i = 0; i < ALPHABET.length; i++)
      INDEXES[ALPHABET[i]] = i;
  }

  public static String encode(@Nonnull byte[] input)
  {
    if (input.length == 0)
      return "";

    input = copyOfRange(input, 0, input.length);

    // Count leading zeroes.
    int zeroCount = 0;
    while (zeroCount < input.length && input[zeroCount] == 0)
      ++zeroCount;

    // The actual encoding.
    final byte[] temp = new byte[input.length * 2];
    int j = temp.length;

    int startAt = zeroCount;
    while (startAt < input.length)
    {
      byte mod = divmod43(input, startAt);
      if (input[startAt] == 0)
        ++startAt;
      temp[--j] = (byte) ALPHABET[mod];
    }

    // Strip extra '1' if there are some after decoding.
    while (j < temp.length && temp[j] == ALPHABET[0])
      ++j;

    // Add as many leading '1' as there were leading zeros.
    while (--zeroCount >= 0)
      temp[--j] = (byte) ALPHABET[0];

    final byte[] output = copyOfRange(temp, j, temp.length);

    return new String(output, Charsets.US_ASCII);
  }

  public static byte[] decode(@Nonnull final String input) throws IllegalArgumentException
  {
    if (input.length() == 0)
      return new byte[0];

    final byte[] input43 = new byte[input.length()];
    // Transform the String to a base43 byte sequence
    for (int i = 0; i < input.length(); ++i)
    {
      final char c = input.charAt(i);

      int digit43 = -1;

      if (c >= 0 && c < 128)
        digit43 = INDEXES[c];

      if (digit43 < 0)
        throw new IllegalArgumentException("Illegal character " + c + " at " + i);

      input43[i] = (byte) digit43;
    }

    // Count leading zeroes
    int zeroCount = 0;
    while (zeroCount < input43.length && input43[zeroCount] == 0)
      ++zeroCount;

    // The encoding
    final byte[] temp = new byte[input.length()];
    int j = temp.length;

    int startAt = zeroCount;
    while (startAt < input43.length)
    {
      byte mod = divmod256(input43, startAt);
      if (input43[startAt] == 0)
        ++startAt;

      temp[--j] = mod;
    }

    // Do no add extra leading zeroes, move j to first non null byte.
    while (j < temp.length && temp[j] == 0)
      ++j;

    return copyOfRange(temp, j - zeroCount, temp.length);
  }

  //
  // number -> number / 43, returns number % 43
  //
  private static byte divmod43(final byte[] number, final int startAt)
  {
    int remainder = 0;
    for (int i = startAt; i < number.length; i++)
    {
      final int digit256 = (int) number[i] & 0xFF;
      final int temp = remainder * 256 + digit256;

      number[i] = (byte) (temp / 43);

      remainder = temp % 43;
    }

    return (byte) remainder;
  }

  //
  // number -> number / 256, returns number % 256
  //
  private static byte divmod256(final byte[] number43, final int startAt)
  {
    int remainder = 0;
    for (int i = startAt; i < number43.length; i++)
    {
      final int digit58 = (int) number43[i] & 0xFF;
      final int temp = remainder * 43 + digit58;

      number43[i] = (byte) (temp / 256);

      remainder = temp % 256;
    }

    return (byte) remainder;
  }

  private static byte[] copyOfRange(final byte[] source, final int from, final int to)
  {
    final byte[] range = new byte[to - from];
    System.arraycopy(source, from, range, 0, range.length);

    return range;
  }
}

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