Base43.java

/**
 * Copyright 2011 Google Inc.
 *
 * 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 devcoin.wallet.util;

import java.nio.charset.Charset;

import javax.annotation.Nonnull;

/**
 * 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, Charset.forName("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;
	}
}