Base58.java

/*
 *  Copyright 2017 Patrick Favre-Bulle
 *
 *  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 at.favre.tools.dice.encode.impl;

import java.math.BigInteger;
import java.util.Arrays;

/**
 * Base58 is a way to encode arbitrary data as alphanumeric strings.
 * This implementation supports the alphabet for Bitcoin, Ripple and Flickr
 * <p>
 * <p>
 * You may want to consider working with VersionedChecksummedBytes instead, which
 * adds support for testing the prefix and suffix bytes commonly found in addresses.
 * <p>
 * Satoshi explains: why base-58 instead of standard base-64 encoding?
 * <ul>
 * <li>Don't want 0OIl characters that look the same in some fonts and
 * could be used to create visually identical looking account numbers.</li>
 * <li>A string with non-alphanumeric characters is not as easily accepted as an account number.</li>
 * <li>E-mail usually won't line-break if there's no punctuation to break at.</li>
 * <li>Doubleclicking selects the whole number as one word if it's all alphanumeric.</li>
 * </ul>
 * <p>
 * However, note that the encoding/decoding runs in O(n&sup2;) time, so it is not useful for large data.
 * <p>
 * The basic idea of the encoding is to treat the data bytes as a large number represented using
 * base-256 digits, convert the number to be represented using base-58 digits, preserve the exact
 * number of leading zeros (which are otherwise lost during the mathematical operations on the
 * numbers), and finally represent the resulting base-58 digits as alphanumeric ASCII characters.
 */
public final class Base58 {
    private static final char[] BITCOIN_ALPHABET = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz".toCharArray();
    private static final char[] RIPPLE_ALPHABET = "rpshnaf39wBUDNEGHJKLM4PQRST7VWXYZ2bcdeCg65jkm8oFqi1tuvAxyz".toCharArray();
    private static final char[] FLICKR_ALPHABET = "123456789abcdefghijkmnopqrstuvwxyzABCDEFGHJKLMNPQRSTUVWXYZ".toCharArray();

    public static Base58 BitcoinEncoder() {
        return new Base58(BITCOIN_ALPHABET);
    }

    public static Base58 RippleEncoder() {
        return new Base58(RIPPLE_ALPHABET);
    }

    public static Base58 FlickrEncoder() {
        return new Base58(FLICKR_ALPHABET);
    }

    private final int[] indexes = new int[128];
    private final char[] alphabet;
    private final char encodedZero;

    private Base58(char[] alphabet) {
        this.alphabet = alphabet;
        this.encodedZero = alphabet[0];

        Arrays.fill(indexes, -1);
        for (int i = 0; i < alphabet.length; i++) {
            indexes[alphabet[i]] = i;
        }
    }

    /**
     * Encodes the given bytes as a base58 string (no checksum is appended).
     *
     * @param input the bytes to encode
     * @return the base58-encoded string
     */
    public String encode(byte[] input) {
        if (input.length == 0) {
            return "";
        }
        // Count leading zeros.
        int zeros = 0;
        while (zeros < input.length && input[zeros] == 0) {
            ++zeros;
        }
        // Convert base-256 digits to base-58 digits (plus conversion to ASCII characters)
        input = Arrays.copyOf(input, input.length); // since we modify it in-place
        char[] encoded = new char[input.length * 2]; // upper bound
        int outputStart = encoded.length;
        for (int inputStart = zeros; inputStart < input.length; ) {
            encoded[--outputStart] = alphabet[divmod(input, inputStart, 256, 58)];
            if (input[inputStart] == 0) {
                ++inputStart; // optimization - skip leading zeros
            }
        }
        // Preserve exactly as many leading encoded zeros in output as there were leading zeros in input.
        while (outputStart < encoded.length && encoded[outputStart] == encodedZero) {
            ++outputStart;
        }
        while (--zeros >= 0) {
            encoded[--outputStart] = encodedZero;
        }
        // Return encoded string (including encoded leading zeros).
        return new String(encoded, outputStart, encoded.length - outputStart);
    }

    /**
     * Decodes the given base58 string into the original data bytes.
     *
     * @param input the base58-encoded string to decode
     * @return the decoded data bytes
     * @throws IllegalArgumentException if the given string is not a valid base58 string
     */
    public byte[] decode(String input) {
        if (input.length() == 0) {
            return new byte[0];
        }
        // Convert the base58-encoded ASCII chars to a base58 byte sequence (base58 digits).
        byte[] input58 = new byte[input.length()];
        for (int i = 0; i < input.length(); ++i) {
            char c = input.charAt(i);
            int digit = c < 128 ? indexes[c] : -1;
            if (digit < 0) {
                throw new IllegalArgumentException("Illegal character " + c + " at position " + i);
            }
            input58[i] = (byte) digit;
        }
        // Count leading zeros.
        int zeros = 0;
        while (zeros < input58.length && input58[zeros] == 0) {
            ++zeros;
        }
        // Convert base-58 digits to base-256 digits.
        byte[] decoded = new byte[input.length()];
        int outputStart = decoded.length;
        for (int inputStart = zeros; inputStart < input58.length; ) {
            decoded[--outputStart] = divmod(input58, inputStart, 58, 256);
            if (input58[inputStart] == 0) {
                ++inputStart; // optimization - skip leading zeros
            }
        }
        // Ignore extra leading zeroes that were added during the calculation.
        while (outputStart < decoded.length && decoded[outputStart] == 0) {
            ++outputStart;
        }
        // Return decoded data (including original number of leading zeros).
        return Arrays.copyOfRange(decoded, outputStart - zeros, decoded.length);
    }

    public BigInteger decodeToBigInteger(String input) {
        return new BigInteger(1, decode(input));
    }

    /**
     * Divides a number, represented as an array of bytes each containing a single digit
     * in the specified base, by the given divisor. The given number is modified in-place
     * to contain the quotient, and the return value is the remainder.
     *
     * @param number     the number to divide
     * @param firstDigit the index within the array of the first non-zero digit
     *                   (this is used for optimization by skipping the leading zeros)
     * @param base       the base in which the number's digits are represented (up to 256)
     * @param divisor    the number to divide by (up to 256)
     * @return the remainder of the division operation
     */
    private static byte divmod(byte[] number, int firstDigit, int base, int divisor) {
        // this is just long division which accounts for the base of the input digits
        int remainder = 0;
        for (int i = firstDigit; i < number.length; i++) {
            int digit = (int) number[i] & 0xFF;
            int temp = remainder * base + digit;
            number[i] = (byte) (temp / divisor);
            remainder = temp % divisor;
        }
        return (byte) remainder;
    }

}