MoneroBase58.java

import java.util.Arrays;
import java.io.*;

public class MoneroBase58 {
    private static final char[] ALPHABET = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz".toCharArray();
    private static final int[] INDEXES = new int[128];
    static {
        Arrays.fill(INDEXES, -1);
        for (int i = 0; i < ALPHABET.length; i++) {
            INDEXES[ALPHABET[i]] = i;
        }
    }

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

        final int unit = 11;

        ByteArrayOutputStream buffer = new ByteArrayOutputStream();
        final int round = input.length() / unit;
        for (int i = 0; i < round; ++i) {
            byte[] data = decodeBlock(input.substring(i*unit, (i+1)*unit));
            buffer.write(data, 0, data.length);
        }

        if (input.length() % unit > 0) {
           byte[] data = decodeBlock(input.substring(round*unit));
           buffer.write(data, 0, data.length);
        }

        return buffer.toByteArray();
    }

    private static int decodeSize(int n) {
        switch (n) {
            case 0: return 0;
            case 2: return 1;
            case 3: return 2;
            case 5: return 3;
            case 6: return 4;
            case 7: return 5;
            case 9: return 6;
            case 10: return 7;
            case 11: return 8;
            default: return -1;
        }
    }

    private static byte[] decodeBlock(String input) throws IllegalArgumentException {
        int outSize = decodeSize(input.length());
        if (outSize < 0) {
            throw new IllegalArgumentException();
        }

        if (outSize == 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();
             }
             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
             }
         }

         // Return decoded data (including original number of leading zeros).
         return Arrays.copyOfRange(decoded, decoded.length-outSize, decoded.length);
    }

    /**
     * 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;
    }
}