Base58.java

package cryptography.encoding.base58;

/*
 * Copyright 2011 Google Inc.
 * Copyright 2018 Andreas Schildbach
 *
 * From https://github.com/bitcoinj/bitcoinj/blob/master/core/src/main/java/org/bitcoinj/core/Base58.java
 * Added encodeFromString and decodeToString functions to use directly strings instead of byte arrays
 *
 * 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.
 */

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

/**
 * Base58 is a way to encode Bitcoin addresses (or arbitrary data) as alphanumeric strings.
 * <p>
 * Note that this is not the same base58 as used by Flickr, which you may find referenced around the Internet.
 * <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 class Base58 {
	public static final char[] ALPHABET = "123456789ABCDEFGHJKLMNPQRSTUVWXYZabcdefghijkmnopqrstuvwxyz".toCharArray();
	private static final char ENCODED_ZERO = ALPHABET[0];
	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;
		}
	}

	/**
	 * Encodes the given bytes as a base58 string (no checksum is appended).
	 *
	 * @param input the bytes to encode
	 * @return the base58-encoded string
	 */
	public static 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] == ENCODED_ZERO) {
			++outputStart;
		}
		while (--zeros >= 0) {
			encoded[--outputStart] = ENCODED_ZERO;
		}
		// 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
	 */
	public static 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 IllegalStateException("InvalidCharacter in base 58");
			}
			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 static BigInteger decodeToBigInteger(String input) {
		return new BigInteger(1, decode(input));
	}

	public static String encodeFromString(String input) {
		return encode(input.getBytes());
	}

	public static String decodeToString(String input) {
		return new String(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;
	}

}