/*
 * Copyright 2021 Christian Pierre MOMON <cmomon@april.org>
 * Copyright 2020 Dr Ian Preston ianopolous
 * Copyright 2018 Andreas Schildbach
 * Copyright 2011 Google Inc.
 *
 * From:
 * https://github.com/multiformats/java-multibase/blob/master/src/main/java/io/ipfs/multibase/Base58.java
 * https://github.com/bitcoinj/bitcoinj/blob/master/core/src/main/java/org/bitcoinj/core/Base58.java
 *
 * 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 org.april.hebdobot.privatebin;

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;
        }
    }

    /**
     * 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(final 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(final 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(final byte[] number, final int firstDigit, final int base, final 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 = number[i] & 0xFF;
            int temp = remainder * base + digit;
            number[i] = (byte) (temp / divisor);
            remainder = temp % divisor;
        }
        return (byte) remainder;
    }

    /**
     * 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);
    }
}