// Copyright (c) 2017 Pieter Wuille
// Copyright (c) 2017-2023 The Bitcoin developers
// Distributed under the MIT software license, see the accompanying
// file COPYING or http://www.opensource.org/licenses/mit-license.php.

#include <cashaddr.h>
#include <util/vector.h>

namespace {

using data = std::vector<uint8_t>;

/**
 * The cashaddr character set for encoding.
 */
const char CHARSET[] = "qpzry9x8gf2tvdw0s3jn54khce6mua7l";

// Ensure basic sanity as per the spec. This should match the strlen() of above.
constexpr const uint8_t PACKED_VAL_LIMIT = 32u;
static_assert(std::size(CHARSET) - 1u == PACKED_VAL_LIMIT);

/**
 * The cashaddr character set for decoding.
 */
const int8_t CHARSET_REV[128] = {
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, -1,
    -1, -1, -1, -1, -1, -1, -1, -1, -1, -1, 15, -1, 10, 17, 21, 20, 26, 30, 7,
    5,  -1, -1, -1, -1, -1, -1, -1, 29, -1, 24, 13, 25, 9,  8,  23, -1, 18, 22,
    31, 27, 19, -1, 1,  0,  3,  16, 11, 28, 12, 14, 6,  4,  2,  -1, -1, -1, -1,
    -1, -1, 29, -1, 24, 13, 25, 9,  8,  23, -1, 18, 22, 31, 27, 19, -1, 1,  0,
    3,  16, 11, 28, 12, 14, 6,  4,  2,  -1, -1, -1, -1, -1};

/**
 * This function will compute what 8 5-bit values to XOR into the last 8 input
 * values, in order to make the checksum 0. These 8 values are packed together
 * in a single 40-bit integer. The higher bits correspond to earlier values.
 */
uint64_t PolyMod(const data &v) {
    /**
     * The input is interpreted as a list of coefficients of a polynomial over F
     * = GF(32), with an implicit 1 in front. If the input is [v0,v1,v2,v3,v4],
     * that polynomial is v(x) = 1*x^5 + v0*x^4 + v1*x^3 + v2*x^2 + v3*x + v4.
     * The implicit 1 guarantees that [v0,v1,v2,...] has a distinct checksum
     * from [0,v0,v1,v2,...].
     *
     * The output is a 40-bit integer whose 5-bit groups are the coefficients of
     * the remainder of v(x) mod g(x), where g(x) is the cashaddr generator, x^8
     * + {19}*x^7 + {3}*x^6 + {25}*x^5 + {11}*x^4 + {25}*x^3 + {3}*x^2 + {19}*x
     * + {1}. g(x) is chosen in such a way that the resulting code is a BCH
     * code, guaranteeing detection of up to 4 errors within a window of 1025
     * characters. Among the various possible BCH codes, one was selected to in
     * fact guarantee detection of up to 5 errors within a window of 160
     * characters and 6 erros within a window of 126 characters. In addition,
     * the code guarantee the detection of a burst of up to 8 errors.
     *
     * Note that the coefficients are elements of GF(32), here represented as
     * decimal numbers between {}. In this finite field, addition is just XOR of
     * the corresponding numbers. For example, {27} + {13} = {27 ^ 13} = {22}.
     * Multiplication is more complicated, and requires treating the bits of
     * values themselves as coefficients of a polynomial over a smaller field,
     * GF(2), and multiplying those polynomials mod a^5 + a^3 + 1. For example,
     * {5} * {26} = (a^2 + 1) * (a^4 + a^3 + a) = (a^4 + a^3 + a) * a^2 + (a^4 +
     * a^3 + a) = a^6 + a^5 + a^4 + a = a^3 + 1 (mod a^5 + a^3 + 1) = {9}.
     *
     * During the course of the loop below, `c` contains the bitpacked
     * coefficients of the polynomial constructed from just the values of v that
     * were processed so far, mod g(x). In the above example, `c` initially
     * corresponds to 1 mod (x), and after processing 2 inputs of v, it
     * corresponds to x^2 + v0*x + v1 mod g(x). As 1 mod g(x) = 1, that is the
     * starting value for `c`.
     */
    uint64_t c = 1;
    for (uint8_t d : v) {
        /**
         * We want to update `c` to correspond to a polynomial with one extra
         * term. If the initial value of `c` consists of the coefficients of
         * c(x) = f(x) mod g(x), we modify it to correspond to
         * c'(x) = (f(x) * x + d) mod g(x), where d is the next input to
         * process.
         *
         * Simplifying:
         * c'(x) = (f(x) * x + d) mod g(x)
         *         ((f(x) mod g(x)) * x + d) mod g(x)
         *         (c(x) * x + d) mod g(x)
         * If c(x) = c0*x^5 + c1*x^4 + c2*x^3 + c3*x^2 + c4*x + c5, we want to
         * compute
         * c'(x) = (c0*x^5 + c1*x^4 + c2*x^3 + c3*x^2 + c4*x + c5) * x + d
         *                                                             mod g(x)
         *       = c0*x^6 + c1*x^5 + c2*x^4 + c3*x^3 + c4*x^2 + c5*x + d
         *                                                             mod g(x)
         *       = c0*(x^6 mod g(x)) + c1*x^5 + c2*x^4 + c3*x^3 + c4*x^2 +
         *                                                             c5*x + d
         * If we call (x^6 mod g(x)) = k(x), this can be written as
         * c'(x) = (c1*x^5 + c2*x^4 + c3*x^3 + c4*x^2 + c5*x + d) + c0*k(x)
         */

        // First, determine the value of c0:
        uint8_t c0 = c >> 35;

        // Then compute c1*x^5 + c2*x^4 + c3*x^3 + c4*x^2 + c5*x + d:
        c = ((c & 0x07ffffffff) << 5) ^ d;

        // Finally, for each set bit n in c0, conditionally add {2^n}k(x):
        if (c0 & 0x01) {
            // k(x) = {19}*x^7 + {3}*x^6 + {25}*x^5 + {11}*x^4 + {25}*x^3 +
            //        {3}*x^2 + {19}*x + {1}
            c ^= 0x98f2bc8e61;
        }

        if (c0 & 0x02) {
            // {2}k(x) = {15}*x^7 + {6}*x^6 + {27}*x^5 + {22}*x^4 + {27}*x^3 +
            //           {6}*x^2 + {15}*x + {2}
            c ^= 0x79b76d99e2;
        }

        if (c0 & 0x04) {
            // {4}k(x) = {30}*x^7 + {12}*x^6 + {31}*x^5 + {5}*x^4 + {31}*x^3 +
            //           {12}*x^2 + {30}*x + {4}
            c ^= 0xf33e5fb3c4;
        }

        if (c0 & 0x08) {
            // {8}k(x) = {21}*x^7 + {24}*x^6 + {23}*x^5 + {10}*x^4 + {23}*x^3 +
            //           {24}*x^2 + {21}*x + {8}
            c ^= 0xae2eabe2a8;
        }

        if (c0 & 0x10) {
            // {16}k(x) = {3}*x^7 + {25}*x^6 + {7}*x^5 + {20}*x^4 + {7}*x^3 +
            //            {25}*x^2 + {3}*x + {16}
            c ^= 0x1e4f43e470;
        }
    }

    /**
     * PolyMod computes what value to xor into the final values to make the
     * checksum 0. However, if we required that the checksum was 0, it would be
     * the case that appending a 0 to a valid list of values would result in a
     * new valid list. For that reason, cashaddr requires the resulting checksum
     * to be 1 instead.
     */
    return c ^ 1;
}

/**
 * Convert to lower case.
 *
 * Assume the input is a character.
 */
inline uint8_t LowerCase(uint8_t c) {
    // ASCII black magic.
    return c | 0x20;
}

/**
 * Expand the address prefix for the checksum computation.
 */
data ExpandPrefix(const std::string &prefix) {
    data ret;
    ret.resize(prefix.size() + 1);
    for (size_t i = 0; i < prefix.size(); ++i) {
        ret[i] = prefix[i] & 0x1f;
    }

    ret[prefix.size()] = 0;
    return ret;
}

/**
 * Verify a checksum.
 */
bool VerifyChecksum(const std::string &prefix, const data &payload) {
    return PolyMod(Cat(ExpandPrefix(prefix), payload)) == 0;
}

/**
 * Create a checksum.
 */
data CreateChecksum(const std::string &prefix, const data &payload) {
    data enc = Cat(ExpandPrefix(prefix), payload);
    // Append 8 zeroes.
    enc.resize(enc.size() + 8);
    // Determine what to XOR into those 8 zeroes.
    uint64_t mod = PolyMod(enc);
    data ret(8);
    for (size_t i = 0; i < 8; ++i) {
        // Convert the 5-bit groups in mod to checksum values.
        ret[i] = (mod >> (5 * (7 - i))) & 0x1f;
    }

    return ret;
}

} // namespace

namespace cashaddr {

/**
 * Encode a cashaddr string.
 */
std::string Encode(const std::string &prefix, const data &payload) {
    data checksum = CreateChecksum(prefix, payload);
    data combined = Cat(payload, checksum);
    std::string ret = prefix + ':';

    ret.reserve(ret.size() + combined.size());
    for (const uint8_t c : combined) {
        ret += CHARSET[c % PACKED_VAL_LIMIT];
    }

    return ret;
}

/**
 * Decode a cashaddr string.
 */
std::pair<std::string, data> Decode(const std::string &str,
                                    const std::string &default_prefix) {
    // Go over the string and do some sanity checks.
    bool lower = false, upper = false, hasNumber = false;
    size_t prefixSize = 0;
    for (size_t i = 0; i < str.size(); ++i) {
        uint8_t c = str[i];
        if (c >= 'a' && c <= 'z') {
            lower = true;
            continue;
        }

        if (c >= 'A' && c <= 'Z') {
            upper = true;
            continue;
        }

        if (c >= '0' && c <= '9') {
            // We cannot have numbers in the prefix.
            hasNumber = true;
            continue;
        }

        if (c == ':') {
            // The separator cannot be the first character, cannot have number
            // and there must not be 2 separators.
            if (hasNumber || i == 0 || prefixSize != 0) {
                return {};
            }

            prefixSize = i;
            continue;
        }

        // We have an unexpected character.
        return {};
    }

    // We can't have both upper case and lowercase.
    if (upper && lower) {
        return {};
    }

    // Get the prefix.
    std::string prefix;
    if (prefixSize == 0) {
        prefix = default_prefix;
    } else {
        prefix.reserve(prefixSize);
        for (size_t i = 0; i < prefixSize; ++i) {
            prefix += LowerCase(str[i]);
        }

        // Now add the ':' in the size.
        prefixSize++;
    }

    // Decode values.
    const size_t valuesSize = str.size() - prefixSize;
    data values(valuesSize);
    for (size_t i = 0; i < valuesSize; ++i) {
        uint8_t c = str[i + prefixSize];
        // We have an invalid char in there.
        if (c > 127 || CHARSET_REV[c] == -1) {
            return {};
        }

        values[i] = CHARSET_REV[c];
    }

    // Verify the checksum.
    if (!VerifyChecksum(prefix, values)) {
        return {};
    }

    return {std::move(prefix), data(values.begin(), values.end() - 8)};
}

} // namespace cashaddr
