// Copyright (c) 2019-2021, The Oxen Project
//
// All rights reserved.
//
// Redistribution and use in source and binary forms, with or without modification, are
// permitted provided that the following conditions are met:
//
// 1. Redistributions of source code must retain the above copyright notice, this list of
//    conditions and the following disclaimer.
//
// 2. Redistributions in binary form must reproduce the above copyright notice, this list
//    of conditions and the following disclaimer in the documentation and/or other
//    materials provided with the distribution.
//
// 3. Neither the name of the copyright holder nor the names of its contributors may be
//    used to endorse or promote products derived from this software without specific
//    prior written permission.
//
// THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND ANY
// EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES OF
// MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL
// THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
// SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
// PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
// INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT,
// STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF
// THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

#pragma once
#include <string>
#include <string_view>
#include <array>
#include <iterator>
#include <cassert>
#include "byte_type.h"

namespace oxenmq {

namespace detail {

/// Compile-time generated lookup tables hex conversion
struct hex_table {
    char from_hex_lut[256];
    char to_hex_lut[16];
    constexpr hex_table() noexcept : from_hex_lut{}, to_hex_lut{} {
        for (unsigned char c = 0; c < 10; c++) {
            from_hex_lut[(unsigned char)('0' + c)] =  0  + c;
            to_hex_lut[  (unsigned char)( 0  + c)] = '0' + c;
        }
        for (unsigned char c = 0; c < 6; c++) {
            from_hex_lut[(unsigned char)('a' + c)] = 10  + c;
            from_hex_lut[(unsigned char)('A' + c)] = 10  + c;
            to_hex_lut[  (unsigned char)(10  + c)] = 'a' + c;
        }
    }
    constexpr char from_hex(unsigned char c) const noexcept { return from_hex_lut[c]; }
    constexpr char to_hex(unsigned char b) const noexcept { return to_hex_lut[b]; }
} constexpr hex_lut;

// This main point of this static assert is to force the compiler to compile-time build the constexpr tables.
static_assert(hex_lut.from_hex('a') == 10 && hex_lut.from_hex('F') == 15 && hex_lut.to_hex(13) == 'd', "");

} // namespace detail

/// Returns the number of characters required to encode a hex string from the given number of bytes.
inline constexpr size_t to_hex_size(size_t byte_size) { return byte_size * 2; }
/// Returns the number of bytes required to decode a hex string of the given size.
inline constexpr size_t from_hex_size(size_t hex_size) { return hex_size / 2; }

/// Iterable object for on-the-fly hex encoding.  Used internally, but also particularly useful when
/// converting from one encoding to another.
template <typename InputIt>
struct hex_encoder final {
private:
    InputIt _it, _end;
    static_assert(sizeof(decltype(*_it)) == 1, "hex_encoder requires chars/bytes input iterator");
    uint8_t c;
    bool second_half = false;
public:
    using iterator_category = std::input_iterator_tag;
    using difference_type = std::ptrdiff_t;
    using value_type = char;
    using reference = value_type;
    using pointer = void;
    hex_encoder(InputIt begin, InputIt end) : _it{std::move(begin)}, _end{std::move(end)} {}

    hex_encoder end() { return {_end, _end}; }

    bool operator==(const hex_encoder& i) { return _it == i._it && second_half == i.second_half; }
    bool operator!=(const hex_encoder& i) { return !(*this == i); }

    hex_encoder& operator++() {
        second_half = !second_half;
        if (!second_half)
            ++_it;
        return *this;
    }
    hex_encoder operator++(int) { hex_encoder copy{*this}; ++*this; return copy; }
    char operator*() {
        return detail::hex_lut.to_hex(second_half
                ? c & 0x0f
                : (c = static_cast<uint8_t>(*_it)) >> 4);
    }
};

/// Creates hex digits from a character sequence given by iterators, writes them starting at `out`.
/// Returns the final value of out (i.e. the iterator positioned just after the last written
/// hex character).
template <typename InputIt, typename OutputIt>
OutputIt to_hex(InputIt begin, InputIt end, OutputIt out) {
    static_assert(sizeof(decltype(*begin)) == 1, "to_hex requires chars/bytes");
    auto it = hex_encoder{begin, end};
    return std::copy(it, it.end(), out);
}

/// Creates a string of hex digits from a character sequence iterator pair
template <typename It>
std::string to_hex(It begin, It end) {
    std::string hex;
    if constexpr (std::is_base_of_v<std::random_access_iterator_tag, typename std::iterator_traits<It>::iterator_category>) {
        using std::distance;
        hex.reserve(to_hex_size(distance(begin, end)));
    }
    to_hex(begin, end, std::back_inserter(hex));
    return hex;
}

/// Creates a hex string from an iterable, std::string-like object
template <typename CharT>
std::string to_hex(std::basic_string_view<CharT> s) { return to_hex(s.begin(), s.end()); }
inline std::string to_hex(std::string_view s) { return to_hex<>(s); }

/// Returns true if the given value is a valid hex digit.
template <typename CharT>
constexpr bool is_hex_digit(CharT c) {
    static_assert(sizeof(CharT) == 1, "is_hex requires chars/bytes");
    return detail::hex_lut.from_hex(static_cast<unsigned char>(c)) != 0 || static_cast<unsigned char>(c) == '0';
}

/// Returns true if all elements in the range are hex characters *and* the string length is a
/// multiple of 2, and thus suitable to pass to from_hex().
template <typename It>
constexpr bool is_hex(It begin, It end) {
    static_assert(sizeof(decltype(*begin)) == 1, "is_hex requires chars/bytes");
    constexpr bool ra = std::is_base_of_v<std::random_access_iterator_tag, typename std::iterator_traits<It>::iterator_category>;
    if constexpr (ra) {
        using std::distance;
        if (distance(begin, end) % 2 != 0)
            return false;
    }

    size_t count = 0;
    for (; begin != end; ++begin) {
        if constexpr (!ra) ++count;
        if (!is_hex_digit(*begin))
            return false;
    }
    if constexpr (!ra)
        return count % 2 == 0;
    return true;
}

/// Returns true if all elements in the string-like value are hex characters
template <typename CharT>
constexpr bool is_hex(std::basic_string_view<CharT> s) { return is_hex(s.begin(), s.end()); }
constexpr bool is_hex(std::string_view s) { return is_hex(s.begin(), s.end()); }

/// Convert a hex digit into its numeric (0-15) value
constexpr char from_hex_digit(unsigned char x) noexcept {
    return detail::hex_lut.from_hex(x);
}

/// Constructs a byte value from a pair of hex digits
constexpr char from_hex_pair(unsigned char a, unsigned char b) noexcept { return (from_hex_digit(a) << 4) | from_hex_digit(b); }

/// Iterable object for on-the-fly hex decoding.  Used internally but also particularly useful when
/// converting from one encoding to another.  Undefined behaviour if the given iterator range is not
/// a valid hex string with even length (i.e. is_hex() should return true).
template <typename InputIt>
struct hex_decoder final {
private:
    InputIt _it, _end;
    static_assert(sizeof(decltype(*_it)) == 1, "hex_encoder requires chars/bytes input iterator");
    char byte;
public:
    using iterator_category = std::input_iterator_tag;
    using difference_type = std::ptrdiff_t;
    using value_type = char;
    using reference = value_type;
    using pointer = void;
    hex_decoder(InputIt begin, InputIt end) : _it{std::move(begin)}, _end{std::move(end)} {
        if (_it != _end)
            load_byte();
    }

    hex_decoder end() { return {_end, _end}; }

    bool operator==(const hex_decoder& i) { return _it == i._it; }
    bool operator!=(const hex_decoder& i) { return _it != i._it; }

    hex_decoder& operator++() {
        if (++_it != _end)
            load_byte();
        return *this;
    }
    hex_decoder operator++(int) { hex_decoder copy{*this}; ++*this; return copy; }
    char operator*() const { return byte; }

private:
    void load_byte() {
        auto a = *_it;
        auto b = *++_it;
        byte = from_hex_pair(static_cast<unsigned char>(a), static_cast<unsigned char>(b));
    }

};

/// Converts a sequence of hex digits to bytes.  Undefined behaviour if any characters are not in
/// [0-9a-fA-F] or if the input sequence length is not even: call `is_hex` first if you need to
/// check.  It is permitted for the input and output ranges to overlap as long as out is no later
/// than begin.  Returns the final value of out (that is, the iterator positioned just after the
/// last written character).
template <typename InputIt, typename OutputIt>
OutputIt from_hex(InputIt begin, InputIt end, OutputIt out) {
    assert(is_hex(begin, end));
    auto it = hex_decoder(begin, end);
    const auto hend = it.end();
    while (it != hend)
        *out++ = static_cast<detail::byte_type_t<OutputIt>>(*it++);
    return out;
}

/// Converts a sequence of hex digits to a string of bytes and returns it.  Undefined behaviour if
/// the input sequence is not an even-length sequence of [0-9a-fA-F] characters.
template <typename It>
std::string from_hex(It begin, It end) {
    std::string bytes;
    if constexpr (std::is_base_of_v<std::random_access_iterator_tag, typename std::iterator_traits<It>::iterator_category>) {
        using std::distance;
        bytes.reserve(from_hex_size(distance(begin, end)));
    }
    from_hex(begin, end, std::back_inserter(bytes));
    return bytes;
}

/// Converts hex digits from a std::string-like object into a std::string of bytes.  Undefined
/// behaviour if any characters are not in [0-9a-fA-F] or if the input sequence length is not even.
template <typename CharT>
std::string from_hex(std::basic_string_view<CharT> s) { return from_hex(s.begin(), s.end()); }
inline std::string from_hex(std::string_view s) { return from_hex<>(s); }

}