oxen-mq/lokimq/hex.h

// Copyright (c) 2019-2020, The Loki Project
//
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#pragma once
#include <string>
#include <array>
#include <iterator>
#include <cassert>

namespace lokimq {

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;

} // namespace detail

/// Creates hex digits from a character sequence.
template <typename InputIt, typename OutputIt>
void to_hex(InputIt begin, InputIt end, OutputIt out) {
    for (; begin != end; ++begin) {
        auto c = *begin;
        *out++ = detail::hex_lut.to_hex((c & 0xf0) >> 4);
        *out++ = detail::hex_lut.to_hex(c & 0x0f);
    }
}

/// Creates a hex string from an iterable, std::string-like object
template <typename String>
std::string to_hex(const String& s) {
    std::string hex;
    hex.reserve(s.size() * 2);
    to_hex(s.begin(), s.end(), std::back_inserter(hex));
    return hex;
}

/// Returns true if all elements in the range are hex characters
template <typename It>
constexpr bool is_hex(It begin, It end) {
    for (; begin != end; ++begin) {
        if (detail::hex_lut.from_hex(*begin) == 0 && *begin != '0')
            return false;
    }
    return true;
}

/// Returns true if all elements in the string-like value are hex characters
template <typename String>
constexpr bool is_hex(const String& 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); }

/// 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.  It is permitted for the input and
/// output ranges to overlap as long as out is no earlier than begin.
template <typename InputIt, typename OutputIt>
void from_hex(InputIt begin, InputIt end, OutputIt out) {
    using std::distance;
    assert(distance(begin, end) % 2 == 0);
    while (begin != end) {
        auto a = *begin++;
        auto b = *begin++;
        *out++ = from_hex_pair(a, b);
    }
}

/// 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 String>
std::string from_hex(const String& s) {
    std::string bytes;
    bytes.reserve(s.size() / 2);
    from_hex(s.begin(), s.end(), std::back_inserter(bytes));
    return bytes;
}

}