c++ - 如何将 boost::multiprecision::cpp_int 转换为字节数组

Question

我需要两个函数:

std::vector<uint8_t> bigint_to_bytes(cpp_int a); 

cpp_int bytes_to_bigint(std::vector<uint8_t> const& a);

google了一下，发现bigint_to_bytes可以通过backend().limbs()来实现，不知道如何实现bytes_to_bigint。

如何通过一个字节数组构造一个cpp_int？

Answer 1

我建议采用简单的方法并使用内置的后端序列化:

Bytes to_bytes(Bigint const& i) {
    namespace io = boost::iostreams;
    namespace ba = boost::archive;

    std::vector<char> chars;
    {
        io::stream_buffer<io::back_insert_device<Chars> > bb(chars);
        ba::binary_oarchive oa(bb, ba::no_header | ba::no_tracking | ba::no_codecvt);
        oa << i;
    }

    return {chars.begin(), chars.end()};
}

那么反过来就是

Bigint to_bigint(Bytes const& v) {
    namespace io = boost::iostreams;
    namespace ba = boost::archive;

    Bigint i;
    {
        std::vector<char> chars { v.begin(), v.end() };
        io::stream_buffer<io::array_source> bb(chars.data(), chars.size());
        ba::binary_iarchive ia(bb, ba::no_header | ba::no_tracking | ba::no_codecvt);
        ia >> i;
    }
    return i;
}

这里唯一的“遗憾”是 Boost 二进制存档不支持具有 char_type uint8_t 的流这意味着您无法复制。

你可以通过引入一些丑陋的 reinterpret_casts 来解决这个问题(并且需要限制有效的序列化大小)。我将把它作为众所周知的练习留给读者。¹

演示

Live On Coliru

#include <boost/multiprecision/cpp_int.hpp>
#include <boost/multiprecision/cpp_int/serialize.hpp>
#include <boost/archive/binary_iarchive.hpp>
#include <boost/archive/binary_oarchive.hpp>
#include <boost/iostreams/device/back_inserter.hpp>
#include <boost/iostreams/device/array.hpp>
#include <boost/iostreams/stream.hpp>

#include <iostream>

namespace {
    using Bigint = boost::multiprecision::cpp_int;
    using Chars = std::vector<char>;
    using Bytes = std::vector<uint8_t>;

    Bytes to_bytes(Bigint const& i) {
        namespace io = boost::iostreams;
        namespace ba = boost::archive;

        std::vector<char> chars;
        {
            io::stream_buffer<io::back_insert_device<Chars> > bb(chars);
            ba::binary_oarchive oa(bb, ba::no_header | ba::no_tracking | ba::no_codecvt);
            oa << i;
        }

        return {chars.begin(), chars.end()};
    }

    Bigint to_bigint(Bytes const& v) {
        namespace io = boost::iostreams;
        namespace ba = boost::archive;

        Bigint i;
        {
            std::vector<char> chars { v.begin(), v.end() };
            io::stream_buffer<io::array_source> bb(chars.data(), chars.size());
            ba::binary_iarchive ia(bb, ba::no_header | ba::no_tracking | ba::no_codecvt);
            ia >> i;
        }
        return i;
   }
}

int main() {
    Bigint i = 77;
    i <<= 33;

    return i == to_bigint(to_bytes(i))? 0 : 1;
}

退出代码为 0。

¹ 反过来更容易:更改为 vector<int8_t>相反:http://coliru.stacked-crooked.com/a/3dbff52c2daab58e