Hashing

The <boost/int128/hash.hpp> header provides specializations of std::hash for uint128_t and int128_t, allowing the library types to be used as keys in std::unordered_map, std::unordered_set, and any other container that relies on std::hash.

#include <boost/int128/hash.hpp>

Specializations

namespace std {

template <>
struct hash<boost::int128::int128_t>
{
    std::size_t operator()(boost::int128::int128_t v) const noexcept;
};

template <>
struct hash<boost::int128::uint128_t>
{
    std::size_t operator()(boost::int128::uint128_t v) const noexcept;
};

} // namespace std

Each 64-bit half of the value is first run through a SplitMix64 finalizer so that every input bit influences the lower bits of the result. This is necessary because std::hash<std::uint64_t> is permitted to truncate to std::size_t, which would lose the upper 32 bits on 32-bit platforms and cause distinct 128-bit values to collide. The two finalized halves are then combined with the boost::hash_combine mixing formula.

Guarantees

Two values comparing equal under operator== produce the same hash.
For any non-zero v, std::hash<int128_t>{}(v) != std::hash<int128_t>{}(-v).
The mixing function is asymmetric, so {high, low} and {low, high} do not collide except by chance.
The hash value is implementation-defined and may differ across platforms, compilers, or library versions. Do not persist hash values across runs.

Example

#include <boost/int128/int128.hpp>
#include <boost/int128/hash.hpp>
#include <unordered_map>

int main()
{
    std::unordered_map<boost::int128::uint128_t, int> counts {};
    counts[boost::int128::uint128_t{1, 0}] = 1;
    counts[boost::int128::uint128_t{0, 1}] = 2;

    return 0;
}

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