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hashCombine.h
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1/*
2 * LSST Data Management System
3 * See COPYRIGHT file at the top of the source tree.
4 *
5 * This product includes software developed by the
6 * LSST Project (http://www.lsst.org/).
7 *
8 * This program is free software: you can redistribute it and/or modify
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21 */
22#ifndef LSST_CPPUTILS_HASH_COMBINE_H
23#define LSST_CPPUTILS_HASH_COMBINE_H
24
25#include <functional>
26
27namespace lsst {
28namespace cpputils {
29
35inline std::size_t hashCombine(std::size_t seed) noexcept { return seed; }
36
59// This implementation is provided by Matteo Italia, https://stackoverflow.com/a/38140932/834250
60// Algorithm described at https://stackoverflow.com/a/27952689
61// WARNING: should not be inline or constexpr; it can cause instantiation-order problems with std::hash<T>
62template <typename T, typename... Rest>
63std::size_t hashCombine(std::size_t seed, const T& value, Rest... rest) noexcept {
64 std::hash<T> hasher;
65 seed ^= hasher(value) + 0x9e3779b9 + (seed << 6) + (seed >> 2);
66 return hashCombine(seed, rest...);
67}
68
90// Note: not an overload of hashCombine to avoid ambiguity with hashCombine(size_t, T1, T2)
91// WARNING: should not be inline or constexpr; it can cause instantiation-order problems with std::hash<T>
92template <typename InputIterator>
93std::size_t hashIterable(std::size_t seed, InputIterator begin, InputIterator end) noexcept {
94 std::size_t result = 0;
95 for (; begin != end; ++begin) {
96 result = hashCombine(result, *begin);
97 }
98 return result;
99}
100
101}
102} // namespace lsst::cpputils
103
104#endif
std::size_t hashCombine(std::size_t seed) noexcept
Combine hashes.
Definition hashCombine.h:35
std::size_t hashIterable(std::size_t seed, InputIterator begin, InputIterator end) noexcept
Combine hashes in an iterable.
Definition hashCombine.h:93