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Merge pull request #2269 from nlohmann/hash

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Hash function without allocation
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Niels Lohmann 2020-07-17 13:57:47 +02:00 committed by GitHub
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6 changed files with 325 additions and 6 deletions
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117
include/nlohmann/detail/hash.hpp Normal file
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@ -0,0 +1,117 @@
#pragma once
#include <cstddef> // size_t, uint8_t
#include <functional> // hash
namespace nlohmann
{
namespace detail
{
// boost::hash_combine
std::size_t combine(std::size_t seed, std::size_t h) noexcept
{
seed ^= h + 0x9e3779b9 + (seed << 6U) + (seed >> 2U);
return seed;
}
/*!
@brief hash a JSON value
The hash function tries to rely on std::hash where possible. Furthermore, the
type of the JSON value is taken into account to have different hash values for
null, 0, 0U, and false, etc.
@tparam BasicJsonType basic_json specialization
@param j JSON value to hash
@return hash value of j
*/
template<typename BasicJsonType>
std::size_t hash(const BasicJsonType& j)
{
using string_t = typename BasicJsonType::string_t;
using number_integer_t = typename BasicJsonType::number_integer_t;
using number_unsigned_t = typename BasicJsonType::number_unsigned_t;
using number_float_t = typename BasicJsonType::number_float_t;
const auto type = static_cast<std::size_t>(j.type());
switch (j.type())
{
case BasicJsonType::value_t::null:
case BasicJsonType::value_t::discarded:
{
return combine(type, 0);
}
case BasicJsonType::value_t::object:
{
auto seed = combine(type, j.size());
for (const auto& element : j.items())
{
const auto h = std::hash<string_t> {}(element.key());
seed = combine(seed, h);
seed = combine(seed, hash(element.value()));
}
return seed;
}
case BasicJsonType::value_t::array:
{
auto seed = combine(type, j.size());
for (const auto& element : j)
{
seed = combine(seed, hash(element));
}
return seed;
}
case BasicJsonType::value_t::string:
{
const auto h = std::hash<string_t> {}(j.template get_ref<const string_t&>());
return combine(type, h);
}
case BasicJsonType::value_t::boolean:
{
const auto h = std::hash<bool> {}(j.template get<bool>());
return combine(type, h);
}
case BasicJsonType::value_t::number_integer:
{
const auto h = std::hash<number_integer_t> {}(j.template get<number_integer_t>());
return combine(type, h);
}
case nlohmann::detail::value_t::number_unsigned:
{
const auto h = std::hash<number_unsigned_t> {}(j.template get<number_unsigned_t>());
return combine(type, h);
}
case nlohmann::detail::value_t::number_float:
{
const auto h = std::hash<number_float_t> {}(j.template get<number_float_t>());
return combine(type, h);
}
case nlohmann::detail::value_t::binary:
{
auto seed = combine(type, j.get_binary().size());
const auto h = std::hash<bool> {}(j.get_binary().has_subtype());
seed = combine(seed, h);
seed = combine(seed, j.get_binary().subtype());
for (const auto byte : j.get_binary())
{
seed = combine(seed, std::hash<std::uint8_t> {}(byte));
}
return seed;
}
default: // LCOV_EXCL_LINE
JSON_ASSERT(false); // LCOV_EXCL_LINE
}
}
} // namespace detail
} // namespace nlohmann

5
include/nlohmann/json.hpp
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@ -51,6 +51,7 @@ SOFTWARE.
#include <nlohmann/detail/conversions/from_json.hpp>
#include <nlohmann/detail/conversions/to_json.hpp>
#include <nlohmann/detail/exceptions.hpp>
#include <nlohmann/detail/hash.hpp>
#include <nlohmann/detail/input/binary_reader.hpp>
#include <nlohmann/detail/input/input_adapters.hpp>
#include <nlohmann/detail/input/lexer.hpp>
@ -8698,9 +8699,7 @@ struct hash<nlohmann::json>
*/
std::size_t operator()(const nlohmann::json& j) const
{
// a naive hashing via the string representation
const auto& h = hash<nlohmann::json::string_t>();
return h(j.dump());
return nlohmann::detail::hash(j);
}
};

123
single_include/nlohmann/json.hpp
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@ -4442,6 +4442,125 @@ class byte_container_with_subtype : public BinaryType
// #include <nlohmann/detail/exceptions.hpp>
// #include <nlohmann/detail/hash.hpp>
#include <cstddef> // size_t, uint8_t
#include <functional> // hash
namespace nlohmann
{
namespace detail
{
// boost::hash_combine
std::size_t combine(std::size_t seed, std::size_t h) noexcept
{
seed ^= h + 0x9e3779b9 + (seed << 6U) + (seed >> 2U);
return seed;
}
/*!
@brief hash a JSON value
The hash function tries to rely on std::hash where possible. Furthermore, the
type of the JSON value is taken into account to have different hash values for
null, 0, 0U, and false, etc.
@tparam BasicJsonType basic_json specialization
@param j JSON value to hash
@return hash value of j
*/
template<typename BasicJsonType>
std::size_t hash(const BasicJsonType& j)
{
using string_t = typename BasicJsonType::string_t;
using number_integer_t = typename BasicJsonType::number_integer_t;
using number_unsigned_t = typename BasicJsonType::number_unsigned_t;
using number_float_t = typename BasicJsonType::number_float_t;
const auto type = static_cast<std::size_t>(j.type());
switch (j.type())
{
case BasicJsonType::value_t::null:
case BasicJsonType::value_t::discarded:
{
return combine(type, 0);
}
case BasicJsonType::value_t::object:
{
auto seed = combine(type, j.size());
for (const auto& element : j.items())
{
const auto h = std::hash<string_t> {}(element.key());
seed = combine(seed, h);
seed = combine(seed, hash(element.value()));
}
return seed;
}
case BasicJsonType::value_t::array:
{
auto seed = combine(type, j.size());
for (const auto& element : j)
{
seed = combine(seed, hash(element));
}
return seed;
}
case BasicJsonType::value_t::string:
{
const auto h = std::hash<string_t> {}(j.template get_ref<const string_t&>());
return combine(type, h);
}
case BasicJsonType::value_t::boolean:
{
const auto h = std::hash<bool> {}(j.template get<bool>());
return combine(type, h);
}
case BasicJsonType::value_t::number_integer:
{
const auto h = std::hash<number_integer_t> {}(j.template get<number_integer_t>());
return combine(type, h);
}
case nlohmann::detail::value_t::number_unsigned:
{
const auto h = std::hash<number_unsigned_t> {}(j.template get<number_unsigned_t>());
return combine(type, h);
}
case nlohmann::detail::value_t::number_float:
{
const auto h = std::hash<number_float_t> {}(j.template get<number_float_t>());
return combine(type, h);
}
case nlohmann::detail::value_t::binary:
{
auto seed = combine(type, j.get_binary().size());
const auto h = std::hash<bool> {}(j.get_binary().has_subtype());
seed = combine(seed, h);
seed = combine(seed, j.get_binary().subtype());
for (const auto byte : j.get_binary())
{
seed = combine(seed, std::hash<std::uint8_t> {}(byte));
}
return seed;
}
default: // LCOV_EXCL_LINE
JSON_ASSERT(false); // LCOV_EXCL_LINE
}
}
} // namespace detail
} // namespace nlohmann
// #include <nlohmann/detail/input/binary_reader.hpp>
@ -24686,9 +24805,7 @@ struct hash<nlohmann::json>
*/
std::size_t operator()(const nlohmann::json& j) const
{
// a naive hashing via the string representation
const auto& h = hash<nlohmann::json::string_t>();
return h(j.dump());
return nlohmann::detail::hash(j);
}
};

1
test/CMakeLists.txt
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@ -112,6 +112,7 @@ set(files
src/unit-deserialization.cpp
src/unit-element_access1.cpp
src/unit-element_access2.cpp
src/unit-hash.cpp
src/unit-inspection.cpp
src/unit-items.cpp
src/unit-iterators1.cpp

1
test/Makefile
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@ -10,6 +10,7 @@ SOURCES = src/unit.cpp \
src/unit-algorithms.cpp \
src/unit-allocator.cpp \
src/unit-alt-string.cpp \
src/unit-assert_macro.cpp \
src/unit-bson.cpp \
src/unit-capacity.cpp \
src/unit-cbor.cpp \

84
test/src/unit-hash.cpp Normal file
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@ -0,0 +1,84 @@
/*
__ _____ _____ _____
__| | __| | | | JSON for Modern C++ (test suite)
| | |__ | | | | | | version 3.8.0
|_____|_____|_____|_|___| https://github.com/nlohmann/json
Licensed under the MIT License <http://opensource.org/licenses/MIT>.
SPDX-License-Identifier: MIT
Copyright (c) 2013-2019 Niels Lohmann <http://nlohmann.me>.
Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:
The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.
THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/
#include "doctest_compatibility.h"
#include <nlohmann/json.hpp>
using json = nlohmann::json;
#include <set>
TEST_CASE("hash")
{
// Collect hashes for different JSON values and make sure that they are distinct
// We cannot compare against fixed values, because the implementation of
// std::hash may differ between compilers.
std::set<std::size_t> hashes;
// null
hashes.insert(std::hash<json> {}(json(nullptr)));
// boolean
hashes.insert(std::hash<json> {}(json(true)));
hashes.insert(std::hash<json> {}(json(false)));
// string
hashes.insert(std::hash<json> {}(json("")));
hashes.insert(std::hash<json> {}(json("foo")));
// number
hashes.insert(std::hash<json> {}(json(0)));
hashes.insert(std::hash<json> {}(json(unsigned(0))));
hashes.insert(std::hash<json> {}(json(-1)));
hashes.insert(std::hash<json> {}(json(0.0)));
hashes.insert(std::hash<json> {}(json(42.23)));
// array
hashes.insert(std::hash<json> {}(json::array()));
hashes.insert(std::hash<json> {}(json::array({1, 2, 3})));
// object
hashes.insert(std::hash<json> {}(json::object()));
hashes.insert(std::hash<json> {}(json::object({{"foo", "bar"}})));
// binary
hashes.insert(std::hash<json> {}(json::binary({})));
hashes.insert(std::hash<json> {}(json::binary({}, 0)));
hashes.insert(std::hash<json> {}(json::binary({}, 42)));
hashes.insert(std::hash<json> {}(json::binary({1, 2, 3})));
hashes.insert(std::hash<json> {}(json::binary({1, 2, 3}, 0)));
hashes.insert(std::hash<json> {}(json::binary({1, 2, 3}, 42)));
// discarded
hashes.insert(std::hash<json> {}(json(json::value_t::discarded)));
CHECK(hashes.size() == 21);
}