Merge pull request #2269 from nlohmann/hash

Hash function without allocation

include/nlohmann/detail/hash.hpp

@@ -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

include/nlohmann/json.hpp

@@ -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
+        return nlohmann::detail::hash(j);
-        const auto& h = hash<nlohmann::json::string_t>();
-        return h(j.dump());
     }
 };
 

single_include/nlohmann/json.hpp

@@ -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
+        return nlohmann::detail::hash(j);
-        const auto& h = hash<nlohmann::json::string_t>();
-        return h(j.dump());
     }
 };
 

test/CMakeLists.txt

@@ -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

test/Makefile

@@ -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 \

test/src/unit-hash.cpp

@@ -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);
+}