Normalize -0.0 in number hashes and test range ends

operator== compares numbers exactly since #5459, so equal numbers
share one value and convert to the same number_float_t. Update the
comment accordingly, map -0.0 to 0.0 before hashing (std::hash need
not do that), and test -0.0 and the ends of the integer ranges. Show
hash(0.0) in the docs example and note the change in the version
history.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

docs/mkdocs/docs/api/basic_json/std_hash.md

@@ -37,3 +37,5 @@ whether they are stored as `#!cpp 0`, `#!cpp 0U`, or `#!cpp 0.0`.
 
 - Added in version 1.0.0.
 - Extended for arbitrary basic_json types in version 3.10.5.
+- Numbers that compare equal hash equally since version 3.13.0; before, `#!cpp 0`, `#!cpp 0U`, and `#!cpp 0.0` had
+  different hash values.

docs/mkdocs/docs/examples/std_hash.cpp

@@ -11,6 +11,7 @@ int main()
               << "hash(false) = " << std::hash<json> {}(json(false)) << '\n'
               << "hash(0) = " << std::hash<json> {}(json(0)) << '\n'
               << "hash(0U) = " << std::hash<json> {}(json(0U)) << '\n'
+              << "hash(0.0) = " << std::hash<json> {}(json(0.0)) << '\n'
               << "hash(\"\") = " << std::hash<json> {}(json("")) << '\n'
               << "hash({}) = " << std::hash<json> {}(json::object()) << '\n'
               << "hash([]) = " << std::hash<json> {}(json::array()) << '\n'

docs/mkdocs/docs/examples/std_hash.output

@@ -2,6 +2,7 @@ hash(null) = 2654435769
 hash(false) = 2654436030
 hash(0) = 2654436221
 hash(0U) = 2654436221
+hash(0.0) = 2654436221
 hash("") = 11160318156688833227
 hash({}) = 2654435832
 hash([]) = 2654435899

include/nlohmann/detail/hash.hpp

@@ -116,14 +116,16 @@ std::size_t hash(const BasicJsonType& j, const std::size_t depth = 0)
         case BasicJsonType::value_t::number_unsigned:
         case BasicJsonType::value_t::number_float:
         {
-            // operator== converts between number_integer, number_unsigned, and
-            // number_float before comparing, so equal numbers of different
-            // internal types (0, 0U, 0.0) must hash the same. Combining a
-            // single shared type tag with the value converted to
-            // number_float_t keeps the hash consistent with operator== for
-            // every pair of numbers it considers equal.
+            // operator== compares numbers by their mathematical value across
+            // number_integer, number_unsigned, and number_float, so equal
+            // numbers of different internal types (0, 0U, 0.0) must hash the
+            // same. Two equal numbers have the same value, which converts to
+            // the same number_float_t, so all numbers share one type tag and
+            // hash that converted value. Adding zero turns -0.0 (equal to 0)
+            // into 0.0, as std::hash need not map both to the same hash.
             const auto number_type = static_cast<std::size_t>(BasicJsonType::value_t::number_float);
-            const auto h = std::hash<number_float_t> {}(j.template get<number_float_t>());
+            const auto value = j.template get<number_float_t>() + static_cast<number_float_t>(0);
+            const auto h = std::hash<number_float_t> {}(value);
             return combine(number_type, h);
         }
 

single_include/nlohmann/json.hpp

@@ -7717,14 +7717,16 @@ std::size_t hash(const BasicJsonType& j, const std::size_t depth = 0)
         case BasicJsonType::value_t::number_unsigned:
         case BasicJsonType::value_t::number_float:
         {
-            // operator== converts between number_integer, number_unsigned, and
-            // number_float before comparing, so equal numbers of different
-            // internal types (0, 0U, 0.0) must hash the same. Combining a
-            // single shared type tag with the value converted to
-            // number_float_t keeps the hash consistent with operator== for
-            // every pair of numbers it considers equal.
+            // operator== compares numbers by their mathematical value across
+            // number_integer, number_unsigned, and number_float, so equal
+            // numbers of different internal types (0, 0U, 0.0) must hash the
+            // same. Two equal numbers have the same value, which converts to
+            // the same number_float_t, so all numbers share one type tag and
+            // hash that converted value. Adding zero turns -0.0 (equal to 0)
+            // into 0.0, as std::hash need not map both to the same hash.
             const auto number_type = static_cast<std::size_t>(BasicJsonType::value_t::number_float);
-            const auto h = std::hash<number_float_t> {}(j.template get<number_float_t>());
+            const auto value = j.template get<number_float_t>() + static_cast<number_float_t>(0);
+            const auto h = std::hash<number_float_t> {}(value);
             return combine(number_type, h);
         }
 

tests/src/unit-hash.cpp

@@ -12,6 +12,7 @@
 using json = nlohmann::json;
 using ordered_json = nlohmann::ordered_json;
 
+#include <limits>
 #include <set>
 #include <string>
 #include <unordered_set>
@@ -147,6 +148,22 @@ TEST_CASE("hash<nlohmann::json>")
     // a std::unordered_set relies on this same consistency between == and hash
     const std::unordered_set<json> numbers {json(0), json(static_cast<unsigned>(0)), json(0.0)};
     CHECK(numbers.size() == 1);
+
+    // -0.0 compares equal to 0 and 0.0
+    CHECK(json(-0.0) == json(0));
+    CHECK(std::hash<json> {}(json(-0.0)) == std::hash<json> {}(json(0)));
+    CHECK(std::hash<json> {}(json(-0.0)) == std::hash<json> {}(json(0.0)));
+
+    // the ends of the integer ranges, which equal floats exactly
+    const auto int_min = (std::numeric_limits<json::number_integer_t>::min)();
+    const auto int_max = (std::numeric_limits<json::number_integer_t>::max)();
+    const auto two_63 = json::number_unsigned_t(1) << 63U;
+    CHECK(json(int_min) == json(-9223372036854775808.0));
+    CHECK(std::hash<json> {}(json(int_min)) == std::hash<json> {}(json(-9223372036854775808.0)));
+    CHECK(json(two_63) == json(9223372036854775808.0));
+    CHECK(std::hash<json> {}(json(two_63)) == std::hash<json> {}(json(9223372036854775808.0)));
+    CHECK(json(json::number_unsigned_t(int_max)) == json(int_max));
+    CHECK(std::hash<json> {}(json(json::number_unsigned_t(int_max))) == std::hash<json> {}(json(int_max)));
 }
 
 TEST_CASE("hash<nlohmann::ordered_json>")