Implement a fast Pointer::Token::property_equals helper

src/core/jsonpointer/include/sourcemeta/core/jsonpointer_token.h

@@ -184,6 +184,39 @@ template <typename PropertyT, typename Hash> class GenericToken {
     return this->hash;
   }
 
+  /// Check whether a JSON Pointer property token equals the given string,
+  /// comparing the precomputed hashes first and only falling back to a string
+  /// comparison when the hash is not perfect. For example:
+  ///
+  /// ```cpp
+  /// #include <sourcemeta/core/json.h>
+  /// #include <sourcemeta/core/jsonpointer.h>
+  /// #include <cassert>
+  ///
+  /// const sourcemeta::core::Pointer::Token token{"foo"};
+  /// assert(token.property_equals(
+  ///     "foo", sourcemeta::core::JSON::Object::hash("foo")));
+  /// ```
+  [[nodiscard]] auto
+  property_equals(const JSON::StringView value,
+                  const typename Hash::hash_type value_hash) const noexcept
+      -> bool {
+    assert(this->is_property());
+    assert(hasher(value.data(), value.size()) == value_hash);
+    if constexpr (requires { hasher.is_perfect(value_hash); }) {
+      // A perfect hash captures the property bytes but not its length, so
+      // two properties that differ only by trailing NUL bytes hash equal.
+      // Comparing sizes disambiguates them without the cost of a full
+      // string comparison
+      return this->hash == value_hash &&
+             (hasher.is_perfect(value_hash)
+                  ? this->to_property().size() == value.size()
+                  : this->to_property() == value);
+    } else {
+      return this->hash == value_hash && this->to_property() == value;
+    }
+  }
+
   /// Get the underlying value of a JSON Pointer object property token
   /// (non-`const` overload). For example:
   ///
@@ -251,8 +284,13 @@ template <typename PropertyT, typename Hash> class GenericToken {
       return false;
     } else if (this->as_property) {
       if constexpr (requires { hasher.is_perfect(this->hash); }) {
+        // A perfect hash captures the property bytes but not its length, so
+        // two properties that differ only by trailing NUL bytes hash equal.
+        // Comparing sizes disambiguates them without the cost of a full
+        // string comparison
         if (hasher.is_perfect(this->hash) && hasher.is_perfect(other.hash)) {
-          return this->hash == other.hash;
+          return this->hash == other.hash &&
+                 this->to_property().size() == other.to_property().size();
         }
       }
 

test/jsonpointer/jsonpointer_token_test.cc

@@ -2,6 +2,7 @@
 #include <sourcemeta/core/jsonpointer.h>
 #include <sourcemeta/core/test.h>
 
+#include <functional>
 #include <type_traits>
 
 TEST(general_traits) {
@@ -93,6 +94,49 @@ TEST(to_json_property) {
   EXPECT_EQ(result.to_string(), "foo");
 }
 
+TEST(equality_short_property_tokens_equal) {
+  const sourcemeta::core::Pointer::Token first{"foo"};
+  const sourcemeta::core::Pointer::Token second{"foo"};
+  EXPECT_EQ(first, second);
+  EXPECT_EQ(second, first);
+}
+
+TEST(equality_short_property_tokens_not_equal) {
+  const sourcemeta::core::Pointer::Token first{"foo"};
+  const sourcemeta::core::Pointer::Token second{"bar"};
+  EXPECT_NE(first, second);
+  EXPECT_NE(second, first);
+}
+
+TEST(equality_long_property_tokens_equal) {
+  const sourcemeta::core::JSON::String property(40, 'a');
+  const sourcemeta::core::Pointer::Token first{property};
+  const sourcemeta::core::Pointer::Token second{property};
+  EXPECT_EQ(first, second);
+  EXPECT_EQ(second, first);
+}
+
+TEST(equality_property_tokens_differing_by_trailing_null_not_equal) {
+  sourcemeta::core::JSON::String property_with_null{"a"};
+  property_with_null.push_back('\x00');
+  const sourcemeta::core::Pointer::Token with_null{property_with_null};
+  const sourcemeta::core::Pointer::Token without_null{"a"};
+  EXPECT_NE(with_null, without_null);
+  EXPECT_NE(without_null, with_null);
+}
+
+TEST(equality_property_tokens_imperfect_hash_collision_not_equal) {
+  const sourcemeta::core::JSON::String stored{
+      sourcemeta::core::JSON::String(31, 'a') + "11111110" + "z"};
+  const sourcemeta::core::JSON::String other{
+      sourcemeta::core::JSON::String(31, 'a') + "22222220" + "z"};
+  const sourcemeta::core::Pointer::Token first{stored};
+  const sourcemeta::core::Pointer::Token second{other};
+  EXPECT_EQ(first.property_hash(), second.property_hash());
+  EXPECT_NE(first, second);
+  EXPECT_NE(second, first);
+}
+
 TEST(property_hash) {
   const sourcemeta::core::Pointer::Token first{"foo"};
   const sourcemeta::core::Pointer::Token second{"bar"};
@@ -102,6 +146,160 @@ TEST(property_hash) {
   EXPECT_EQ(first.property_hash(), third.property_hash());
 }
 
+TEST(property_equals_perfect_match) {
+  const sourcemeta::core::Pointer::Token token{"foo"};
+  EXPECT_TRUE(token.property_equals(
+      "foo", sourcemeta::core::JSON::Object::hash("foo")));
+}
+
+TEST(property_equals_perfect_mismatch_same_size) {
+  const sourcemeta::core::Pointer::Token token{"foo"};
+  EXPECT_FALSE(token.property_equals(
+      "bar", sourcemeta::core::JSON::Object::hash("bar")));
+}
+
+TEST(property_equals_perfect_mismatch_different_size) {
+  const sourcemeta::core::Pointer::Token token{"foo"};
+  EXPECT_FALSE(token.property_equals(
+      "foobar", sourcemeta::core::JSON::Object::hash("foobar")));
+}
+
+TEST(property_equals_character_token_match) {
+  const sourcemeta::core::Pointer::Token token{'a'};
+  EXPECT_TRUE(
+      token.property_equals("a", sourcemeta::core::JSON::Object::hash("a")));
+}
+
+TEST(property_equals_empty_property_match) {
+  const sourcemeta::core::Pointer::Token token{""};
+  EXPECT_TRUE(
+      token.property_equals("", sourcemeta::core::JSON::Object::hash("")));
+}
+
+TEST(property_equals_empty_property_mismatch) {
+  const sourcemeta::core::Pointer::Token token{""};
+  EXPECT_FALSE(
+      token.property_equals("a", sourcemeta::core::JSON::Object::hash("a")));
+}
+
+TEST(property_equals_empty_value_mismatch) {
+  const sourcemeta::core::Pointer::Token token{"a"};
+  EXPECT_FALSE(
+      token.property_equals("", sourcemeta::core::JSON::Object::hash("")));
+}
+
+TEST(property_equals_property_with_trailing_null_not_matched_as_prefix) {
+  sourcemeta::core::JSON::String property_with_null{"a"};
+  property_with_null.push_back('\x00');
+  const sourcemeta::core::Pointer::Token token{property_with_null};
+  EXPECT_FALSE(
+      token.property_equals("a", sourcemeta::core::JSON::Object::hash("a")));
+}
+
+TEST(property_equals_value_with_trailing_null_not_matched_as_prefix) {
+  sourcemeta::core::JSON::String value_with_null{"a"};
+  value_with_null.push_back('\x00');
+  const sourcemeta::core::Pointer::Token token{"a"};
+  EXPECT_FALSE(token.property_equals(
+      value_with_null, sourcemeta::core::JSON::Object::hash(value_with_null)));
+}
+
+TEST(property_equals_property_with_trailing_null_match) {
+  sourcemeta::core::JSON::String property_with_null{"a"};
+  property_with_null.push_back('\x00');
+  const sourcemeta::core::Pointer::Token token{property_with_null};
+  EXPECT_TRUE(token.property_equals(
+      property_with_null,
+      sourcemeta::core::JSON::Object::hash(property_with_null)));
+}
+
+TEST(property_equals_longest_perfect_property_match) {
+  const sourcemeta::core::JSON::String property(31, 'a');
+  const sourcemeta::core::Pointer::Token token{property};
+  EXPECT_TRUE(token.property_equals(
+      property, sourcemeta::core::JSON::Object::hash(property)));
+}
+
+TEST(property_equals_shortest_imperfect_property_match) {
+  const sourcemeta::core::JSON::String property(32, 'a');
+  const sourcemeta::core::Pointer::Token token{property};
+  EXPECT_TRUE(token.property_equals(
+      property, sourcemeta::core::JSON::Object::hash(property)));
+}
+
+TEST(property_equals_perfect_property_against_imperfect_prefix_value) {
+  const sourcemeta::core::JSON::String property(31, 'a');
+  const sourcemeta::core::JSON::String value(32, 'a');
+  const sourcemeta::core::Pointer::Token token{property};
+  EXPECT_FALSE(token.property_equals(
+      value, sourcemeta::core::JSON::Object::hash(value)));
+}
+
+TEST(property_equals_imperfect_property_against_perfect_prefix_value) {
+  const sourcemeta::core::JSON::String property(32, 'a');
+  const sourcemeta::core::JSON::String value(31, 'a');
+  const sourcemeta::core::Pointer::Token token{property};
+  EXPECT_FALSE(token.property_equals(
+      value, sourcemeta::core::JSON::Object::hash(value)));
+}
+
+TEST(property_equals_long_property_match) {
+  const sourcemeta::core::JSON::String property(40, 'a');
+  const sourcemeta::core::Pointer::Token token{property};
+  EXPECT_TRUE(token.property_equals(
+      property, sourcemeta::core::JSON::Object::hash(property)));
+}
+
+TEST(property_equals_long_property_mismatch) {
+  const sourcemeta::core::JSON::String property(40, 'a');
+  const sourcemeta::core::JSON::String value(40, 'b');
+  const sourcemeta::core::Pointer::Token token{property};
+  EXPECT_FALSE(token.property_equals(
+      value, sourcemeta::core::JSON::Object::hash(value)));
+}
+
+TEST(property_equals_imperfect_hash_collision_rejected) {
+  const sourcemeta::core::JSON::String property{
+      sourcemeta::core::JSON::String(31, 'a') + "11111110" + "z"};
+  const sourcemeta::core::JSON::String value{
+      sourcemeta::core::JSON::String(31, 'a') + "22222220" + "z"};
+  const sourcemeta::core::Pointer::Token token{property};
+  EXPECT_EQ(sourcemeta::core::JSON::Object::hash(property),
+            sourcemeta::core::JSON::Object::hash(value));
+  EXPECT_FALSE(token.property_equals(
+      value, sourcemeta::core::JSON::Object::hash(value)));
+}
+
+TEST(property_equals_weak_token_perfect_match) {
+  const std::string property{"foo"};
+  const sourcemeta::core::WeakPointer::Token token{std::cref(property)};
+  EXPECT_TRUE(token.property_equals(
+      "foo", sourcemeta::core::JSON::Object::hash("foo")));
+}
+
+TEST(property_equals_weak_token_perfect_mismatch) {
+  const std::string property{"foo"};
+  const sourcemeta::core::WeakPointer::Token token{std::cref(property)};
+  EXPECT_FALSE(token.property_equals(
+      "bar", sourcemeta::core::JSON::Object::hash("bar")));
+}
+
+TEST(property_equals_weak_token_property_with_trailing_null_not_matched) {
+  std::string property_with_null{"a"};
+  property_with_null.push_back('\x00');
+  const sourcemeta::core::WeakPointer::Token token{
+      std::cref(property_with_null)};
+  EXPECT_FALSE(
+      token.property_equals("a", sourcemeta::core::JSON::Object::hash("a")));
+}
+
+TEST(property_equals_weak_token_long_property_match) {
+  const std::string property(40, 'a');
+  const sourcemeta::core::WeakPointer::Token token{std::cref(property)};
+  EXPECT_TRUE(token.property_equals(
+      property, sourcemeta::core::JSON::Object::hash(property)));
+}
+
 TEST(at_property_with_hash) {
   const auto document = sourcemeta::core::parse_json(R"JSON({
     "foo": 1,