Remove specialization of std::hash for Timestamp (#3797)

Author: var-const

Firestore/core/include/firebase/firestore/timestamp.h

@@ -236,18 +236,4 @@ std::chrono::time_point<Clock, Duration> Timestamp::ToTimePoint() const {
 
 }  // namespace firebase
 
-namespace std {
-/** Convenience-only specialization of `std::hash`. */
-template <>
-struct hash<firebase::Timestamp> {
-  /**
-   * Hashes the given `timestamp`.
-   *
-   * @note: specialization of `std::hash` is provided for convenience only. The
-   * implementation is subject to change.
-   */
-  size_t operator()(const firebase::Timestamp& timestamp) const;
-};
-}  // namespace std
-
 #endif  // FIRESTORE_CORE_INCLUDE_FIREBASE_FIRESTORE_TIMESTAMP_H_

Firestore/core/src/firebase/firestore/model/snapshot_version.cc

@@ -18,6 +18,8 @@
 
 #include <ostream>
 
+#include "Firestore/core/src/firebase/firestore/util/hashing.h"
+
 namespace firebase {
 namespace firestore {
 namespace model {
@@ -37,7 +39,7 @@ util::ComparisonResult SnapshotVersion::CompareTo(
 }
 
 size_t SnapshotVersion::Hash() const {
-  return std::hash<Timestamp>{}(timestamp_);
+  return util::Hash(timestamp_.seconds(), timestamp_.nanoseconds());
 }
 
 std::string SnapshotVersion::ToString() const {

Firestore/core/src/firebase/firestore/timestamp.cc

@@ -138,13 +138,3 @@ void Timestamp::ValidateBounds() const {
 }
 
 }  // namespace firebase
-
-namespace std {
-size_t hash<firebase::Timestamp>::operator()(
-    const firebase::Timestamp& timestamp) const {
-  // Note: if sizeof(size_t) == 4, this discards high-order bits of seconds.
-  return 37 * static_cast<size_t>(timestamp.seconds()) +
-         static_cast<size_t>(timestamp.nanoseconds());
-}
-
-}  // namespace std

Firestore/core/test/firebase/firestore/timestamp_test.cc

@@ -274,18 +274,6 @@ TEST(Timestamp, Comparison) {
   EXPECT_NE(Timestamp(123, 123456789), Timestamp(123, 123456780));
 }
 
-TEST(Timestamp, Hash) {
-  const Timestamp foo1{123, 456000000};
-  const Timestamp foo2 = foo1;
-  const Timestamp foo3 =
-      Timestamp::FromTimePoint(TimePoint{Sec(123) + Ms(456)});
-  EXPECT_EQ(std::hash<Timestamp>()(foo1), std::hash<Timestamp>()(foo2));
-  EXPECT_EQ(std::hash<Timestamp>()(foo2), std::hash<Timestamp>()(foo3));
-
-  const Timestamp bar{123, 456};
-  EXPECT_NE(std::hash<Timestamp>()(foo1), std::hash<Timestamp>()(bar));
-}
-
 TEST(Timestamp, InvalidArguments) {
   // Negative nanoseconds.
   ASSERT_ANY_THROW(Timestamp(0, -1));