Define a general hashing utility in C++ (#1195)

This is good enough to make it possible for the new C++ code to
interoperate with existing Objective-C code where `-hash` is required if
you override `-isEqual:`.

3 files changed, 8 insertions, 11 deletions

diff --git a/Firestore/core/src/firebase/firestore/model/base_path.h b/Firestore/core/src/firebase/firestore/model/base_path.h
index bc1f89d..59a9b5a 100644
--- a/Firestore/core/src/firebase/firestore/model/base_path.h
+++ b/Firestore/core/src/firebase/firestore/model/base_path.h
@@ -25,6 +25,7 @@
 #include <vector>
 
 #include "Firestore/core/src/firebase/firestore/util/firebase_assert.h"
+#include "Firestore/core/src/firebase/firestore/util/hashing.h"
 
 namespace firebase {
 namespace firestore {
@@ -162,13 +163,8 @@ class BasePath {
 #if defined(__OBJC__)
   // For Objective-C++ hash; to be removed after migration.
   // Do NOT use in C++ code.
-  NSUInteger Hash() const {
-    std::hash<std::string> hash_fn;
-    NSUInteger hash_result = 0;
-    for (const std::string& segment : segments_) {
-      hash_result = hash_result * 31u + hash_fn(segment);
-    }
-    return hash_result;
+  size_t Hash() const {
+    return util::Hash(segments_);
   }
 #endif  // defined(__OBJC__)
 
diff --git a/Firestore/core/src/firebase/firestore/model/database_id.h b/Firestore/core/src/firebase/firestore/model/database_id.h
index 0c0e0ec..c432b8f 100644
--- a/Firestore/core/src/firebase/firestore/model/database_id.h
+++ b/Firestore/core/src/firebase/firestore/model/database_id.h
@@ -20,6 +20,7 @@
 #include <cstdint>
 #include <string>
 
+#include "Firestore/core/src/firebase/firestore/util/hashing.h"
 #include "absl/strings/string_view.h"
 
 namespace firebase {
@@ -62,9 +63,8 @@ class DatabaseId {
 #if defined(__OBJC__)
   // For objective-c++ hash; to be removed after migration.
   // Do NOT use in C++ code.
-  NSUInteger Hash() const {
-    std::hash<std::string> hash_fn;
-    return hash_fn(project_id_) * 31u + hash_fn(database_id_);
+  size_t Hash() const {
+    return util::Hash(project_id_, database_id_);
   }
 #endif  // defined(__OBJC__)
 
diff --git a/Firestore/core/src/firebase/firestore/model/document_key.h b/Firestore/core/src/firebase/firestore/model/document_key.h
index 4bdc04b..7f6478f 100644
--- a/Firestore/core/src/firebase/firestore/model/document_key.h
+++ b/Firestore/core/src/firebase/firestore/model/document_key.h
@@ -26,6 +26,7 @@
 #endif  // defined(__OBJC__)
 
 #include "Firestore/core/src/firebase/firestore/model/resource_path.h"
+#include "Firestore/core/src/firebase/firestore/util/hashing.h"
 #include "absl/strings/string_view.h"
 
 namespace firebase {
@@ -61,7 +62,7 @@ class DocumentKey {
   }
 
   NSUInteger Hash() const {
-    return std::hash<std::string>{}(ToString());
+    return util::Hash(ToString());
   }
 #endif