Add "if_contains" to parallel_hash_map so the value does not need to be copied to be read in a thread-safe manner

Add tests for parallel_hash_map thread-safe functions.

parallel_hashmap/phmap.h

@@ -3408,6 +3408,16 @@ public:
         return true;
     }
 
+    template <class K = key_type, class F>
+    bool if_contains(const key_arg<K>& key, F&& f) {
+        typename Lockable::SharedLock m;
+        auto it = const_cast<parallel_hash_map*>(this)->find(key, hash(key), m);
+        if (it == this->end())
+            return false;
+        std::forward<F>(f)(Policy::value(&*it));
+        return true;
+    }
+
     template <class K = key_type, class P = Policy, K* = nullptr>
     MappedReference<P> operator[](key_arg<K>&& key) {
         return Policy::value(&*try_emplace(std::forward<K>(key)).first);

tests/flat_hash_map_test.cc

@@ -244,6 +244,7 @@ TEST(THIS_TEST_NAME, MergeExtractInsert) {
   m.insert(std::move(node));
   EXPECT_THAT(m, UnorderedElementsAre(Pair(1, 17), Pair(2, 9)));
 }
+
 #if !defined(__ANDROID__) && !defined(__APPLE__) && !defined(__EMSCRIPTEN__) && defined(PHMAP_HAVE_STD_ANY)
 TEST(THIS_TEST_NAME, Any) {
   ThisMap<int, std::any> m;

tests/parallel_flat_hash_map_test.cc

@@ -2,3 +2,4 @@
 #define THIS_TEST_NAME ParallelFlatHashMap
 
 #include "flat_hash_map_test.cc"
+#include "parallel_hash_map_test.cc"

tests/parallel_hash_map_test.cc

@@ -0,0 +1,32 @@
+#ifndef THIS_HASH_MAP
+    #define THIS_HASH_MAP  parallel_flat_hash_map
+    #define THIS_TEST_NAME ParallelFlatHashMap
+#endif
+
+namespace phmap {
+namespace container_internal {
+namespace {
+
+TEST(THIS_TEST_NAME, ThreadSafeContains) {
+    // We can't test mutable keys, or non-copyable keys with ThisMap.
+    // Test that the nodes have the proper API.
+    ThisMap<int, int> m = { {1, 7}, {2, 9} };
+    auto val = 0;
+    EXPECT_TRUE(m.contains(1, val));
+    EXPECT_EQ(val, 7);
+
+    EXPECT_FALSE(m.contains(3, val));
+
+#if __cplusplus > 199711L
+    auto func = [&val](int& v) { val = v; };
+    EXPECT_TRUE(m.if_contains(2, func));
+    EXPECT_EQ(val, 9);
+
+    EXPECT_FALSE(m.if_contains(3, func));
+#endif
+
+}
+
+}  // namespace
+}  // namespace container_internal
+}  // namespace phmap

tests/parallel_node_hash_map_test.cc

@@ -2,3 +2,4 @@
 #define THIS_TEST_NAME ParallelNodeHashMap
 
 #include "flat_hash_map_test.cc"
+#include "parallel_hash_map_test.cc"