Make IntSet hashable.

So it can be used with flat_hash_set/map.

Author: garretrieger

common/BUILD

@@ -113,5 +113,6 @@ cc_test(
         ":mocks",
         "@googletest//:gtest_main",
         "@abseil-cpp//absl/container:btree",
+        "@abseil-cpp//absl/hash:hash_test",
     ],
 )

common/int_set.h

@@ -140,10 +140,14 @@ class IntSet {
     return a == this->end();
   }
 
-  // TODO(garretrieger): add union, intersection etc.
+  template <typename H>
+  friend H AbslHashValue(H h, const IntSet& set) {
+    // Utilize the existing harfbuzz hashing function.
+    unsigned harfbuzz_hash = hb_set_hash(set.set_.get());
+    return H::combine(std::move(h), harfbuzz_hash);
+  }
 
-  // TODO(garretrieger): add absl hashing support so we can use these in
-  // hash_sets/maps
+  // TODO(garretrieger): add union, intersection etc.
 
   iterator begin() { return iterator(set_.get()); }
 

common/int_set_test.cc

@@ -2,6 +2,7 @@
 
 #include "common/hb_set_unique_ptr.h"
 #include "gtest/gtest.h"
+#include "absl/hash/hash_testing.h"
 
 namespace common {
 
@@ -200,6 +201,31 @@ TEST_F(IntSetTest, UseInBtreeSet) {
   ASSERT_EQ(*it, empty);
 }
 
-// TODO(garretrieger): test use in hash and btree, sets and maps.
+TEST_F(IntSetTest, UseInHashSet) {
+  absl::flat_hash_set<IntSet> sets{{7, 8, 11}, {7, 8}, {7, 8, 12}, {}};
+
+  IntSet empty{};
+  IntSet a{7, 8};
+  IntSet b{7, 8, 11};
+  IntSet c{7, 8, 12};
+  IntSet d{8, 11};
+
+  ASSERT_TRUE(sets.contains(empty));
+  ASSERT_TRUE(sets.contains(a));
+  ASSERT_TRUE(sets.contains(b));
+  ASSERT_TRUE(sets.contains(c));
+  ASSERT_FALSE(sets.contains(d));
+}
+
+TEST_F(IntSetTest, SupportsAbslHash) {
+  EXPECT_TRUE(absl::VerifyTypeImplementsAbslHashCorrectly({
+      IntSet {},
+      IntSet {7, 8},
+      IntSet {7, 8, 11},
+      IntSet {7, 8, 12},
+      IntSet {8, 11},
+      IntSet {7, 8, 12},
+  }));
+}
 
 }  // namespace common