folly/Hash.h add a test to ensure different hashes

Summary:
"Wouldn't this test case still pass if the hash function didn't depend on the value?

It would be nice to verify that different tuple values produce different hash values (or maybe there's another test that verifies that), especially since this is a tricky thing to get right."

I think there's not much chance of this failing since it's just leveraging hash_combine and that's already tested. But it's easy to add another sanity check, so here we are.

Test Plan: runtests

Reviewed By: tulloch@fb.com

FB internal diff: D890448

Blame Revision: D888796

diff --git a/folly/test/HashTest.cpp b/folly/test/HashTest.cpp
index 6ed9a280a1d9ea0a3c58b651be730e94e9650966..5b8840f5f24b6d3bc18725a37af7406b3635d0e2 100644 (file)
--- a/folly/test/HashTest.cpp
+++ b/folly/test/HashTest.cpp
@@ -237,3 +237,15 @@ TEST(Hash, std_tuple) {
   m[t] = "bar";
   EXPECT_EQ("bar", m[t]);
 }
+
+TEST(Hash, std_tuple_different_hash) {
+  typedef std::tuple<int64_t, std::string, int32_t> tuple3;
+  tuple3 t1(42, "foo", 1);
+  tuple3 t2(9, "bar", 3);
+  tuple3 t3(42, "foo", 3);
+
+  EXPECT_NE(std::hash<tuple3>()(t1),
+            std::hash<tuple3>()(t2));
+  EXPECT_NE(std::hash<tuple3>()(t1),
+            std::hash<tuple3>()(t3));
+}