Better hash function for pairs (#1157)

CVC4 was computing hashes for pairs of objects by simply XORing the
hashes of the two objects. This commit implements a better way of
combining hashes based on the FNV-1a hash algorithm. The algorithm is
public domain.

1 files changed, 18 insertions, 1 deletions

diff --git a/src/util/hash.h b/src/util/hash.h
index b04fb8bb5..aa373b0ca 100644
--- a/src/util/hash.h
+++ b/src/util/hash.h
@@ -20,6 +20,7 @@
 #ifndef __CVC4__HASH_H
 #define __CVC4__HASH_H
 
+#include <cstdint>
 #include <functional>
 #include <string>
 
@@ -40,11 +41,27 @@ struct hash<uint64_t> {
 
 namespace CVC4 {
 
+namespace fnv1a {
+
+/**
+ * FNV-1a hash algorithm for 64-bit numbers.
+ *
+ * More details here: http://www.isthe.com/chongo/tech/comp/fnv/index.html
+ */
+inline uint64_t fnv1a_64(uint64_t v, uint64_t hash = 14695981039346656037U) {
+  hash ^= v;
+  // Compute (hash * 1099511628211)
+  return hash + (hash << 1) + (hash << 4) + (hash << 5) + (hash << 7) +
+         (hash << 8) + (hash << 40);
+}
+
+}  // namespace fnv1a
 
 template <class T, class U, class HashT = std::hash<T>, class HashU = std::hash<U> >
 struct PairHashFunction {
   size_t operator()(const std::pair<T, U>& pr) const {
-    return HashT()(pr.first) ^ HashU()(pr.second);
+    uint64_t hash = fnv1a::fnv1a_64(HashT()(pr.first));
+    return static_cast<size_t>(fnv1a::fnv1a_64(HashU()(pr.second), hash));
   }
 };/* struct PairHashFunction */