Merge pull request #5302 from rocallahan/commutative-hash

Improve commutative hashing.

kernel/hashlib.h

@@ -12,6 +12,7 @@
 #ifndef HASHLIB_H
 #define HASHLIB_H
 
+#include <array>
 #include <stdexcept>
 #include <algorithm>
 #include <optional>
@@ -127,6 +128,7 @@ private:
 		*this = hash_ops<T>::hash_into(t, *this);
 	}
 
+	[[deprecated]]
 	void commutative_eat(hash_t t) {
 		state ^= t;
 	}
@@ -356,6 +358,29 @@ template<typename K, int offset = 0, typename OPS = hash_ops<K>> class idict;
 template<typename K, typename OPS = hash_ops<K>> class pool;
 template<typename K, typename OPS = hash_ops<K>> class mfp;
 
+// Computes the hash value of an unordered set of elements.
+// See https://www.preprints.org/manuscript/201710.0192/v1/download.
+// This is the Sum(4) algorithm from that paper, which has good collision resistance,
+// much better than Sum(1) or Xor(1) (and somewhat better than Xor(4)).
+class commutative_hash {
+public:
+	commutative_hash() {
+		buckets.fill(0);
+	}
+	void eat(Hasher h) {
+		Hasher::hash_t v = h.yield();
+		size_t index = v & (buckets.size() - 1);
+		buckets[index] += v;
+	}
+	[[nodiscard]] Hasher hash_into(Hasher h) const {
+		for (auto b : buckets)
+			h.eat(b);
+		return h;
+	}
+private:
+	std::array<Hasher::hash_t, 4> buckets;
+};
+
 template<typename K, typename T, typename OPS>
 class dict {
 	struct entry_t
@@ -801,14 +826,14 @@ public:
 	}
 
 	[[nodiscard]] Hasher hash_into(Hasher h) const {
+		commutative_hash comm;
 		for (auto &it : entries) {
 			Hasher entry_hash;
 			entry_hash.eat(it.udata.first);
 			entry_hash.eat(it.udata.second);
-			h.commutative_eat(entry_hash.yield());
+			comm.eat(entry_hash);
 		}
-		h.eat(entries.size());
-		return h;
+		return comm.hash_into(h);
 	}
 
 	void reserve(size_t n) { entries.reserve(n); }
@@ -1184,11 +1209,11 @@ public:
 	}
 
 	[[nodiscard]] Hasher hash_into(Hasher h) const {
+		commutative_hash comm;
 		for (auto &it : entries) {
-			h.commutative_eat(ops.hash(it.udata).yield());
+			comm.eat(ops.hash(it.udata));
 		}
-		h.eat(entries.size());
-		return h;
+		return comm.hash_into(h);
 	}
 
 	void reserve(size_t n) { entries.reserve(n); }

kernel/yosys_common.h

@@ -20,6 +20,7 @@
 #ifndef YOSYS_COMMON_H
 #define YOSYS_COMMON_H
 
+#include <array>
 #include <map>
 #include <set>
 #include <tuple>

tests/unit/kernel/hashTest.cc

@@ -0,0 +1,67 @@
+#include <gtest/gtest.h>
+#include "kernel/yosys_common.h"
+
+#include <unordered_set>
+
+YOSYS_NAMESPACE_BEGIN
+
+static Hasher hash(int x)
+{
+	Hasher h;
+	h.eat(x);
+	return h;
+}
+
+TEST(CommutativeTest, basic)
+{
+	hashlib::commutative_hash comm1;
+	comm1.eat(hash(1));
+	comm1.eat(hash(2));
+	hashlib::commutative_hash comm2;
+	comm2.eat(hash(2));
+	comm2.eat(hash(1));
+	EXPECT_EQ(comm1.hash_into(Hasher()).yield(), comm2.hash_into(Hasher()).yield());
+}
+
+TEST(PoolHashTest, collisions)
+{
+	uint64_t collisions = 0;
+	std::unordered_set<Hasher::hash_t> hashes;
+	for (int i = 0; i < 1000; ++i) {
+		for (int j = i + 1; j < 1000; ++j) {
+			pool<int> p1;
+			p1.insert(i);
+			p1.insert(j);
+			auto h = p1.hash_into(Hasher()).yield();
+			if (!hashes.insert(h).second) {
+				++collisions;
+			}
+		}
+	}
+	std::cout << "pool<int> collisions: " << collisions << std::endl;
+	EXPECT_LT(collisions, 10'000);
+}
+
+TEST(PoolHashTest, subset_collisions)
+{
+	uint64_t collisions = 0;
+	std::unordered_set<Hasher::hash_t> hashes;
+	for (int i = 0; i < 1000 * 1000; ++i) {
+
+		pool<int> p1;
+		for (int b = 0; i >> b; ++b) {
+			if ((i >> b) & 1) {
+				p1.insert(b);
+			}
+		}
+		auto h = p1.hash_into(Hasher()).yield();
+		if (!hashes.insert(h).second) {
+			++collisions;
+		}
+
+	}
+	std::cout << "pool<int> subset collisions: " << collisions << std::endl;
+	EXPECT_LT(collisions, 100);
+}
+
+YOSYS_NAMESPACE_END