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Improve commutative hashing.

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The simple XOR `commutative_eat()` implementation produces a lot of collisions.
https://www.preprints.org/manuscript/201710.0192/v1/download is a useful reference on this topic.

Running the included `hashTest.cc` without the hashlib changes, I get 49,580,349 collisions.
The 49,995,000 (i,j) pairs (0 <= i < 10000, i < j < 10000) hash into only 414,651 unique hash values.
We get simple collisions like (0,1) colliding with (2,3).

With the hashlib changes, we get only 707,099 collisions and 49,287,901 unique hash values.
Much better! The `commutative_hash` implementation corresponds to `Sum(4)` in the paper
mentioned above.
This commit is contained in:
Robert O'Callahan 2025-08-19 03:21:54 +00:00
parent b0d709f6cf
commit 3a5742ffd2
3 changed files with 78 additions and 7 deletions
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45
tests/unit/kernel/hashTest.cc Normal file
View file

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#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 < 10000; ++i) {
for (int j = i + 1; j < 10000; ++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, 1000000);
}
YOSYS_NAMESPACE_END