I am working on a capability to target requests to a mongos based on the mongod server hosting the data within a sharded cluster.

For "normal" indexes (ascending and descending) this is easy enough and shows promise in improving performance.

For hashed indexes I ran into the issue that there was exactly 1 test case that I could find to ensure my hashing of the index is consistent with MongoDB's.

struct HasherUnitTest : public StartupTest {
    void run() {
        // Hard-coded check to ensure the hash function is consistent across platforms
        BSONObj o = BSON( "check" << 42 );
        verify( BSONElementHasher::hash64( o.firstElement(), 0 ) == -944302157085130861LL );
    }
} hasherUnitTest;

This seems a little "under specified" to me. I created a more complete test suite of "test vectors" and figured it is probably a useful thing to have in the baseline to ensure future changes that modify the hashing indexes will be more likely to be caught.

Pull Request coming soon...