static int hash(int h) {
        // This function ensures that hashCodes that differ only by
        // constant multiples at each bit position have a bounded
        // number of collisions (approximately 8 at default load factor).
        h = h * h;
        h ^= (h >>> 20) ^ (h >>> 12);
        return h ^ (h >>> 7) ^ (h >>> 4);
    }
Applies a supplemental hash function to a given hashCode, which defends
against poor quality hash functions. This is critical because
SafelyHashMap uses power-of-two length hash tables, that otherwise
encounter collisions for hashCodes that do not differ in lower bits.
Note: Null keys always map to hash 0, thus index 0.
src/main/java/com/alibaba/fastjson/util/AntiCollisionHashMap.java