prime_growth_policy Class Reference

Grow the hash table by using prime numbers as bucket count. More...

Public Member Functions
	prime_growth_policy (std::size_t &min_bucket_count_in_out)
std::size_t	bucket_for_hash (std::size_t hash) const noexcept
std::size_t	next_bucket_count () const
std::size_t	max_bucket_count () const
void	clear () noexcept

Detailed Description

Grow the hash table by using prime numbers as bucket count.

Slower than tsl::rh::power_of_two_growth_policy in general but will probably distribute the values around better in the buckets with a poor hash function.

To allow the compiler to optimize the modulo operation, a lookup table is used with constant primes numbers.

With a switch the code would look like:

switch(iprime) { // iprime is the current prime of the hash table
    case 0: hash % 5ul;
            break;
    case 1: hash % 17ul;
            break;
    case 2: hash % 29ul;
            break;
    ...
}

Due to the constant variable in the modulo the compiler is able to optimize the operation by a series of multiplications, substractions and shifts.

The 'hash % 5' could become something like 'hash - (hash * 0xCCCCCCCD) >> 34)

• 5' in a 64 bits environment.

Definition at line 364 of file robin_growth_policy.h.

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The documentation for this class was generated from the following file:

• pxr/base/tf/pxrTslRobinMap/robin_growth_policy.h