Public Member Functions | List of all members
tsl::sh::prime_growth_policy Class Reference

#include <sparse_growth_policy.h>

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::sh::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;
case 1: hash % 17ul;
case 2: hash % 29ul;

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 environement.

Constructor & Destructor Documentation

◆ prime_growth_policy()

tsl::sh::prime_growth_policy::prime_growth_policy ( std::size_t &  min_bucket_count_in_out)

Member Function Documentation

◆ bucket_for_hash()

std::size_t tsl::sh::prime_growth_policy::bucket_for_hash ( std::size_t  hash) const

◆ clear()

void tsl::sh::prime_growth_policy::clear ( )

◆ max_bucket_count()

std::size_t tsl::sh::prime_growth_policy::max_bucket_count ( ) const

◆ next_bucket_count()

std::size_t tsl::sh::prime_growth_policy::next_bucket_count ( ) const

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