Editing Mask (computing) (section)

===Hash tables===
To create a hashing function for a [[hash table]], often a function is used that has a large domain. To create an index from the output of the function, a modulo can be taken to reduce the size of the domain to match the size of the array; however, it is often faster on many processors to restrict the size of the hash table to powers of two sizes and use a bitmask instead.

An example of both modulo and masking in C:

<syntaxhighlight lang="c">
#include <stdint.h>
#include <string.h>

int main(void) {
    const uint32_t NUM_BUCKETS = 0xFFFFFFFF;  // 2^32 - 1
    const uint32_t MAX_RECORDS = 1<<10;  // 2^10
    const uint32_t HASH_BITMASK = 0x3FF;  // (2^10)-1

    char **token_array = NULL;
    // Handle memory allocation for token_array…

    char token[] = "some hashable value";
    uint32_t hashed_token = hash_function(token, strlen(token), NUM_BUCKETS);

    // Using modulo
    size_t index = hashed_token % MAX_RECORDS;

    // OR

    // Using bitmask
    size_t index = hashed_token & HASH_BITMASK;

    *(token_array+index) = token;

    // Free the memory from token_array …
    return 0;
}
</syntaxhighlight>