Hash function

A hash function is a function that returns the hash value of a string. Hashing means to convert a string into a number.

Process

This is an example hashing process:

idx(si) return si(Let's assume that a~z is 1~26)
hash[i]=(hash[i-1]*idx(si)) % mod
p and mod are both prime numbers, where p<mod
Assume that p=13, mod=101
Now let's hash the string "abc". We need an array to store the hash process (or a variable might be enough.)
hash[0]=1;
hash[1]=(hash[0]*13+2)%101=15;
hash[2]=(hash[1]*13+3)%101=97;
So, "abc"'s hash value is 97.

Sometimes, the hashing process will return the same value from different strings. Refer to https://www.planetmath.org/goodhashtableprimes for more information.

C example

unsigned hash(const char a[static 1])
{
    unsigned h = 0;
    for (size_t i = 0; a[i] != '\0'; i++)
        h = 65 * h + a[i];
    return h;
}