enum { nb = 1024 };
struct { int k, v; } hash[nb];
// 0 is an invalid key
void incr(int k)
{
int i = k % nb, j = i;
do {
int k2 = hash[i].k;
if (k2 && k2 != k) {
i = (i+1) % nb;
continue;
}
hash[i].k = k;
hash[i].v++;
return;
} while (i != j);
abort();
}
Apologies for the telegraphic variable names and weird control flow. I wrote this on my cellphone. Lacking these 15 lines are what keep you from writing C?
There's a nice tutorial on hash tables in K&R, and I can also recommend learning about Chris Wellons's "MSI" hash table design, described in C at https://nullprogram.com/blog/2022/08/08/. He shows a more full featured hash table in about 30 lines of code, depending on what functionality you need. It's eminently practical, and usually a lot faster than generic algorithms.
That's not an exceptionally simple hash table either. One night I hurriedly wrote a rather long-winded implementation also on my cellphone (strings, with separate chaining and a djb-style hash function) and it also came to about 30 lines of code: http://canonical.org/~kragen/sw/dev3/justhash.c
In the interest of correctness, I just wrote a quick test of the above code at http://canonical.org/~kragen/sw/dev3/intcount.c. Perhaps surprisingly, it works, including the error handling, which is not always the case when you write a bunch of untested C after midnight on your cellphone. But a hash table is evidently simple enough.
