Hashmap: Do not insert duplicate keys

[boki1]

Description
While looking into the table for a spot for the new key, search
until an empty one is found and do not overwrite tombstones.

Reported as #53.

Test

I was not able to find FNV colliding strings so instead I replaced the hash function and used a "dumb" one instead:

static uint64_t dumb_hash(char *s, int len) {
    if (len < 5)
        return 0;
    return 1;
}

Then using the following test case I reproduced the issue:

  hashmap_put(map, "123", (void *)(size_t)3);
  hashmap_put(map, "345", (void *)(size_t)3);
  assert(hashmap_get(map, "123") == (void *)(size_t) 3);
  assert(hashmap_get(map, "345") == (void *)(size_t) 3);
  hashmap_delete(map, "123");
  hashmap_put(map, "345", (void *)(size_t)3);
  assert(hashmap_get(map, "345") == (void *)(size_t) 3);
  hashmap_delete(map, "345");
  assert(hashmap_get(map, "345") == (void *)(size_t) 3);

The test passes when using 2c523f.

Signed-off-by: Kristiyan Stoimenov kristoimenov@gmail.com