Put and Get

Time to assemble the map itself: a heap-allocated array of bucket heads plus a length counter.

struct hashmap {
	struct hm_entry **buckets;  /* array of nbuckets chain heads */
	size_t nbuckets;
	size_t len;                 /* number of stored entries */
};

hm_get is the two helpers from last lesson glued together: hash, index, walk the chain — the whole lookup is just this pipeline:

hm_put has more decisions in it:

• Overwrite, don't duplicate. If the key already exists, replace its value in place; len doesn't change. Only a genuinely new key allocates a node and bumps len.
• Own your keys. The caller's string may be a stack buffer that dies at the next }. The map must copy the key into memory it owns (allocate strlen + 1 bytes and copy — this is what non-standard strdup does).
• Report allocation failure. malloc returns NULL when memory runs out; a library that crashes instead of returning an error is a library you can't ship. Return 0 on success, -1 on failure.

Returning int * (a pointer to the stored value) from hm_get instead of int kills two birds: NULL cleanly signals "not found", and callers can update a value through the pointer without a second lookup.