reduce duplicate hash computation in json_object_object_add()

This can be a very considerable performance saver.

json_object.c

@@ -403,10 +403,11 @@ void json_object_object_add(struct json_object* jso, const char *key,
 	// and re-adding it, so the existing key remains valid.
 	json_object *existing_value = NULL;
 	struct lh_entry *existing_entry;
-	existing_entry = lh_table_lookup_entry(jso->o.c_object, (void*)key);
+	const unsigned long hash = lh_get_hash(jso->o.c_object, (void*)key);
+	existing_entry = lh_table_lookup_entry_w_hash(jso->o.c_object, (void*)key, hash);
 	if (!existing_entry)
 	{
-		lh_table_insert(jso->o.c_object, strdup(key), val);
+		lh_table_insert_w_hash(jso->o.c_object, strdup(key), val, hash);
 		return;
 	}
 	existing_value = (void *)existing_entry->v;
@@ -415,6 +416,7 @@ void json_object_object_add(struct json_object* jso, const char *key,
 	existing_entry->v = val;
 }
 
+
 int json_object_object_length(struct json_object *jso)
 {
 	return lh_table_length(jso->o.c_object);

linkhash.c

@@ -490,14 +490,13 @@ void lh_table_free(struct lh_table *t)
 }
 
 
-int lh_table_insert(struct lh_table *t, void *k, const void *v)
+int lh_table_insert_w_hash(struct lh_table *t, void *k, const void *v, const unsigned long h)
 {
-	unsigned long h, n;
+	unsigned long n;
 
 	t->inserts++;
 	if(t->count >= t->size * LH_LOAD_FACTOR) lh_table_resize(t, t->size * 2);
 
-	h = t->hash_fn(k);
 	n = h % t->size;
 
 	while( 1 ) {
@@ -522,11 +521,14 @@ int lh_table_insert(struct lh_table *t, void *k, const void *v)
 
 	return 0;
 }
-
-
-struct lh_entry* lh_table_lookup_entry(struct lh_table *t, const void *k)
+int lh_table_insert(struct lh_table *t, void *k, const void *v)
+{
+	return lh_table_insert_w_hash(t, k, v, lh_get_hash(t, k));
+}
+
+
+struct lh_entry* lh_table_lookup_entry_w_hash(struct lh_table *t, const void *k, const unsigned long h)
 {
-	unsigned long h = t->hash_fn(k);
 	unsigned long n = h % t->size;
 	int count = 0;
 
@@ -541,6 +543,10 @@ struct lh_entry* lh_table_lookup_entry(struct lh_table *t, const void *k)
 	return NULL;
 }
 
+struct lh_entry* lh_table_lookup_entry(struct lh_table *t, const void *k)
+{
+	return lh_table_lookup_entry_w_hash(t, k, lh_get_hash(t, k));
+}
 
 const void* lh_table_lookup(struct lh_table *t, const void *k)
 {

linkhash.h

@@ -231,6 +231,20 @@ extern void lh_table_free(struct lh_table *t);
 extern int lh_table_insert(struct lh_table *t, void *k, const void *v);
 
 
+/**
+ * Insert a record into the table. This one accepts the key's hash in additon
+ * to the key. This is an exension to support functions that need to calculate
+ * the hash several times and allows them to do it just once and then pass
+ * in the hash to all utility functions. Depending on use case, this can be a 
+ * very considerate performance improvement.
+ * @param t the table to insert into.
+ * @param k a pointer to the key to insert.
+ * @param v a pointer to the value to insert.
+ * @param h hash value of the key to insert
+ */
+extern int lh_table_insert_w_hash(struct lh_table *t, void *k, const void *v, const unsigned long h);
+
+
 /**
  * Lookup a record into the table.
  * @param t the table to lookup
@@ -239,6 +253,19 @@ extern int lh_table_insert(struct lh_table *t, void *k, const void *v);
  */
 extern struct lh_entry* lh_table_lookup_entry(struct lh_table *t, const void *k);
 
+/**
+ * Lookup a record into the table. This one accepts the key's hash in additon
+ * to the key. This is an exension to support functions that need to calculate
+ * the hash several times and allows them to do it just once and then pass
+ * in the hash to all utility functions. Depending on use case, this can be a 
+ * very considerate performance improvement.
+ * @param t the table to lookup
+ * @param k a pointer to the key to lookup
+ * @param h hash value of the key to lookup
+ * @return a pointer to the record structure of the value or NULL if it does not exist.
+ */
+extern struct lh_entry* lh_table_lookup_entry_w_hash(struct lh_table *t, const void *k, const unsigned long h);
+
 /**
  * Lookup a record into the table
  * @param t the table to lookup
@@ -285,6 +312,22 @@ extern int lh_table_length(struct lh_table *t);
 void lh_abort(const char *msg, ...);
 void lh_table_resize(struct lh_table *t, int new_size);
 
+
+/**
+ * Calculate the hash of a key for a given table.
+ * This is an exension to support functions that need to calculate
+ * the hash several times and allows them to do it just once and then pass
+ * in the hash to all utility functions. Depending on use case, this can be a 
+ * very considerate performance improvement.
+ * @param t the table (used to obtain hash function)
+ * @param k a pointer to the key to lookup
+ * @return the key's hash
+ */
+static inline unsigned long lh_get_hash(const struct lh_table *t, const void *k)
+{
+	return t->hash_fn(k);
+}
+
 #ifdef __cplusplus
 }
 #endif