Add json_object_array_shrink() (and array_list_shrink()) and use it in json_tokener to minimize the amount of memory used. This results in a 39%-50% reduction in memory use (peak RSS, peak heap usage) on the jc-bench benchmark and 9% shorter runtime.

Also add the json_object_new_array_ext, array_list_new2, and array_list_shrink functions.

ChangeLog

@@ -4,7 +4,7 @@ Next Release 0.15
 
 Deprecated and removed features:
 --------------------------------
-...none yet...
+* array_list_new() has been deprecated in favor of array_list_new2()
 
 Other changes
 --------------
@@ -19,6 +19,12 @@ Other changes
    less memory usage.
   Memory used just for json_object structures decreased 27%, so use cases
    with fewer arrays and/or strings would benefit more.
+* Minimize memory usage in array handling in json_tokener by shrinking
+   arrays to the exact number of elements parsed.  On bench/ benchmark:
+   9% faster test time, 39%(max RSS)-50%(peak heap) less memory usage.
+   Add json_object_array_shrink() and array_list_shrink() functions.
+* Add json_object_new_array_ext(int) and array_list_new_2(int) to allow
+   arrays to be allocated with the exact size needed, when known.
 
 
 ***

arraylist.c

@@ -37,13 +37,18 @@
 #include "arraylist.h"
 
 struct array_list *array_list_new(array_list_free_fn *free_fn)
+{
+	return array_list_new2(free_fn, ARRAY_LIST_DEFAULT_SIZE);
+}
+
+struct array_list *array_list_new2(array_list_free_fn *free_fn, int initial_size)
 {
 	struct array_list *arr;
 
 	arr = (struct array_list *)malloc(sizeof(struct array_list));
 	if (!arr)
 		return NULL;
-	arr->size = ARRAY_LIST_DEFAULT_SIZE;
+	arr->size = initial_size;
 	arr->length = 0;
 	arr->free_fn = free_fn;
 	if (!(arr->array = (void **)malloc(arr->size * sizeof(void *))))
@@ -96,6 +101,26 @@ static int array_list_expand_internal(struct array_list *arr, size_t max)
 	return 0;
 }
 
+int array_list_shrink(struct array_list *arr, size_t empty_slots)
+{
+	void *t;
+	size_t new_size;
+
+	new_size = arr->length + empty_slots;
+	if (new_size == arr->size)
+		return 0;
+	if (new_size > arr->size)
+		return array_list_expand_internal(arr, new_size);
+	if (new_size == 0)
+		new_size = 1;
+
+	if (!(t = realloc(arr->array, new_size * sizeof(void *))))
+		return -1;
+	arr->array = (void **)t;
+	arr->size = new_size;
+	return 0;
+}
+
 //static inline int _array_list_put_idx(struct array_list *arr, size_t idx, void *data)
 int array_list_put_idx(struct array_list *arr, size_t idx, void *data)
 {
@@ -165,6 +190,8 @@ int array_list_del_idx(struct array_list *arr, size_t idx, size_t count)
 		return -1;
 	for (i = idx; i < stop; ++i)
 	{
+		// Because put_idx can skip entries, we need to check if
+		// there's actually anything in each slot we're erasing.
 		if (arr->array[i])
 			arr->free_fn(arr->array[i]);
 	}

arraylist.h

@@ -37,8 +37,27 @@ struct array_list
 };
 typedef struct array_list array_list;
 
+/**
+ * Allocate an array_list of the default size (32).
+ * @deprecated Use array_list_new2() instead.
+ */
 extern struct array_list *array_list_new(array_list_free_fn *free_fn);
 
+/**
+ * Allocate an array_list of the desired size.
+ *
+ * If possible, the size should be chosen to closely match
+ * the actual number of elements expected to be used.
+ * If the exact size is unknown, there are tradeoffs to be made:
+ * - too small - the array_list code will need to call realloc() more
+ *   often (which might incur an additional memory copy).
+ * - too large - will waste memory, but that can be mitigated
+ *   by calling array_list_shrink() once the final size is known.
+ *
+ * @see array_list_shrink
+ */
+extern struct array_list *array_list_new2(array_list_free_fn *free_fn, int initial_size);
+
 extern void array_list_free(struct array_list *al);
 
 extern void *array_list_get_idx(struct array_list *al, size_t i);
@@ -56,6 +75,14 @@ extern void *array_list_bsearch(const void **key, struct array_list *arr,
 
 extern int array_list_del_idx(struct array_list *arr, size_t idx, size_t count);
 
+
+/**
+ * Shrink the array list to just enough to fit the number of elements in it,
+ * plus empty_slots.
+ */
+extern int array_list_shrink(struct array_list *arr, size_t empty_slots);
+
+
 #ifdef __cplusplus
 }
 #endif

json_object.c

@@ -1431,11 +1431,15 @@ static void json_object_array_delete(struct json_object *jso)
 }
 
 struct json_object *json_object_new_array(void)
+{
+	return json_object_new_array_ext(ARRAY_LIST_DEFAULT_SIZE);
+}
+struct json_object *json_object_new_array_ext(int initial_size)
 {
 	struct json_object_array *jso = JSON_OBJECT_NEW(array);
 	if (!jso)
 		return NULL;
-	jso->c_array = array_list_new(&json_object_array_entry_free);
+	jso->c_array = array_list_new2(&json_object_array_entry_free, initial_size);
 	if (jso->c_array == NULL)
 	{
 		free(jso);
@@ -1523,6 +1527,13 @@ static int json_array_equal(struct json_object *jso1, struct json_object *jso2)
 	return 1;
 }
 
+int json_object_array_shrink(struct json_object *jso, int empty_slots)
+{
+	if (empty_slots < 0)
+		json_abort("json_object_array_shrink called with negative empty_slots");
+	return array_list_shrink(JC_ARRAY(jso)->c_array, empty_slots);
+}
+
 struct json_object *json_object_new_null(void)
 {
 	return NULL;

json_object.h

@@ -500,10 +500,16 @@ JSON_EXPORT void json_object_object_del(struct json_object *obj, const char *key
 /* Array type methods */
 
 /** Create a new empty json_object of type json_type_array
+ * If you know the array size you'll need ahead of time, use
+ * json_object_new_array_ext() instead.
+ * @see json_object_new_array_ext()
+ * @see json_object_array_shrink()
  * @returns a json_object of type json_type_array
  */
 JSON_EXPORT struct json_object *json_object_new_array(void);
 
+JSON_EXPORT struct json_object *json_object_new_array_ext(int initial_size);
+
 /** Get the arraylist of a json_object of type json_type_array
  * @param obj the json_object instance
  * @returns an arraylist
@@ -595,6 +601,15 @@ JSON_EXPORT struct json_object *json_object_array_get_idx(const struct json_obje
  */
 JSON_EXPORT int json_object_array_del_idx(struct json_object *obj, size_t idx, size_t count);
 
+/**
+ * Shrink the internal memory allocation of the array to just
+ * enough to fit the number of elements in it, plus empty_slots.
+ *
+ * @param jso the json_object instance, must be json_type_array
+ * @param empty_slots the number of empty slots to leave allocated
+ */
+JSON_EXPORT int json_object_array_shrink(struct json_object *jso, int empty_slots);
+
 /* json_bool type methods */
 
 /** Create a new empty json_object of type json_type_boolean

json_tokener.c

@@ -964,6 +964,9 @@ struct json_object *json_tokener_parse_ex(struct json_tokener *tok, const char *
 		case json_tokener_state_array:
 			if (c == ']')
 			{
+				// Minimize memory usage; assume parsed objs are unlikely to be changed
+				json_object_array_shrink(current, 0);
+
 				if (state == json_tokener_state_array_after_sep &&
 				    (tok->flags & JSON_TOKENER_STRICT))
 				{
@@ -997,6 +1000,9 @@ struct json_object *json_tokener_parse_ex(struct json_tokener *tok, const char *
 		case json_tokener_state_array_sep:
 			if (c == ']')
 			{
+				// Minimize memory usage; assume parsed objs are unlikely to be changed
+				json_object_array_shrink(current, 0);
+
 				saved_state = json_tokener_state_finish;
 				state = json_tokener_state_eatws;
 			}