util/array: Add array_realloc for wl_array

array_realloc will grow the array for the target size like wl_insert_add, but
will also shrink the array if the target size is sufficiently smaller than the
current allocation.

include/util/array.h

@@ -27,4 +27,9 @@ bool set_remove(uint32_t values[], size_t *len, size_t cap, uint32_t target);
  */
 void array_remove_at(struct wl_array *arr, size_t offset, size_t size);
 
+/**
+ * Grow or shrink the array to fit the specifized size.
+ */
+bool array_realloc(struct wl_array *arr, size_t size);
+
 #endif

util/array.c

@@ -55,3 +55,32 @@ void array_remove_at(struct wl_array *arr, size_t offset, size_t size) {
 	memmove(&data[offset], &data[offset + size], arr->size - offset - size);
 	arr->size -= size;
 }
+
+bool array_realloc(struct wl_array *arr, size_t size) {
+	// If the size is less than 1/4th of the allocation size, we shrink it.
+	// 1/4th is picked to provide hysteresis, without which an array with size
+	// arr->alloc would constantly reallocate if an element is added and then
+	// removed continously.
+	size_t alloc;
+	if (arr->alloc > 0 && size > arr->alloc / 4) {
+		alloc = arr->alloc;
+	} else {
+		alloc = 16;
+	}
+
+	while (alloc < size) {
+		alloc *= 2;
+	}
+
+	if (alloc == arr->alloc) {
+		return true;
+	}
+
+	void *data = realloc(arr->data, alloc);
+	if (data == NULL) {
+		return false;
+	}
+	arr->data = data;
+	arr->alloc = alloc;
+	return true;
+}