Add some helpers for resizing dynamic arrays to allow them to both not grow if they're at capacity or shrink if significantly below capacity.

git-svn-id: https://www.unprompted.com/svn/projects/tildefriends/trunk@3902 ed5197a5-7fde-0310-b194-c3ffbd925b24

src/mem.c

@@ -154,6 +154,32 @@ char* tf_strdup(const char* string)
 	return buffer;
 }
 
+void* tf_resize_vec(void* ptr, size_t size)
+{
+	void* alloc_ptr = ptr ? (void*)((intptr_t)ptr - sizeof(size_t)) : NULL;
+	size_t alloc_size = 0;
+	if (alloc_ptr)
+	{
+		memcpy(&alloc_size, alloc_ptr, sizeof(size_t));
+	}
+
+	if (alloc_size >= 16 * size + sizeof(size_t))
+	{
+		/* If we've dropped significantly in size, resize down. */
+		return tf_realloc(ptr, size);
+	}
+	else if (alloc_size >= size + sizeof(size_t))
+	{
+		/* Otherwise, if we're big enough, stay the same size. */
+		return ptr;
+	}
+	else
+	{
+		/* If we need to grow, overallocate 2x to give room to continue growing. */
+		return tf_realloc(ptr, size * 2);
+	}
+}
+
 size_t tf_mem_get_tf_malloc_size()
 {
 	return s_tf_malloc_size;