first draft of arena resize

arena_allocator.c

@@ -1,5 +1,6 @@
 #include "cig.h"
 #include <stdbool.h>
+#include <assert.h>
 
 static void arena_reset(arena_allocator_t *this) {
 	if (0 < this->bytes_outside_data) {
@@ -31,8 +32,41 @@ static void *arena_alloc(arena_allocator_t *this, size_t bytes) {
 }
 
 static void *arena_resize(arena_allocator_t *this, void *old_ptr, size_t bytes) {
-	bool is_old_ptr_in_data = this->data <= old_ptr && old_ptr < &this->data[this->size]
+	uint8_t *old_ptr_b = (uint8_t *) old_ptr;
-	// TODO: implement this
+
+	bool is_old_ptr_in_data =
+		this->data <= old_ptr_b &&
+		old_ptr_b < &this->data[this->capacity];
+
+	if (is_old_ptr_in_data) {
+		assert(
+			old_ptr_b < &this->data[this->size] &&
+			"You cannot resize a pointer you got from before this allocator was reset.\n"
+			"You should not store pointer gotten from this allocator across resets.\n"
+		);
+	}
+
+	if (!is_old_ptr_in_data) {
+		// just a guess for how many extra bytes were allocated.
+		this->bytes_outside_data += bytes / 2;
+		return allocator_resize(this->allocator, old_ptr, bytes);
+	}
+
+	uint8_t *new_ptr_b = arena_alloc(this, bytes);
+	if (new_ptr_b == NULL) {
+		return NULL;
+	}
+
+	// sizeof not necessary but ill do it anyways.
+	size_t old_index = ((size_t)old_ptr - (size_t)this->data) / sizeof(*this->data);
+	size_t bytes_til_buffer_end = old_index + this->size;
+	size_t max_bytes_to_copy = bytes < bytes_til_buffer_end
+		? bytes
+		: bytes_til_buffer_end;
+	for (size_t i = old_index; i < old_index + max_bytes_to_copy; i++) {
+		new_ptr_b[i] = this->data[i];
+	}
+	return new_ptr_b;
 }
 
 // typedef struct arena_allocator {