diff --git a/arena_allocator.c b/arena_allocator.c
index 6a12df1..526df1d 100644
--- a/arena_allocator.c
+++ b/arena_allocator.c
@@ -57,14 +57,14 @@ static void *arena_resize(arena_allocator_t *this, void *old_ptr, size_t bytes)
 		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];
+	// Calculate how many bytes from old_ptr to the end of currently used arena space
+	size_t old_offset = old_ptr_b - this->data;
+	size_t bytes_from_old_to_end = this->size - old_offset;
+	// Copy the smaller of: new allocation size or remaining old data
+	size_t bytes_to_copy = bytes < bytes_from_old_to_end ? bytes : bytes_from_old_to_end;
+	
+	for (size_t i = 0; i < bytes_to_copy; i++) {
+		new_ptr_b[i] = old_ptr_b[i];
 	}
 	return new_ptr_b;
 }
diff --git a/test_arena_allocator.c b/test_arena_allocator.c
index 3fb3e55..be5a4b3 100644
--- a/test_arena_allocator.c
+++ b/test_arena_allocator.c
@@ -1,8 +1,9 @@
 #include <criterion/criterion.h>
 #include <criterion/internal/assert.h>
+#include <string.h>
 #include "cig.h"
 
-Test(arena_allocator, test) {
+Test(arena_allocator, basic_allocation) {
 	with_borrow(backing) {
 		allocator_t arena = arena_allocator_create(backing, 4 * MB);
 		for ( size_t i = 0; i < 1000; i++ ) {
@@ -77,3 +78,246 @@ Test(arena_allocator, test) {
 		cr_assert_eq(concrete_arena->capacity, expected_capacity);
 	}
 }
+
+Test(arena_allocator, resize_preserves_data) {
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 1 * KB);
+		allocator_reset(arena);
+		
+		// Allocate a buffer and fill it with data
+		int *data = allocator_alloc(arena, sizeof(int) * 10);
+		for (int i = 0; i < 10; i++) {
+			data[i] = i * 100;
+		}
+		
+		// Resize to larger size - should preserve existing data
+		int *resized = allocator_resize(arena, data, sizeof(int) * 20);
+		
+		// Verify old data is preserved
+		for (int i = 0; i < 10; i++) {
+			cr_assert_eq(resized[i], i * 100, "Expected resized[%d] = %d, got %d", i, i * 100, resized[i]);
+		}
+		
+		// Fill the new space
+		for (int i = 10; i < 20; i++) {
+			resized[i] = i * 100;
+		}
+		
+		// Verify all data
+		for (int i = 0; i < 20; i++) {
+			cr_assert_eq(resized[i], i * 100);
+		}
+	}
+}
+
+Test(arena_allocator, resize_smaller_preserves_partial_data) {
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 1 * KB);
+		allocator_reset(arena);
+		
+		// Allocate a buffer and fill it with data
+		int *data = allocator_alloc(arena, sizeof(int) * 20);
+		for (int i = 0; i < 20; i++) {
+			data[i] = i * 7;
+		}
+		
+		// Resize to smaller size - should preserve first N elements
+		int *resized = allocator_resize(arena, data, sizeof(int) * 10);
+		
+		// Verify first 10 elements are preserved
+		for (int i = 0; i < 10; i++) {
+			cr_assert_eq(resized[i], i * 7, "Expected resized[%d] = %d, got %d", i, i * 7, resized[i]);
+		}
+	}
+}
+
+Test(arena_allocator, resize_with_zero_capacity) {
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 1 * KB);
+		allocator_reset(arena);
+		
+		// Simulate dyn_array with capacity 0 behavior
+		typedef struct {
+			size_t size;
+			size_t capacity;
+			size_t itemsize;
+			allocator_t allocator;
+			uint8_t bytes[];
+		} test_header_t;
+		
+		// Allocate just the header (capacity = 0)
+		test_header_t *header = allocator_alloc(arena, sizeof(test_header_t));
+		header->size = 0;
+		header->capacity = 0;
+		header->itemsize = sizeof(int);
+		
+		// Resize to add space for 1 element
+		header = allocator_resize(arena, header, sizeof(test_header_t) + sizeof(int) * 1);
+		cr_assert_neq(header, NULL);
+		header->capacity = 1;
+		
+		// Add an element
+		int *data = (int*)header->bytes;
+		data[0] = 42;
+		header->size = 1;
+		
+		// Resize again to capacity 2
+		header = allocator_resize(arena, header, sizeof(test_header_t) + sizeof(int) * 2);
+		cr_assert_neq(header, NULL);
+		header->capacity = 2;
+		
+		// Verify first element is preserved
+		data = (int*)header->bytes;
+		cr_assert_eq(data[0], 42);
+		
+		// Add second element
+		data[1] = 99;
+		header->size = 2;
+		
+		// Verify both elements
+		cr_assert_eq(data[0], 42);
+		cr_assert_eq(data[1], 99);
+	}
+}
+
+Test(arena_allocator, dyn_array_resize_pattern) {
+	// Test the actual pattern used by dynamic arrays
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 10 * KB);
+		allocator_reset(arena);
+		
+		int *arr = dyn_array_create(arena, int);
+		
+		// Add 100 elements, forcing multiple resizes
+		for (int i = 0; i < 100; i++) {
+			dyn_array_append(arr, i * 3);
+		}
+		
+		// Verify all elements
+		cr_assert_eq(dyn_array_length(arr), 100);
+		for (int i = 0; i < 100; i++) {
+			cr_assert_eq(arr[i], i * 3, "Expected arr[%d] = %d, got %d", i, i * 3, arr[i]);
+		}
+	}
+}
+
+Test(arena_allocator, multiple_arrays_resize) {
+	// Test multiple dynamic arrays resizing independently
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 20 * KB);
+		allocator_reset(arena);
+		
+		int *arr1 = dyn_array_create(arena, int);
+		int *arr2 = dyn_array_create(arena, int);
+		
+		// Add to arr1
+		for (int i = 0; i < 50; i++) {
+			dyn_array_append(arr1, i);
+		}
+		
+		// Add to arr2
+		for (int i = 0; i < 30; i++) {
+			dyn_array_append(arr2, i * 2);
+		}
+		
+		// Add more to arr1
+		for (int i = 50; i < 100; i++) {
+			dyn_array_append(arr1, i);
+		}
+		
+		// Verify arr1
+		cr_assert_eq(dyn_array_length(arr1), 100);
+		for (int i = 0; i < 100; i++) {
+			cr_assert_eq(arr1[i], i);
+		}
+		
+		// Verify arr2
+		cr_assert_eq(dyn_array_length(arr2), 30);
+		for (int i = 0; i < 30; i++) {
+			cr_assert_eq(arr2[i], i * 2);
+		}
+	}
+}
+
+Test(arena_allocator, resize_with_string_data) {
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 1 * KB);
+		allocator_reset(arena);
+		
+		// Allocate and fill with string data
+		char *str = allocator_alloc(arena, 10);
+		strcpy(str, "Hello");
+		
+		// Resize to larger
+		str = allocator_resize(arena, str, 20);
+		
+		// Verify string is preserved
+		cr_assert_str_eq(str, "Hello");
+		
+		// Append more
+		strcat(str, " World!");
+		cr_assert_str_eq(str, "Hello World!");
+	}
+}
+
+Test(arena_allocator, nested_structures_with_resize) {
+	// Test resizing with nested pointer structures (like 2D arrays)
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 10 * KB);
+		allocator_reset(arena);
+		
+		// Create array of int pointers (like dyn_array of dyn_arrays)
+		int **rows = dyn_array_create(arena, int*);
+		
+		// Add 5 rows
+		for (int i = 0; i < 5; i++) {
+			int *row = dyn_array_create(arena, int);
+			for (int j = 0; j < 10; j++) {
+				dyn_array_append(row, i * 10 + j);
+			}
+			dyn_array_append(rows, row);
+		}
+		
+		// Verify data
+		cr_assert_eq(dyn_array_length(rows), 5);
+		for (int i = 0; i < 5; i++) {
+			cr_assert_eq(dyn_array_length(rows[i]), 10);
+			for (int j = 0; j < 10; j++) {
+				cr_assert_eq(rows[i][j], i * 10 + j);
+			}
+		}
+	}
+}
+
+Test(arena_allocator, large_resize_chain) {
+	// Test many consecutive resizes
+	with_borrow(backing) {
+		allocator_t arena = arena_allocator_create(backing, 50 * KB);
+		allocator_reset(arena);
+		
+		int *data = allocator_alloc(arena, sizeof(int) * 1);
+		data[0] = 123;
+		
+		// Resize many times, each time doubling
+		for (int iteration = 0; iteration < 10; iteration++) {
+			size_t old_size = 1 << iteration;
+			size_t new_size = 1 << (iteration + 1);
+			
+			data = allocator_resize(arena, data, sizeof(int) * new_size);
+			
+			// Verify first element is still preserved
+			cr_assert_eq(data[0], 123);
+			
+			// Fill new elements
+			for (size_t i = old_size; i < new_size; i++) {
+				data[i] = (int)i;
+			}
+		}
+		
+		// Final size should be 1024
+		cr_assert_eq(data[0], 123);
+		for (int i = 1; i < 1024; i++) {
+			cr_assert_eq(data[i], i);
+		}
+	}
+}