It works, basic but works, need to move out logic to places, like ASTNode array shouldn't be on the lexer or future logic for fractions and error handling in the evaluator. Just the things at the top of my head

test/test_evaluator.c

@@ -0,0 +1,32 @@
+#include "lexer.h"
+#include "parser.h"
+#include "evaluator.h"
+#include <stdarg.h>
+#include <stdbool.h>
+#include <stddef.h>
+#include <stdint.h>
+#include <setjmp.h>
+#include <cmocka.h>
+#include <stdlib.h>
+
+static void test_basic_evaluation(void** state) {
+    (void) state;
+
+    char expr[256] = "2 + 4 * 40 / 2";
+    ASTNodeArray context;
+
+    tokenize(expr, &context);
+    AST tree = parse(&context);
+    uint64_t value = evaluate(tree.head);
+
+    assert_int_equal(value, 82);
+}
+
+int main(void) {
+    const struct CMUnitTest tests[] = {
+        cmocka_unit_test(test_basic_evaluation),
+    };
+
+    cmocka_run_group_tests(tests, NULL, NULL);
+    return EXIT_SUCCESS;
+}