diff --git a/src/lib_test.ts b/src/lib_test.ts
new file mode 100644
index 0000000..7d19b41
--- /dev/null
+++ b/src/lib_test.ts
@@ -0,0 +1,72 @@
+import { assertEquals } from "jsr:@std/assert";
+import { gcd } from "./lib.ts";
+
+Deno.test("gcd - basic calculations", () => {
+  assertEquals(gcd(12, 8), 4);
+  assertEquals(gcd(54, 24), 6);
+  assertEquals(gcd(48, 18), 6);
+  assertEquals(gcd(100, 50), 50);
+});
+
+Deno.test("gcd - zero values", () => {
+  assertEquals(gcd(0, 5), 5);
+  assertEquals(gcd(5, 0), 5);
+  assertEquals(gcd(0, 0), 0);
+});
+
+Deno.test("gcd - negative numbers", () => {
+  // gcd should handle negative numbers by converting to absolute values
+  assertEquals(gcd(-12, 8), 4);
+  assertEquals(gcd(12, -8), 4);
+  assertEquals(gcd(-12, -8), 4);
+  assertEquals(gcd(-54, 24), 6);
+});
+
+Deno.test("gcd - same numbers", () => {
+  assertEquals(gcd(7, 7), 7);
+  assertEquals(gcd(1, 1), 1);
+  assertEquals(gcd(100, 100), 100);
+});
+
+Deno.test("gcd - coprime numbers", () => {
+  // Numbers with no common divisor except 1
+  assertEquals(gcd(13, 17), 1);
+  assertEquals(gcd(7, 11), 1);
+  assertEquals(gcd(25, 49), 1);
+});
+
+Deno.test("gcd - one is 1", () => {
+  assertEquals(gcd(1, 5), 1);
+  assertEquals(gcd(5, 1), 1);
+});
+
+Deno.test("gcd - large numbers", () => {
+  assertEquals(gcd(1000, 500), 500);
+  assertEquals(gcd(1071, 462), 21);
+  assertEquals(gcd(270, 192), 6);
+});
+
+Deno.test("gcd - decimal numbers (should be rounded)", () => {
+  // The function rounds inputs, so decimals should work
+  // 12.7 rounds to 13, 8.3 rounds to 8 -> gcd(13, 8) = 1
+  assertEquals(gcd(12.7, 8.3), 1);
+  // 54.9 rounds to 55, 24.1 rounds to 24 -> gcd(55, 24) = 1
+  assertEquals(gcd(54.9, 24.1), 1);
+  // 10.5 rounds to 11, 5.5 rounds to 6 -> gcd(11, 6) = 1
+  assertEquals(gcd(10.5, 5.5), 1);
+  // 12.4 rounds to 12, 8.4 rounds to 8 -> gcd(12, 8) = 4
+  assertEquals(gcd(12.4, 8.4), 4);
+});
+
+Deno.test("gcd - prime numbers", () => {
+  assertEquals(gcd(17, 19), 1);
+  assertEquals(gcd(23, 29), 1);
+  assertEquals(gcd(11, 13), 1);
+});
+
+Deno.test("gcd - powers of 2", () => {
+  assertEquals(gcd(16, 8), 8);
+  assertEquals(gcd(32, 16), 16);
+  assertEquals(gcd(64, 32), 32);
+  assertEquals(gcd(128, 256), 128);
+});