diff --git a/test.ts b/src/index.test.ts
similarity index 97%
rename from test.ts
rename to src/index.test.ts
index 2941e2a..73b8917 100644
--- a/test.ts
+++ b/src/index.test.ts
@@ -1,10 +1,11 @@
 import { describe, it, expect } from 'vitest';
 import { request } from 'undici';
-import { diffInYears } from './src/utils';
+import { diffInYears } from './utils';
 
-const Personnummer = process.env.FILE?.includes('cjs')
-  ? require(process.env.FILE)
-  : (await import(process.env.FILE)).default;
+const file = process.env.FILE || '.';
+const Personnummer = file.includes('cjs')
+  ? require(file)
+  : (await import(file)).default;
 
 const availableListFormats = [
   'long_format',
diff --git a/src/utils.test.ts b/src/utils.test.ts
new file mode 100644
index 0000000..6763bc7
--- /dev/null
+++ b/src/utils.test.ts
@@ -0,0 +1,104 @@
+import { describe, it, expect } from 'vitest';
+import { compareAsc } from 'date-fns';
+import { diffInYears, luhn, testDate } from './utils';
+
+describe('diffInYears', () => {
+  it('should return 0 if both dates are the same', () => {
+    const date = new Date();
+    expect(diffInYears(date, date)).toBe(0);
+  });
+
+  it('should return 1 if the second date is exactly one year ahead of the first date', () => {
+    const dateLeft = new Date(2021, 1, 1);
+    const dateRight = new Date(2020, 1, 1);
+    expect(diffInYears(dateLeft, dateRight)).toBe(1);
+  });
+
+  it('should return -1 if the second date is exactly one year behind the first date', () => {
+    const dateLeft = new Date(2020, 1, 1);
+    const dateRight = new Date(2021, 1, 1);
+    expect(diffInYears(dateLeft, dateRight)).toBe(-1);
+  });
+
+  it('should return the correct difference for dates with different years', () => {
+    const dateLeft = new Date(2023, 1, 1);
+    const dateRight = new Date(2020, 1, 1);
+    expect(diffInYears(dateLeft, dateRight)).toBe(3);
+  });
+
+  it('should return the correct difference for dates with the same year but different months', () => {
+    const dateLeft = new Date(2020, 0, 1);
+    const dateRight = new Date(2020, 11, 31);
+    expect(diffInYears(dateLeft, dateRight)).toBe(0);
+  });
+
+  it('should return the correct difference for dates with the same year and month but different days', () => {
+    const dateLeft = new Date(2020, 0, 1);
+    const dateRight = new Date(2020, 0, 31);
+    expect(diffInYears(dateLeft, dateRight)).toBe(0);
+  });
+});
+
+describe('luhn', () => {
+  it('should return 0 for an empty string', () => {
+    expect(luhn('')).toBe(0);
+  });
+
+  it('should return 0 for a non-numeric string', () => {
+    expect(luhn('abcd')).toBe(0);
+  });
+
+  it('should return 0 for a string with non-numeric characters', () => {
+    expect(luhn('1234abcd')).toBe(0);
+  });
+
+  it('should return the correct check digit for a valid number', () => {
+    expect(luhn('1212121212')).toBe(0);
+  });
+
+  it('should return the correct check digit for a long valid number', () => {
+    expect(luhn('1234567812345670')).toBe(0);
+  });
+})
+
+describe('testDate', () => {
+  it('should return true for a valid date', () => {
+    expect(testDate(2024, 2, 14)).toBe(true);
+  });
+
+  it('should return false for an invalid date with a non-existent month', () => {
+    expect(testDate(2024, 13, 14)).toBe(false);
+  });
+
+  it('should return false for an invalid date with a non-existent day in February', () => {
+    expect(testDate(2024, 2, 30)).toBe(false);
+  });
+
+  it('should return false for an invalid date with a non-existent day in April', () => {
+    expect(testDate(2024, 4, 31)).toBe(false);
+  });
+
+  it('should return false for an invalid date with a non-existent day in June', () => {
+    expect(testDate(2024, 6, 31)).toBe(false);
+  });
+
+  it('should return false for an invalid date with a non-existent day in September', () => {
+    expect(testDate(2024, 9, 31)).toBe(false);
+  });
+
+  it('should return false for an invalid date with a non-existent day in November', () => {
+    expect(testDate(2024, 11, 31)).toBe(false);
+  });
+
+  it('should return true for a valid date in a leap year February', () => {
+    expect(testDate(2024, 1, 28)).toBe(true);
+  });
+
+  it('should return true for a valid date in a non-leap year February', () => {
+    expect(testDate(2023, 1, 28)).toBe(true);
+  });
+
+  it('should return true for a valid date in a leap year February', () => {
+    expect(testDate(2024, 2, 29)).toBe(true);
+  });
+})
diff --git a/src/utils.ts b/src/utils.ts
index 77a03d0..e3f83d5 100644
--- a/src/utils.ts
+++ b/src/utils.ts
@@ -52,7 +52,7 @@ export const luhn = (str: string): number => {
     sum += v;
   }
 
-  return Math.ceil(sum / 10) * 10 - sum;
+  return Math.ceil(sum / 10) * 10 - sum || 0;
 };
 
 /**
diff --git a/vitest.config.mjs b/vitest.config.mjs
deleted file mode 100644
index d0822a1..0000000
--- a/vitest.config.mjs
+++ /dev/null
@@ -1,7 +0,0 @@
-import { defineConfig } from 'vitest/config';
-
-export default defineConfig({
-  test: {
-    include: ['./test.ts'],
-  },
-});