diff --git a/lib/utils/math-utils.js b/lib/utils/math-utils.js
new file mode 100644
--- /dev/null
+++ b/lib/utils/math-utils.js
@@ -0,0 +1,13 @@
+// @flow
+
+function leastPositiveResidue(number: number, modulus: number): number {
+  // the smallest positive of x is integer k such that
+  // x is congruent to k (mod n)
+  // in our case we only consider n > 0
+  if (modulus <= 0) {
+    throw new Error(`modulus must be greater than 0, but was ${modulus}`);
+  }
+  return number - Math.floor(number / modulus) * modulus;
+}
+
+export { leastPositiveResidue };
diff --git a/lib/utils/math-utils.test.js b/lib/utils/math-utils.test.js
new file mode 100644
--- /dev/null
+++ b/lib/utils/math-utils.test.js
@@ -0,0 +1,19 @@
+// @flow
+
+import { leastPositiveResidue } from './math-utils';
+
+describe('leastPositiveResidue', () => {
+  it('x should be equal to 2 for x ≡ 8 (mod 3)', () => {
+    expect(leastPositiveResidue(8, 3)).toEqual(2);
+  });
+
+  it('x should be equal to 5 for x ≡ 19 (mod 7)', () => {
+    expect(leastPositiveResidue(19, 7)).toEqual(5);
+  });
+
+  it('function should throw error', () => {
+    expect(() => leastPositiveResidue(5, -1)).toThrow(
+      'modulus must be greater than 0, but was -1',
+    );
+  });
+});