Fix bug with primality test.

Author: trekhleb

src/algorithms/math/primality-test/README.md

@@ -1,6 +1,16 @@
 # Primality Test
 
-A primality test is an algorithm for determining whether an input 
+A **prime number** (or a **prime**) is a natural number greater than `1` that 
+cannot be formed by multiplying two smaller natural numbers. A natural number 
+greater than `1` that is not prime is called a composite number. For 
+example, `5` is prime because the only ways of writing it as a 
+product, `1 × 5` or `5 × 1`, involve `5` itself. However, `6` is 
+composite because it is the product of two numbers `(2 × 3)` that are 
+both smaller than `6`. 
+
+![Prime Numbers](https://upload.wikimedia.org/wikipedia/commons/f/f0/Primes-vs-composites.svg)
+
+A **primality test** is an algorithm for determining whether an input 
 number is prime. Among other fields of mathematics, it is used 
 for cryptography. Unlike integer factorization, primality tests 
 do not generally give prime factors, only stating whether the 
@@ -11,4 +21,5 @@ size of the input).
 
 ## References
 
-[Wikipedia](https://en.wikipedia.org/wiki/Primality_test)
+- [Prime Numbers on Wikipedia](https://en.wikipedia.org/wiki/Prime_number)
+- [Primality Test on Wikipedia](https://en.wikipedia.org/wiki/Primality_test)

src/algorithms/math/primality-test/__test__/trialDivision.test.js

@@ -23,6 +23,11 @@ function primalityTest(testFunction) {
   expect(testFunction(192)).toBeFalsy();
   expect(testFunction(200)).toBeFalsy();
   expect(testFunction(400)).toBeFalsy();
+
+  // It should also deal with floats.
+  expect(testFunction(0.5)).toBeFalsy();
+  expect(testFunction(1.3)).toBeFalsy();
+  expect(testFunction(10.5)).toBeFalsy();
 }
 
 describe('trialDivision', () => {

src/algorithms/math/primality-test/trialDivision.js

@@ -3,6 +3,11 @@
  * @return {boolean}
  */
 export default function trialDivision(number) {
+  // Check if number is integer.
+  if (number % 1 !== 0) {
+    return false;
+  }
+
   if (number <= 1) {
     // If number is less than one then it isn't prime by definition.
     return false;