Update README.

Author: trekhleb

README.md

@@ -31,6 +31,7 @@
   * [Fibonacci Number](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/fibonacci)
   * [Cartesian Product](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/cartesian-product)
   * [Power Set](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/power-set)
+  * [Primality Tests](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/math/primality-tests)
   * Collatz Conjecture algorithm
   * Extended Euclidean algorithm
   * Euclidean algorithm to calculate the Greatest Common Divisor (GCD)
@@ -39,7 +40,6 @@
   * Greatest Difference
   * Least Common Multiple
   * Newton's square
-  * Primality Test
   * Shannon Entropy
 * **String**
   * [String Permutations](https://github.com/trekhleb/javascript-algorithms/tree/master/src/algorithms/string/permutations)

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

@@ -0,0 +1,14 @@
+# Primality Test
+
+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 
+input number is prime or not. Factorization is thought to be 
+a computationally difficult problem, whereas primality testing 
+is comparatively easy (its running time is polynomial in the 
+size of the input). 
+
+## References
+
+[Wikipedia](https://en.wikipedia.org/wiki/Primality_test)

src/algorithms/math/primality-tests/__test__/primalityTest.js

@@ -0,0 +1,22 @@
+/**
+ * @param {function(n: number)} testFunction
+ */
+export default function primalityTest(testFunction) {
+  expect(testFunction(1)).toBeTruthy();
+  expect(testFunction(2)).toBeTruthy();
+  expect(testFunction(3)).toBeTruthy();
+  expect(testFunction(5)).toBeTruthy();
+  expect(testFunction(11)).toBeTruthy();
+  expect(testFunction(191)).toBeTruthy();
+  expect(testFunction(191)).toBeTruthy();
+  expect(testFunction(199)).toBeTruthy();
+
+  expect(testFunction(-1)).toBeFalsy();
+  expect(testFunction(0)).toBeFalsy();
+  expect(testFunction(4)).toBeFalsy();
+  expect(testFunction(6)).toBeFalsy();
+  expect(testFunction(12)).toBeFalsy();
+  expect(testFunction(192)).toBeFalsy();
+  expect(testFunction(200)).toBeFalsy();
+  expect(testFunction(400)).toBeFalsy();
+}

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

@@ -0,0 +1,8 @@
+import trialDivision from '../trialDivision';
+import primalityTest from './primalityTest';
+
+describe('trialDivision', () => {
+  it('should detect prime numbers', () => {
+    primalityTest(trialDivision);
+  });
+});

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

@@ -0,0 +1,28 @@
+/**
+ * @param {number} number
+ * @return {boolean}
+ */
+export default function trialDivision(number) {
+  if (number <= 0) {
+    // If number is less then one then it isn't prime by definition.
+    return false;
+  } else if (number <= 3) {
+    // All numbers from 1 to 3 are prime.
+    return true;
+  }
+
+  // If the number is not divided by 2 then we may eliminate all further even dividers.
+  if (number % 2 === 0) {
+    return false;
+  }
+
+  // If there is no dividers up to square root of n then there is no higher dividers as well.
+  const dividerLimit = Math.sqrt(number);
+  for (let divider = 3; divider < dividerLimit; divider += 2) {
+    if (number % divider === 0) {
+      return false;
+    }
+  }
+
+  return true;
+}