Add algorithm IsPrimeNumber (#562)

Author: khanhkhanhlele

Algorithms.Tests/Numeric/PrimeNumberTest.cs

@@ -0,0 +1,37 @@
+using Algorithms.Numeric;
+
+namespace Algorithms.Tests.Numeric;
+
+public static class PrimeNumberTests
+{
+    /// <summary>
+    ///     Tests the PrimeChecker.IsPrime method with various inputs (primes, composites, edge cases)
+    ///     to ensure the result is correct.
+    /// </summary>
+    [TestCase(-5, ExpectedResult = false)] // Negative number
+    [TestCase(0, ExpectedResult = false)] // Zero
+    [TestCase(1, ExpectedResult = false)] // One
+    [TestCase(2, ExpectedResult = true)] // Smallest prime
+    [TestCase(3, ExpectedResult = true)] // Prime
+    [TestCase(4, ExpectedResult = false)] // Composite (2*2)
+    [TestCase(7, ExpectedResult = true)] // Prime
+    [TestCase(9, ExpectedResult = false)] // Composite (3*3)
+    [TestCase(13, ExpectedResult = true)] // Prime
+    [TestCase(15, ExpectedResult = false)] // Composite (3*5)
+    [TestCase(25, ExpectedResult = false)] // Composite (5*5)
+    [TestCase(29, ExpectedResult = true)] // Prime
+    [TestCase(35, ExpectedResult = false)] // Composite (5*7)
+    [TestCase(49, ExpectedResult = false)] // Composite (7*7)
+    [TestCase(97, ExpectedResult = true)] // Larger prime
+    [TestCase(100, ExpectedResult = false)] // Larger composite
+    public static bool IsPrime_ResultIsCorrect(int number)
+    {
+        // Arrange is implicit here
+
+        // Act
+        var result = PrimeChecker.IsPrime(number);
+
+        // Assert
+        return result;
+    }
+}

Algorithms/Numeric/PrimeChecker.cs

@@ -0,0 +1,55 @@
+namespace Algorithms.Numeric;
+
+/// <summary>
+///     A prime number (or a prime) is a natural number greater than 1 that is not a product of two smaller natural numbers.
+/// </summary>
+public static class PrimeChecker
+{
+    /// <summary>
+    ///     Checks if a number is a prime number or not using optimized trial division.
+    ///     This method avoids checking multiples of 2 and 3, optimizing the loop by checking
+    ///     divisors of the form 6k ± 1 up to the square root of the number.
+    /// </summary>
+    /// <param name="number">The integer number to check.</param>
+    /// <returns>True if the number is prime; False otherwise.</returns>
+    public static bool IsPrime(int number)
+    {
+        // Numbers less than or equal to 1 are not prime.
+        if (number <= 1)
+        {
+            return false;
+        }
+
+        // 2 and 3 are the first two prime numbers.
+        if (number <= 3)
+        {
+            return true;
+        }
+
+        // Check for divisibility by 2 and 3.
+        if (number % 2 == 0 || number % 3 == 0)
+        {
+            return false;
+        }
+
+        // Check for divisibility by numbers of the form 6k ± 1 up to sqrt(number).
+        // The loop increments by 6 to skip known non-prime divisors.
+        for (int i = 5; i <= number / i; i = i + 6)
+        {
+            // Check 6k - 1
+            if (number % i == 0)
+            {
+                return false;
+            }
+
+            // Check 6k + 1
+            if (number % (i + 2) == 0)
+            {
+                return false;
+            }
+        }
+
+        // If no divisors are found, the number is prime.
+        return true;
+    }
+}