using System.Collections.Generic;

using NUnit.Framework;

​

public class CharacterCounter

{

    public static Dictionary<char, int> CountCharacterOccurrences(string input)

    {

        // Input validation: Check for null or empty input

        if (string.IsNullOrEmpty(input))

        {

            return new Dictionary<char, int>(); // Return an empty dictionary for null or empty input

        }

​

        Dictionary<char, int> charCounts = new Dictionary<char, int>();

​

        // Iterate through the string and count character occurrences

        foreach (char c in input)

        {

            // Convert character to lowercase to count uppercase and lowercase characters together (optional)

            char lowerC = char.ToLower(c);

​

            if (charCounts.ContainsKey(lowerC))

            {

                charCounts[lowerC]++;

            }

            else

            {

                charCounts[lowerC] = 1;

            }

        }

​

        return charCounts;

    }

}

​

[TestFixture]

public class CharacterCounterTests

{

    [Test]

    public void CountCharacterOccurrences_EmptyString_ReturnsEmptyDictionary()

    {

        Dictionary<char, int> result = CharacterCounter.CountCharacterOccurrences("");

        Assert.That(result, Is.Empty);

    }

​

    [Test]

    public void CountCharacterOccurrences_NullString_ReturnsEmptyDictionary()

    {

        Dictionary<char, int> result = CharacterCounter.CountCharacterOccurrences(null);

        Assert.That(result, Is.Empty);

    }

​

    [Test]

    public void CountCharacterOccurrences_SingleCharacter_ReturnsDictionaryWithCountOne()

    {

        Dictionary<char, int> result = CharacterCounter.CountCharacterOccurrences("a");

        Assert.That(result, Is.EqualTo(new Dictionary<char, int>() { { 'a', 1 } }));

    }

​

    [Test]

    public void CountCharacterOccurrences_SimpleString_ReturnsCorrectCounts()

    {

        Dictionary<char, int> result = CharacterCounter.CountCharacterOccurrences("hello");

        Assert.That(result, Is.EqualTo(new Dictionary<char, int>() { { 'h', 1 }, { 'e', 1 }, { 'l', 2 }, { 'o', 1 } }));

    }

​

    [Test]

    public void CountCharacterOccurrences_StringWithSpaces_ReturnsCorrectCounts()

    {

        Dictionary<char, int> result = CharacterCounter.CountCharacterOccurrences("hello world");

        Assert.That(result, Is.EqualTo(new Dictionary<char, int>() { { 'h', 1 }, { 'e', 1 }, { 'l', 3 }, { 'o', 2 }, { ' ', 1 }, { 'w', 1 }, { 'r', 1 }, { 'd', 1 } }));

    }

​

     [Test]

    public void CountCharacterOccurrences_CaseInsensitiveString_ReturnsCorrectCounts()

    {

        Dictionary<char, int> result = CharacterCounter.CountCharacterOccurrences("Hello");

        Assert.That(result, Is.EqualTo(new Dictionary<char, int>() { { 'h', 1 }, { 'e', 1 }, { 'l', 2 }, { 'o', 1 } }));

    }

​

    [Test]

    public void CountCharacterOccurrences_StringWithSpecialCharacters_ReturnsCorrectCounts()

    {

        Dictionary<char, int> result = CharacterCounter.CountCharacterOccurrences("a!@#a");

        Assert.That(result, Is.EqualTo(new Dictionary<char, int>() { { 'a', 2 }, { '!', 1 }, { '@', 1 }, { '#', 1 } }));

    }

}

​

public class Program

{

    public static void Main()

    {

        new NUnitLite.AutoRun().Execute(["--noc" /*no colored output for .Net Fiddle*/]);

    }

}