using System.Text;

using Xunit;

​

​

namespace Jetstream_Code_Test

{

    public class StringCompressor

    {

        public static string? Compress(string? input)

        {

            if (string.IsNullOrEmpty(input))

            {

                return input;

            }

​

            var compressed = new StringBuilder();

            var currentChar = input[0];

            var currentCharCount = 1;

​

            for (var i = 1; i < input.Length; i++)

            {

                if (input[i] == currentChar)

                {

                    currentCharCount++;

                }

                else

                {

                    AppendCompressedSequenceToResult(compressed, currentChar, currentCharCount);

​

                    currentChar = input[i];

                    currentCharCount = 1;

                }

            }

​

            AppendCompressedSequenceToResult(compressed, currentChar, currentCharCount);

​

            return compressed.ToString();

        }

​

        private static void AppendCompressedSequenceToResult(StringBuilder compressed, char currentChar, int currentCharCount)

        {

            compressed.Append(currentChar);

            if (currentCharCount > 1)

            {

                compressed.Append(currentCharCount);

            }

        }

    }

}

​

​

​

​

namespace Jetstream_Code_Test

{

    public class StringCompressorTests

    {

        [Fact]

        public void Compress_Null_ReturnsNull()

        {

            var result = StringCompressor.Compress(null);

​

            Assert.Null(result);

        }

​

​

        [Theory]

        [InlineData("", "")]

        [InlineData("a", "a")]

        [InlineData("aa", "a2")]

        [InlineData("aaabccccdd", "a3bc4d2")]

        [InlineData("yyyyyzzzzzzzzzz", "y5z10")]

        [InlineData("abcd", "abcd")]

        [InlineData("hhhccchhhccc", "h3c3h3c3")]

        [InlineData("aaAAbbBB__A1", "a2A2b2B2_2A1")] // case sensitive

        [InlineData("a3bc4d2", "a3bc4d2")] // already compressed

​

        [InlineData(" ", " ")]

        [InlineData("  ", " 2")]

        [InlineData("  aaa", " 2a3")]

        [InlineData("  aaa  ", " 2a3 2")]

        [InlineData("11", "12")]

        [InlineData("112223333", "122334")]

​

​

        public void Compress_GivenString_ReturnsExpectedResult(string? input, string expected)

        {

            // Act

            var result = StringCompressor.Compress(input);

​

            // Assert

            Assert.Equal(expected, result);

        }

​

    }

}

​

​