#nullable enable

using System;

using System.Text;

using System.Linq;

​

string str = $"   I\tam a \t\t\t\r\n   string\rwith \r\r\n\r\n many  spaces.   ";

Console.WriteLine($"\"{str.ConsolidateWhiteSpace()}\"");

Console.WriteLine($"\"{str.ConsolidateWhiteSpace2()}\"");

​

public static class StringExtensions {

public static string ConsolidateWhiteSpace(this string? value) => value?.Length switch 

    {

        null => string.Empty,

        1 => value,

        >1 => value.Aggregate((builder: new StringBuilder(value.Length), lastChar: 'A'),

                                   (tuple, c) => !char.IsWhiteSpace(c) ? (tuple.builder.Append(c), tuple.lastChar = c) 

                                                                       : !char.IsWhiteSpace(tuple.lastChar) ? (tuple.builder.Append(' '), tuple.lastChar = c) 

                                                                                                            : (tuple.builder, tuple.lastChar = c),

                                   tuple => tuple.builder.ToString()),

        _ => throw new InvalidOperationException($"Invalid sting length {value.Length}"),

    };

public static string ConsolidateWhiteSpace2(this string? instr) => instr?.Length switch 

    {

        null => string.Empty,

        <=1 => instr,

        >1 => char.IsWhiteSpace(instr[0]) switch {

                false => instr[0] + instr.Substring(1).ConsolidateWhiteSpace2(),

                true => char.IsWhiteSpace(instr[1]) ? instr.Substring(1).ConsolidateWhiteSpace2() : " " + instr.Substring(1).ConsolidateWhiteSpace2()

        }

    };

}