Implementation of the `UnityEditor.ObjectNames.NicifyVariableName` algorithm

TextUtils.cs

using System.Text;

namespace QuickEye.Utility
{
    public static class TextUtils
    {
        public static string NicifyVariableName(string input)
        {
            var result = new StringBuilder(input.Length * 2);

            var prevIsLetter = false;
            var prevIsLetterUpper = false;
            var prevIsDigit = false;
            var prevIsStartOfWord = false;
            var prevIsNumberWord = false;

            var firstCharIndex = 0;
            if (input.StartsWith('_'))
                firstCharIndex = 1;
            else if (input.StartsWith("m_"))
                firstCharIndex = 2;

            for (var i = input.Length - 1; i >= firstCharIndex; i--)
            {
                var currentChar = input[i];
                var currIsLetter = char.IsLetter(currentChar);
                if (i == firstCharIndex && currIsLetter)
                    currentChar = char.ToUpper(currentChar);
                var currIsLetterUpper = char.IsUpper(currentChar);
                var currIsDigit = char.IsDigit(currentChar);
                var currIsSpacer = currentChar == ' ' || currentChar == '_';

                var addSpace = (currIsLetter && !currIsLetterUpper && prevIsLetterUpper) ||
                               (currIsLetter && prevIsLetterUpper && prevIsStartOfWord) ||
                               (currIsDigit && prevIsStartOfWord) ||
                               (!currIsDigit && prevIsNumberWord) ||
                               (currIsLetter && !currIsLetterUpper && prevIsDigit);

                if (!currIsSpacer && addSpace)
                {
                    result.Insert(0, ' ');
                }

                result.Insert(0, currentChar);
                prevIsStartOfWord = currIsLetter && currIsLetterUpper && prevIsLetter && !prevIsLetterUpper;
                prevIsNumberWord = currIsDigit && prevIsLetter && !prevIsLetterUpper;
                prevIsLetterUpper = currIsLetter && currIsLetterUpper;
                prevIsLetter = currIsLetter;
                prevIsDigit = currIsDigit;
            }

            return result.ToString();
        }
    }
}

TextUtilsTests.cs

using System.Collections;
using System.Collections.Generic;
using System.Linq;
using NUnit.Framework;

namespace QuickEye.Utility.Tests.Editor
{
    public class TextUtilsTests
    {
        [TestCaseSource(nameof(GetTestCases))]
        [TestCaseSource(nameof(GetTestCases2))]
        public void Should_OutputMatchUnityEditorMethod_When_NicifyClassName(string input)
        {
            var expected = UnityEditor.ObjectNames.NicifyVariableName(input);
            var actual = TextUtils.NicifyVariableName(input);
            Assert.AreEqual(expected, actual);
        }

        private static IEnumerable GetTestCases()
        {
            yield return "className2Number";
            yield return "className2number";
            yield return "_className2number_";
            yield return "m_className2number_";
            yield return "_m_className2number_";
            yield return " _className2number_ ";
            yield return "_ className2number _";
            yield return "className22Number";
            yield return "className222Number";
            yield return "className22number";
            yield return "className222number";
            yield return "className23";
            yield return "clasSName23";
            yield return "clasSDKName23";
            yield return "Class_Name_23";
            yield return "SDKName";
            yield return "SDK_Name";
            yield return "sDK_Name";

            yield return "G2A";
            yield return "SDK2";
            yield return "SDK22";
            yield return "SDK2a";
            yield return "SDK22a";

            yield return "SDk2";
            yield return "SDk22";
            yield return "SDk2a";
            yield return "SDk22a";

            yield return "SDK22a2a";
            yield return "SDK2a2a";
            yield return "SDk2";
            yield return "SDk22";
            yield return "SDk22a";
            yield return "SD K2";
            yield return "SD k2";
            yield return "SDK2a";
            yield return "SDk2A";
            yield return "SD2A";
            yield return "2 A2 a";
        }

        private static IEnumerable<string> GetTestCases2()
        {
            var str = "m_AAA02_";
            var results =
                from e in Enumerable.Range(0, 1 << str.Length)
                let p = from b in Enumerable.Range(0, str.Length)
                    select (e & (1 << b)) == 0 ? (char?)null : str[b]
                select string.Join(string.Empty, p);
            return results;
        }
    }
}