NysiisEncoder.cs

using System.Globalization;
using System.Linq;
using System.Text;

namespace Algorithms.Encoders
{
    /// <summary>
    ///     Class for NYSIIS encoding strings.
    /// </summary>
    public class NysiisEncoder
    {
        private static readonly char[] Vowels = { 'A', 'E', 'I', 'O', 'U' };

        /// <summary>
        ///     Encodes a string using the NYSIIS Algorithm.
        /// </summary>
        /// <param name="text">The string to encode.</param>
        /// <returns>The NYSIIS encoded string (all uppercase).</returns>
        public string Encode(string text)
        {
            text = text.ToUpper(CultureInfo.CurrentCulture);
            text = TrimSpaces(text);
            text = StartReplace(text);
            text = EndReplace(text);

            for (var i = 1; i < text.Length; i++)
            {
                text = ReplaceStep(text, i);
            }

            text = RemoveDuplicates(text);
            return TrimEnd(text);
        }

        private string TrimSpaces(string text) => text.Replace(" ", string.Empty);

        private string RemoveDuplicates(string text)
        {
            var sb = new StringBuilder();
            sb.Append(text[0]);
            foreach (var c in text)
            {
                if (sb[^1] != c)
                {
                    sb.Append(c);
                }
            }

            return sb.ToString();
        }

        private string TrimEnd(string text)
        {
            var checks = new (string from, string to)?[]
            {
                ("S", string.Empty),
                ("AY", "Y"),
                ("A", string.Empty),
            };
            var replacement = checks.FirstOrDefault(t => text.EndsWith(t!.Value.from));
            if (replacement is { })
            {
                var (from, to) = replacement!.Value;
                text = Replace(text, text.Length - from.Length, from.Length, to);
            }

            return text;
        }

        private string ReplaceStep(string text, int i)
        {
            (string from, string to)[] replacements =
            {
                ("EV", "AF"),
                ("E", "A"),
                ("I", "A"),
                ("O", "A"),
                ("U", "A"),
                ("Q", "G"),
                ("Z", "S"),
                ("M", "N"),
                ("KN", "NN"),
                ("K", "C"),
                ("SCH", "SSS"),
                ("PH", "FF"),
            };
            var replaced = TryReplace(text, i, replacements, out text);
            if (replaced)
            {
                return text;
            }

            // H[vowel] or [vowel]H -> text[i-1]
            if (text[i] == 'H')
            {
                if (!Vowels.Contains(text[i - 1]))
                {
                    return ReplaceWithPrevious();
                }

                if (i < text.Length - 1 && !Vowels.Contains(text[i + 1]))
                {
                    return ReplaceWithPrevious();
                }
            }

            // [vowel]W -> [vowel]
            if (text[i] == 'W' && Vowels.Contains(text[i - 1]))
            {
                return ReplaceWithPrevious();
            }

            return text;

            string ReplaceWithPrevious() => Replace(text, i, 1, text[i - 1].ToString());
        }

        private bool TryReplace(string text, int index, (string, string)[] opts, out string result)
        {
            for (var i = 0; i < opts.Length; i++)
            {
                var check = opts[i].Item1;
                var repl = opts[i].Item2;
                if (text.Length >= index + check.Length && text.Substring(index, check.Length) == check)
                {
                    result = Replace(text, index, check.Length, repl);
                    return true;
                }
            }

            result = text;
            return false;
        }

        private string StartReplace(string start)
        {
            var checks = new (string from, string to)?[]
            {
                ("MAC", "MCC"),
                ("KN", "NN"),
                ("K", "C"),
                ("PH", "FF"),
                ("PF", "FF"),
                ("SCH", "SSS"),
            };
            var replacement = checks.FirstOrDefault(t => start.StartsWith(t!.Value.from));
            if (replacement is { })
            {
                var (from, to) = replacement!.Value;
                start = Replace(start, 0, from.Length, to);
            }

            return start;
        }

        private string EndReplace(string end)
        {
            var checks = new (string from, string to)?[]
            {
                ("EE", "Y"),
                ("IE", "Y"),
                ("DT", "D"),
                ("RT", "D"),
                ("NT", "D"),
                ("ND", "D"),
            };
            var replacement = checks.FirstOrDefault(t => end.EndsWith(t!.Value.from));
            if (replacement is { })
            {
                var (from, to) = replacement!.Value;
                end = Replace(end, end.Length - from.Length, from.Length, to);
            }

            return end;
        }

        private string Replace(string text, int index, int length, string substitute) =>
            text[..index] + substitute + text[(index + length) ..];
    }
}