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

namespace LanguageGame
{
    public static class Translator
    {
        /// <summary>
        /// Translates from English to Pig Latin. Pig Latin obeys a few simple following rules:
        /// - if word starts with vowel sounds, the vowel is left alone, and most commonly 'yay' is added to the end;
        /// - if word starts with consonant sounds or consonant clusters, all letters before the initial vowel are
        ///   placed at the end of the word sequence. Then, "ay" is added.
        /// Note: If a word begins with a capital letter, then its translation also begins with a capital letter,
        /// if it starts with a lowercase letter, then its translation will also begin with a lowercase letter.
        /// </summary>
        /// <param name="phrase">Source phrase.</param>
        /// <returns>Phrase in Pig Latin.</returns>
        /// <exception cref="ArgumentException">Thrown if phrase is null or empty.</exception>
        /// <example>
        /// "apple" -> "appleyay"
        /// "Eat" -> "Eatyay"
        /// "explain" -> "explainyay"
        /// "Smile" -> "Ilesmay"
        /// "Glove" -> "Oveglay"
        /// </example>
        public static string TranslateToPigLatin(string phrase)
        {
            if (string.IsNullOrWhiteSpace(phrase))
            {
                throw new ArgumentException("given phrase is null or empty", nameof(phrase));
            }

            char[] alphabet = "abcdefghijklmnopqrstuvwxyz’ABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray();
            char[] vowels = "aeiouAEIOU".ToCharArray();
            int found = phrase.IndexOfAny(alphabet);
            string result = phrase;
            while (found != -1)
            {
                int endOfWord = IndexNotOf(result, alphabet, found);
                int vowelIndex = result.IndexOfAny(vowels, found);
                if (found == vowelIndex)
                {
                    result = result.Insert(endOfWord, "yay");
                    found = result.IndexOfAny(alphabet, endOfWord + 3);
                    continue;
                }

                if (endOfWord < vowelIndex)
                {
                    result = result.Insert(endOfWord, "ay");
                    found = result.IndexOfAny(alphabet, endOfWord + 2);
                    continue;
                }

                StringBuilder builder = new StringBuilder(result);
                builder.Remove(found, endOfWord - found);
                builder.Insert(found, result[vowelIndex..endOfWord] + result[found..vowelIndex] + "ay");
                if (char.IsUpper(result[found]))
                {
                    builder[found] = char.ToUpper(builder[found], CultureInfo.InvariantCulture);
                    builder[endOfWord - vowelIndex + found] = char.ToLower(builder[endOfWord - vowelIndex + found], CultureInfo.InvariantCulture);
                }

                result = builder.ToString();
                found = result.IndexOfAny(alphabet, endOfWord + 2);
            }

            return result;
        }

        private static int IndexNotOf(string source, char[] array, int startIndex)
        {
            bool notFound = false;
            for (int i = startIndex; i < source.Length; i++)
            {
                for (int j = 0; j < array.Length; j++)
                {
                    if (source[i] == array[j])
                    {
                        notFound = false;
                        break;
                    }

                    notFound = true;
                }

                if (notFound)
                {
                    return i;
                }
            }

            return source.Length;
        }
    }
}