using System;
using System.Buffers;
using System.Globalization;

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)
        {
            int position = 0;
            int endOfWord = EndOfWord(phrase, position);
            string result = string.Empty;
            do
            {
                result += ToPigWord(phrase[position..endOfWord]);
                position = IndexOfLetter(phrase, endOfWord);
                result += phrase[endOfWord..position];
                endOfWord = EndOfWord(phrase, position);
            }
            while (position != -1 || endOfWord != -1);

            return result;
        }

        private static bool IsVowel(char symbol) => symbol is 'a' || symbol is 'A' ||
                                                    symbol is 'o' || symbol is 'O' ||
                                                    symbol is 'e' || symbol is 'E' ||
                                                    symbol is 'i' || symbol is 'I' ||
                                                    symbol is 'u' || symbol is 'U';

        private static string ToPigWord(string phrase)
        {
            if (IsVowel(phrase[0]))
            {
                return phrase + "yay";
            }

            int position = VowelFirstPosition(phrase);

            return char.IsUpper(phrase[0])
                ? char.ToUpper(phrase[position], CultureInfo.InvariantCulture) + phrase[(position + 1) ..] +
                  char.ToLower(phrase[0], CultureInfo.InvariantCulture) + phrase[1..position] + "ay"
                : phrase[position..] + phrase[..position] + "ay";
        }

        private static int VowelFirstPosition(string phrase, int position = 0)
        {
            for (int i = position; i < phrase.Length; i++)
            {
                if (IsVowel(phrase[i]))
                {
                    return i;
                }
            }

            return -1;
        }

        private static int EndOfWord(string phrase, int position = 0)
        {
            for (int i = position; i < phrase.Length; i++)
            {
                if (!char.IsLetter(phrase[i]))
                {
                    return i;
                }
            }

            return -1;
        }

        private static int IndexOfLetter(string phrase, int position = 0)
        {
            for (int i = position; i < phrase.Length; i++)
            {
                if (char.IsLetter(phrase[i]))
                {
                    return i;
                }
            }

            return -1;
        }
    }
}