using System;
using System.Collections.Generic;
using System.Linq;

namespace MarkovGrams
{
	/// <summary>
	/// An unweighted character-based markov chain.
	/// </summary>
	public class UnweightedMarkovChain
	{
		/// <summary>
		/// The random number generator
		/// </summary>
		private Random rand = new Random();

		/// <summary>
		/// The ngrams that this markov chain currently contains.
		/// </summary>
		private List<string> ngrams;

		/// <summary>
		/// Whether to always start generating a new word from an n-gram that starts with
		/// an uppercase letter.
		/// </summary>
		public bool StartOnUppercase = false;

		/// <summary>
		/// The generation mode to use when running the Markov Chain.
		/// </summary>
		/// <remarks>
		/// The input n-grams must have been generated using the same mode specified here.
		/// </remarks>
		public GenerationMode Mode { get; private set; } = GenerationMode.CharacterLevel;

		/// <summary>
		/// Creates a new character-based markov chain.
		/// </summary>
		/// <param name="inNgrams">The ngrams to populate the new markov chain with.</param>
		public UnweightedMarkovChain(IEnumerable<string> inNgrams, GenerationMode inMode)
		{
			ngrams = new List<string>(inNgrams);
			Mode = inMode;
		}

		/// <summary>
		/// Returns a random ngram that's currently loaded into this UnweightedMarkovChain.
		/// </summary>
		/// <returns>A random ngram from this UnweightMarkovChain's cache of ngrams.</returns>
		public string RandomNgram()
		{
			IEnumerable<string> validNGrams = StartOnUppercase ? ngrams.Where((ngram) => char.IsUpper(ngram[0])) : ngrams;
			if (validNGrams.Count() == 0)
				throw new Exception($"Error: No valid starting ngrams were found (StartOnUppercase: {StartOnUppercase}).");
			return validNGrams.ElementAt(rand.Next(0, validNGrams.Count()));
		}

		/// <summary>
		/// Generates a new random string from the currently stored ngrams.
		/// </summary>
		/// <param name="length">
		/// The length of ngram to generate.
		/// Note that this is a target, not a fixed value - e.g. passing 2 when the n-gram order is 3 will
		/// result in a string of length 3. Also, depending on the current ngrams this markov chain contains,
		/// it may end up being cut short. 
		/// </param>
		/// <returns>A new random string.</returns>
		public string Generate(int length)
		{
			string result = RandomNgram();
			string lastNgram = result;
			while(result.Length < length)
			{
				// The substring that the next ngram in the chain needs to start with
				string nextStartsWith = Mode == GenerationMode.CharacterLevel ? lastNgram.Substring(1) : lastNgram.Split(' ')[0];
				// Get a list of possible n-grams we could choose from next
				List<string> nextNgrams = ngrams.FindAll(gram => gram.StartsWith(nextStartsWith));
				// If there aren't any choices left, we can't exactly keep adding to the new string any more :-(
				if(nextNgrams.Count == 0)
					break;
				// Pick a random n-gram from the list
				string nextNgram = nextNgrams.ElementAt(rand.Next(0, nextNgrams.Count));
				// Add the last character from the n-gram to the string we're building
				if (Mode == GenerationMode.CharacterLevel)
					result += nextNgram[nextNgram.Length - 1];
				else
					result += string.Join(" ", nextNgram.Split(' ').Skip(1));
				lastNgram = nextNgram;
			}

			return result;
		}
	}
}