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

namespace niceware
{
    public static partial class Niceware
    {
        /// <summary>Maximum number of bytes for a passphrase</summary>
        public const int MaxPassphraseSize = 1024;

        /// <summary>Convert an array of bytes into a passphrase</summary>
        /// <param name="bytes">Bytes to convert</param>
        /// <returns>A passphrase</returns>
        /// <remarks>Must be a even size array of bytes</remarks>
        public static List<string> ToPassphrase(this byte[] bytes)
        {
            if (bytes.Length % 2 == 1)
            {
                throw new ArgumentException("Only even-sized byte arrays are supported");
            }

            var result = new List<string>();
            for (var i = 0; i < bytes.Length; i = i + 2)
            {
                result.Add(WordList[bytes[i] * 256 + bytes[i + 1]]);
            }
            return result;
        }

        /// <summary>Generates a random passphrase with the specified number of bytes</summary>
        /// <param name="size">Number of random bytes to use.</param>
        /// <returns>A passphrase</returns>
        /// <remarks>Size must be an even number</remarks>
        public static List<string> GeneratePassphrase(int size)
        {
            return Generate(size).Item1;
        }

        /// <summary>Generates random bytes corresponding passphrase with the specified number of bytes</summary>
        /// <param name="size">Number of random bytes to use.</param>
        /// <returns>Tuple of the passphrase and generated bytes</returns>
        /// <remarks>Size must be an even number and less than <see cref="MaxPassphraseSize" /></remarks>
        public static Tuple<List<string>, byte[]> Generate(int size)
        {
            if (size < 0 || size > MaxPassphraseSize)
            {
                throw new ArgumentException($"Size must be between 0 and {MaxPassphraseSize} bytes");
            }

            var bytes = new Byte[size];
            var rng = System.Security.Cryptography.RandomNumberGenerator.Create();
            rng.GetBytes(bytes);
            rng.Dispose();
            return new Tuple<List<string>, byte[]>(bytes.ToPassphrase(), bytes);
        }

        /// <summary>Converts a phrase back into the original byte array</summary>
        /// <param name="passphrase">Passphrase</param>
        /// <returns>Array of bytes</returns>
        /// <remarks>Throws an ArgumentException if a word is not in the list</remarks>
        public static byte[] PassphraseToBytes(this IEnumerable<string> passphrase)
        {
            if (passphrase == null)
            {
                return new byte[0];
            }

            var result = new byte[passphrase.Count() * 2];
            for (int i = 0; i < passphrase.Count(); i++)
            {
                int wordIndex = -1;
                var word = passphrase.ElementAtOrDefault(i)?.ToLower();
                if (word != null)
                {
                    wordIndex = WordList.IndexOf(word);
                }

                if (wordIndex == -1)
                {
                    throw new ArgumentException($"Invalid word {word}");
                }
                result[2 * i] = (byte)Math.Floor(wordIndex / 256d);
                result[2 * i + 1] = (byte)(wordIndex % 256);
            }
            return result;
        }
    }
}