using System;

namespace MongoAuth
{
    /* Based off http://stackoverflow.com/a/7135008 */
    internal static class Base32Encoding
    {
        public static byte[] ToBytes(string input)
        {
            if (string.IsNullOrEmpty(input))
            {
                throw new ArgumentNullException(nameof(input));
            }

            input = input.TrimEnd('='); //remove padding characters
            int byteCount = input.Length * 5 / 8; //this must be TRUNCATED
            var returnArray = new byte[byteCount];

            byte curByte = 0,
                 bitsRemaining = 8;

            int arrayIndex = 0;

            foreach (char c in input)
            {
                int cValue = CharToValue(c);

                int mask;
                if (bitsRemaining > 5)
                {
                    mask = cValue << (bitsRemaining - 5);
                    curByte = (byte)(curByte | mask);
                    bitsRemaining -= 5;
                }
                else
                {
                    mask = cValue >> (5 - bitsRemaining);
                    curByte = (byte)(curByte | mask);
                    returnArray[arrayIndex++] = curByte;
                    curByte = (byte)(cValue << (3 + bitsRemaining));
                    bitsRemaining += 3;
                }
            }

            //if we didn't end with a full byte
            if (arrayIndex != byteCount)
            {
                returnArray[arrayIndex] = curByte;
            }

            return returnArray;
        }

        public static string ToString(byte[] input)
        {
            if (input == null || input.Length == 0)
            {
                throw new ArgumentNullException(nameof(input));
            }

            int charCount = (int)Math.Ceiling(input.Length / 5d) * 8;
            var returnArray = new char[charCount];

            byte nextChar = 0,
                 bitsRemaining = 5;
            int arrayIndex = 0;

            foreach (byte b in input)
            {
                nextChar = (byte)(nextChar | (b >> (8 - bitsRemaining)));
                returnArray[arrayIndex++] = ValueToChar(nextChar);

                if (bitsRemaining < 4)
                {
                    nextChar = (byte)((b >> (3 - bitsRemaining)) & 31);
                    returnArray[arrayIndex++] = ValueToChar(nextChar);
                    bitsRemaining += 5;
                }

                bitsRemaining -= 3;
                nextChar = (byte)((b << bitsRemaining) & 31);
            }

            //if we didn't end with a full char
            if (arrayIndex != charCount)
            {
                returnArray[arrayIndex++] = ValueToChar(nextChar);
                while (arrayIndex != charCount)
                    returnArray[arrayIndex++] = '='; //padding
            }

            return new string(returnArray);
        }

        private static int CharToValue(char c)
        {
            int value = c;

            //65-90 == uppercase letters
            if (value < 91 && value > 64)
            {
                return value - 65;
            }
            //50-55 == numbers 2-7
            if (value < 56 && value > 49)
            {
                return value - 24;
            }
            //97-122 == lowercase letters
            if (value < 123 && value > 96)
            {
                return value - 97;
            }

            throw new ArgumentException("Character is not a Base32 character.", nameof(c));
        }

        private static char ValueToChar(byte b)
        {
            if (b < 26)
            {
                return (char)(b + 65);
            }

            if (b < 32)
            {
                return (char)(b + 24);
            }

            throw new ArgumentException("Byte is not a value Base32 value.", nameof(b));
        }
    }
}