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

namespace AusPostCode {
	public static class BaseConversion {
		// honestly surprised c# doesn't do this out of the box

		private static readonly char[] Characters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz-=".ToCharArray();

		// thanks to https://stackoverflow.com/a/35004409
		private static readonly Dictionary<char, int> CharacterIndices =
			Characters.Select((c, i) => new {Char = c, Index = i}).ToDictionary(c => c.Char, c => c.Index);

		public static string ToBase(int num, int numberBase) {
			if (numberBase > Characters.Length) {
				throw new ArgumentOutOfRangeException(
					nameof(numberBase),
					$"Base must be below {Characters.Length}"
				);
			}

			if (Math.Abs(num) == 0) {
				return num.ToString();
			}

			var digits = new List<char>();
			var negative = num < 0;

			while (num > 0) {
				digits.Add(Characters[num % numberBase]);
				num /= numberBase;
			}

			digits.Reverse();
			return (negative ? "-" : "") + string.Join("", digits);
		}

		public static int FromBase(string num, int numberBase) {
			if (numberBase > Characters.Length) {
				throw new ArgumentOutOfRangeException(
					nameof(numberBase),
					$"Base must be below {Characters.Length}"
				);
			}

			var negative = false;

			if (num.StartsWith("-")) {
				// it's a negative number
				negative = true;
				num = num.Substring(1);
			}

			var x = num.ToCharArray();
			var result = 0;

			// thanks to https://stackoverflow.com/a/35004409 again for this 0uo
			for (var i = 0; i < x.Length; i++) {
				result += CharacterIndices[x[i]] * (int) Math.Pow(numberBase, x.Length - i - 1);
			}

			if (negative) {
				result *= -1;
			}

			return result;
		}
	}
}