using System;
using System.Collections.Generic;
using System.Numerics;
using System.Text.Json;
using System.Text.Json.Serialization;

namespace WTelegram
{
	public static class Helpers
	{
		/// <summary>Callback for logging a line (string) with its associated <see href="https://docs.microsoft.com/en-us/dotnet/api/microsoft.extensions.logging.loglevel">severity level</see> (int)</summary>
		public static Action<int, string> Log { get; set; } = DefaultLogger;

		/// <summary>For serializing indented Json with fields included</summary>
		public static readonly JsonSerializerOptions JsonOptions = new() { IncludeFields = true, WriteIndented = true, IgnoreReadOnlyProperties = true };

		public static V GetOrCreate<K, V>(this Dictionary<K, V> dictionary, K key) where V : new()
			=> dictionary.TryGetValue(key, out V value) ? value : dictionary[key] = new V();

		private static readonly ConsoleColor[] LogLevelToColor = new[] { ConsoleColor.DarkGray, ConsoleColor.DarkCyan, ConsoleColor.Cyan,
			ConsoleColor.Yellow, ConsoleColor.Red, ConsoleColor.Magenta, ConsoleColor.DarkBlue };
		private static void DefaultLogger(int level, string message)
		{
			Console.ForegroundColor = LogLevelToColor[level];
			Console.WriteLine(message);
			Console.ResetColor();
		}

		/// <summary>Get a cryptographic random 64-bit value</summary>
		public static long RandomLong()
		{
#if NETCOREAPP2_1_OR_GREATER
			Span<long> span = stackalloc long[1];
			System.Security.Cryptography.RandomNumberGenerator.Fill(System.Runtime.InteropServices.MemoryMarshal.AsBytes(span));
			return span[0];
#else
			var span = new byte[8];
			Encryption.RNG.GetBytes(span);
			return BitConverter.ToInt64(span, 0);
#endif
		}

		internal static byte[] ToBigEndian(ulong value) // variable-size buffer
		{
			int i;
			var temp = value;
			for (i = 1; (temp >>= 8) != 0; i++) ;
			var result = new byte[i];
			while (--i >= 0) { result[i] = (byte)value; value >>= 8; }
			return result;
		}

		internal static ulong FromBigEndian(byte[] bytes) // variable-size buffer
		{
			if (bytes.Length > 8) throw new ArgumentException($"expected bytes length <= 8 but got {bytes.Length}");
			ulong result = 0;
			foreach (byte b in bytes)
				result = (result << 8) + b;
			return result;
		}

		internal static byte[] To256Bytes(this BigInteger bi)
		{
			var bigEndian = bi.ToByteArray(true, true);
			if (bigEndian.Length == 256) return bigEndian;
			var result = new byte[256];
			bigEndian.CopyTo(result, 256 - bigEndian.Length);
			return result;
		}

		internal static ulong PQFactorize(ulong pq) // ported from https://github.com/tdlib/td/blob/master/tdutils/td/utils/crypto.cpp#L90
		{
			if (pq < 2) return 1;
			var random = new Random();
			ulong g = 0;
			for (int i = 0, iter = 0; i < 3 || iter < 1000; i++)
			{
				ulong q = (ulong)random.Next(17, 32) % (pq - 1);
				ulong x = ((ulong)random.Next() + (ulong)random.Next() << 31) % (pq - 1) + 1;
				ulong y = x;
				int lim = 1 << (Math.Min(5, i) + 18);
				for (int j = 1; j < lim; j++)
				{
					iter++;
					ulong a = x;
					ulong b = x;
					ulong c = q;

					// c += a * b
					while (b != 0)
					{
						if ((b & 1) != 0)
						{
							c += a;
							if (c >= pq)
								c -= pq;
						}
						a += a;
						if (a >= pq)
							a -= pq;
						b >>= 1;
					}

					x = c;
					ulong z = x < y ? pq + x - y : x - y;
					g = gcd(z, pq);
					if (g != 1)
						break;

					if ((j & (j - 1)) == 0)
						y = x;
				}
				if (g > 1 && g < pq)
					break;
			}
			if (g != 0)
			{
				ulong other = pq / g;
				if (other < g)
					g = other;
			}
			return g;

			static ulong gcd(ulong a, ulong b)
			{
				if (a == 0) return b;
				if (b == 0) return a;

				int shift = 0;
				while ((a & 1) == 0 && (b & 1) == 0)
				{
					a >>= 1;
					b >>= 1;
					shift++;
				}

				while (true)
				{
					while ((a & 1) == 0)
						a >>= 1;
					while ((b & 1) == 0)
						b >>= 1;
					if (a > b)
						a -= b;
					else if (b > a)
						b -= a;
					else
						return a << shift;
				}
			}
		}

		public static int MillerRabinIterations { get; set; } = 64; // 64 is OpenSSL default for 2048-bits numbers
		private static readonly HashSet<BigInteger> GoodPrimes = new();
		/// <summary>Miller–Rabin primality test</summary>
		/// <param name="n">The number to check for primality</param>
		public static bool IsProbablePrime(this BigInteger n)
		{
			var n_minus_one = n - BigInteger.One;
			if (n_minus_one.Sign <= 0) return false;
			if (GoodPrimes.Contains(n)) return true;

			int s;
			var d = n_minus_one;
			for (s = 0; d.IsEven; s++) d >>= 1;

			var bitLen = n.GetBitLength();
			var randomBytes = new byte[bitLen / 8 + 1];
			var lastByteMask = (byte)((1 << (int)(bitLen % 8)) - 1);
			BigInteger a;
			if (MillerRabinIterations < 15) // 15 is the minimum recommended by Telegram
				Log(3, $"MillerRabinIterations ({MillerRabinIterations}) is below the minimal level of safety (15)");
			for (int i = 0; i < MillerRabinIterations; i++)
			{
				do
				{
					Encryption.RNG.GetBytes(randomBytes);
					randomBytes[^1] &= lastByteMask; // we don't want more bits than necessary
					a = new BigInteger(randomBytes);
				}
				while (a < 3 || a >= n_minus_one);
				a--;

				var x = BigInteger.ModPow(a, d, n);
				if (x.IsOne || x == n_minus_one) continue;

				int r;
				for (r = s - 1; r > 0; r--)
				{
					x = BigInteger.ModPow(x, 2, n);
					if (x.IsOne) return false;
					if (x == n_minus_one) break;
				}
				if (r == 0) return false;
			}
			GoodPrimes.Add(n);
			return true;
		}
	}
}