using System;
using System.Diagnostics;

namespace Org.BouncyCastle.Math.EC.Custom.Sec
{
    internal class SecP256K1Field
    {
        // 2^256 - 2^32 - 2^9 - 2^8 - 2^7 - 2^6 - 2^4 - 1
        internal static readonly uint[] P = new uint[]{ 0xFFFFFC2F, 0xFFFFFFFE, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
            0xFFFFFFFF, 0xFFFFFFFF };
        private const uint P7 = 0xFFFFFFFF;
        private static readonly uint[] PExt = new uint[]{ 0x000E90A1, 0x000007A2, 0x00000001, 0x00000000, 0x00000000,
            0x00000000, 0x00000000, 0x00000000, 0xFFFFF85E, 0xFFFFFFFD, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
            0xFFFFFFFF, 0xFFFFFFFF };
        private const uint PExt15 = 0xFFFFFFFF;
        private static readonly ulong PInv = 0x00000001000003D1UL;

        public static void Add(uint[] x, uint[] y, uint[] z)
        {
            uint c = Nat256.Add(x, y, z);
            if (c != 0 || (z[7] == P7 && Nat256.Gte(z, P)))
            {
                Nat256.AddDWord(PInv, z, 0);
            }
        }

        public static void AddExt(uint[] xx, uint[] yy, uint[] zz)
        {
            uint c = Nat256.AddExt(xx, yy, zz);
            if (c != 0 || (zz[15] == PExt15 && Nat256.GteExt(zz, PExt)))
            {
                Nat256.SubExt(zz, PExt, zz);
            }
        }

        public static void AddOne(uint[] x, uint[] z)
        {
            Array.Copy(x, 0, z, 0, 8);
            uint c = Nat256.Inc(z, 0);
            if (c != 0 || (z[7] == P7 && Nat256.Gte(z, P)))
            {
                Nat256.AddDWord(PInv, z, 0);
            }
        }

        public static uint[] FromBigInteger(BigInteger x)
        {
            uint[] z = Nat256.FromBigInteger(x);
            if (z[7] == P7 && Nat256.Gte(z, P))
            {
                Nat256.AddDWord(PInv, z, 0);
            }
            return z;
        }

        public static void Half(uint[] x, uint[] z)
        {
            if ((x[0] & 1) == 0)
            {
                Nat256.ShiftDownBit(x, 0, z);
            }
            else
            {
                uint c = Nat256.Add(x, P, z);
                Nat256.ShiftDownBit(z, c, z);
            }
        }

        public static void Multiply(uint[] x, uint[] y, uint[] z)
        {
            uint[] tt = Nat256.CreateExt();
            Nat256.Mul(x, y, tt);
            Reduce(tt, z);
        }

        public static void Negate(uint[] x, uint[] z)
        {
            if (Nat256.IsZero(x))
            {
                Nat256.Zero(z);
            }
            else
            {
                Nat256.Sub(P, x, z);
            }
        }

        public static void Reduce(uint[] tt, uint[] z)
        {
            long extra = -(long)tt[8];
            extra += (long)Nat256.MulWordAddExt((uint)PInv, tt, 8, tt, 0);
            extra += (long)Nat256.AddToExt(tt, 8, tt, 1) << 32;
            extra += (long)tt[8];

            ulong c = Nat256.MulWordDwordAdd((uint)PInv, (ulong)extra, tt, 0);
            c += Nat256.AddDWord((ulong)extra, tt, 1);

            Debug.Assert(c == 0 || c == 1);

            if (c != 0 || (tt[7] == P7 && Nat256.Gte(tt, P)))
            {
                Nat256.AddDWord(PInv, tt, 0);
            }

            Array.Copy(tt, 0, z, 0, 8);
        }

        public static void Square(uint[] x, uint[] z)
        {
            uint[] tt = Nat256.CreateExt();
            Nat256.Square(x, tt);
            Reduce(tt, z);
        }

        public static void Subtract(uint[] x, uint[] y, uint[] z)
        {
            int c = Nat256.Sub(x, y, z);
            if (c != 0)
            {
                Nat256.SubDWord(PInv, z);
            }
        }

        public static void SubtractExt(uint[] xx, uint[] yy, uint[] zz)
        {
            int c = Nat256.SubExt(xx, yy, zz);
            if (c != 0)
            {
                Nat256.AddExt(zz, PExt, zz);
            }
        }

        public static void Twice(uint[] x, uint[] z)
        {
            uint c = Nat256.ShiftUpBit(x, 0, z);
            if (c != 0 || (z[7] == P7 && Nat256.Gte(z, P)))
            {
                Nat256.AddDWord(PInv, z, 0);
            }
        }
    }
}