using System;
using System.Diagnostics;

namespace Org.BouncyCastle.Math.EC.Custom.Sec
{
    internal class SecP521R1Field
    {
        // 2^521 - 1
        internal static readonly uint[] P = new uint[]{ 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
            0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x1FF };
        private const int P16 = 0x1FF;

        public static void Add(uint[] x, uint[] y, uint[] z)
        {
            uint c = Nat.Add(16, x, y, z) + x[16] + y[16];
            if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
            {
                c += Nat.Inc(16, z, 0);
                c &= P16;
            }
            z[16] = c;
        }

        public static void AddOne(uint[] x, uint[] z)
        {
            Array.Copy(x, 0, z, 0, 16);
            uint c = Nat.Inc(16, z, 0) + z[16];
            if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
            {
                c += Nat.Inc(16, z, 0);
                c &= P16;
            }
            z[16] = c;
        }

        public static uint[] FromBigInteger(BigInteger x)
        {
            uint[] z = Nat.FromBigInteger(521, x);
            if (Nat.Eq(17, z, P))
            {
                Nat.Zero(17, z);
            }
            return z;
        }

        public static void Half(uint[] x, uint[] z)
        {
            uint c0 = x[0] & 1, x16 = x[16], c512 = x16 & 1;
            Nat.ShiftDownBit(16, x, c512, z);
            z[16] = (x16 >> 1) | (c0 << 8);
        }

        public static void Multiply(uint[] x, uint[] y, uint[] z)
        {
            uint[] tt = Nat.Create(34);
            Nat.Mul(17, x, y, tt);
            Reduce(tt, z);
        }

        public static void Negate(uint[] x, uint[] z)
        {
            if (Nat.IsZero(17, x))
            {
                Nat.Zero(17, z);
            }
            else
            {
                Nat.Sub(17, P, x, z);
            }
        }

        public static void Reduce(uint[] xx, uint[] z)
        {
            Debug.Assert(xx[33] == 0);
            Debug.Assert(xx[32] >> 18 == 0);
            uint xx32 = xx[32];
            uint c = Nat.ShiftDownBitsExt(16, xx, 16, 9, xx32, z) >> 23;
            c += xx32 >> 9;
            c += Nat.Add(16, z, xx, z);
            if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
            {
                c += Nat.Inc(16, z, 0);
                c &= P16;
            }
            z[16] = c;
        }

        public static void Reduce23(uint[] z)
        {
            uint z16 = z[16];
            uint c = Nat.AddWord(16, z16 >> 9, z) + (z16 & P16);
            if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
            {
                c += Nat.Inc(16, z, 0);
                c &= P16;
            }
            z[16] = c;
        }

        public static void Square(uint[] x, uint[] z)
        {
            uint[] tt = Nat.Create(34);
            Nat.Square(17, x, tt);
            Reduce(tt, z);
        }

        public static void SquareN(uint[] x, int n, uint[] z)
        {
            Debug.Assert(n > 0);
            uint[] tt = Nat.Create(34);
            Nat.Square(17, x, tt);
            Reduce(tt, z);

            while (--n > 0)
            {
                Nat.Square(17, z, tt);
                Reduce(tt, z);
            }
        }

        public static void Subtract(uint[] x, uint[] y, uint[] z)
        {
            int c = Nat.Sub(16, x, y, z) + (int)(x[16] - y[16]);
            if (c < 0)
            {
                c += Nat.Dec(16, z, 0);
                c &= P16;
            }
            z[16] = (uint)c;
        }

        public static void Twice(uint[] x, uint[] z)
        {
            uint c = Nat.ShiftUpBit(16, x, 0, z) | (x[16] << 1);
            if (c > P16 || (c == P16 && Nat.Eq(16, z, P)))
            {
                c += Nat.Inc(16, z, 0);
                c &= P16;
            }
            z[16] = c;
        }
    }
}