using System;
using System.Diagnostics;

using Org.BouncyCastle.Crypto.Utilities;
using Org.BouncyCastle.Utilities;

namespace Org.BouncyCastle.Math.EC
{
    internal abstract class Mod
    {
        public static void Invert(uint[] p, uint[] x, uint[] z)
        {
            int len = p.Length;
            if (Nat.IsZero(len, x))
                throw new ArgumentException("cannot be 0", "x");
            if (Nat.IsOne(len, x))
            {
                Array.Copy(x, 0, z, 0, len);
                return;
            }

            uint[] u = Nat.Copy(len, x);
            uint[] a = Nat.Create(len);
            a[0] = 1;
            int ac = 0;

            if ((u[0] & 1) == 0)
            {
                InversionStep(p, u, len, a, ref ac);
            }
            if (Nat.IsOne(len, u))
            {
                InversionResult(p, ac, a, z);
                return;
            }

            uint[] v = Nat.Copy(len, p);
            uint[] b = Nat.Create(len);
            int bc = 0;

            int uvLen = len;

            for (;;)
            {
                while (u[uvLen - 1] == 0 && v[uvLen - 1] == 0)
                {
                    --uvLen;
                }

                if (Nat.Gte(len, u, v))
                {
                    Nat.Sub(len, u, v, u);
                    Debug.Assert((u[0] & 1) == 0);
                    ac += Nat.Sub(len, a, b, a) - bc;
                    InversionStep(p, u, uvLen, a, ref ac);
                    if (Nat.IsOne(len, u))
                    {
                        InversionResult(p, ac, a, z);
                        return;
                    }
                }
                else
                {
                    Nat.Sub(len, v, u, v);
                    Debug.Assert((v[0] & 1) == 0);
                    bc += Nat.Sub(len, b, a, b) - ac;
                    InversionStep(p, v, uvLen, b, ref bc);
                    if (Nat.IsOne(len, v))
                    {
                        InversionResult(p, bc, b, z);
                        return;
                    }
                }
            }
        }

        public static uint[] Random(uint[] p)
        {
            int len = p.Length;
            Random rand = new Random();
            uint[] s = Nat.Create(len);

            uint m = p[len - 1];
            m |= m >> 1;
            m |= m >> 2;
            m |= m >> 4;
            m |= m >> 8;
            m |= m >> 16;

            do
            {
                byte[] bytes = new byte[len << 2];
                rand.NextBytes(bytes);
                Pack.BE_To_UInt32(bytes, 0, s);
                s[len - 1] &= m;
            }
            while (Nat.Gte(len, s, p));

            return s;
        }

        public static void Subtract(uint[] p, uint[] x, uint[] y, uint[] z)
        {
            int len = p.Length;
            int c = Nat.Sub(len, x, y, z);
            if (c != 0)
            {
                Nat.Add(len, z, p, z);
            }
        }

        private static void InversionResult(uint[] p, int ac, uint[] a, uint[] z)
        {
            if (ac < 0)
            {
                Nat.Add(p.Length, a, p, z);
            }
            else
            {
                Array.Copy(a, 0, z, 0, p.Length);
            }
        }

        private static void InversionStep(uint[] p, uint[] u, int uLen, uint[] x, ref int xc)
        {
            int len = p.Length;
            int count = 0;
            while (u[0] == 0)
            {
                Nat.ShiftDownWord(uLen, u, 0);
                count += 32;
            }

            {
                int zeroes = GetTrailingZeroes(u[0]);
                if (zeroes > 0)
                {
                    Nat.ShiftDownBits(uLen, u, zeroes, 0);
                    count += zeroes;
                }
            }

            for (int i = 0; i < count; ++i)
            {
                if ((x[0] & 1) != 0)
                {
                    if (xc < 0)
                    {
                        xc += (int)Nat.Add(len, x, p, x);
                    }
                    else
                    {
                        xc += Nat.Sub(len, x, p, x);
                    }
                }

                Debug.Assert(xc == 0 || xc == -1);
                Nat.ShiftDownBit(len, x, (uint)xc);
            }
        }

        private static int GetTrailingZeroes(uint x)
        {
            Debug.Assert(x != 0);
            int count = 0;
            while ((x & 1) == 0)
            {
                x >>= 1;
                ++count;
            }
            return count;
        }
    }
}