package org.bouncycastle.math.ec.custom.sec;

import java.math.BigInteger;

import org.bouncycastle.math.ec.Nat;

public class SecP521R1Field
{
    // 2^521 - 1
    static final int[] P = new int[]{ 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF,
        0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0xFFFFFFFF, 0x1FF };
    private static final int P16 = 0x1FF;

    public static void add(int[] x, int[] y, int[] z)
    {
        int 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(int[] x, int[] z)
    {
        Nat.copy(16, x, z);
        int c = Nat.inc(16, z, 0) + x[16];
        if (c > P16 || (c == P16 && Nat.eq(16, z, P)))
        {
            c += Nat.inc(16, z, 0);
            c &= P16;
        }
        z[16] = c;
    }

    public static int[] fromBigInteger(BigInteger x)
    {
        int[] z = Nat.fromBigInteger(521, x);
        if (Nat.eq(17, z, P))
        {
            Nat.zero(17, z);
        }
        return z;
    }

    public static void half(int[] x, int[] z)
    {
        int x16 = x[16];
        int c = Nat.shiftDownBit(16, x, x16, z);
        z[16] = (x16 >>> 1) | (c >>> 23);
    }

    public static void multiply(int[] x, int[] y, int[] z)
    {
        int[] tt = Nat.create(33);
        implMultiply(x, y, tt);
        reduce(tt, z);
    }

    public static void negate(int[] x, int[] z)
    {
        if (Nat.isZero(17, x))
        {
            Nat.zero(17, z);
        }
        else
        {
            Nat.sub(17, P, x, z);
        }
    }

    public static void reduce(int[] xx, int[] z)
    {
//        assert xx[32] >>> 18 == 0;

        int xx32 = xx[32];
        int c = Nat.shiftDownBits(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(int[] z)
    {
        int z16 = z[16];
        int c = Nat.addWord(16, z16 >>> 9, z, 0) + (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(int[] x, int[] z)
    {
        int[] tt = Nat.create(33);
        implSquare(x, tt);
        reduce(tt, z);
    }

    public static void squareN(int[] x, int n, int[] z)
    {
//        assert n > 0;

        int[] tt = Nat.create(33);
        implSquare(x, tt);
        reduce(tt, z);

        while (--n > 0)
        {
            implSquare(z, tt);
            reduce(tt, z);
        }
    }

    public static void subtract(int[] x, int[] y, int[] z)
    {
        int c = Nat.sub(16, x, y, z) + x[16] - y[16];
        if (c < 0)
        {
            c += Nat.dec(16, z, 0);
            c &= P16;
        }
        z[16] = c;
    }

    public static void twice(int[] x, int[] z)
    {
        int 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;
    }

    protected static void implMultiply(int[] x, int[] y, int[] zz)
    {
        Nat512.mul(x, y, zz);

        int x16 = x[16], y16 = y[16];
        zz[32] = Nat.mul31BothAdd(16, x16, y, y16, x, zz, 16) + (x16 * y16);
    }

    protected static void implSquare(int[] x, int[] zz)
    {
        Nat512.square(x, zz);

        int x16 = x[16];
        zz[32] = Nat.mulWordAddTo(16, x16 << 1, x, 0, zz, 16) + (x16 * x16);
    }
}

@@ -3195,188 +3195,86 @@
 int 
-c = Nat.shiftUpBit(16, x, 0, z) %7C (x%5B16%5D %3C%3C 1);%0A        if (c %3E P16 %7C%7C (c == P16 && Nat.eq
+x16 = x%5B16%5D;%0A        int c = Nat.shiftUpBit
 (16, 
-z, P)))%0A        %7B%0A            c += Nat.inc(16, z, 0);%0A            c &= P16;%0A        %7D
+x, x16 %3C%3C 23, z) %7C (x16 %3C%3C 1);
 %0A   
@@ -3279,32 +3279,38 @@
        z%5B16%5D = c
+ & P16
 ;%0A    %7D%0A%0A    pro

Simplify twice()