private SimpleBigDecimal(SimpleBigDecimal limBigDec)
{
bigInt = limBigDec.bigInt;
scale = limBigDec.scale;
}
public SimpleBigDecimal Add(SimpleBigDecimal b)
{
CheckScale(b);
return(new SimpleBigDecimal(bigInt.Add(b.bigInt), scale));
}
public BigInteger Round()
{
SimpleBigDecimal oneHalf = new SimpleBigDecimal(BigInteger.One, 1);
return(Add(oneHalf.AdjustScale(scale)).Floor());
}
public int CompareTo(SimpleBigDecimal val)
{
CheckScale(val);
return(bigInt.CompareTo(val.bigInt));
}
public SimpleBigDecimal Multiply(SimpleBigDecimal b)
{
CheckScale(b);
return(new SimpleBigDecimal(bigInt.Multiply(b.bigInt), scale + scale));
}
public SimpleBigDecimal Subtract(SimpleBigDecimal b)
{
return(Add(b.Negate()));
}
/**
* Rounds an element <code>λ</code> of <code><b>R</b>[τ]</code>
* to an element of <code><b>Z</b>[τ]</code>, such that their difference
* has minimal norm. <code>λ</code> is given as
* <code>λ = λ<sub>0</sub> + λ<sub>1</sub>τ</code>.
* @param lambda0 The component <code>λ<sub>0</sub></code>.
* @param lambda1 The component <code>λ<sub>1</sub></code>.
* @param mu The parameter <code>μ</code> of the elliptic curve. Must
* equal 1 or -1.
* @return The rounded element of <code><b>Z</b>[τ]</code>.
* @throws ArgumentException if <code>lambda0</code> and
* <code>lambda1</code> do not have same scale.
*/
public static ZTauElement Round(SimpleBigDecimal lambda0,
SimpleBigDecimal lambda1, sbyte mu)
{
int scale = lambda0.Scale;
if (lambda1.Scale != scale)
{
throw new ArgumentException("lambda0 and lambda1 do not have same scale");
}
if (!((mu == 1) || (mu == -1)))
{
throw new ArgumentException("mu must be 1 or -1");
}
BigInteger f0 = lambda0.Round();
BigInteger f1 = lambda1.Round();
SimpleBigDecimal eta0 = lambda0.Subtract(f0);
SimpleBigDecimal eta1 = lambda1.Subtract(f1);
// eta = 2*eta0 + mu*eta1
SimpleBigDecimal eta = eta0.Add(eta0);
if (mu == 1)
{
eta = eta.Add(eta1);
}
else
{
// mu == -1
eta = eta.Subtract(eta1);
}
// check1 = eta0 - 3*mu*eta1
// check2 = eta0 + 4*mu*eta1
SimpleBigDecimal threeEta1 = eta1.Add(eta1).Add(eta1);
SimpleBigDecimal fourEta1 = threeEta1.Add(eta1);
SimpleBigDecimal check1;
SimpleBigDecimal check2;
if (mu == 1)
{
check1 = eta0.Subtract(threeEta1);
check2 = eta0.Add(fourEta1);
}
else
{
// mu == -1
check1 = eta0.Add(threeEta1);
check2 = eta0.Subtract(fourEta1);
}
sbyte h0 = 0;
sbyte h1 = 0;
// if eta >= 1
if (eta.CompareTo(BigInteger.One) >= 0)
{
if (check1.CompareTo(MinusOne) < 0)
{
h1 = mu;
}
else
{
h0 = 1;
}
}
else
{
// eta < 1
if (check2.CompareTo(BigInteger.Two) >= 0)
{
h1 = mu;
}
}
// if eta < -1
if (eta.CompareTo(MinusOne) < 0)
{
if (check1.CompareTo(BigInteger.One) >= 0)
{
h1 = (sbyte)-mu;
}
else
{
h0 = -1;
}
}
else
{
// eta >= -1
if (check2.CompareTo(MinusTwo) < 0)
{
h1 = (sbyte)-mu;
}
}
BigInteger q0 = f0.Add(BigInteger.ValueOf(h0));
BigInteger q1 = f1.Add(BigInteger.ValueOf(h1));
return(new ZTauElement(q0, q1));
}
