/** Calculate the integral power of an IntNum.
* @param x the value (base) to exponentiate
* @param y the exponent (must be non-negative)
*/
public static IntNum power(IntNum x, int y)
{
if (y <= 0)
{
if (y == 0)
return one ();
else
throw new Exception ("negative exponent");
}
if (x.isZero ())
return x;
int plen = x.words == null ? 1 : x.ival; // Length of pow2.
int blen = ((x.intLength () * y) >> 5) + 2 * plen;
bool negative = x.isNegative () && (y & 1) != 0;
int[] pow2 = new int [blen];
int[] rwords = new int [blen];
int[] work = new int [blen];
x.getAbsolute (pow2); // pow2 = abs(x);
int rlen = 1;
rwords[0] = 1; // rwords = 1;
for (;;) // for (i = 0; ; i++)
{
// pow2 == x**(2**i)
// prod = x**(sum(j=0..i-1, (y>>j)&1))
if ((y & 1) != 0)
{ // r *= pow2
MPN.mul (work, pow2, plen, rwords, rlen);
int[] tempx = work; work = rwords; rwords = tempx;
rlen += plen;
while (rwords[rlen-1] == 0) rlen--;
}
y >>= 1;
if (y == 0)
break;
// pow2 *= pow2;
MPN.mul (work, pow2, plen, pow2, plen);
int[] temp = work; work = pow2; pow2 = temp; // swap to avoid a copy
plen *= 2;
while (pow2[plen-1] == 0) plen--;
}
if (rwords[rlen-1] < 0)
rlen++;
if (negative)
negate (rwords, rwords, rlen);
return IntNum.make (rwords, rlen);
}