harvey/sys/src/libc/port/pow10.c

#include	<u.h>
#include	<libc.h>

/*
 * this table might overflow 127-bit exponent representations.
 * in that case, truncate it after 1.0e38.
 * it is important to get all one can from this
 * routine since it is used in atof to scale numbers.
 * the presumption is that C converts fp numbers better
 * than multipication of lower powers of 10.
 */
static
double	tab[] =
{
	1.0e0,  1.0e1,  1.0e2,  1.0e3,  1.0e4,  1.0e5,  1.0e6,  1.0e7,  1.0e8,  1.0e9, 
	1.0e10, 1.0e11, 1.0e12, 1.0e13, 1.0e14, 1.0e15, 1.0e16, 1.0e17, 1.0e18, 1.0e19, 
	1.0e20, 1.0e21, 1.0e22, 1.0e23, 1.0e24, 1.0e25, 1.0e26, 1.0e27, 1.0e28, 1.0e29, 
	1.0e30, 1.0e31, 1.0e32, 1.0e33, 1.0e34, 1.0e35, 1.0e36, 1.0e37, 1.0e38, 1.0e39, 
	1.0e40, 1.0e41, 1.0e42, 1.0e43, 1.0e44, 1.0e45, 1.0e46, 1.0e47, 1.0e48, 1.0e49, 
	1.0e50, 1.0e51, 1.0e52, 1.0e53, 1.0e54, 1.0e55, 1.0e56, 1.0e57, 1.0e58, 1.0e59, 
	1.0e60, 1.0e61, 1.0e62, 1.0e63, 1.0e64, 1.0e65, 1.0e66, 1.0e67, 1.0e68, 1.0e69, 
	1.0e70, 1.0e71, 1.0e72, 1.0e73, 1.0e74, 1.0e75, 1.0e76, 1.0e77, 1.0e78, 1.0e79, 
	1.0e80, 1.0e81, 1.0e82, 1.0e83, 1.0e84, 1.0e85, 1.0e86, 1.0e87, 1.0e88, 1.0e89, 
	1.0e90, 1.0e91, 1.0e92, 1.0e93, 1.0e94, 1.0e95, 1.0e96, 1.0e97, 1.0e98, 1.0e99, 
	1.0e100,1.0e101,1.0e102,1.0e103,1.0e104,1.0e105,1.0e106,1.0e107,1.0e108,1.0e109,
	1.0e110,1.0e111,1.0e112,1.0e113,1.0e114,1.0e115,1.0e116,1.0e117,1.0e118,1.0e119,
	1.0e120,1.0e121,1.0e122,1.0e123,1.0e124,1.0e125,1.0e126,1.0e127,1.0e128,1.0e129,
	1.0e130,1.0e131,1.0e132,1.0e133,1.0e134,1.0e135,1.0e136,1.0e137,1.0e138,1.0e139,
	1.0e140,1.0e141,1.0e142,1.0e143,1.0e144,1.0e145,1.0e146,1.0e147,1.0e148,1.0e149,
	1.0e150,1.0e151,1.0e152,1.0e153,1.0e154,1.0e155,1.0e156,1.0e157,1.0e158,1.0e159,
};

double
pow10(int n)
{
	int m;

	if(n < 0) {
		n = -n;
		if(n < sizeof(tab)/sizeof(tab[0]))
			return 1/tab[n];
		m = n/2;
		return 1/(pow10(m) * pow10(n-m));
	}
	if(n < sizeof(tab)/sizeof(tab[0]))
		return tab[n];
	m = n/2;
	return pow10(m) * pow10(n-m);
}