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head     1.2;
access   ;
symbols  ;
locks    bin:1.2;
comment  @ * @;


1.2
date     90.04.23.22.19.30;  author bin;  state Exp;
branches ;
next	1.1;

1.1
date     90.04.23.22.10.04;  author bin;  state Exp;
branches ;
next	;


desc
@Initial MWC RCS revision.
Renamed _pow10.c, used to be pow10.c.
@


1.2
log
@steve 4/23/90
Replaced old COHERENT libc _pow10.c with modified version of MSDOS source.
The modifications are a conditionalization on IEEE; the range of DECVAX
exponents is much smaller than IEEE, so much of the big exponent table
is unnecessary for DECVAX.
@
text
@/*
 * C general utilities library internals.
 * _pow10()
 * Compute 10.0^n.
 * Assumes -319 <= DBL_MIN_10_EXP and DBL_MAX_10_EXP <= 319,
 * which is true for IEEE-format doubles.
 * Modified for MS-DOS from the standard ANSI library source.
 */

/*
 * There are lots of ways to do this, with varying accuracy, size, and speed.
 * This version is nonrecursive and fast but uses somewhat bulky tables.
 * It does the common cases -16 < exp < 16 by table lookup with no fp arithmetic.
 * It does 16 <= exp <= DBL_MAX_10_EXP with one fp multiply.
 * It does DBL_MIN_10_EXP <= exp <= -16 with one fp divide.
 */

#include <math.h>

#if	IEEE
#define	DBL_MAX_10_EXP	308
#define DBL_MIN_10_EXP	-307
#else
#define	DBL_MAX_10_EXP	38
#define DBL_MIN_10_EXP	-38
#endif

static const double powtab0[] = {
	1e0,	1e-1,	1e-2,	1e-3,	1e-4,	1e-5,	1e-6,	1e-7,
	1e-8,	1e-9,	1e-10,	1e-11,	1e-12,	1e-13,	1e-14,	1e-15
};

static const double powtab1[] = {
	1e0,	1e1,	1e2,	1e3,	1e4,	1e5,	1e6,	1e7,
	1e8,	1e9,	1e10,	1e11,	1e12,	1e13,	1e14,	1e15
};

static const double powtab2[] = {
#ifdef	IEEE
	1e16,	1e32,	1e48,	1e64,	1e80,	1e96,	1e112,	1e128,
	1e144,	1e160,	1e176,	1e192,	1e208,	1e224,	1e240,	1e256,
	1e272,	1e288,	1e304
#else
	1e16,	1e32
#endif
};

double
_pow10(exp) register int exp;
{
	if (exp < 0) {
		if ((exp = -exp) < 16)
			return powtab0[exp];
		else if (exp <= -DBL_MIN_10_EXP)
			return powtab0[exp & 15] / powtab2[(exp >> 4) - 1];
		else
			return 0.0;		/* exponent underflow */
	}
	else if (exp < 16)
		return powtab1[exp];
	else if (exp <= DBL_MAX_10_EXP)
		return powtab1[exp & 15] * powtab2[(exp >> 4) - 1];
	else
		return HUGE_VAL;		/* exponent overflow */
}
@


1.1
log
@Initial revision
@
text
@d2 6
a7 3
 * Compute 10**exp, as a double.
 * Called from dtefg.c.
 * This assumes that the exponent is in the range [-511,511].
d9 39
d49 1
a49 2
_pow10(exp)
register int	exp;
a50 7
	register double	d;

	static readonly double _powtab[] = {
		1e0,	1e1,	1e2,	1e3,	1e4,	1e5,
		1e6,	1e7,	1e8,	1e9,	1e10,	1e11,
		1e12,	1e13,	1e14,	1e15 };

d52 6
a57 1
		return (1.0 / _pow10(-exp));
d59 6
a64 26
	d = 1.0;
	if (exp >= 256) {
		exp -= 256;
		d *= 1e256;
	}
	if (exp >= 128) {
		exp -= 128;
		d *= 1e128;
	}
	if (exp >= 64) {
		exp -= 64;
		d *= 1e64;
	}
	if (exp >= 32) {
		exp -= 32;
		d *= 1e32;
	}
	if (exp >= 16) {
		exp -= 16;
		d *= 1e16;
	}
	d *= _powtab[exp];
	return (d);
/*
	return (d * _powtab[ exp]);
*/
@

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