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/*
 * 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>
#include <float.h>

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[] = {
#if	_IEEE
	1e16,	1e32,	1e48,	1e64,	1e80,	1e96,	1e112,	1e128,
	1e144,	1e160,	1e176,	1e192,	1e208,	1e224,	1e240,	1e256,
	1e272,	1e288,	1e304
#endif
#if	_DECVAX
	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 */
}

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