# code: 9ferno

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```/* derived from /netlib/fdlibm */

/* @(#)s_expm1.c 1.3 95/01/18 */
/*
* ====================================================
*
* Developed at SunSoft, a Sun Microsystems, Inc. business.
* Permission to use, copy, modify, and distribute this
* software is freely granted, provided that this notice
* is preserved.
* ====================================================
*/

/* expm1(x)
* Returns exp(x)-1, the exponential of x minus 1.
*
* Method
*   1. Argument reduction:
*	Given x, find r and integer k such that
*
*               x = k*ln2 + r,  |r| <= 0.5*ln2 ~ 0.34658
*
*      Here a correction term c will be computed to compensate
*	the error in r when rounded to a floating-point number.
*
*   2. Approximating expm1(r) by a special rational function on
*	the interval [0,0.34658]:
*	Since
*	    r*(exp(r)+1)/(exp(r)-1) = 2+ r^2/6 - r^4/360 + ...
*	we define R1(r*r) by
*	    r*(exp(r)+1)/(exp(r)-1) = 2+ r^2/6 * R1(r*r)
*	That is,
*	    R1(r**2) = 6/r *((exp(r)+1)/(exp(r)-1) - 2/r)
*		     = 6/r * ( 1 + 2.0*(1/(exp(r)-1) - 1/r))
*		     = 1 - r^2/60 + r^4/2520 - r^6/100800 + ...
*      We use a special Reme algorithm on [0,0.347] to generate
* 	a polynomial of degree 5 in r*r to approximate R1. The
*	maximum error of this polynomial approximation is bounded
*	by 2**-61. In other words,
*	    R1(z) ~ 1.0 + Q1*z + Q2*z**2 + Q3*z**3 + Q4*z**4 + Q5*z**5
*	where 	Q1  =  -1.6666666666666567384E-2,
* 		Q2  =   3.9682539681370365873E-4,
* 		Q3  =  -9.9206344733435987357E-6,
* 		Q4  =   2.5051361420808517002E-7,
* 		Q5  =  -6.2843505682382617102E-9;
*  	(where z=r*r, and the values of Q1 to Q5 are listed below)
*	with error bounded by
*	    |                  5           |     -61
*	    | 1.0+Q1*z+...+Q5*z   -  R1(z) | <= 2
*	    |                              |
*
*	expm1(r) = exp(r)-1 is then computed by the following
* 	specific way which minimize the accumulation rounding error:
*			       2     3
*			      r     r    [ 3 - (R1 + R1*r/2)  ]
*	      expm1(r) = r + --- + --- * [--------------------]
*		              2     2    [ 6 - r*(3 - R1*r/2) ]
*
*	To compensate the error in the argument reduction, we use
*		expm1(r+c) = expm1(r) + c + expm1(r)*c
*			   ~ expm1(r) + c + r*c
*	Thus c+r*c will be added in as the correction terms for
*	expm1(r+c). Now rearrange the term to avoid optimization
* 	screw up:
*		        (      2                                    2 )
*		        ({  ( r    [ R1 -  (3 - R1*r/2) ]  )  }    r  )
*	 expm1(r+c)~r - ({r*(--- * [--------------------]-c)-c} - --- )
*	                ({  ( 2    [ 6 - r*(3 - R1*r/2) ]  )  }    2  )
*                      (                                             )
*
*		   = r - E
*   3. Scale back to obtain expm1(x):
*	From step 1, we have
*	   expm1(x) = either 2^k*[expm1(r)+1] - 1
*		    = or     2^k*[expm1(r) + (1-2^-k)]
*   4. Implementation notes:
*	(A). To save one multiplication, we scale the coefficient Qi
*	     to Qi*2^i, and replace z by (x^2)/2.
*	(B). To achieve maximum accuracy, we compute expm1(x) by
*	  (i)   if x < -56*ln2, return -1.0, (raise inexact if x!=inf)
*	  (ii)  if k=0, return r-E
*	  (iii) if k=-1, return 0.5*(r-E)-0.5
*        (iv)	if k=1 if r < -0.25, return 2*((r+0.5)- E)
*	       	       else	     return  1.0+2.0*(r-E);
*	  (v)   if (k<-2||k>56) return 2^k(1-(E-r)) - 1 (or exp(x)-1)
*	  (vi)  if k <= 20, return 2^k((1-2^-k)-(E-r)), else
*	  (vii) return 2^k(1-((E+2^-k)-r))
*
* Special cases:
*	expm1(INF) is INF, expm1(NaN) is NaN;
*	expm1(-INF) is -1, and
*	for finite argument, only expm1(0)=0 is exact.
*
* Accuracy:
*	according to an error analysis, the error is always less than
*	1 ulp (unit in the last place).
*
* Misc. info.
*	For IEEE double
*	    if x >  7.09782712893383973096e+02 then expm1(x) overflow
*
* Constants:
* The hexadecimal values are the intended ones for the following
* constants. The decimal values may be used, provided that the
* compiler will convert from decimal to binary accurately enough
* to produce the hexadecimal values shown.
*/

#include "fdlibm.h"

static const double
one		= 1.0,
Huge		= 1.0e+300,
tiny		= 1.0e-300,
o_threshold	= 7.09782712893383973096e+02,/* 0x40862E42, 0xFEFA39EF */
ln2_hi		= 6.93147180369123816490e-01,/* 0x3fe62e42, 0xfee00000 */
ln2_lo		= 1.90821492927058770002e-10,/* 0x3dea39ef, 0x35793c76 */
invln2		= 1.44269504088896338700e+00,/* 0x3ff71547, 0x652b82fe */
/* scaled coefficients related to expm1 */
Q1  =  -3.33333333333331316428e-02, /* BFA11111 111110F4 */
Q2  =   1.58730158725481460165e-03, /* 3F5A01A0 19FE5585 */
Q3  =  -7.93650757867487942473e-05, /* BF14CE19 9EAADBB7 */
Q4  =   4.00821782732936239552e-06, /* 3ED0CFCA 86E65239 */
Q5  =  -2.01099218183624371326e-07; /* BE8AFDB7 6E09C32D */

double expm1(double x)
{
double y,hi,lo,c,t,e,hxs,hfx,r1;
int k,xsb;
unsigned hx;

hx  = __HI(x);	/* high word of x */
xsb = hx&0x80000000;		/* sign bit of x */
if(xsb==0) y=x; else y= -x;	/* y = |x| */
hx &= 0x7fffffff;		/* high word of |x| */

/* filter out Huge and non-finite argument */
if(hx >= 0x4043687A) {			/* if |x|>=56*ln2 */
if(hx >= 0x40862E42) {		/* if |x|>=709.78... */
if(hx>=0x7ff00000) {
if(((hx&0xfffff)|__LO(x))!=0)
return x+x; 	 /* NaN */
else return (xsb==0)? x:-1.0;/* exp(+-inf)={inf,-1} */
}
if(x > o_threshold) return Huge*Huge; /* overflow */
}
if(xsb!=0) { /* x < -56*ln2, return -1.0 with inexact */
if(x+tiny<0.0)		/* raise inexact */
return tiny-one;	/* return -1 */
}
}

/* argument reduction */
if(hx > 0x3fd62e42) {		/* if  |x| > 0.5 ln2 */
if(hx < 0x3FF0A2B2) {	/* and |x| < 1.5 ln2 */
if(xsb==0)
{hi = x - ln2_hi; lo =  ln2_lo;  k =  1;}
else
{hi = x + ln2_hi; lo = -ln2_lo;  k = -1;}
} else {
k  = invln2*x+((xsb==0)?0.5:-0.5);
t  = k;
hi = x - t*ln2_hi;	/* t*ln2_hi is exact here */
lo = t*ln2_lo;
}
x  = hi - lo;
c  = (hi-x)-lo;
}
else if(hx < 0x3c900000) {  	/* when |x|<2**-54, return x */
t = Huge+x;	/* return x with inexact flags when x!=0 */
return x - (t-(Huge+x));
}
else k = 0;

/* x is now in primary range */
hfx = 0.5*x;
hxs = x*hfx;
r1 = one+hxs*(Q1+hxs*(Q2+hxs*(Q3+hxs*(Q4+hxs*Q5))));
t  = 3.0-r1*hfx;
e  = hxs*((r1-t)/(6.0 - x*t));
if(k==0) return x - (x*e-hxs);		/* c is 0 */
else {
e  = (x*(e-c)-c);
e -= hxs;
if(k== -1) return 0.5*(x-e)-0.5;
if(k==1)
if(x < -0.25) return -2.0*(e-(x+0.5));
else 	      return  one+2.0*(x-e);
if (k <= -2 || k>56) {   /* suffice to return exp(x)-1 */
y = one-(e-x);
__HI(y) += (k<<20);	/* add k to y's exponent */
return y-one;
}
t = one;
if(k<20) {
__HI(t) = 0x3ff00000 - (0x200000>>k);  /* t=1-2^-k */
y = t-(e-x);
__HI(y) += (k<<20);	/* add k to y's exponent */
} else {
__HI(t)  = ((0x3ff-k)<<20);	/* 2^-k */
y = x-(e+t);
y += one;
__HI(y) += (k<<20);	/* add k to y's exponent */
}
}
return y;
}
```