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/* log2.c
*
* Base 2 logarithm
*
*
*
* SYNOPSIS:
*
* double x, y, log2();
*
* y = log2( x );
*
*
*
* DESCRIPTION:
*
* Returns the base 2 logarithm of x.
*
* The argument is separated into its exponent and fractional
* parts. If the exponent is between -1 and +1, the base e
* logarithm of the fraction is approximated by
*
* log(1+x) = x - 0.5 x**2 + x**3 P(x)/Q(x).
*
* Otherwise, setting z = 2(x-1)/x+1),
*
* log(x) = z + z**3 P(z)/Q(z).
*
*
*
* ACCURACY:
*
* Relative error:
* arithmetic domain # trials peak rms
* IEEE 0.5, 2.0 30000 2.0e-16 5.5e-17
* IEEE exp(+-700) 40000 1.3e-16 4.6e-17
*
* In the tests over the interval [exp(+-700)], the logarithms
* of the random arguments were uniformly distributed.
*
* ERROR MESSAGES:
*
* log2 singularity: x = 0; returns -INFINITY
* log2 domain: x < 0; returns NAN
*/
/*
Cephes Math Library Release 2.8: June, 2000
Copyright 1984, 1995, 2000 by Stephen L. Moshier
*/
#include <math.h>
static char fname[] = {"log2"};
/* Coefficients for log(1+x) = x - x**2/2 + x**3 P(x)/Q(x)
* 1/sqrt(2) <= x < sqrt(2)
*/
#ifdef UNK
static double P[] = {
1.01875663804580931796E-4,
4.97494994976747001425E-1,
4.70579119878881725854E0,
1.44989225341610930846E1,
1.79368678507819816313E1,
7.70838733755885391666E0,
};
static double Q[] = {
/* 1.00000000000000000000E0, */
1.12873587189167450590E1,
4.52279145837532221105E1,
8.29875266912776603211E1,
7.11544750618563894466E1,
2.31251620126765340583E1,
};
#define LOG2EA 0.44269504088896340735992
#endif
#ifdef DEC
static unsigned short P[] = {
0037777,0127270,0162547,0057274,
0041001,0054665,0164317,0005341,
0041451,0034104,0031640,0105773,
0041677,0011276,0123617,0160135,
0041701,0126603,0053215,0117250,
0041420,0115777,0135206,0030232,
};
static unsigned short Q[] = {
/*0040200,0000000,0000000,0000000,*/
0041220,0144332,0045272,0174241,
0041742,0164566,0035720,0130431,
0042246,0126327,0166065,0116357,
0042372,0033420,0157525,0124560,
0042271,0167002,0066537,0172303,
0041730,0164777,0113711,0044407,
};
static unsigned short L[5] = {0037742,0124354,0122560,0057703};
#define LOG2EA (*(double *)(&L[0]))
#endif
#ifdef IBMPC
static unsigned short P[] = {
0x1bb0,0x93c3,0xb4c2,0x3f1a,
0x52f2,0x3f56,0xd6f5,0x3fdf,
0x6911,0xed92,0xd2ba,0x4012,
0xeb2e,0xc63e,0xff72,0x402c,
0xc84d,0x924b,0xefd6,0x4031,
0xdcf8,0x7d7e,0xd563,0x401e,
};
static unsigned short Q[] = {
/*0x0000,0x0000,0x0000,0x3ff0,*/
0xef8e,0xae97,0x9320,0x4026,
0xc033,0x4e19,0x9d2c,0x4046,
0xbdbd,0xa326,0xbf33,0x4054,
0xae21,0xeb5e,0xc9e2,0x4051,
0x25b2,0x9e1f,0x200a,0x4037,
};
static unsigned short L[5] = {0x0bf8,0x94ae,0x551d,0x3fdc};
#define LOG2EA (*(double *)(&L[0]))
#endif
#ifdef MIEEE
static unsigned short P[] = {
0x3f1a,0xb4c2,0x93c3,0x1bb0,
0x3fdf,0xd6f5,0x3f56,0x52f2,
0x4012,0xd2ba,0xed92,0x6911,
0x402c,0xff72,0xc63e,0xeb2e,
0x4031,0xefd6,0x924b,0xc84d,
0x401e,0xd563,0x7d7e,0xdcf8,
};
static unsigned short Q[] = {
/*0x3ff0,0x0000,0x0000,0x0000,*/
0x4026,0x9320,0xae97,0xef8e,
0x4046,0x9d2c,0x4e19,0xc033,
0x4054,0xbf33,0xa326,0xbdbd,
0x4051,0xc9e2,0xeb5e,0xae21,
0x4037,0x200a,0x9e1f,0x25b2,
};
static unsigned short L[5] = {0x3fdc,0x551d,0x94ae,0x0bf8};
#define LOG2EA (*(double *)(&L[0]))
#endif
/* Coefficients for log(x) = z + z**3 P(z)/Q(z),
* where z = 2(x-1)/(x+1)
* 1/sqrt(2) <= x < sqrt(2)
*/
#ifdef UNK
static double R[3] = {
-7.89580278884799154124E-1,
1.63866645699558079767E1,
-6.41409952958715622951E1,
};
static double S[3] = {
/* 1.00000000000000000000E0,*/
-3.56722798256324312549E1,
3.12093766372244180303E2,
-7.69691943550460008604E2,
};
/* log2(e) - 1 */
#define LOG2EA 0.44269504088896340735992
#endif
#ifdef DEC
static unsigned short R[12] = {
0140112,0020756,0161540,0072035,
0041203,0013743,0114023,0155527,
0141600,0044060,0104421,0050400,
};
static unsigned short S[12] = {
/*0040200,0000000,0000000,0000000,*/
0141416,0130152,0017543,0064122,
0042234,0006000,0104527,0020155,
0142500,0066110,0146631,0174731,
};
/* log2(e) - 1 */
#define LOG2EA 0.44269504088896340735992L
#endif
#ifdef IBMPC
static unsigned short R[12] = {
0x0e84,0xdc6c,0x443d,0xbfe9,
0x7b6b,0x7302,0x62fc,0x4030,
0x2a20,0x1122,0x0906,0xc050,
};
static unsigned short S[12] = {
/*0x0000,0x0000,0x0000,0x3ff0,*/
0x6d0a,0x43ec,0xd60d,0xc041,
0xe40e,0x112a,0x8180,0x4073,
0x3f3b,0x19b3,0x0d89,0xc088,
};
#endif
#ifdef MIEEE
static unsigned short R[12] = {
0xbfe9,0x443d,0xdc6c,0x0e84,
0x4030,0x62fc,0x7302,0x7b6b,
0xc050,0x0906,0x1122,0x2a20,
};
static unsigned short S[12] = {
/*0x3ff0,0x0000,0x0000,0x0000,*/
0xc041,0xd60d,0x43ec,0x6d0a,
0x4073,0x8180,0x112a,0xe40e,
0xc088,0x0d89,0x19b3,0x3f3b,
};
#endif
#ifdef ANSIPROT
extern double frexp ( double, int * );
extern double ldexp ( double, int );
extern double polevl ( double, void *, int );
extern double p1evl ( double, void *, int );
extern int isnan ( double );
extern int isfinite ( double );
#else
double frexp(), ldexp(), polevl(), p1evl();
int isnan(), isfinite();
#endif
#define SQRTH 0.70710678118654752440
extern double LOGE2, INFINITY, NAN;
double log2(x)
double x;
{
int e;
double y;
VOLATILE double z;
#ifdef DEC
short *q;
#endif
#ifdef NANS
if( isnan(x) )
return(x);
#endif
#ifdef INFINITIES
if( x == INFINITY )
return(x);
#endif
/* Test for domain */
if( x <= 0.0 )
{
if( x == 0.0 )
{
mtherr( fname, SING );
return( -INFINITY );
}
else
{
mtherr( fname, DOMAIN );
return( NAN );
}
}
/* separate mantissa from exponent */
#ifdef DEC
q = (short *)&x;
e = *q; /* short containing exponent */
e = ((e >> 7) & 0377) - 0200; /* the exponent */
*q &= 0177; /* strip exponent from x */
*q |= 040000; /* x now between 0.5 and 1 */
#endif
/* Note, frexp is used so that denormal numbers
* will be handled properly.
*/
#ifdef IBMPC
x = frexp( x, &e );
/*
q = (short *)&x;
q += 3;
e = *q;
e = ((e >> 4) & 0x0fff) - 0x3fe;
*q &= 0x0f;
*q |= 0x3fe0;
*/
#endif
/* Equivalent C language standard library function: */
#ifdef UNK
x = frexp( x, &e );
#endif
#ifdef MIEEE
x = frexp( x, &e );
#endif
/* logarithm using log(x) = z + z**3 P(z)/Q(z),
* where z = 2(x-1)/x+1)
*/
if( (e > 2) || (e < -2) )
{
if( x < SQRTH )
{ /* 2( 2x-1 )/( 2x+1 ) */
e -= 1;
z = x - 0.5;
y = 0.5 * z + 0.5;
}
else
{ /* 2 (x-1)/(x+1) */
z = x - 0.5;
z -= 0.5;
y = 0.5 * x + 0.5;
}
x = z / y;
z = x*x;
y = x * ( z * polevl( z, R, 2 ) / p1evl( z, S, 3 ) );
goto ldone;
}
/* logarithm using log(1+x) = x - .5x**2 + x**3 P(x)/Q(x) */
if( x < SQRTH )
{
e -= 1;
x = ldexp( x, 1 ) - 1.0; /* 2x - 1 */
}
else
{
x = x - 1.0;
}
z = x*x;
#if DEC
y = x * ( z * polevl( x, P, 5 ) / p1evl( x, Q, 6 ) ) - ldexp( z, -1 );
#else
y = x * ( z * polevl( x, P, 5 ) / p1evl( x, Q, 5 ) ) - ldexp( z, -1 );
#endif
ldone:
/* Multiply log of fraction by log2(e)
* and base 2 exponent by 1
*
* ***CAUTION***
*
* This sequence of operations is critical and it may
* be horribly defeated by some compiler optimizers.
*/
z = y * LOG2EA;
z += x * LOG2EA;
z += y;
z += x;
z += e;
return( z );
}
|
