116 lines
1.7 KiB
C
116 lines
1.7 KiB
C
#include <u.h>
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#include <libc.h>
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static int
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isodd(double v)
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{
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double iv;
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if(modf(v, &iv) != 0)
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return 0;
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return (vlong)iv & 1;
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}
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double
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pow(double x, double y) /* return x ^ y (exponentiation) */
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{
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double xy, y1, ye;
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long i;
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int ex, ey, flip;
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/*
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* Special cases.
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* Checking early here prevents an infinite loop.
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* We need to test if !isNaN() here because otherwise
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* we trap.
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*/
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if(!isNaN(x) && x == 1.0)
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return 1.0;
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if(!isNaN(y) && y == 0.0)
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return 1.0;
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if(isNaN(x) || isNaN(y))
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return NaN();
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if(isInf(x, 1)){
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if(y < 0)
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return 0.0;
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else
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return Inf(1);
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}else if(isInf(x, -1)){
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if(y < 0)
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return isodd(y) ? -0.0 : 0.0;
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else if(y > 0)
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return isodd(y) ? Inf(-1) : Inf(1);
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}
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if(isInf(y, 1)){
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if(x == -1.0)
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return 1.0;
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else if(fabs(x) < 1.0)
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return 0.0;
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else if(fabs(x) > 1.0)
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return Inf(1);
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}else if(isInf(y, -1)){
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if(x == -1.0)
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return 1.0;
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else if(fabs(x) < 1.0)
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return Inf(1);
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else if(fabs(x) > 1.0)
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return 0.0;
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}
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flip = 0;
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if(y < 0.){
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y = -y;
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flip = 1;
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}
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y1 = modf(y, &ye);
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if(y1 != 0.0){
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if(x <= 0.)
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goto zreturn;
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if(y1 > 0.5) {
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y1 -= 1.;
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ye += 1.;
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}
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xy = exp(y1 * log(x));
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}else
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xy = 1.0;
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if(ye > 0x7FFFFFFF){ /* should be ~0UL but compiler can't convert double to ulong */
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if(x <= 0.){
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zreturn:
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if(x==0. && !flip)
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return 0.;
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return NaN();
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}
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if(flip){
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if(y == .5)
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return 1./sqrt(x);
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y = -y;
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}else if(y == .5)
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return sqrt(x);
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return exp(y * log(x));
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}
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x = frexp(x, &ex);
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ey = 0;
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i = ye;
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if(i)
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for(;;){
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if(i & 1){
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xy *= x;
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ey += ex;
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}
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i >>= 1;
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if(i == 0)
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break;
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x *= x;
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ex <<= 1;
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if(x < .5){
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x += x;
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ex -= 1;
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}
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}
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if(flip){
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xy = 1. / xy;
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ey = -ey;
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}
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return ldexp(xy, ey);
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}
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