gonzui

    1: /* s_cbrtf.c -- float version of s_cbrt.c.
    2:  * Conversion to float by Ian Lance Taylor, Cygnus Support, ian@cygnus.com.
    3:  */
    4: 
    5: /*
    6:  * ====================================================
    7:  * Copyright (C) 1993 by Sun Microsystems, Inc. All rights reserved.
    8:  *
    9:  * Developed at SunPro, a Sun Microsystems, Inc. business.
   10:  * Permission to use, copy, modify, and distribute this
   11:  * software is freely granted, provided that this notice 
   12:  * is preserved.
   13:  * ====================================================
   14:  */
   15: 
   16: #include "math.h"
   17: #include "math_private.h"
   18: 
   19: /* cbrtf(x)
   20:  * Return cube root of x
   21:  */
   22: static const unsigned 
   23: B1 = 709958130, /* B1 = (84+2/3-0.03306235651)*2**23 */
   24: B2 = 642849266; /* B2 = (76+2/3-0.03306235651)*2**23 */
   25: 
   26: static const float
   27: C =  5.4285717010e-01, /* 19/35     = 0x3f0af8b0 */
   28: D = -7.0530611277e-01, /* -864/1225 = 0xbf348ef1 */
   29: E =  1.4142856598e+00, /* 99/70     = 0x3fb50750 */
   30: F =  1.6071428061e+00, /* 45/28     = 0x3fcdb6db */
   31: G =  3.5714286566e-01; /* 5/14      = 0x3eb6db6e */
   32: 
   33: float
   34: cbrtf(float x) 
   35: {
   36:         float r,s,t;
   37:         int32_t hx;
   38:         u_int32_t sign;
   39:         u_int32_t high;
   40: 
   41:         GET_FLOAT_WORD(hx,x);
   42:         sign=hx&0x80000000;            /* sign= sign(x) */
   43:         hx  ^=sign;
   44:         if(hx>=0x7f800000) return(x+x); /* cbrt(NaN,INF) is itself */
   45:         if(hx==0) 
   46:             return(x);         /* cbrt(0) is itself */
   47: 
   48:         SET_FLOAT_WORD(x,hx);  /* x <- |x| */
   49:     /* rough cbrt to 5 bits */
   50:         if(hx<0x00800000)              /* subnormal number */
   51:           {SET_FLOAT_WORD(t,0x4b800000); /* set t= 2**24 */
   52:            t*=x; GET_FLOAT_WORD(high,t); SET_FLOAT_WORD(t,high/3+B2);
   53:           }
   54:         else
   55:           SET_FLOAT_WORD(t,hx/3+B1);
   56: 
   57: 
   58:     /* new cbrt to 23 bits */
   59:         r=t*t/x;
   60:         s=C+r*t;
   61:         t*=G+F/(s+E+D/s);      
   62: 
   63:     /* retore the sign bit */
   64:         GET_FLOAT_WORD(high,t);
   65:         SET_FLOAT_WORD(t,high|sign);
   66:         return(t);
   67: }