Subversion Repositories planix.SVN

#include <u.h>
#include <libc.h>

static  long    sqtab[64] =
{
        0x6cdb2, 0x726d4, 0x77ea3, 0x7d52f, 0x82a85, 0x87eb1, 0x8d1c0, 0x923bd,
        0x974b2, 0x9c4a8, 0xa13a9, 0xa61be, 0xaaeee, 0xafb41, 0xb46bf, 0xb916e,
        0xbdb55, 0xc247a, 0xc6ce3, 0xcb495, 0xcfb95, 0xd41ea, 0xd8796, 0xdcca0,
        0xe110c, 0xe54dd, 0xe9818, 0xedac0, 0xf1cd9, 0xf5e67, 0xf9f6e, 0xfdfef,
        0x01fe0, 0x05ee6, 0x09cfd, 0x0da30, 0x11687, 0x1520c, 0x18cc8, 0x1c6c1,
        0x20000, 0x2388a, 0x27068, 0x2a79e, 0x2de32, 0x3142b, 0x3498c, 0x37e5b,
        0x3b29d, 0x3e655, 0x41989, 0x44c3b, 0x47e70, 0x4b02b, 0x4e16f, 0x51241,
        0x542a2, 0x57296, 0x5a220, 0x5d142, 0x60000, 0x62e5a, 0x65c55, 0x689f2,
};

double
sqrt(double arg)
{
        int e, ms;
        double a, t;
        union
        {
                double  d;
                struct
                {
                        long    ms;
                        long    ls;
                };
        } u;

        u.d = arg;
        ms = u.ms;

        /*
         * sign extend the mantissa with
         * exponent. result should be > 0 for
         * normal case.
         */
        e = ms >> 20;
        if(e <= 0) {
                if(e == 0)
                        return 0;
                return NaN();
        }

        /*
         * pick up arg/4 by adjusting exponent
         */
        u.ms = ms - (2 << 20);
        a = u.d;

        /*
         * use 5 bits of mantissa and 1 bit
         * of exponent to form table index.
         * insert exponent/2 - 1.
         */
        e = (((e - 1023) >> 1) + 1022) << 20;
        u.ms = *(long*)((char*)sqtab + ((ms >> 13) & 0xfc)) | e;
        u.ls = 0;

        /*
         * three laps of newton
         */
        e = 1 << 20;
        t = u.d;
        u.d = t + a/t;
        u.ms -= e;              /* u.d /= 2; */
        t = u.d;
        u.d = t + a/t;
        u.ms -= e;              /* u.d /= 2; */
        t = u.d;

        return t + a/t;
}

/*
 * this is the program that generated the table.
 * it calls sqrt by some other means.
 * 
 * void
 * main(void)
 * {
 *      int i;
 *      union   U
 *      {
 *              double  d;
 *              struct
 *              {
 *                      long    ms;
 *                      long    ls;
 *              };
 *      } u;
 * 
 *      for(i=0; i<64; i++) {
 *              u.ms = (i<<15) | 0x3fe04000;
 *              u.ls = 0;
 *              u.d = sqrt(u.d);
 *              print(" 0x%.5lux,", u.ms & 0xfffff);
 *      }
 *      print("\n");
 *      exits(0);
 * }
 */