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- #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);
- * }
- */
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