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author Shinji KONO Mon, 25 May 2020 18:13:55 +0900 1830386684a0
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```
/* Compute x^2 + y^2 - 1, without large cancellation error.
Copyright (C) 2012-2018 Free Software Foundation, Inc.
This file is part of the GNU C Library.

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Lesser General Public
version 2.1 of the License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Lesser General Public License for more details.

You should have received a copy of the GNU Lesser General Public
License along with the GNU C Library; if not, see

/* Calculate X + Y exactly and store the result in *HI + *LO.  It is
given that |X| >= |Y| and the values are small enough that no
overflow occurs.  */

static inline void
add_split (__float128 *hi, __float128 *lo, __float128 x, __float128 y)
{
/* Apply Dekker's algorithm.  */
*hi = x + y;
*lo = (x - *hi) + y;
}

/* Compare absolute values of floating-point values pointed to by P
and Q for qsort.  */

static int
compare (const void *p, const void *q)
{
__float128 pld = fabsq (*(const __float128 *) p);
__float128 qld = fabsq (*(const __float128 *) q);
if (pld < qld)
return -1;
else if (pld == qld)
return 0;
else
return 1;
}

/* Return X^2 + Y^2 - 1, computed without large cancellation error.
It is given that 1 > X >= Y >= epsilon / 2, and that X^2 + Y^2 >=
0.5.  */

__float128
{
__float128 vals[5];
SET_RESTORE_ROUNDF128 (FE_TONEAREST);
mul_splitq (&vals[1], &vals[0], x, x);
mul_splitq (&vals[3], &vals[2], y, y);
vals[4] = -1;
qsort (vals, 5, sizeof (__float128), compare);
/* Add up the values so that each element of VALS has absolute value
at most equal to the last set bit of the next nonzero
element.  */
for (size_t i = 0; i <= 3; i++)
{
add_split (&vals[i + 1], &vals[i], vals[i + 1], vals[i]);
qsort (vals + i + 1, 4 - i, sizeof (__float128), compare);
}
/* Now any error from this addition will be small.  */
return vals[4] + vals[3] + vals[2] + vals[1] + vals[0];
}
```