round.c

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/*                                                     round.c
 *
 *     Round double to nearest or even integer valued double
 *
 *
 *
 * SYNOPSIS:
 *
 * double x, y, round();
 *
 * y = round(x);
 *
 *
 *
 * DESCRIPTION:
 *
 * Returns the nearest integer to x as a double precision
 * floating point result.  If x ends in 0.5 exactly, the
 * nearest even integer is chosen.
 * 
 *
 *
 * ACCURACY:
 *
 * If x is greater than 1/(2*MACHEP), its closest machine
 * representation is already an integer, so rounding does
 * not change it.
 */

/*
 * Cephes Math Library Release 2.1:  January, 1989
 * Copyright 1984, 1987, 1989 by Stephen L. Moshier
 * Direct inquiries to 30 Frost Street, Cambridge, MA 02140
 */
 
#include "mconf.h"
 
double round(double x)
{
    double y, r;
 
    /* Largest integer <= x */
    y = floor(x);
 
    /* Fractional part */
    r = x - y;
 
    /* Round up to nearest. */
    if (r > 0.5)
        goto rndup;
 
    /* Round to even */
    if (r == 0.5) {
        r = y - 2.0 * floor(0.5 * y);
        if (r == 1.0) {
          rndup:
            y += 1.0;
        }
    }
 
    /* Else round down. */
    return (y);
}