/* origin: FreeBSD /usr/src/lib/msun/src/s_fmaf.c */

/*-

 * Copyright (c) 2005-2011 David Schultz <das@FreeBSD.ORG>

 * All rights reserved.

 *

 * Redistribution and use in source and binary forms, with or without

 * modification, are permitted provided that the following conditions

 * are met:

 * 1. Redistributions of source code must retain the above copyright

 *    notice, this list of conditions and the following disclaimer.

 * 2. Redistributions in binary form must reproduce the above copyright

 *    notice, this list of conditions and the following disclaimer in the

 *    documentation and/or other materials provided with the distribution.

 *

 * THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND

 * ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE

 * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE

 * ARE DISCLAIMED.  IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE

 * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL

 * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS

 * OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)

 * HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT

 * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY

 * OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF

 * SUCH DAMAGE.

 */

use core::f32;

use core::ptr::read_volatile;

use super::fenv::{

    FE_INEXACT, FE_TONEAREST, FE_UNDERFLOW, feclearexcept, fegetround, feraiseexcept, fetestexcept,

};

/*

 * Fused multiply-add: Compute x * y + z with a single rounding error.

 *

 * A double has more than twice as much precision than a float, so

 * direct double-precision arithmetic suffices, except where double

 * rounding occurs.

 */

/// Floating multiply add (f32)

///

/// Computes `(x*y)+z`, rounded as one ternary operation:

/// Computes the value (as if) to infinite precision and rounds once to the result format,

/// according to the rounding mode characterized by the value of FLT_ROUNDS.

#[cfg_attr(all(test, assert_no_panic), no_panic::no_panic)]

pub fn fmaf(x: f32, y: f32, mut z: f32) -> f32 {

    let xy: f64;

    let mut result: f64;

    let mut ui: u64;

    let e: i32;

    xy = x as f64 * y as f64;

    result = xy + z as f64;

    ui = result.to_bits();

    e = (ui >> 52) as i32 & 0x7ff;

    /* Common case: The double precision result is fine. */

    if (

        /* not a halfway case */

        ui & 0x1fffffff) != 0x10000000 ||

        /* NaN */

        e == 0x7ff ||

        /* exact */

        (result - xy == z as f64 && result - z as f64 == xy) ||

        /* not round-to-nearest */

        fegetround() != FE_TONEAREST

    {

        /*

            underflow may not be raised correctly, example:

            fmaf(0x1p-120f, 0x1p-120f, 0x1p-149f)

        */

        if e < 0x3ff - 126 && e >= 0x3ff - 149 && fetestexcept(FE_INEXACT) != 0 {

            feclearexcept(FE_INEXACT);

            // prevent `xy + vz` from being CSE'd with `xy + z` above

            let vz: f32 = unsafe { read_volatile(&z) };

            result = xy + vz as f64;

            if fetestexcept(FE_INEXACT) != 0 {

                feraiseexcept(FE_UNDERFLOW);

            } else {

                feraiseexcept(FE_INEXACT);

            }

        }

        z = result as f32;

        return z;

    }

    /*

     * If result is inexact, and exactly halfway between two float values,

     * we need to adjust the low-order bit in the direction of the error.

     */

    let neg = ui >> 63 != 0;

    let err = if neg == (z as f64 > xy) { xy - result + z as f64 } else { z as f64 - result + xy };

    if neg == (err < 0.0) {

        ui += 1;

    } else {

        ui -= 1;

    }

    f64::from_bits(ui) as f32

}

#[cfg(test)]

mod tests {

    #[test]

    fn issue_263() {

        let a = f32::from_bits(1266679807);

        let b = f32::from_bits(1300234242);

        let c = f32::from_bits(1115553792);

        let expected = f32::from_bits(1501560833);

        assert_eq!(super::fmaf(a, b, c), expected);

    }

}