ParagonIE_Sodium_Core32_Int64::ctSelect

    public static function ctSelect(
        ParagonIE_Sodium_Core32_Int64 $A,
        ParagonIE_Sodium_Core32_Int64 $B
    ) {
        $a = clone $A;
        $b = clone $B;
        /** @var int $aNeg */
        $aNeg = ($a->limbs[0] >> 15) & 1;
        /** @var int $bNeg */
        $bNeg = ($b->limbs[0] >> 15) & 1;
        /** @var int $m */
        $m = (-($aNeg & $bNeg)) | 1;
        /** @var int $swap */
        $swap = $bNeg & ~$aNeg;
        /** @var int $d */
        $d = -$swap;

        /*
        if ($bNeg && !$aNeg) {
            $a = clone $int;
            $b = clone $this;
        } elseif($bNeg && $aNeg) {
            $a = $this->mulInt(-1);
            $b = $int->mulInt(-1);
        }
         */
        $x = $a->xorInt64($b)->mask64($d, $d);
        return array(
            $a->xorInt64($x)->mulInt($m),
            $b->xorInt64($x)->mulInt($m)
        );
    }