ParagonIE_Sodium_Core32_Int64::addInt64

    public function addInt64(ParagonIE_Sodium_Core32_Int64 $addend)
    {
        $i0 = $this->limbs[0];
        $i1 = $this->limbs[1];
        $i2 = $this->limbs[2];
        $i3 = $this->limbs[3];
        $j0 = $addend->limbs[0];
        $j1 = $addend->limbs[1];
        $j2 = $addend->limbs[2];
        $j3 = $addend->limbs[3];

        $r3 = $i3 + ($j3 & 0xffff);
        $carry = $r3 >> 16;

        $r2 = $i2 + ($j2 & 0xffff) + $carry;
        $carry = $r2 >> 16;

        $r1 = $i1 + ($j1 & 0xffff) + $carry;
        $carry = $r1 >> 16;

        $r0 = $i0 + ($j0 & 0xffff) + $carry;
        $carry = $r0 >> 16;

        $r0 &= 0xffff;
        $r1 &= 0xffff;
        $r2 &= 0xffff;
        $r3 &= 0xffff;

        $return = new ParagonIE_Sodium_Core32_Int64(
            array($r0, $r1, $r2, $r3)
        );
        $return->overflow = $carry;
        $return->unsignedInt = $this->unsignedInt;
        return $return;
    }