Aug 19

C++17 and Fold Expressions

So I’ve just come across this very cool feature in C++17 that I plan on using called ‘fold expressions’. This is basically a way of expanding a bunch of operations. It is also very hard for me to wrap my head around, so we’re going to go through a (very simple) example here, since people don’t seem to explain what exactly happens.

This simple example prints out everything that is passed into the method to stdout:

template <typename... argn>
void fold_test_types( argn... params ){
    (std::cout << ... << params);
}

int main(){
    fold_test_types( 3, 4, "hi", 8);
}

The output is 34hi8, because we forgot to add any newlines or spaces, but that’s fine for this example.

Now what really tripped me up here is the fact that in the fold expression, we’re using the << operator to distinguish things, and it makes it very confusing as to what is actually happening. With fold expressions, the data between the parentheses is expanded, kinda like a template. Let’s look at the rules real quick:

( pack op ... ) (1)
( ... op pack ) (2)
( pack op ... op init ) (3)
( init op ... op pack ) (4)


1) unary right fold
2) unary left fold
3) binary right fold
4) binary left fold


op – any of the following 32 binary operators: + – * / % ^ & | = < > << >> += -= *= /= %= ^= &= |= <<= >>= == != <= >= && || , .* ->*. In a binary fold, both ops must be the same.
pack – an expression that contains an unexpanded parameter pack and does not contain an operator with precedence lower than cast at the top level (formally, a cast-expression)
init – an expression that does not contain an unexpanded parameter pack and does not contain an operator with precedence lower than cast at the top level (formally, a cast-expression)
Note that the open and closing parentheses are part of the fold expression.

https://en.cppreference.com/w/cpp/language/fold

This is a bit hard to understand, but the … is basically a keyword when it is in the parentheses like this. ‘pack’ is the name of the parameters, e.g. in our example it is ‘params’.

Looking at these rules, the rule that we are following in this example is #4, the binary left fold. Let’s look at the fold expression again, with some comments:

(std::cout << ... << params);
|    |      |  |   |    |  ^----ending paren
|    |      |  |   |    ^---- pack(params) - see definition above
|    |      |  |   ^---- operator(<<)
|    |      |  ^---- ...(keyword?)
|    |      ^---- operator(<<)
|    ^--- init(std::cout) - see definition above
^----Starting paren   

With these broken out, it should now be clear why this is #4, the binary left hold. The rules on how it folds are as follows:

1) Unary right fold (E op …) becomes (E1 op (… op (EN-1 op EN)))
2) Unary left fold (… op E) becomes (((E1 op E2) op …) op EN)
3) Binary right fold (E op … op I) becomes (E1 op (… op (EN−1 op (EN op I))))
4) Binary left fold (I op … op E) becomes ((((I op E1) op E2) op …) op EN)

https://en.cppreference.com/w/cpp/language/fold

Since we are using #4, apply the rule for #4. When the compiler runs, the code effectively becomes the following with the template substitution and the fold expression expansion:

void fold_test_types( int arg1, int arg2, std::string arg3, int arg4 ){
    ((((std::cout << arg1) << arg2) << arg3) << arg4);
}

int main(){
    fold_test_types( 3, 4, "hi", 8);
}

Jul 28

Thoughts on Nuclear Power

A few weeks ago on Slashdot, there was an article on safer nuclear reactors.  It sounded interesting, but of course since it was on Slashdot the comments turned into a not quite a flame-fest, but certainly there tends to be an undercurrent of “I’m right and you’re wrong” whenever something like this comes up(see also: systemd).

Anyway, while I don’t have anything against nuclear power per se, there are a bunch of problems that make it at least somewhat impractical.  First of all, what are the good things about nuclear power?

  • It’s low carbon – see this Wikipedia article for the exact numbers, but suffice to say that once it is running, it is low carbon.
  • It is cheap electricity once it is running.

The ‘once it is running’ is a rather large caveat though – as we’ve seen in Georgia, building new plants is expensive.  Currently, about 27 billion dollars.  That’s quite a lot of money.  Even once it is on-line, there are very expensive long-term costs from personnel to long-term storage of nuclear waste.  A brother of a family friend of mine works at a nuclear power plant, and he’s now retired at ~50.  He still goes back as a contractor, and apparently makes >$100/hour.

One other problem that probably can’t be solved easily is the NIMBY problem – people are afraid of nuclear power due to the accidents that have happened.  I don’t see a way to solve this, since it is a similar problem to people not liking to fly.  Even though flying is the safest mode of transportation, there are many people who don’t like flying.  My suspicion is this is due to the lack of feeling in control, as well as the fear that if something bad does happen, you feel like you will certainly die.  Most plane accidents that people probably think about are the ones where pretty much everybody dies, so they are afraid that there’s no way out if something bad does happen.

The next question of course, assuming that we don’t have nuclear power, is can we actually generate all of our power from renewable sources such as wind and solar?  The answer may be yes, given that Scotland can(under favorable conditions) generate enough power to power it twice over.

Let’s take the above example of the Vogtle nuclear plant, and assume we were to put all of that $27 billion into wind turbines as well.  Assuming that each turbine costs $4 million to install and has a capacity of 2 MW, that means we could install 6,750 turbines for the cost of the nuclear plant, giving us an installed capacity of 13,500 MW.  Actual numbers will probably be lower, given that the wind does not blow all the time, but that’s the theoretical maximum.  According to Wikipedia, the new reactors will have a capacity of 1117 MW each, giving us  a grand total of 2234 MW capacity.

Assuming my math here is correct, it makes much more sense to build wind turbines than to build a nuclear plant(at least in the US).  I suspect(but don’t have the numbers to back it up) that long-term costs are also lower for turbines, since I don’t think that they need much maintenance, plus you don’t have a problem with guarding spent fuel.

If we throw solar in the mix as well, that also has some interesting numbers.  According to Dominion Energy(pg 19), a solar plant array lifetime is 35 years, with one year for construction/destruction takes it to 37 years.  Given that building the Vogtle plant is at least an 8+ year project, it doesn’t seem very feasible to continue on the path of nuclear.

Conclusion: nuclear, while not a bad source of power, has quite a few practical problems compared to modern wind and solar energy.  Of course, each power source has its own pros and cons.  This is not intended to be a fully exhaustive comparison of all energy sources, so you may want to take this with a grain of salt.

Apr 25

Fun with templates

As you may be aware, I maintain dbus-cxx, and I’ve been working on it lately to get it ready for a new release. Most of that work is not adding new features, but updating the code generator to work correctly. However, this post today is not about that work, it is about the work that I am doing on the next major version of dbus-cxx(2.0). Part of this work involves using new C++ features, due to libsigc++ needing C++17 to compile now. With the new variadic templates that C++ has(actually since C++11), we can have more than 7 template parameters to a function. (This limit of 7 is arbitrarily chosen by libsigc++, it’s not a C++ limitation.)

Because of this however, some of the code in dbus-cxx needs to change in order to work correctly. The main portion that I’m working on right now has to do with getting the DBus signatures to work properly. Here’s a small piece of code that is currently in dbus_signal.h(after running dbus_signal.h.m4 through m4):

  /** Returns a DBus XML description of this interface */
  virtual std::string introspect(int space_depth=0) const
  {
    std::ostringstream sout;
    std::string spaces;
    for (int i=0; i < space_depth; i++ ) spaces += " ";
    sout << spaces << "<signal name=\"" << name() << "\">\n";

    T_arg1 arg1;
    sout << spaces << "  <arg name=\"" << m_arg_names[1-1] << "\" type=\"" << signature(arg1) << "\"/>\n";
    T_arg2 arg2;
    sout << spaces << "  <arg name=\"" << m_arg_names[2-1] << "\" type=\"" << signature(arg2) << "\"/>\n";
    sout << spaces << "</signal>\n";
    return sout.str();
  }

This method is created once for each overloaded type that we have. The important part is that T_arg is created once for each argument that we have. With variadic templates, this is impossible to do. The way to get the template arguments out of the variadic call is to do recursion.

Recursion + templates is not something that I’m very familiar with, so this took me a while to figure out. However, I present the following sample code for getting the signature of a DBus method:

  inline std::string signature( uint8_t )     { return "y"; }
  inline std::string signature( bool )        { return "b"; }
  inline std::string signature( int16_t )     { return "n"; }
  inline std::string signature( uint16_t )    { return "q"; }
  inline std::string signature( int32_t )     { return "i"; }
  inline std::string signature( uint32_t )    { return "u"; }

  template<typename... argn> class sig;

   template<> class sig<>{
   public:
   std::string sigg() const {
     return "";
   }
   };

   template<typename arg1, typename... argn>
   class sig<arg1, argn...> : public sig<argn...> {
   public:
   std::string sigg() const{
     arg1 arg;
     return signature(arg) + sig<argn...>::sigg();
   }
   };

int main(int argc, char** argv){
  std::cout << sig<uint32_t,uint32_t,bool,int64_t>().sigg() << std::endl;
}

Output: 
uubx

This took me a few hours to figure out exactly how to do it, so I’m at least a little proud of it! The other confusing part that I had to work out was how to use a recursive call(with signature()) also with the recursive call for the templates, which leads us to the following observation:

It’s recursion all the way down.

Mar 15

One large program or many little programs?

Earlier this week, I came across this question on softwareengineering.stackexchange.com.  This has some relevance for me, since at work two of our main projects follow the two sides of this design philosophy: one project is a more monolithic application, and the other follows more of a microservices model(e.g. many applications).  There are some reasons for this which I will now attempt to explain.

Option 1: Monolithic Application

The first project that I will explain here is our project that has a more monolithic application.  First of all, a brief overview of how this project works.  Custom hardware(running Linux) collects information from sensors(both built-in and often third-party over Modbus) and aggregates it and classifies it.  The classification is due to the nature of the project – sensor data falls into one of several classes(temperature, voltage, etc.).  This information is saved off periodically to a local database, and then synchronized with a custom website for viewing.  The project, for the most part, can be split into these two main parts:

  • Data collector
  • Web viewer
  • Local viewer(separate system, talks over ethernet)

Due to the nature of the hardware, there is no web interface on the hardware directly, it is on a cloud server.

Now, the data collector application is a mostly monolithic application.  However, it is structured similarly to the Linux kernel in that we have specific ‘drivers’ that talk with different pieces of equipment, so the core parts of the application don’t know what hardware they are talking to, they are just talking with specific interfaces that we have defined.

In this case, why did we choose to go with a monolithic application?  Well, there are a few reasons and advantages.

Reason 1: As primarily a data collector device, there’s no real need to have different applications send data to each other.

Reason 2: The development of the system is much easier, since you don’t need to debug interactions between different programs.

Reason 3: Following from the first two, we often have a need to talk with multiple devices on the same serial link using Modbus.  This has to be siphoned in through a single point of entry to avoid contention on the bus, since you can only have one modbus message in-flight at a time.

Reason 4: All of the data comes in on one processor, there is no need to talk with another processor.  Note that this is not the same as talking with other devices.

Reason 5: It’s a lot simpler to pass data around and think about it conceptually when it is all in the same process.

Now that we have some reasons, what are some disadvantages to this scheme?

Disadvantage 1: Bugs.  Since our application is in C++(the ability to use C libraries is important), a single segfault can crash the entire application.

Disadvantage 2: The build can take a long time; the incremental build and linking isn’t bad, but a clean build can take a few minutes.  A test build on Jenkins will take >10 minutes, and it can still take several minutes to compile on a dev machine if you don’t do parallel make.

Overall, the disadvantages are not show-stoppers(except for number 1, there is some bad memory management happening somewhere but I haven’t figured out where yet).  The separation into three basic parts(data collection, local GUI, web GUI) gives us a good separation of concerns.  We do blend in a little bit of option 2 with multiple applications, but that is to allow certain core functionality to function even if the main application is down – what we use that for is to talk with our local cell modem.  Given that the data collection hardware may not be easily accessible, ensuring that the cellular communications are free from bugs in our main application is important.

Option 2: Multiple Applications

If you don’t want to make a monolithic application, you may decide to do a lot of small applications.  One of my other primary projects uses this approach, and the reason is due to the nature of the hardware and how things need to interact.

In our project with multiple applications, we have both multiple compute units and very disparate sensor readings that we are taking in.  Unlike the monolithic application where data is easily classified into categories, this project has even more disparate data.  Moreover, we take in a lot of different kinds of data.  This data can come in on any processor, so there is no ‘master’ application per se.  This data also needs to be replicated to all displays, which may(or may not) be smart displays.  We also want to insulate ourselves from failure in any one application.  A single bug should not take down the entire system.

To handle this, we essentially have a common data bus that connects all of the processors together.  We don’t use RabbitMQ, but the concept is similar to their federation plugin, in that you can publish a message on any processor and it will be replicated to all connected processors.  This makes adding new processors extremely easy.  All of the data is basically made on a producer/consumer model.

Advantage 1: Program resiliency.  With multiple applications running, a bug in one application will not cause the others to exit.

Advantage 2: You can easily add more processors.  This is not really a problem for us, but since data is automatically synchronized between processors, adding a new consumer of data becomes very simple.

Advantage 3: Data can come and go from any connected system, you need not know in advance which processor is giving out information.

This design is not without some caveats though.

Disadvantage 1: Debugging becomes much harder.  Since you can have more than one processor in the system, your producer and your consumer can be on different processors, or you could have multiple consumers.

Disadvantage 2: Because this is a producer/consumer system(it’s the only way that I can see to effectively scale), there’s no way to get data directly from an application(e.g. there’s no remote procedure call easily possible over the network).

Conclusion

There are two very different use cases for these two designs.  From my experience, here’s a quick rundown:

Monolithic Application

  • Generally easier to develop, since you don’t have to figure out program<->program interactions
  • Often needed if you need to control access to a resource(e.g. physical serial port)
  • Works best if you only have to run on one computer at a time

Multiple Applications

  • Harder to develop due to program<->program interactions
  • Better at scaling across multiple computers
  • Individual applications are generally simpler

Due to the nature of engineering, there’s no one way to do this that is best.  There are often multiple ways to solve a given problem, and very rarely is one of them unequivocally the best solution.

Jan 31

Counting lines of code

So a few years ago, me and a coworker had to figure out how many lines of code we had. This was either for metrics or for because we were curious, I can’t really remember why. I came across the script again today while going through some old code. Here it is in all its glory:

#!/bin/bash

let n=0; for x in "$@"; do temp1=`find $x | grep '\\.cpp$\|\\.c$\|\\.java$\|\\.cc$\|\\.h$\|\\.xml$\|\\.sh$\|\\.pl$\|\\.bash$\|\\.proto$'`; temp=`cat /dev/null $temp1 | grep -c [.]*`; let n=$n+$temp; if [ $temp -gt 0 ]; then printf "%s: " $x ; echo $temp; fi; done ; echo Total: $n

This took us at least an hour(or perhaps more), I’m not really sure.

Anyway, it was after we had done all of that work that we realized that wc exists.

Oct 17

Law Abiding Citizen

A few weeks ago, I came across this comment on Reddit about Law Abiding Citizen, and how some people think that the ending was bad.  So I would just like to say that first of all, I do like the ending as it stands(see the parent comment).  It fits with the movie, especially since earlier in the movie Clyde explicitly told Nick that his objective wasn’t to kill him, it was to teach him a lesson.  It’s a bit hard to teach somebody a lesson if they are dead!

However, the two alternative endings are interesting, so here are some thoughts on how those might play out:

The same ending happens, but during the meeting with the city officials the Mayor basically sanctions higher ups to stop Shelton by any means necessary because of how politically damaging it is becoming. She basically gives the go ahead to have them kill him and make it look like an accident, but unbeknownst to them the same feed Shelton was watching in prison is being live-streamed to media new outlets around the world, thereby showing the world that the people that swore to uphold justice will wilfully abandon their morals to save themselves, thus “bring the whole fuckin’ diseased, corrupt temple down…”.

This would be kinda cool, in a revenge sort of way, to perhaps show that corruption goes all the way up.  But I don’t really see how this could be a good ending with teaching Nick a lesson – I think it would have to end with the feed going out, but the bomb still going off, to show that once you break the rules there are consequences.

The original ending plays out the same, except at the end Foxx is sitting at his daughters recital pleased as punch that he beat Butler, even though it was by straight up murdering him letting him die, and his tie suddenly tightens and chokes him to death (the same method that was foreshadowed by the CIA agent earlier in the film). Clyde still dies, but Foxx learns that even the DA is not exempt from ‘action without consequence’ so it’s a little easier to swallow.

So what would also make this very cool would be to have an ending similar to Inception.  You don’t show the actual act, you have the tie start to visibly tighten and then show Nick’s hand going up to his neck to play with the tie – and then cut out. Again though, I don’t think that this fits with the theme of teaching Nick a lesson, since it’s hard to teach him a lesson if he’s dead.  It would be a cool ending though.

Apr 28

English sentences and punctuation

Barbara Bush died a few days ago, and The Onion had this to say about it:

Barbara Bush Passes Away Surrounded By Loved Ones, Jeb

This got me thinking a bit, how can we change the meaning of this sentence just by making some very minor edits to it?  As it stands right no, the comma at the end of the headline make two different groups, Barbara Bush’s loved ones and Jeb.  These groups are separate, and the headline would be the same if the comma was replaced with an ampersand(&).  What happens if we change the comma to a colon though?

Barbara Bush Passes Away Surrounded By Loved Ones: Jeb

As a headline in this case, this is saying that Jeb stated Barbara passed away.  There’s no relationship between Jeb and her loved ones.  Now what would happen if we add more people to the end?

Barbara Bush Passes Away Surrounded By Loved Ones: Jeb, George

By having more than one person here, we are now defining who the loved ones are of Barbara.  At least that’s what first comes to mind for me.

 

Anyway, I just thought that this was interesting.  And quite possibly confusing to people who are just learning English, as the punctuation makes a big difference in this case.

Apr 09

Bitcoin Mining for a Useful Purpose?

So I was thinking about this the other day when I came across this article on Slashdot that points out that GPU prices are high due to the demand for Bitcoin(and other cryptocurrencies) mining.  This got me thinking, what’s the point for this?  What if we could do something useful(well, more useful) than mining for virtual currency?  I’ve been running BOINC for probably about 12+ years now, doing calculations for SETI@Home.  Originally, I wasn’t even using the BOINC client, SETI@Home had their own standalone software that has now been superseded by BOINC.  Which given that the original software was used until 2005, means that I have probably actually been doing this for 15+ years at this point(logging into the SETI website indicates I have been a member since December 2005)…

But I digress.  The question really is, could we mine cryptocurrency as part of the normal BOINC process?  It seems like this would have a number of benefits for people:

  • For mining people, they can still mine coins
  • For projects on BOINC, they gain computing power from people wanting to mine coins at the same time
  • This could result in more computing power for a “good” cause as well, instead of what is(in my mind at least) a rather useless endeavor

I’m not exactly sure how this would work, as I don’t really know anything about blockchain.  Could perhaps Ethereum be used to provide people with “compute credits” that would allow this scheme to work?  It could also provide a good way of storing the calculation results, and have them verifiable.