Day 23: Christmas-oriented design and implementation

Elves graduating from the community college, in front of a Santa Statue

Every year by the beginning of the school year, which starts by January 8th in the North Pole, after every version of the Christmas gift-giving spirit has made their rounds, Santa needs to sit down to schedule the classes of the North Pole Community College. These elves need continuous education, and they need to really learn about those newfangled toys, apart from the tools and skills of the trade.

Plus it’s a good thing to have those elves occupied during the whole year in something practical and useful, so that they don’t start to invent practical jokes and play them on each other.

Since there are over one million elves, the NPCC is huge. But there’s also a huge problem assigning courses to classrooms. Once registration for classes is open, they talk to each other about what’s the ultimate blow off class, which one gives you extra credit for winning snowball fights. So you can’t just top up enrollment: every year, you need to check the available classrooms, and then match it to the class that will be the most adequate for it.

Here are the available classrooms:

Kris, 33
Kringle, 150
Santa, 120
Claus, 120
Niklaas, 110
Reyes Magos, 60
Olentzero, 50
Papa Noël, 30

They’re named after gift-giving spirits from all over the world, with the biggest class obviously named Kringle. In any given year, this could be the enrollment after the registration period is over.

Woodworking 101, 130
Toymaking 101, 120
Wrapping 210, 40
Reindeer speed driving 130, 30
ToyOps 310, 45
Ha-ha-haing 401, 33

They love woodworking 101, because it’s introductory, and they get to keep whatever assignment they do during the year. Plus you get all the wood parings for burning in your stove, something immensely useful in a place that’s cold all year long.

So Santa created this script to take care of it, using a bit of point free programming and Perl being Perl, the whippipitude and dwimmability of the two sister languages, Perl and Raku.

sub read-and-sort( $file where .IO.e ) {
      ==> map( *.split( /","\s+/) )
      ==> sort( { -$_[1].Int } )
      ==> map( { |@_ ) } )

say (read-and-sort( "classes.csv") Z read-and-sort( "courses.csv"))
    .map( {  $ { .key } ).join( "\t→\t") }  )
    .join( "\n" )

The subroutine reads the file given its name, checking that it exists before, splits it by the comma, sorts it in decreasing number, and then creates a pair out of it. The other command uses the Z operator to zip the two lists together in decreasing order of elves, and produce a list just like this one:

Kringle	→	Woodworking 101
Santa	→	Toymaking 101
Claus	→	ToyOps 310
Niklaas	→	Wrapping 210
Reyes Magos	→	Ha-ha-haing 401
Olentzero	→	Reindeer speed driving 130

So the Kringle lecture hall gets woodworking, and it goes down from there. The Kris and Papa Noël classroom get nothing, having been eliminated but kept there to be used for extra-curricular activities such as carol singing and origami.

All this works well while it does: as long as you remember where’re the files, what the script did, nothing changes name or capacity, and the files are not lost. But those are a lot of ifs, and Santa is not getting any younger

As a matter of fact, not getting any older either.

So Santa and its ToyOps team will need a more systematic approach to this scheduling, by creating an object oriented application from requirements. After learning all about TWEAKS and roles, now it’s the time to stand back and put it to work from the very beginning.

Agile scheduling

The cold that pervades the North Pole makes everything a little less agile. But no worries, we can still be agile when we create something for IT operations there. First thing we need are user stories. Who wants to create a schedule and what is it? So let’s sit down and write them down.

  • [US1] As a NPCC dean, given I have a list of classrooms (and their capacity) and a list of courses (and their enrolment), I want to assign classrooms to courses in the best way possible.

OK, we got something to work on here, so we can apply a tiny bit of domain driven design. We have a couple of entities, classrooms and courses, and a few value objects: single classrooms and single courses. Let’s go and write them. Using Comma, of course.


Using classes for classes is only natural. But looking at the two defined classes, Santa couldn’t say which was which. At the end of the day, something with a name and a capacity is something with a name and a capacity. This begs for a factoring out of the common code, following the DRY (don’t repeat yourself) principle.

Besides, we have a prototype above that pretty much says that whatever we use for classrooms and courses, our life is going to be easier if we can sort it in the same way. So it’s probably best if we spin off a role with the common behavior. Let’s make an issue out of that. Pretty much the same as the US, but the protagonist is going to be a programmer:

  • As a programmer, I need to sort classrooms and courses by capacity using the same.

Let’s call it Capped, as in having a certain capacity. Since both objects will have the self-same method, capacity, we can call it to sort them out. Our example above shows that we need to create an object out of a couple of elements, so that’s another issue

  • As a programmer, I need to build an object using positional arguments that are strings.

So finally our role will have all these things:

unit role Cap;

has Str $!name is required;
has Int $!capacity is required;

submethod new( Str $name, Int $capacity ) {
    self.bless( :$name, :$capacity )

submethod BUILD( :$!name, :$!capacity ) {};

method name() { $!name }
method capacity() { $!capacity }

plus handy accessors for name and capacity, all the while keeping these privates, also implying that they are immutable. Value objects are things that simply get a value, there’s not much business logic to them.

We could already create a function that does the sorting out of a list of classrooms/courses, that is, Caps, but in OO design we should try and put into classes (from which we spawn objects) as many entities from the original problem as possible. These entities will be the ones actually doing the heavy lifting.

It would be great, again, if we could create kinda the same things, because we will able to handle them uniformly. But there’s the conundrum: one of them will contain a list of Courses, another a list of Classrooms. They both do Cap, so in principle we could create a class that hosts lists of Caps. But this controller class will have some business logic: it will create objects of that class; we can’t simply use Roles to create classes that compose them. So we will use a curried Role, a parametrized role that uses, as a parameter, the role we’ll be instantiating it with. This will be Cap-List:

unit role Cap-List[::T];

has T @!list;

submethod new( $file where .IO.e ) {
            ==> map( *.split( /","\s+/) )
            ==> map( { @_[0], +@_[1] ) } )
            ==> sort( { -$_.capacity } )
            ==> my @list;
    self.bless( :@list );
submethod BUILD( :@!list ) {}

method list() { @!list }

This code is familiar and similar to what we’ve done above, except we’re swapping the object creation and sorting list, and we’re use .capacity to sort the list. We create a list and bless it into the object. Out of that, we create a couple of classes:

unit class Classroom-List does Cap-List[Classroom];
unit class Course-List does Cap-List[Course];

We don’t need any more logic; that’s all there is. It’s essentially the same thing, same business logic, but we’re working in a type-safe way. We have also tested the whole thing, so we’ve frozen the API and protected it from future evolution. Which Santa approves.

So we’re almost there. Let’s write the assignment function with this:

my $courses = "docs/courses.csv");
my $classes = "docs/classes.csv");
say ($classes.list Z $courses.list )
        .map( {  $ { .name } ).join( "\t→\t") }  )
        .join( "\n" );

This returns the same thing as we had before. But we’ve hidden all business logic (sorting, and anything else we might want) in the object capsule.

But, have we?

Not actually. Assignment should also be encapsulated in some class, and thoroughly tested. That’s, however, left for another occasion.

Published by jjmerelo

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2 thoughts on “Day 23: Christmas-oriented design and implementation

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