=head1 Data First OOP

=head2 Introduction

It's pretty common to hear people extolling the virtues of Object Oriented Programming, or OOP.  Usually they talk about the wonders of inheritance and polymorphism.  This is great if you have a background in computer science and know what these things are and why they are desirable. 

My take on OOP is different. OOP is great because it makes it easy to work with data.  Everything else is just a side benefit.

In my view, the most important thing about OOP is that B<Objects tie data to behaviors>. 


=head2 Caught in a BIND


Before I did a lot of OOP, I often found myself with a big nested data structure or two at the heart my code. These things are such a pain that I find the nickname BIND (BIg Nested Data) appropriate.

If you've ever tried to work with a hash of arrays of hashes or something similar, you know how unpleasant it can be.  Now go two or three levels deeper.  Add some non-uniform layers where some elements are hashes, others arrays or scalars.  It very quickly becomes tedious to write routines  operate on BINDs.  What routine has to reach into what part? Oh no, I need to restructure a bit, and I have 15 subroutines to modify now!  Not to mention all the debugging when I absent-mindedly use the wrong subscript or access the wrong part of the BIND.

This is where objects come to the rescue.


=head2 Example Acme Cab Company

Rather than go into a big theoretical discussion, I'll start with an example.

Let's consider an application that generates reports for a taxi company.  At the end of each shift, each driver turns in a log with a list of his fares.  We are putting together a program to read in these logs and produce some reports.


    my $shift_data = {
        'DAY' => {
            'George Smith' => [
                [   pickup  => [ '08:12:30', '123 Anystreet'   ]
                    dropoff => [ '08:45:19', '432 Otherstreet' ]
                    fare    =>  '$25.62',
                    source  => 'dispatch',
                ],
                [   pickup  => [ '09:02:05', '23 Maple Ave'   ]
                    dropoff => [ '10:45:19', '32 Box Bvd' ]
                    fare    =>  '$175.20',
                    source  => 'flagged',
                ],
            ],

            'Ralph Stone' => [
                [   pickup  => [ '08:12:30', '123 Anystreet'   ]
                    dropoff => [ '08:45:19', '432 Otherstreet' ]
                    fare    =>  '$25.62',
                    source  => 'dispatch',
                ],
                [   pickup  => [ '09:02:05', '23 Maple Ave'   ]
                    dropoff => [ '10:45:19', '32 Box Bvd' ]
                    fare    =>  '$175.20',
                    source  => 'flagged',
                ],
            ],
        },
        'SWING' => {
            'Frank Franklin' => [
                [   pickup  => [ '08:12:30', '123 Anystreet'   ]
                    dropoff => [ '08:45:19', '432 Otherstreet' ]
                    fare    =>  '$25.62',
                    source  => 'dispatch',
                ],
                [   pickup  => [ '09:02:05', '23 Maple Ave'   ]
                    dropoff => [ '10:45:19', '32 Box Bvd' ]
                    fare    =>  '$175.20',
                    source  => 'flagged',
                ],
            ],

            'Jack Jones' => [
                [   pickup  => [ '08:12:30', '123 Anystreet'   ]
                    dropoff => [ '08:45:19', '432 Otherstreet' ]
                    fare    =>  '$25.62',
                    source  => 'dispatch',
                ],
                [   pickup  => [ '09:02:05', '23 Maple Ave'   ]
                    dropoff => [ '10:45:19', '32 Box Bvd' ]
                    fare    =>  '$175.20',
                    source  => 'flagged',
                ],
            ],
        },
        'GRAVEYARD' => {
            'Amy Ericson' => [
                [   pickup  => [ '08:12:30', '123 Anystreet'   ]
                    dropoff => [ '08:45:19', '432 Otherstreet' ]
                    fare    =>  '$25.62',
                    source  => 'dispatch',
                ],
                [   pickup  => [ '09:02:05', '23 Maple Ave'   ]
                    dropoff => [ '10:45:19', '32 Box Bvd' ]
                    fare    =>  '$175.20',
                    source  => 'flagged',
                ],
            ],

            'Patrick Brown' => [
                [   pickup  => [ '08:12:30', '123 Anystreet'   ]
                    dropoff => [ '08:45:19', '432 Otherstreet' ]
                    fare    =>  '$25.62',
                    source  => 'dispatch',
                ],
                [   pickup  => [ '09:02:05', '23 Maple Ave'   ]
                    dropoff => [ '10:45:19', '32 Box Bvd' ]
                    fare    =>  '$175.20',
                    source  => 'flagged',
                ],
            ],
        },

    };


A nice clean structure.  Easy enough to see that for a given day we have three shifts.  Each shift has a few drivers who have a few fares.  Each fare has some information about where and when it started and ended, as well as the money collected and whether the fare came from central dispatch or from being flagged down on the street.

On the downside, it is complicated enough that it will be tricky to write code to dig into it.

This is where we turn the structure into a group of objects and save ourselves some confusion.

How do we break this BIND into objects? The same way we broke it down when reading it.  I like to do this kinds of conversion from the deepest layer out to the top layer.

Our deepest data structure is an array of two values labeled as either C<pickup> or C<dropoff>.  What are pickups and dropoffs?  They are times when the cab stops.  So let's call the object 'Stop'.  We now have a name, what information is important about a stop?  Well, we have a location and a time.  So we define a stop as an object with a location and a time.

package AcmeCab::Stop;
use strict;
use warnings;

sub new {
    my $class = shift;

    my $self = {};
    bless $self, $class;

    # Initialize any attributes
    $self->$_( $arg{$_} ) for keys %arg;

    return $self;
}

sub location {
    my $self = shift;

    if( @_ ) {
        my $loc = shift;
        
        # Add some validation here.

        $self->{location} = $loc;
    }

    return $self->{location};
}

sub time {
    my $self = shift;

    if( @_ ) {
        my $time = shift;
        
        # Add some validation here.

        $self->{time} = $time;
    }

    return $self->{time};
}

1;

Now a fare can be thought of as:

    [   pickup  => AcmeCab::Stop->new( time => '08:12:30', location => '123 Anystreet' ),
        dropoff => AcmeCab::Stop->new( time => '08:45:19', location => '432 Otherstreet' ),
        fare    =>  '$25.62',
        source  => 'dispatch',
    ],

Which brings us to the next level of the structure: The Fare.  So let's make AcmeCab::Fare.  Each fare has two stops, an amount charged and source.  So we write:

   
package AcmeCab::Fare;
use strict;
use warnings;
use Carp qw(croak);

use constant SOURCES => qw( dispatch flagged cabstand );
my %SOURCES; @SOURCES{ SOURCES() } = ();

sub new {
    my $class = shift;

    my $self = {};
    bless $self, $class;

    # Initialize any attributes
    $self->$_( $arg{$_} ) for keys %arg;

    return $self;
}

sub pickup {
    my $self = shift;

    if( @_ ) {
        my $pu = shift;

        croak 'pickup must be an AcmeCab::Stop object'
            unless $pu->isa('AcmeCab::Stop');

        $self->{pickup} = $pu;
    }

    return $self->{pickup};
}

sub dropoff {
    my $self = shift;

    if( @_ ) {
        my $do = shift;

        croak 'dropoff must be an AcmeCab::Stop object'
            unless $do->isa('AcmeCab::Stop');

        $self->{dropoff} = $do;
    }

    return $self->{dropoff};
}

sub source {
    my $self = shift;

    if( @_ ) {
        my $source = shift;
        
        croak 'source must be one of ' . join(",", map " '$_'", SOURCES)
            unless exists $SOURCES{$source};

        $self->{source} = $source;
    }

    return $self->{source};
}

sub fare {
    my $self = shift;

    if( @_ ) {
        my $fare = shift;
        
        $self->{fare} = $fare;
    }

    return $self->{fare};
}

1;

What else do we want know about a fare?  What about how long it lasted?  Add a subroutine/method called 'duration' and you've got it.

=head2 Results

"So what?" you may be asking. There's a lot of code, and it doesn't do anything.

Let's consider what would be needed to calculate total earning time for each driver on a given shift.

First with the bind:

for my $shift ( qw( DAY NIGHT GRAVEYARD ) ) {
}