This parking lot command line utility accepts a file as an argument and processes the contents of the file (basically on a line by line basis). Each line is basically a command to be executed with certain parameters. Right now there are four commands which could be accepted.

1. create_parking_lot {capacity}
   This command creates a parking lot with the capacity of slots provided as a parameter. It accepts integer as the parameter
2. park {reg_number} This command parks a vehicle in any of the nearest slot available from the entrance.
3. leave {reg_number} {hours} This command vacates a vehicle from parking lot and computes the parking fee for number of hours provided.
4. status This command doesn't needs any parameter and prints the status of the parked vehicles.

The application is designed by implementing Command design pattern. The four possible command's translate into different command classes which have a common base class. These classes are

1. Command::BaseCommand
2. Command::Status
3. Command::Create
4. Command::Park
5. Command::Leave

In order to read input file and process it's contents line by line following classes are used.

1. Processor::CommandProcessor
2. Processor::CommandFactory

A CommandProcessor is initialised through a script which takes a file-name as an argument. The CommandProcessor's process method take a line of the file as an argument and processes it. It in turn breaks it up into commands and parameters and then delegates the task of building the command to CommandFactory's make method. CommandFactory's make method takes the command_key, parameters and current_context as the arguments. Any command works over a context and holds the reference to it. Once the command is executed we get the message as output. This forms our output. Also, after executing the command from it the context is fetched so that this represent the current state of the system.

The idea behind encapsulating all model classes and parameters in a context is to separate state from behaviour. A Context essentially represents the current state of the system. Any command that needs to mutate the state has to create a new context and then returns the new context to the CommandProcessor. The CommandProcessor retains this context so that current context forms the basis for the next command execution.

The model classes are fairly simple to understand. I am listing those over here

1. Vehicle (base class for any type of vehicle)
2. Car (sub class of vehicle)
3. Slot (A place to park a vehicle)
4. Parking Lot (A place with many slots)
5. Context (A class to hold the reference to the ParkingLot and any parameters, which are passed for the execution of a command)

First, install Ruby. Then run the following commands under the main dir.

parking-lot-problem $ ruby -v # confirm Ruby present
ruby 3.0.0p0 (2020-12-25 revision 95aff21468) [x86_64-darwin19]
parking-lot-problem $ gem install bundler # install bundler to manage dependencies
Fetching bundler-2.2.15.gem
Successfully installed bundler-2.2.15
Parsing documentation for bundler-2.2.15
Installing ri documentation for bundler-2.2.15
Done installing documentation for bundler after 2 seconds
1 gem installed
parking-lot-problem $ bundle install # install dependencies
Using bundler 2.2.15
Fetching rspec-support 3.7.1
Fetching diff-lcs 1.3
Fetching method_source 0.9.2
Fetching coderay 1.1.2
Installing method_source 0.9.2
Installing rspec-support 3.7.1
Installing diff-lcs 1.3
Installing coderay 1.1.2
Fetching rspec-core 3.7.1
Fetching rspec-expectations 3.7.0
Fetching rspec-mocks 3.7.0
Fetching pry 0.12.2
Installing rspec-mocks 3.7.0
Installing rspec-expectations 3.7.0
Installing rspec-core 3.7.1
Installing pry 0.12.2
Fetching rspec 3.7.0
Installing rspec 3.7.0
Bundle complete! 3 Gemfile dependencies, 10 gems now installed.
Use `bundle info [gemname]` to see where a bundled gem is installed.
functional_spec $ 

You can run the full suite from parking_lot_2.0.0 by doing

parking_lot $ rspec 
...........................

Finished in 0.01671 seconds (files took 0.4255 seconds to load)
27 examples, 0 failures

You can run the application using bin/parking_lot by doing

parking-lot-problem $ ./bin/parking_lot fixtures/file_input.txt

• Hussain Pithawala