# Parking Lot Problem

## Info

This Parking Lot Problem is a popular programming assignment at various companies, like Google and Amazon. We are to utilize the appropriate data structures to solve a straightforward yet intricate problem of assigning vehicle slots, in Golang. The problem statement and its solution are first described. Finally, a code refactoring exercise, which is an extension to the original problem, is posed at the bottom of this page.

## Problem Statement

Mr Zorro owns a multi-storey parking lot that can hold up to n vehicles at any given point in time. The parking slots are numbered, beginning at 1 and increases with increasing distance from the entry point in steps of one. Mr Zorro has requested your help to design an automated ticketing system for his parking lot.

When a vehicle enters the parking lot, its vehicle registration number (i.e., number plate) and colour are noted. Then, an available parking slot is allocated. Following are the rules of parking slot ticket issuance:

• Each customer should be allocated the nearest available parking slot to the entry point.
• Upon exiting the parking lot, the customer returns the ticket which marks their previously allocated lot as now available.
• The system should provide the ability to determine:
• Registration numbers of all cars of a particular colour.
• Slot number in which a car with a given registration number is parked.
• Slot numbers of all slots where a car of a particular colour is parked.

The ticketing system should be operable via two modes of input, namely, interactive commands and commands from a file. In other words, the ticketing system should be an executable which accepts:

1. Interactive commands from an interactive command prompt shell
2. A filename as an input argument at the command prompt and executes the commands from the given file

Example below includes all the commands which need to be supported.

Example: File Input

To run the code so it accepts input from a file:

$bin/parking_lot file_inputs.txt  Input (contents of file): create_parking_lot 6 park KA-01-HH-1234 White park KA-01-HH-9999 White park KA-01-BB-0001 Black park KA-01-HH-7777 Red park KA-01-HH-2701 Blue park KA-01-HH-3141 Black leave 4 status park KA-01-P-333 White park DL-12-AA-9999 White registration_numbers_for_cars_with_colour White slot_numbers_for_cars_with_colour White slot_number_for_registration_number KA-01-HH-3141 slot_number_for_registration_number MH-04-AY-  Output (to STDOUT): Created a parking lot with 6 slots Allocated slot number: 1 Allocated slot number: 2 Allocated slot number: 3 Allocated slot number: 4 Allocated slot number: 5 Allocated slot number: 6 Slot number 4 is free Slot No. Registration No Colour 1 KA-01-HH-1234 White 2 KA-01-HH-9999 White 3 KA-01-BB-0001 Black 5 KA-01-HH-2701 Blue 6 KA-01-HH-3141 Black Allocated slot number: 4 Sorry, parking lot is full KA-01-HH-1234, KA-01-HH-9999, KA-01-P-333 1, 2, 4 6 Not found  Example: Interactive To run the code, launch the shell, and to accept interactive input from the shell: $ bin/parking_lot


Assuming a parking lot with n=6 slots, the following commands should be run in sequence by typing them in at a prompt and should produce output as described below the command. Note that exit terminates the process and returns control to the shell.

$create_parking_lot 6 Created a parking lot with 6 slots$ park KA-01-HH-1234 White
Allocated slot number: 1
$park KA-01-HH-9999 White Allocated slot number: 2$ park KA-01-BB-0001 Black
Allocated slot number: 3
$park KA-01-HH-7777 Red Allocated slot number: 4$ park KA-01-HH-2701 Blue
Allocated slot number: 5
$park KA-01-HH-3141 Black Allocated slot number: 6$ leave 4
Slot number 4 is free
$status Slot No. Registration No Colour 1 KA-01-HH-1234 White 2 KA-01-HH-9999 White 3 KA-01-BB-0001 Black 5 KA-01-HH-2701 Blue 6 KA-01-HH-3141 Black$ park KA-01-P-333 White
Allocated slot number: 4
$park DL-12-AA-9999 White Sorry, parking lot is full$ registration_numbers_for_cars_with_colour White
KA-01-HH-1234, KA-01-HH-9999, KA-01-P-333
$slot_numbers_for_cars_with_colour White 1, 2, 4$ slot_number_for_registration_number KA-01-HH-3141
6
$slot_number_for_registration_number MH-04-AY-1111 Not found$ exit


## Repository

The repository contains the following:

• Golang solution code

## Learning Outcome

At the end of this project, we should be able to:

• solve the Parking Lot problem with data structures optimized for complexity
• write comprehensive unit tests in Golang
• perform functional testing in Golang
• utilize heap, hash map, and slice, data structures in Go
• pretty print slices, arrays, and strings, using an interface

## Instructions

1. Setup Go
• Install Go following the instructions here.
• Set GOROOT which is the location of your Go installation. Assuming it is installed at $HOME/go2.x, execute:  export GOROOT=$HOME/go2.x
export PATH=$PATH:$GOROOT/bin

• Set GOPATH environment variable which specifies the location of your Go workspace. It defaults to $HOME/go on Unix/Linux.  export GOPATH=$HOME/go

• Set GOBIN path for generation of binary file when go install is run.
  export GOBIN=$GOPATH/bin export PATH=$PATH:$GOPATH/bin  2. Source code • Git clone the project into $GOPATH/src/parking_lot folder in your computer.
  git clone https://github.com/Adaickalavan/Parking-Lot-Problem.git $GOPATH/src/parking_lot  Here, git clone syntax follows the pattern: git clone <repo> <local folder-name>. 3. Executable • To create an executable in the $GOPATH/bin/ directory, execute
  go install parking_lot

4. Unit test and functional test
• To run complete test suite, run
  go test -v parking_lot


Here, -v is the verbose command flag.

• To run specific test, run
  go test -v parking_lot -run xxx


Here, xxx is the name of test function.

• Test coverage: 94.1% of statements
5. Running
• Launch interactive user input mode by executing
  $GOPATH/bin/parking_lot  • Launch file input mode by executing $GOPATH/bin/parking_lot.exe $GOPATH/src/parking_lot/inputFile.txt  Here, $GOPATH/src/parking_lot/inputFile.txt refers to the input file with complete path.

## Project structure

The project structure is as follows:

project                               # folder containing all project files
├── bin                               # contains executable commands
│   ├── setup                         # place your commands to build/compile the Go code here in setup file
│   └── parking_lot.exe               # .exe file for parking_lot generated by go install command
└── src                               # contains Go source files
└── parking_lot                   # main folder
├── vendor                    # folder containing dependencies
│   ├── minheap               # dependant package minheap
│   │   ├── item.go           # element of heap
│   │   └── priorityQueue.go  # min heap implementation
│   └── pretty                # dependant package pretty
│       └── printer.go        # pretty prints array, slice, string
├── car.go                    # element of carpark
├── carpark.go                # carpark struct and pointer receiver methods
├── carpark_test.go           # unit tests of the carpark.go code
├── main.go                   # main file of Go code
├── main_test.go              # functional test of the main code
├── inputFile.txt             # sample input file for testing
└── inputInteractive.txt      # sample interactive input for testing


## Notes on solution

1. Data structures
• A hash map and a min heap was used to solve the parking lot problem.
2. Complexity
• To park a car: O(log(n1)). Here, n1 is the size of the min heap.
• To remove a car: O(log(n1)). Here, n1 is the size of the min heap.
• To retrieve a car by colour: O(n2). Here, n2 is the size of the hash map.
• To retrieve a car by registration number: O(n2). Here, n2 is the size of the hash map.
• To get status: O(n2). Here, n2 is the size of the hash map.
3. Assumptions and rationale for choice of data structures to optimize complexity
• Car parking and removing operations will be more frequent compared to retrieving car by colour/registration number or status requests.
• A hash map with slot number as key is used to store all the cars parked in the carpark. Complexity O(1) of hash map simplifies insertion and removal of cars by slot number.
• A min heap is used to store previoulsy-occupied-but-now-empty slots in ordered sequence with complexity O(log(n1)) for push and pop operations. Here, empty slots n1 refer only to slots which were previously occupied but is now free. It does not refer to the total number of free slots in the carpark.
4. Alternative solutions to reduce complexity at the expense of increased memory
• To achieve complexity O(1) in retrieving a car by colour, implement an additional hash map with colour as key to store all the cars parked in the carpark.
• To achieve complexity O(1) in retrieving a car by registration number, implement an additional hash map with registration number as key to store all the cars parked in the carpark.

## Code Refactoring

This code refactoring problem is provided as an exercise for the readers.

Thanks to you, Mr Zorro’s multi­storey parking lot is functioning efficiently. Mr Zorro has now expanded his business by building two additional multistorey parking lots in adjacent plots of land. The multistorey parking lots are numbered in an increasing order (i.e., 1, 2, 3, etc). He needs your help to refactor your original ticketing system to manage all three multistorey parking lots.

1. The expanded ticketing system should provide the same ability to determine
• registration numbers of all cars of a particular colour,
• slot number in which a car with a given registration number is parked, and
• slot numbers of all slots where a car of a particular colour is parked,

across all three parking lots. For example, the query slot_numbers_for_cars_with_colour white should return all white cars irrespective of the parking lot they are parked in.

2. The expanded ticketing system should distribute customers among the three parking lots following either one of two rules. The ticketing system should accept rule changes dynamically during operation.
• Rule A - Even Distribution: Vehicles are evenly distributed among the three parking lots based on the their occupancy rate. Occupancy rate is defined as percentage of parking slots filled within a multistorey parking lot. Multistorey parking lot with lower occupancy rate is filled first. If multiple multistorey parking lots have the same occupancy rate, the lowest numbered multistorey parking lot is chosen.
• Rule B - Sequential Allocation: Vehicles are assigned to the lowest numbered multistorey parking lot which has available parking slots. That is to say, vehicles fill up multistorey parking lot 1 before filling parking lot 2, and parking lot 2 is filled before parking lot 3.
3. Remember that each customer should still be allocated the nearest available parking slot to the entry point within the multistorey parking lot to which they were assigned to.

4. Vehicle registration number (i.e., number plate) and colour are noted upon ticket issuance. Upon exiting the parking lot, the customer returns the ticket which marks their previously allocated lot as now available.

Example

The following commands run in sequence should produce output as shown below:

Input Output
create_parking_lot 5 Created a parking lot with 5 slots
create_parking_lot 3 Created a parking lot with 3 slots
create_parking_lot 6 Created a parking lot with 6 slots
dispatch_rule even_distribution Dispatcher is now using the Even Distribution rule

Updated: