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Optimizing the Capacitated Vehicle Routing Problem using Genetic Algorithms

Info

Authors

  • Vrund Parikh
  • Manu Puduvalli
  • Lok Kwong
  • Glen George
  • Samuel Yuen

Objective

This project was created to fulfill the term-project requirement for Dr. Khaled Rasheed's CSCI 4560 Evolutionary Computing Applications class at the University of Georgia.

Introduction

Problem

The Capacitated Vehicle Routing Problem (CVRP) is an NP-Hard combinatorial optimization problem. Similar to the travelling salesperson problem, several nodes are placed on a 2D Euclidean space which represent customers. Each node has a numerical value associated with it representing a pickup demand of packages or goods. At the center of a grid space exists a depot responsible for sending out vehicles and all vehicles have a homogenous capacity. Every vehicle must visit each customer and pickup their packages without exceeding the vehicles capacity. The goal is to find the most optimal route for each vehicle with respect to capacity and overall distance.

Research

We solve this problem using evolutionary approaches, specifically genetic algorithms, to optimize the problem set into its best known solution.

Abstract

This project was submitted in conjunction with a conference-like research paper.

This paper studies the effect of optimized heuristic crossover operations by utilizing a Genetic Algorithm to optimize the Capacitated Vehicle Routing Problem (CVRP) and understand the effect on optimized crossovers on diversity maintenance. Best-Route Crossover showed promising results within 5% of the optimal solution on various well-known CVRP data sets. An optimized type of Cycle Crossover exhibited a solution to maintain population diversity and prevent premature convergence.

Index Terms

Best-Route Crossover, Capacitated Vehicle Routing Problem, Cycle Crossover, Genetic Algorithm, Genetic Vehicle Representation, Optimization, Travelling Salesperson Problem, Vehicle Routing Problem

Problem Sets

Problem sets are organized in a custom format called .ocvrp for easier data processing. It's a simple format that is easy to use. It contains 5 required headers and 1 optional COMMENTS header:

  1. Name of the problem set - (str)
  2. Comments for the problem set - (str) (optional)
  3. The dimension of the problem set (including the depot) - (int)
  4. The maximum capacity each vehicle is able to hold (int)
  5. The optimal value for the data set (int)
  6. Nodes for data set

Node data is organized in tabular format with 4 elements per row. These elements are:

  1. Node number - (int)
  2. Node x-coordinate - (int) (float)
  3. Node y-coordinate - (int) (float)
  4. Node service demand - (int) (float)

Header values follow the format:

HEADER: value

And for Node headers:

HEADER:
value
value
...

Ordering of headers, spacing in between a header and value, or spacing in between sets of headers are irrelevant. The numeric values of nodes must be positioned below the NODES header value. Headers are case-insensitive although convention is to use all-CAPS. The first node must be the depot location and comments or any other unrecognizable characters are forbidden. An example of the format is shown below.

NAME: A-n54-k7
COMMENTS: Augerat 1995 Set A
DIM: 54
CAPACITY: 100
OPTIMAL: 1167
NODES:
1 61 5 0
2 85 53 24
3 17 57 9
4 49 93 15
5 69 11 17

...

Execution

Requirements

  • Python 3 - version must be >= 3.6.0
  • Python pip or pipenv

Setup

There are two options to run the algorithm:

  1. With pip 
    python install pip
pip install matplotlib
python driver.py
  2. With pipenv 
    python install pip
pip install pipenv
pipenv install
pipenv run python driver.py

Running without command line arguments runs the program with the default arguments:

  • Population Size: 600
  • Selection Size: 5
  • Number of Generations: 100000
  • Mutation Probability: 0.15
  • Crossover Probability: 0.85
  • Crossover: best_route_xo
  • Mutation: inversion_mut

Command Line

To run the program on your command line, view the arguments by running python driver.py -h or python driver.py --help

usage: driver.py [-h] [-o] [-p] [-s] [-g] [-m] [-c] [-r] [-B | -C | -E | -O]
                 [-I | -W | -G] [-i ] [-P] [-A] [-S] [-R] [-M]
                 file

Runs the CVRP with any of the optional arguments

positional arguments:
  file                the path to the problem set

optional arguments:
  -h, --help          show this help message and exit
  -o , --output       the path to output the results (creates the path if it does not exist)
  -p , --pop          the population size
  -s , --sel          the selection size
  -g , --ngen         the generation size
  -m , --mutpb        the mutation probability
  -c , --cxpb         the crossover probability
  -r , --run          the number of times to run the problem
  -B, --brxo          use best route crossover
  -C, --cxo           use cycle crossover
  -E, --erxo          use edge recombination crossover
  -O, --oxo           use order crossover
  -I, --vmt           use inversion mutation
  -W, --swmt          use swap mutation
  -G, --gvmt          use GVR based scramble mutation
  -i [], --indent []  the indentation amount of the result string
  -P, --pgen          prints the current generation
  -A, --agen          prints the average fitness every 1000 generations
  -S, --save          saves the results to a file
  -R, --routes        adds every route (verbose) of the best individual to the result
  -M, --plot          plot average fitness across generations with matplotlib

Saving Results

If the -S option is specified to save the results to a file, the output is stored in a results directory as a JSON file. If the results directory does not exist, one will be created. If the -o option is specified with -S, the results are saved to a path. The path is created if it does not exist.

The file naming convention for saving results are as follows:

CROSSOVER ALGORITHM_GENERATION SIZE_CROSSOVER PROBABILITY_DATA SET__YYYYMMDD__HH_MM_SSAM/PM

An example is:

best_route_xo_100000_0.85_F-n45-k4__20201213__06_03_29PM

If the -M option is specified to save matplotlib results ta file, the output is stored in a results directory similar to saving the results to a file. If the results directory does not exist, one will be created for you. The file naming convention for matplotlib results are as follows:

CROSSOVER ALGORITHM_GENERATION SIZE_CROSSOVER PROBABILITY_DATA SET__YYYYMMDD__HH_MM_SSAM/PM__RUN NUMBER__FITNESS VALUE

An example is:

best_route_xo_100000_0.85_F-n45-k4__20201213__06_03_29PM__RUN1__FIT732

Using Package

For non-terminal based runs and integration, a CVRP object can be created and run by calling the run() function.

import json
from ocvrp import algorithms as algo
from ocvrp.cvrp import CVRP
from ocvrp.util import CVRPEncoder

# The path to the .ocvrp file is the problem set for this instance
cvrp = CVRP("./data/A-n54-k7.ocvrp", cxpb=0.75, ngen=50_000, pgen=True, plot=True, cx_algo=algo.edge_recomb_xo)

# Result contains a dict of information about the run which includes the best individual found 
result = cvrp.run()

# Save the matplotlib object to a file (only if plot=True)
result['mat_plot'].savefig("A-n54-k7-Run1.png", bbox_inches='tight')

js_res = json.dumps(obj=result, cls=CVRPEncoder, indent=2)
print(js_res)

cvrp.reset()

CVRP.run() will return a dictionary object of the run summary. An example of the object is provided:

{
'name': 'CVRP', 
'problem_set_name': 'F-n45-k4', 
'problem_set_optimal': 724, 
'time': '522.453125 seconds', 
'vehicles': 4, 
'vehicle_capacity': 2010, 
'dimension': 44, 
'population_size': 800, 
'selection_size': 5, 
'generations': 100000, 
'cxpb': 0.85, 
'mutpb': 0.15, 
'cx_algorithm': 'best_route_xo', 
'mut_algorithm': 'inversion_mut', 
'mat_plot': <module 'matplotlib.pyplot'>, 
'best_individual_fitness': 729
}

If verbose_routes is set to True for the CVRP instance, the exact route of the best individual will be added to the dictionary object. To convert the dictionary to a JSON object, a CVRPEncoder class is provided to specify to the json.dumps function.

Testing

A PowerShell script template has been provided under the testing directory for batch processing algorithm runs. There are two versions:

  1. CVRP_Test.ps1
  2. CVRP_TestThreadedJob.ps1

The first option runs a single-threaded job. The second option runs a multi-threaded job but requires PowerShell version 7. To check your PowerShell version, run the following command on your PowerShell terminal:

Get-Host | Select-Object Version

For more information on PowerShell Jobs visit:
https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/start-job?view=powershell-7.1 https://docs.microsoft.com/en-us/powershell/module/threadjob/start-threadjob?view=powershell-7.1 https://docs.microsoft.com/en-us/powershell/module/microsoft.powershell.core/get-job?view=powershell-7.1

We encourage you to modify the script template to meet your needs.