Generating PDFs for Problem Statements

LCOJ can export problem statements to PDF, which is useful for onsite contests where contestants receive printed problem statements.

Note:

• This feature is optional
• This guide covers bare metal installs
• With Docker, you need to set up Pdfoid separately on the host or in another container

Installing Pdfoid

Pdfoid is a service that converts HTML to PDF.

Step 1: Install dependencies

apt update
apt install chromium-driver exiftool

Step 2: Clone Pdfoid

git clone https://github.com/DMOJ/pdfoid.git
cd pdfoid

Step 3: Create a virtual environment

python3 -m venv env
source env/bin/activate
pip install -e .

Step 4: Run Pdfoid

export CHROME_PATH=/usr/bin/chromium
export CHROMEDRIVER_PATH=/usr/bin/chromedriver
export EXIFTOOL_PATH=/usr/bin/exiftool
env/bin/pdfoid --port=8888

If these programs are already in your $PATH, you do not need the exports.

Configuring LCOJ

Add to local_settings.py:

# Pdfoid URL
DMOJ_PDF_PDFOID_URL = 'http://localhost:8888'

# Timeout (seconds)
DMOJ_PDF_PROBLEM_TIMEOUT = 30

Restart

Docker:

docker compose restart site

Bare metal:

supervisorctl restart site

Usage

Generate a PDF for a problem

Go to: https://luyencode.net/problem/<problem_code>/pdf

Example: https://luyencode.net/problem/APLUSB/pdf

Generate PDFs for multiple problems

To generate PDFs for all problems in a contest:

1. Go to the contest page
2. Click Download problems as PDF
3. Select the problems to download
4. Click Generate PDF

Running Pdfoid with Supervisor

Create the file /etc/supervisor/conf.d/pdfoid.conf:

[program:pdfoid]
command=/path/to/pdfoid/env/bin/pdfoid --port=8888
directory=/path/to/pdfoid
user=pdfoid
environment=CHROME_PATH="/usr/bin/chromium",CHROMEDRIVER_PATH="/usr/bin/chromedriver",EXIFTOOL_PATH="/usr/bin/exiftool"
autostart=true
autorestart=true
redirect_stderr=true
stdout_logfile=/var/log/pdfoid.log

Start it:

supervisorctl update
supervisorctl start pdfoid

Customizing PDFs

Custom CSS

Add PDF-specific CSS in local_settings.py:

DMOJ_PDF_PROBLEM_EXTRA_CSS = """
@page {
    size: A4;
    margin: 2cm;
}
body {
    font-family: "Times New Roman", serif;
    font-size: 12pt;
}
"""

Header/Footer

DMOJ_PDF_PROBLEM_HEADER = """
<div style="text-align: center; font-size: 10pt;">
    LuyenCode Online Judge
</div>
"""

DMOJ_PDF_PROBLEM_FOOTER = """
<div style="text-align: center; font-size: 10pt;">
    Page <span class="pageNumber"></span> / <span class="totalPages"></span>
</div>
"""

Troubleshooting

PDF generation fails:

• Check that Pdfoid is running: curl http://localhost:8888
• Check that Chrome/Chromium is installed
• View Pdfoid logs (Docker): docker compose logs -f pdfoid (if running in Docker)
• View Pdfoid logs (bare metal): supervisorctl tail -f pdfoid

PDF has font issues:

• Install the required fonts:

apt install fonts-liberation fonts-dejavu

Timeout:

• Increase DMOJ_PDF_PROBLEM_TIMEOUT
• Check that the server has enough RAM

Images do not appear:

• Make sure images use absolute URLs (not relative paths)
• Check that the images are accessible from the server

Optimization

Cache PDFs

To avoid regenerating PDFs repeatedly:

DMOJ_PDF_PROBLEM_CACHE = '/home/lcoj/pdf_cache'
DMOJ_PDF_PROBLEM_CACHE_TIME = 3600  # 1 hour

Create the directory:

mkdir -p /home/lcoj/pdf_cache
chown www-data:www-data /home/lcoj/pdf_cache

Reduce PDF size

DMOJ_PDF_PROBLEM_COMPRESS = True

Parallel processing

If you need to generate many PDFs at once, run multiple Pdfoid instances:

# Instance 1
env/bin/pdfoid --port=8888

# Instance 2
env/bin/pdfoid --port=8889

Configure load balancing in local_settings.py:

DMOJ_PDF_PDFOID_URLS = [
    'http://localhost:8888',
    'http://localhost:8889',
]

Printing PDFs

Print settings

When printing PDFs, we recommend that you:

• Choose A4 paper
• Set 2cm margins on each side
• Print double-sided to save paper
• Check the print preview before printing

Quantity

Calculate the number of copies needed:

• Number of contestants × Number of problems
• Add 10% as a buffer
• Add copies for the judges

Example Workflow

Preparing an onsite contest

1. Create a contest with the problems
2. Check that the problem statements render correctly
3. Generate a PDF for each problem
4. Review the PDFs
5. Print the PDFs
6. Package the problem statements

Automation script

#!/bin/bash
CONTEST="contest_key"
PROBLEMS=("APLUSB" "SORTING" "GRAPH")

for problem in "${PROBLEMS[@]}"; do
    curl "https://luyencode.net/problem/$problem/pdf" \
         -o "${problem}.pdf"
    echo "Downloaded $problem.pdf"
done