IMO/download_script/download.py

# -----------------------------------------------------------------------------
# Author: Jiawei Liu
# Date: 2025-10-25
# -----------------------------------------------------------------------------
'''
Download script for APMO
To run: 
`python IMO/download_script/download.py`
'''

import re
import requests
from bs4 import BeautifulSoup
from tqdm import tqdm
from pathlib import Path
from requests.adapters import HTTPAdapter
from urllib3.util.retry import Retry
from urllib.parse import urljoin


def build_session(
    max_retries: int = 3,
    backoff_factor: int = 2,
    session: requests.Session = None
) -> requests.Session:
    """
    Build a requests session with retries

    Args:
        max_retries (int, optional): Number of retries. Defaults to 3.
        backoff_factor (int, optional): Backoff factor. Defaults to 2.
        session (requests.Session, optional): Session object. Defaults to None.
    """
    session = session or requests.Session()
    adapter = HTTPAdapter(max_retries=Retry(total=max_retries, backoff_factor=backoff_factor))
    session.mount("http://", adapter)
    session.mount("https://", adapter)
    session.headers.update({
        "User-Agent": "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.3"
    })

    return session


def main():
    base_url = "https://web.evanchen.cc/problems.html"
    req_session = build_session()

    output_dir = Path(__file__).parent.parent / "raw"
    output_dir.mkdir(parents=True, exist_ok=True)

    resp = req_session.get(base_url)
    soup = BeautifulSoup(resp.text, 'html.parser')

    imo_exams = soup.find_all("a", href=lambda h: h and re.search(r"IMO\-\d{4}\-notes.pdf$", h))
    imo_pdf_links = [urljoin(base_url, link["href"]) for link in imo_exams]

    for link in tqdm(imo_pdf_links):
        output_file = output_dir / f"en-{Path(link).name}"

        # Check if the file already exists
        if output_file.exists():
            continue

        pdf_resp = req_session.get(link)

        if pdf_resp.status_code != 200:
            print(f"Failed to download {link}")
            continue

        output_file.write_bytes(pdf_resp.content)


if __name__ == "__main__":
    main()