Filtering CLIST API over a serverless function.

index.js

// URLS: 
// Codeforces: https://filtered-cal.vercel.app/api/codeforces
// Codechef: https://filtered-cal.vercel.app/api/codechef
// Atcoder: https://filtered-cal.vercel.app/api/atcoder

const express = require("express");

const app = express();

/**
 * Keyword filters (lowercase)
 */
const FILTERS = {
  codeforces: ["icpc"],
  codechef: ["dsa challenge"],
  atcoder: ["heuristic"]
};

/**
 * Source calendars
 */
const SOURCES = {
  codeforces:
    "https://calendar.google.com/calendar/ical/br1o1n70iqgrrbc875vcehacjg@group.calendar.google.com/public/basic.ics",
  codechef:
    "https://calendar.google.com/calendar/ical/ogc7qt4hlg454ggkj9o6ttqnq8@group.calendar.google.com/public/basic.ics",
  atcoder:
    "https://calendar.google.com/calendar/ical/vdobhfoodrv59522b684lhfs7c@group.calendar.google.com/public/basic.ics"
};

app.get("/api/:platform", async (req, res) => {
  const { platform } = req.params;
  const url = SOURCES[platform];
  const filters = FILTERS[platform];

  if (!url || !filters) {
    return res.status(404).send("unknown platform");
  }

  const upstream = await fetch(url);

  // forward important headers
  [
    "content-type",
    "cache-control",
    "pragma",
    "expires"
  ].forEach(h => {
    const v = upstream.headers.get(h);
    if (v) res.setHeader(h, v);
  });

  const text = await upstream.text();
  const lines = text.split(/\r?\n/);

  let out = [];
  let buf = [];
  let inEvent = false;
  let drop = false;

  for (const line of lines) {
    if (line === "BEGIN:VEVENT") {
      inEvent = true;
      drop = false;
      buf = [line];
      continue;
    }

    if (line === "END:VEVENT") {
      buf.push(line);
      if (!drop) out.push(...buf);
      inEvent = false;
      continue;
    }

    if (inEvent) {
      buf.push(line);
      const lower = line.toLowerCase();
      if (filters.some(w => lower.includes(w))) {
        drop = true;
      }
    } else {
      out.push(line);
    }
  }

  res.send(out.join("\r\n"));
});

app.listen(3000, () => {
  console.log("ICS filter running on :3000");
});

package.json

{
  "name": "filtered-cal",
  "version": "1.0.0",
  "description": "",
  "main": "index.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "keywords": [],
  "author": "",
  "license": "ISC",
  "type": "commonjs",
  "dependencies": {
    "express": "^5.2.1"
  }
}

vercel.json

{
    "builds": [
        {
            "src": "index.js",
            "use": "@vercel/node"
        }
    ],
    "routes": [
        {
          "src": "/(.*)",
          "dest": "index.js"
        }
    ]
}