Introduction

// 1. Use Hashing(ensures same url gets same shortened URL)
const crypto = require("crypto");
 
function hashUrl(url, length = 7) {
  const hash = crypto.createHash("sha256").update(url).digest("base64url");
  return hash.slice(0, length);
}
 
// 2. Use Id
const urlToId = new Map();
const idToUrl = new Map();
let counter = 1000;
 
const BASE62 = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
function toBase62(num) {
  let str = "";
  while (num > 0) {
    str = BASE62[num % 62] + str;
    num = Math.floor(num / 62);
  }
  return str || "0";
}
 
function shortenUrl(url) {
  if (urlToId.has(url)) {
    return toBase62(urlToId.get(url));
  }
 
  const id = counter++;
  urlToId.set(url, id);
  idToUrl.set(id, url);
  return toBase62(id);
}
 
function expandShortCode(shortCode) {
  const id = parseInt(shortCode, 62);
  return idToUrl.get(id);
}

function flipSelective(obj) {
  const flipped = {};
  for (const [key, value] of Object.entries(obj)) {
    if (typeof value === "number" || typeof value === "string") {
      flipped[value] = key;
    } else {
      flipped[key] = value;
    }
  }
  return flipped;
}

const express = require("express");
const app = express();
 
const urlMap = {
  "uid-123": "https://example.com/some-long-url",
  "uid-456": "https://google.com",
};
 
app.get("/:uid", (req, res) => {
  const { uid } = req.params;
  const redirectUrl = urlMap[uid];
 
  if (redirectUrl) {
    return res.redirect(302, redirectUrl);
  }
 
  res.status(404).send("URL not found");
});
 
const PORT = process.env.PORT || 3000;
app.listen(PORT, () => {
  console.log(`Server running on port ${PORT}`);
});