app.js

const http = require('http');
const fs = require('fs');
const crypto  = require('crypto');

const random = new Date();

const hostname = `127.0.0.1`;
const port = 3000;


const server = http.createServer((req, res) => {
	const url = req.url;
	const method = req.method;

	if(url == '/api' && method == "GET"){
		getResource(res);
	}
	if(url == '/api' && method == "POST"){
		createResource(res);
	}
	if(url == '/api' && method == "PUT"){
		editResource(res, req);
	}
});


const createHash = () => {
	const hash = crypto.createHash('sha1').update(random.toString()).digest('hex');
	fs.writeFileSync('hash.txt', hash, (err) => {
		if(err) throw err;
		console.log("Hash generated and saved");
	});
	return hash;
}

const getHash = () => {
	const hash = fs.readFileSync('hash.txt', 'utf-8');
	return hash;
}

const checkHash = (hash) => {
	const data = fs.readFileSync('hash.txt', 'utf-8');
	if(data===hash) return true;
	return false;

}


const readData = (fd, res) => {
	const readStream = fs.createReadStream("", {fd: fd});
	let output = "";
	readStream.on('data', function(chunk) {
		output += chunk;
	});

	readStream.on('end', function() {
		res.statusCode = 200;
		res.statusMessage = "Read successful";
		res.setHeader('Content-Type', 'text/json');
		res.setHeader('E-Tag', getHash());
		res.write(JSON.stringify({
			"message": output,
		}));
		res.end(() => {
			console.log("ReadData function reads: ", output);
		})
		
	});

	readStream.on('error', function(err) {
		res.statusCode = 500;
		res.setHeader('Content-Type', 'text/plain');
		res.end("Error");
		throw err;
	});	
} 


const writeData = (fd, data, res) => {
	
	fs.writeFile('resource', data, (err) => {
		if(err) {
			res.statusCode = 500;
			res.setHeader('Content-Type', 'text/plain');
			res.end("Error");
		}
		
		res.statusCode = 200;
		res.statusMessage = "Read successful";
		res.setHeader('Content-Type', 'text/json');
		res.setHeader('E-Tag', createHash());
		res.write(JSON.stringify({
			"message": data,
		}))
		res.end(() => {
			console.log("Written Successfully in the file");
		})
		
	})	
}

const getResource = (res) => {
	/* check if the resource exists or in our case, resource.json exists
		if yes then return the resource else throw a 404 error
	*/

	fs.open('resource', 'r', (err, fd) => {
		if(err){
			if(err.code == 'ENOENT'){
				res.statusCode = 500;
				res.setHeader('Content-Type', 'text/plain');
				res.end("Error");
			}
		throw err;
		}
		readData(fd, res);
	});

	console.log("GET is called");

}

const createResource = (res) => {
	/* check if the resource exists or in our case, resource.json exists
		if not then create it with dummy data, else return forbidden header
	*/
	fs.open('resource', 'wx', (err, fd) => {
		if(err){
			if(err.code == 'EEXIST'){
				console.error("File already exists");
			}
		throw err;
		}
		writeData(fd, 'Hello there, I am a resource \n', res);
	});
	console.log("POST is called");
}

const editResource = (res, req) => {
	/* check if the resource exists or in our case, resource.json exists
		if yes then edit the resource with modified value else throw a 404 error
	*/
	fs.open('resource', 'r', (err, fd) => {
		if(err){
			throw err;
		}
		// check if E-Tag is same as in the hash_cache 
		if(checkHash(req.headers['e-tag'])){
			writeData(fd, 'Hello there, I am a resource changed \n', res);
		}else {
			throw Error("You have an outdated copy of the material");
		}
		console.log("req.headers", req.headers['e-tag']);
		
	});
	console.log("PUT is called");


}




server.listen(port, hostname, ()=> {
	console.log(`server listening at http://${hostname}:${port}/`);
});