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

// Function to sign a file
function signFile(filePath, privateKey, algorithm = 'SHA256') {
  return new Promise((resolve, reject) => {
    // Create Sign object
    const sign = crypto.createSign(algorithm);
    
    // Create read stream
    const readStream = fs.createReadStream(filePath);
    
    // Handle stream events
    readStream.on('data', (data) => {
      sign.update(data);
    });
    
    readStream.on('end', () => {
      // Create signature
      const signature = sign.sign(privateKey, 'hex');
      resolve(signature);
    });
    
    readStream.on('error', (error) => {
      reject(error);
    });
  });
}

// Generate a keypair for this example
const { privateKey, publicKey } = crypto.generateKeyPairSync('rsa', {
  modulusLength: 2048,
  publicKeyEncoding: {
    type: 'spki',
    format: 'pem'
  },
  privateKeyEncoding: {
    type: 'pkcs8',
    format: 'pem'
  }
});

// Save the public key for verification
fs.writeFileSync('public_key_file.pem', publicKey);

// Example usage (adjust file path as needed)
const filePath = 'example_to_sign.txt';

// Create a test file if it doesn't exist
if (!fs.existsSync(filePath)) {
  fs.writeFileSync(filePath, 'This is a test file for digital signature.\n'.repeat(100));
  console.log(`Created test file: ${filePath}`);
}

// Sign the file
signFile(filePath, privateKey)
  .then(signature => {
    console.log(`File: ${filePath}`);
    console.log(`Signature: ${signature}`);
    
    // Save the signature for later verification
    fs.writeFileSync(`${filePath}.sig`, signature);
    console.log(`Signature saved to: ${filePath}.sig`);
  })
  .catch(error => {
    console.error('Error signing file:', error.message);
  });