Entropy Calculator

function shannonEntropy(str) {
  if (!str) return 0;

  // Count character frequencies
  const freq = {};
  for (const char of str) {
    freq[char] = (freq[char] || 0) + 1;
  }

  // Calculate Shannon entropy: H = -Σ p(x) × log₂(p(x))
  let entropy = 0;
  const len = str.length;
  for (const count of Object.values(freq)) {
    const p = count / len;
    entropy -= p * Math.log2(p);
  }

  return {
    shannonBitsPerChar: entropy,
    totalShannonBits: entropy * str.length,
    uniqueChars: Object.keys(freq).length,
    totalChars: len,
    charDistribution: freq
  };
}

// Usage
const result = shannonEntropy('k9mPx2nLq');
console.log(result.shannonBitsPerChar); // ~3.17 bits/char
console.log(result.totalShannonBits);   // ~28.5 bits
console.log(result.uniqueChars);        // 9

function analyzePasswordEntropy(password) {
  // Shannon entropy (actual information content)
  const freq = {};
  for (const char of password) freq[char] = (freq[char] || 0) + 1;
  let shannon = 0;
  for (const count of Object.values(freq)) {
    const p = count / password.length;
    shannon -= p * Math.log2(p);
  }

  // Pool-based entropy (theoretical maximum)
  const hasLower = /[a-z]/.test(password);
  const hasUpper = /[A-Z]/.test(password);
  const hasDigit = /[0-9]/.test(password);
  const hasSymbol = /[^a-zA-Z0-9]/.test(password);
  let poolSize = 0;
  if (hasLower) poolSize += 26;
  if (hasUpper) poolSize += 26;
  if (hasDigit) poolSize += 10;
  if (hasSymbol) poolSize += 33;
  const poolEntropy = password.length * Math.log2(poolSize || 1);

  // Crack time estimates
  const hashesPerSec = 1e12; // 1 trillion (large GPU cluster)
  const combinations = Math.pow(poolSize, password.length);
  const secondsToCrack = combinations / 2 / hashesPerSec;

  return {
    length: password.length,
    uniqueChars: Object.keys(freq).length,
    shannonBitsPerChar: shannon.toFixed(2),
    totalShannonBits: (shannon * password.length).toFixed(1),
    poolBasedBits: poolEntropy.toFixed(1),
    poolSize,
    crackTime: formatTime(secondsToCrack),
    strength: poolEntropy > 80 ? 'Very Strong' : poolEntropy > 60 ? 'Strong' : poolEntropy > 40 ? 'Moderate' : 'Weak'
  };
}

function formatTime(seconds) {
  if (seconds < 1) return 'Instant';
  if (seconds < 60) return seconds.toFixed(1) + ' seconds';
  if (seconds < 3600) return (seconds / 60).toFixed(1) + ' minutes';
  if (seconds < 86400) return (seconds / 3600).toFixed(1) + ' hours';
  if (seconds < 31536000) return (seconds / 86400).toFixed(1) + ' days';
  return (seconds / 31536000).toExponential(1) + ' years';
}

// Usage
console.log(analyzePasswordEntropy('password123'));
// { strength: "Weak", totalShannonBits: "28.5", poolBasedBits: "52.5" }

console.log(analyzePasswordEntropy('k9$mPx2#nLq!vR7@'));
// { strength: "Very Strong", totalShannonBits: "~65", poolBasedBits: "105.1" }