NanoID Generator

const ALPHABET = 'ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_';

function generateNanoID(length = 21, alphabet = ALPHABET) {
  const bytes = new Uint8Array(length);
  crypto.getRandomValues(bytes);

  // Rejection sampling to avoid modulo bias
  const maxValid = Math.floor(256 / alphabet.length) * alphabet.length;
  let id = '';
  let i = 0;
  while (id.length < length) {
    if (bytes[i] < maxValid) {
      id += alphabet[bytes[i] % alphabet.length];
    }
    i++;
    if (i >= bytes.length) {
      crypto.getRandomValues(bytes);
      i = 0;
    }
  }
  return id;
}

// Default: 21 chars, 126 bits
console.log(generateNanoID());

// Short: 10 chars, 60 bits
console.log(generateNanoID(10));

// Numeric only: 8 digits
console.log(generateNanoID(8, '0123456789'));

function nanoIDEntropy(length, alphabetSize) {
  const entropy = length * Math.log2(alphabetSize);
  const combinations = Math.pow(alphabetSize, length);
  const crackTime = combinations / 2 / 1e9; // seconds at 1B guesses/sec

  return {
    length,
    alphabetSize,
    entropyBits: entropy.toFixed(1),
    totalCombinations: combinations.toExponential(2),
    crackTimeSeconds: crackTime.toExponential(2),
    crackTimeFormatted: formatTime(crackTime)
  };
}

function formatTime(seconds) {
  if (seconds < 1) return 'instant';
  if (seconds < 60) return seconds.toFixed(0) + ' sec';
  if (seconds < 3600) return (seconds / 60).toFixed(0) + ' min';
  if (seconds < 86400) return (seconds / 3600).toFixed(0) + ' hours';
  return (seconds / 86400).toExponential(1) + ' days';
}

// Common configurations
console.log(nanoIDEntropy(21, 64)); // Default: 126 bits
console.log(nanoIDEntropy(10, 64)); // Short: 60 bits
console.log(nanoIDEntropy(8, 36));  // Lowercase+digits: 41.4 bits