strongPasswordChecker.js

function computeRopeCosts(password, additionsNeeded, removalsNeeded) {
  let ropes = []

  let lastChar;
  let lastCharIndex;
  for (let i = 0; i < password.length; ++i) {
    const char = password[i];
    if (char != lastChar) {
      if (lastChar && i - lastCharIndex >= 3) {
        ropes.push(i - lastCharIndex)
      }
      lastChar = char
      lastCharIndex = i
    }
  }
  if (lastChar && password.length - lastCharIndex >= 3) {
    ropes.push(password.length - lastCharIndex)
  }

  ropes = ropes.map(count => {
    if (count - 2 <= removalsNeeded) {
      removalsNeeded -= count - 2
      return count - 2
    } else {
      return count
    }
  })

  ropes = ropes.map(count => {
    if (removalsNeeded > 0) {
      if (count % 3 == 0) {
        --removalsNeeded
        return count - 1
      }
    } else if (additionsNeeded > 0) {
      if (count % 3 == 0 || count % 3 == 1) {
        --additionsNeeded
        return count - 2
      }
    }

    return count
  })

  ropes = ropes.filter(count => count >= 3)

  let additions = 0
  let modifications = 0
  let removals = 0

  for (let count of ropes) {
    modifications += Math.floor(count / 3)
  }

  return { additions, modifications, removals }
}

function CheckStrongPassword(password) {
  let digits = 0
  let lowercase = 0
  let uppercase = 0

  for (const char of password) {
    if (char.match(/^[a-z]$/)) {
      ++lowercase
    } else if (char.match(/^[A-Z]$/)) {
      ++uppercase
    } else if (char.match(/^[0-9]$/)) {
      ++digits
    }
  }

  const additionsNeeded = Math.max(0, 6 - password.length)
  const removalsNeeded = Math.max(0, password.length - 20)

  // Regardless of everything else, we always have to make enough modifications
  // or additions to match the char type requirements.
  let minMidAdds = (digits == 0) + (lowercase == 0) + (uppercase == 0)
  minMidAdds = Math.max(0, minMidAdds - additionsNeeded)

  console.log('additionsNeeded', additionsNeeded)
  console.log('removalsNeeded', removalsNeeded)
  console.log('minMidAdds', minMidAdds)

  const { additions, modifications, removals } =
      computeRopeCosts(password, additionsNeeded, removalsNeeded)

  console.log(additions, modifications, removals)

  return additions + modifications + removals
      + Math.max(0, removalsNeeded - removals)
      + Math.max(0, additionsNeeded - additions)
      + Math.max(0, minMidAdds - (additions + modifications))
}