The Best Time to Party

partytime.js

// (Impure) Functional javascript kata inspired by
// Puzzle 2: The Best Time to Party
// MIT 6.S095 Programming for the Puzzled, IAP 2018
// https://youtu.be/a1RaIqkdG0c

// Assuming that the list of celebrities can be very large
// and the number of hours are limited (small)
// This solution only traverses the list once

// Celebrities arrival and departure times
const celebrities = [
  [6, 7],
  [7, 9],
  [10, 11],
  [10, 12],
  [8, 10],
  [9, 11],
  [6, 8]
]

// inc: increment array element using 0 for empty
const inc = mutableArray =>
  index =>
    (mutableArray[index] = (mutableArray[index] || 0) + 1)

// count: reducer to count celebrities present at any hour
const count = (accAry, [arrival, departure]) => {
  // console.log(arrive, depart)
  const incInAry = inc(accAry)
  for (var t = arrival; t < departure; t++) {
    incInAry(t)
  }
  // console.log(accAry)
  return accAry
}

let time
const max = (acc, n, i) =>
  (n || 0) > acc
    ? ((time = i), n)
    : acc

console.log('Result')
console.log(
  'There will be ' +
  celebrities
    .reduce(count, [])
    .reduce(max, 0) +
    ' celebrities present')
console.log('if you arrive at ' + time)