// 1
fun calculateHour(h: Int, period: String?): Int =
  period?.let {
    when(it) {
      "am" -> if (h == HOURS_PER_PERIOD) 0 else h
      "pm" -> (h % HOURS_PER_PERIOD) + HOURS_PER_PERIOD
      else -> h // should not happen
    }
  } ?: h
}

// 2
fun calculateHour(h: Int, period: String?): Int =
  if (period == null) {
    h
  } else when (period) {
    "am" -> if (h == HOURS_PER_PERIOD) 0 else h
    "pm" -> (h % HOURS_PER_PERIOD) + HOURS_PER_PERIOD
    else -> h // should not happen
  }
}

but what if

when (period) {
    null -> h
    <http://Period.AM|Period.AM> -> if (h == HOURS_PER_PERIOD) 0 else h
    <http://Period.PM|Period.PM> -> (h % HOURS_PER_PERIOD) + HOURS_PER_PERIOD
}

But of course! For some reason it didn’t occur to me that

null

could be a

when

value. Thank you.

to be abundantly clear, my example will only compile if you make

Period

an enum (or sealed class technically but you probably don’t need one to denote AM/PM)

Of course. Only part of my brain was malfunctioning.

The naming is probably suboptimal, it’s not calculating anything, it’s formatting, right?