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0030

The Night Shift Bug

Basics
Easy
time
arithmetic
boundaries

The Night Shift Bug

The parcel depot runs at night: it opens at 22:00 and closes at 06:00 the next morning. The first "are we open?" check was copied from a daytime shop, so it insists that 01:00 is outside a 22:00–06:00 window and that the night shift is minus sixteen hours long. Your job is three small procedures that get windows crossing midnight right — and that refuse the undefined time 0T with a clear message instead of a runtime crash.

Requirements

Create a codeunit named "Depot Hours" with three public procedures:

procedure IsOpen(At: Time; OpensAt: Time; ClosesAt: Time): Boolean
procedure ShiftMinutes(StartTime: Time; EndTime: Time): Integer
procedure MinutesUntilClose(At: Time; OpensAt: Time; ClosesAt: Time): Integer

Rules:

  1. An opening window runs from OpensAt to ClosesAt. When ClosesAt is later on the clock than OpensAt it is a daytime window (08:00–17:00). When ClosesAt is earlier on the clock than OpensAt the window crosses midnight: 22:00–06:00 covers 22:00 up to midnight and midnight up to 06:00.
  2. IsOpen returns true when At lies inside the window. The opening instant is open and the closing instant is closed: for 22:00–06:00, 22:00 exactly is open, 23:30 and 01:00 are open, 06:00 exactly is closed, 07:00 is closed.
  3. When OpensAt equals ClosesAt the depot is open round the clock: IsOpen returns true for every At, including At equal to that shared time.
  4. ShiftMinutes returns the length of a shift in whole minutes from StartTime to EndTime, wrapping past midnight when EndTime is earlier on the clock: 22:00 to 06:00 is 480, 08:00 to 16:30 is 510, 23:59 to 00:01 is 2. Leftover seconds are dropped, never rounded up: 08:15:15 to 09:00:00 is 44. A shift whose EndTime equals its StartTime is 0 minutes long — a shift is a duration, so equal times mean nothing happened, unlike the round-the-clock rule for a window.
  5. MinutesUntilClose returns 0 when the depot is closed at At (rule 2 decides, so exactly at ClosesAt it is 0). Otherwise it returns the whole minutes from At until the clock next reads ClosesAt, counting past midnight when needed: for 22:00–06:00, 23:30 gives 390 and 01:00 gives 300. "Next" means strictly in the future, so in a round-the-clock window At equal to ClosesAt gives a full 1440.
  6. 0T is the undefined time: adding to it or subtracting it is a runtime error, and the runtime's own message is not what a user should see. Every procedure must check its Time parameters first and, when any of them is 0T, call Error with a message that contains the text undefined time (0T) — for example StartTime is an undefined time (0T).

What you need from AL: Time values compare with the ordinary <, >= and = operators; Time - Time gives the difference in milliseconds as an Integer, negative when the left value is earlier on the clock; Time + Integer moves a time forward by that many milliseconds; div divides two integers and throws the remainder away. A minute is 60 × 1000 milliseconds and a day is 24 × 60 × 60 × 1000.

Pick object IDs in the 50100–50199 range and reference other objects by name, never by ID.

What the tests check

The IsOpen tests use the 22:00–06:00 window at 22:00, 23:30, 01:00, 06:00 and 07:00, an 08:00–17:00 window at exactly 08:00 (open), at a generated minute inside it, at exactly 17:00 (closed) and at 06:00 and 19:00 outside it, and two round-the-clock windows: 00:00–00:00 at a generated minute and 06:00–06:00 at exactly 06:00. ShiftMinutes is checked with a generated daytime length (08:00 plus 1–600 minutes), 22:00–06:00 (480), 23:59–00:01 (2), a generated time paired with itself (0) and 08:15:15–09:00:00 (44 — seconds dropped, not rounded). MinutesUntilClose expects 390 at 23:30 and 300 at 01:00 for 22:00–06:00, 0 at 06:00 and at 07:00, the exact remaining minutes at a generated minute inside 08:00–17:00 and 0 at exactly 17:00, 180 at 21:00 in a 00:00–00:00 window and 1440 at 06:00 in a 06:00–06:00 window. Finally, every Time parameter of every procedure is tried with 0T in turn (At, OpensAt and ClosesAt for IsOpen and MinutesUntilClose; StartTime and EndTime for ShiftMinutes), with the other arguments defined, and each call must fail with an error message that contains undefined time (0T) — the test shows the actual error text when it does not match.

Learn More

Hint 1
Decide the shape of the window first by comparing OpensAt with ClosesAt. A daytime window needs At to be at or after the opening AND before the closing; a window that crosses midnight is satisfied by either half — at or after the opening OR before the closing. Look closely at which branch the equal-times case lands in: that decides whether it means round the clock or never open.
Hint 2
Time - Time is an Integer of milliseconds and goes negative when the end is earlier on the clock. A negative difference is really 'the rest of the day', so add a full day of milliseconds to it. Dividing with div by the milliseconds in a minute gives whole minutes and silently drops the seconds — exactly what rule 4 asks for.
Hint 3
Compare each parameter with the literal 0T before any arithmetic and raise the message yourself with Error and a Label that carries a %1 placeholder for the parameter name. MinutesUntilClose can lean on IsOpen for the closed case and the 0T check; only the closing-minus-now difference is left, and there a zero must be treated like a negative, because an open depot is never zero minutes from closing.
ALBusiness Central 28.5
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