Question

I have a set of appointments on a specific work day that goes from 8am to 6pm:

Appointment 1: 9am-11am
Appointment 2: 2pm-5pm

I am looking for an efficient way to locate the free time. So in this case, the available time would be:

8am-9am
11am-2pm
5pm-6pm

So I have an TimeBlock class

class TimeBlock {
  public DateTime start
  public DateTime end
}

var appointments = new List<TimeBlock>();
var freeTimeBlocks = new List<TimeBlock>();

' add appointments
appointments.Add(new TimeBlock{start...
appointments.Add(new TimeBlock{start...

I am looking for an efficient way to find free time, because the algorithm will run on a pretty big data set.

Answer 1

Try this, it assumes there’s no overlapping appointments:

var orderedAppointments = appointments.OrderBy(a => a.start).ToArray();

freeTimeBlocks.Clear();

for(int i = 0; i < orderedAppointments.Length - 1; i++)
{
    freeTimeBlocks.Add(new TimeBlock(){ start = orderedAppointments[i].end; end = orderedAppointments[i + 1].start });
}
var firstAppointment = orderedAppointments.First();
var lastAppointment = orderedAppointments.Last();

if(firstAppointment.start.Hour > 8)
   freeTimeBlocks.Add(new TimeBlock() { start = firstAppointment.Today.AddHours(8); end = firstAppointment.start });
if(lastAppointment.end.Hour < 18)
   freeTimeBlocks.Add(new TimeBlock() { start = lastAppointment.end; end = lastAppointment.Today.AddHours(18) });