Question

how can i get a list of DateTime objects which match the following criteria:

• they are between two instances of DateTime
• they fall on a fraction of an hour/min. eg. they are a full quarter of an hour

with active support a possible solution is:

(my_datetime_ob_a.to_i .. my_datetime_ob_b.to_i).each { |timestamp|
  puts timestamp if (timestamp % (3600/4) == 0)
}

this is not nice: too many iterations and too much conversion (you would need to reconvert timestamp to a DateTime object.

Answer 1

Here is an alternative approach:

• Find the first timestamp >= my_datetime_ob_a which is a full quarter of an hour (i.e. round up to 15 minute interval)
• While this timestamp is <= my_datetime_ob_b put it in your list, add 15 minutes and loop

Implementation:

def round_to_quarter_hour(datetime)
  secs_past_quarter_hour = datetime.to_i % 15.minutes
  if secs_past_quarter_hour == 0
    datetime
  else
    secs_to_advance_to_next_quarter_hour = 15.minutes - 
      secs_past_quarter_hour
    datetime.advance :seconds => secs_to_advance_to_next_quarter_hour
  end
end

def quarter_hours_between(datetime_a, datetime_b)
  results = []
  datetime = round_to_quarter_hour(datetime_a)
  while datetime <= datetime_b
    results << datetime
    datetime = datetime.advance :minutes => 15
  end
  results
end

Example:

>> d1
=> Sat, 24 Jul 2010 17:28:06 +0100
>> d2
=> Sat, 24 Jul 2010 18:03:00 +0100
>> quarter_hours_between(d1, d2)
=> [Sat, 24 Jul 2010 17:30:00 +0100, Sat, 24 Jul 2010 17:45:00 +0100,
Sat, 24 Jul 2010 18:00:00 +0100]