Finding the gaps in a sequence of dates

Somewhere earlier this week I had to find the gaps in a sequence of dates. Admittedly, my first action was to search Stackoverflow for a clean solution. But since no one asked the question there yet, I had to implement it myself.

The solution comes in the form of an extension method on IEnumerable<DateTime>, which takes a lower bound and an upper bound, and returns an enumerable of dates.

public static IEnumerable<DateTime> GetGaps(
    this IEnumerable<DateTime> sequence, DateTime lowerbound, DateTime upperbound)
{          
    var completeSequence = new List<DateTime>();           
    var tmpDay = lowerbound.Date;

    while (tmpDay <= upperbound.Date)
    {
        completeSequence.Add(tmpDay);
        tmpDay = tmpDay.AddDays(1);
    }
    
    var gaps = completeSequence.Except(sequence.Select(d => d.Date));
    
    return gaps;
}

I start with building a second - from lower bound to upper bound, thus gapless - sequence. Then I use the LINQ Except method to compare the complete sequence to the input sequence, and produce a difference set.

It was only after a few not so successful iterations that I found this - for me - satisfactory solution though. If I hadn't intellistumbled upon the LINQ Except method, I probably would have ended up with something a lot less elegant.

This extension method is backed up by a few tests.

[TestMethod()]
public void Given_an_empty_sequence_everything_between_bounds_is_a_gap()
{
    var sequence = new List<DateTime>();            
 
    var actual = sequence.GetGaps(new DateTime(2012, 1, 6), new DateTime(2012, 1, 9));

    Assert.IsTrue(actual.Count() == 4);
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 6)));
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 7)));
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 8)));
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 9)));
}

[TestMethod()]        
public void Given_sequence_with_duplicate_dates_one_is_not_returned_as_gap()
{
    var sequence = new[] 
    { 
        new DateTime(2012, 1, 6, 10, 0, 0), // double date
        new DateTime(2012, 1, 6, 11, 0, 0), // double date
        new DateTime(2012, 1, 7) 
    };

    var actual = sequence.GetGaps(new DateTime(2012, 1, 6), new DateTime(2012, 1, 9));

    Assert.IsTrue(actual.Count() == 2);
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 8)));
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 9)));
}

[TestMethod()]
public void Given_sequence_with_gaps_the_gaps_are_returned()
{
    var sequence = new[] 
    { 
        // gap
        new DateTime(2012, 1, 4), 
        new DateTime(2012, 1, 5), 
        // gap
        new DateTime(2012, 1, 7, 11, 10, 1) 
        // gap
    };

    var actual = sequence.GetGaps(new DateTime(2012, 1, 3), new DateTime(2012, 1, 8));

    Assert.IsTrue(actual.Count() == 3);
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 3)));
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 6)));
    Assert.IsTrue(actual.Contains(new DateTime(2012, 1, 8)));
}

[TestMethod()]
public void Given_sequence_with_no_gaps_an_empty_enumerable_is_returned()
{
    var sequence = new[] 
    {                
        new DateTime(2012, 1, 4), 
        new DateTime(2012, 1, 5),                 
        new DateTime(2012, 1, 6)                 
    };

    var actual = sequence.GetGaps(new DateTime(2012, 1, 4), new DateTime(2012, 1, 6));

    Assert.IsFalse(actual.Any());
}

Do you know of a better way to do this?