Count days within overlapping date ranges in VB.NET - vb.net

Question

I have a large amount of date FROM and TO pairs, and I need to count all the days within them. But if two ranges are overlapping then the overlapping days should not be counted twice.
Here's the code that counts all days that I have:
Dim total_days_used = 0
For Each row As DataRow In MY_DATA.Tables(0).Rows
Dim total_days As Double = 0
Dim date_from = MY_FROM_DATE_FROM_DATA
Dim date_to = MY_TO_DATE_FROM_DATA
Dim span = date_to - date_from
total_days = span.TotalDays '=4
total_days_used += total_days
Next
I can't figure out an easy way to subtract the overlapping days though or even keep track of them. I guess that would be one way to go, the other would be to combine overlapping ranges until I end up with a set of ranges where there are no overlaps but that seems way too complicated as well. There should be a simple way to do this?

Answer 1

Something like this should work.
We first order the ranges to know if the last overlaps the current.
And then count the overlapping days and substract that from the total.
+1 days if you want the range to be inclusive otherwise remove that.
Private Sub Main()
Dim ranges = New List(Of Range)() From { _
{New Range(New DateTime(2000, 1, 1), New DateTime(2000, 1, 30))}, _
{New Range(New DateTime(2000, 1, 28), New DateTime(2000, 2, 3))} _
}
CountNonOverlappingsDays(ranges).Dump() '34 days
End Sub
Private Function CountNonOverlappingsDays(ranges As IEnumerable(Of Range)) As Integer
Dim isFirst = True
Dim last As Range = Nothing
Dim overlapping As Integer = 0
Dim total As Integer = 0
For Each current In ranges.OrderBy(Function(r) r.[To])
total += CInt((current.[To] - current.From).TotalDays) + 1 '+1 if we want Inclusive count
If isFirst Then
isFirst = False
last = current
Continue For
End If
If (last.From <= current.[To]) AndAlso (last.[To] >= current.From) Then
Dim start = current.From
Dim [end] = last.[To]
overlapping += CInt(([end] - start).TotalDays) + 1 '+1 if we want Inclusive count
End If
last = current
Next
Return total - overlapping
End Function
Public Class Range
Public Sub New([from] As DateTime, [to] As DateTime)
[From] = [from]
[To] = [to]
End Sub
Public Property [From]() As DateTime
Get
Return m_From
End Get
Set
m_From = Value
End Set
End Property
Private m_From As DateTime
Public Property [To]() As DateTime
Get
Return m_To
End Get
Set
m_To = Value
End Set
End Property
Private m_To As DateTime
End Class

Answer 2

Use the following (or similar DateRange class).
Class DateRange
Implements IEnumerable(Of DateTime)
Public Sub New(startDate As DateTime, endDate As DateTime)
me.StartDate = startDate
me.EndDate = endDate
End Sub
Public ReadOnly Property StartDate() As DateTime
Public ReadOnly Property EndDate() As DateTime
Public Function GetEnumerator() As IEnumerator(Of DateTime) Implements IEnumerable(of DateTime).GetEnumerator
Return Enumerable.Range(0, 1 + EndDate.Subtract(StartDate).Days).[Select](Function(offset) StartDate.AddDays(offset)).GetEnumerator()
End Function
Private Function IEnumerable_GetEnumerator() As IEnumerator Implements IEnumerable.GetEnumerator
Return GetEnumerator()
End Function
End Class
The important part of this class is that it is enumerable. That is, it returns the sequence of dates between the start and end dates (inclusive), when used in a for each loop (among other things).
Then you can use code like this to get what you want:
Dim ranges = New List(Of DateRange)()
ranges.Add(New DateRange(#2017/1/1#,#2017/1/10#))
ranges.Add(New DateRange(#2017/1/8#,#2017/1/20#))
Dim merged = ranges.SelectMany(Function(r) r.AsEnumerable()).Distinct().OrderBy(Function(dt) dt)
Console.WriteLine($"{merged.Count()} days: ")
For Each [date] As DateTime In merged
Console.WriteLine([date].ToShortDateString())
Next
Console.ReadLine()
This uses the LINQ SelectMany function to flatten the lists of dates (created by the IEnuemrable DateRange) from all the DateRange instances in the list to a single list of DateTime. It then gets the distinct (unique) values, and sorts the list.
The output shows output from a list containing 2 DateRange instances. The first is from Jan 1, 2017 to Jan 10, 2017, the second from Jan 8, 2017 to Jan 20, 2017. These ranges overlap on the 8th, 9th, and 10th. As you can see, those overlapped dates are only included once.
The following output is produced:
20 days:
1/1/2017
1/2/2017
1/3/2017
1/4/2017
1/5/2017
1/6/2017
1/7/2017
1/8/2017
1/9/2017
1/10/2017
1/11/2017
1/12/2017
1/13/2017
1/14/2017
1/15/2017
1/16/2017
1/17/2017
1/18/2017
1/19/2017
1/20/2017

Answer 3

Try this:
Dim range As New List(Of DateTime)()
For Each row As DataRow In MY_DATA.Tables(0).Rows
Dim date_from = MY_FROM_DATE_FROM_DATA
Dim date_to = MY_TO_DATE_FROM_DATA
range.AddRange(Enumerable.Range(0, (date_from-date_to).TotalDays).Select(d => date_from.AddDays(d))
Next
Dim total_days_used As Integer = range.Distinct().Count()
The magic is in two parts. The first part uses Enumerable.Range() to actually project all of the dates in each range, so we can add them to a list. The second part then just takes the distinct members of that list and counts them.