How to determine when multiple(n) datetime ranges overlap each other

Question

I'm looking for a function that I can call that will tell me what datetime ranges all (p) people are available at the same time. Please help me for Objective-c or Swift language.

p1: start: "2016-01-01 12:00", end: "2016-05-01 03:00"
p2: start: "2016-01-01 03:00", end: "2016-05-01 03:00"
p3: start: "2016-01-01 03:00", end: "2016-04-30 13:31"

In the above example the answer should be:

start: 2016-04-30 12:00, end: 2016-04-30 13:31

Answer 1

Convert your pairs of dates to NSDateInterval objects and take their intersection:

https://developer.apple.com/documentation/foundation/nsdateinterval/1641645-intersectionwithdateinterval

The docs even provide a rather nice diagram:

Answer 2

You need to do the following steps:

1. Convert your date string to Date objects.
2. Create DateIntervals with your start and end date objects.
3. Loop through the intervals and check for intersection.

Here is a quick code I can come up with in swift:

func answer()  {
    let dateFormat = "yyyy-MM-dd HH:mm Z"
    // Date ranges
    let times = [["start": "2016-01-01 12:00 +0000", "end": "2016-05-01 03:00 +0000"],
                 ["start": "2016-01-01 03:00 +0000", "end": "2016-05-01 03:00 +0000"],
                 ["start": "2016-01-01 03:00 +0000", "end": "2016-04-30 13:31 +0000"]]

    var intervals = [DateInterval]()
    // Loop through date ranges to convert them to date intervals
    for item in times {
        if let start = convertStringToDate(string: item["start"]!, withFormat: dateFormat),
            let end = convertStringToDate(string: item["end"]!, withFormat: dateFormat) {
            intervals.append(DateInterval(start: start, end: end))
        }
    }

    // Check for intersection
    let intersection = intersect(intervals: intervals)
    print(intersection)
}

// Converts the string to date with given format
func convertStringToDate(string: String, withFormat format: String)  -> Date? {
    let dateFormatter = DateFormatter()
    dateFormatter.dateFormat = format
    return dateFormatter.date(from: string)
}

// Cehck for intersection among the intervals in the given array and return
    // the interval if found.
    func intersect(intervals: [DateInterval]) -> DateInterval? {
        // Algorithm:
        // We will compare first two intervals.
        // If an intersection is found, we will save the resultant interval
        // and compare it with the next interval in the array.
        // If no intersection is found at any iteration
        // it means the intervals in the array are disjoint. Break the loop and return nil
        // Otherwise return the last intersection.

        var previous = intervals.first
        for (index, element) in intervals.enumerated() {
            if index == 0 {
                continue
            }

            previous = previous?.intersection(with: element)

            if previous == nil {
                break
            }
        }

        return previous
    }

Note: Please test with several examples. I tested with above date ranges and its working fine.