A function to help you get the longest contiguous sequence of days from an array of dates. For example, can be used in the habit tracker.

CheckDaysStreak.swift

func checkStreak(of dateArray: [Date]) -> Int{
    let dates = dateArray.sorted()
    // Check if the array contains more than 0 dates, otherwise return 0
    guard dates.count > 0 else { return 0 }
    // Get full day value of first date in array
    let referenceDate = Calendar.current.startOfDay(for: dates.first!)
    // Get an array of (non-decreasing) integers
    let dayDiffs = dates.map { (date) -> Int in
        Calendar.current.dateComponents([.day], from: referenceDate, to: date).day!
    }
    // Return max streak
    return maximalConsecutiveNumbers(in: dayDiffs)
}


// Find maximal length of a subsequence of consecutive numbers in the array.
// It is assumed that the array is sorted in non-decreasing order.
// Consecutive equal elements are ignored.

func maximalConsecutiveNumbers(in array: [Int]) -> Int{
    var longest = 0 // length of longest subsequence of consecutive numbers
    var current = 1 // length of current subsequence of consecutive numbers

    for (prev, next) in zip(array, array.dropFirst()) {
        if next > prev + 1 {
            // Numbers are not consecutive, start a new subsequence.
            current = 1
        } else if next == prev + 1 {
            // Numbers are consecutive, increase current length
            current += 1
        }
        if current > longest {
            longest = current
        }
    }
    return longest
}