Group a Swift Model by day with ascending or descending order (Swift 5) E.g.: for a UITableView with sections

Sequence+GroupByDate.swift

import UIKit

extension Sequence {
    func groupByDay(dateKey: (Iterator.Element) -> Date, order: ComparisonResult = .orderedAscending) -> [[Iterator.Element]] {
        var groups: [[Iterator.Element]] = []
        for element in self {
            let key = dateKey(element)
            guard let dayIndex = groups.firstIndex(where: { $0.contains(where: { Calendar.current.isDate(dateKey($0), inSameDayAs: key) }) }) else {
                guard let nextIndex = groups.firstIndex(where: { $0.contains(where: { dateKey($0).compare(key) == order }) }) else {
                    groups.append([element])
                    continue
                }
                groups.insert([element], at: nextIndex)
                continue
            }
            guard let nextIndex = groups[dayIndex].firstIndex(where: { dateKey($0).compare(key) == order }) else {
                groups[dayIndex].append(element)
                continue
            }
            groups[dayIndex].insert(element, at: nextIndex)
        }
        return groups
    }
}


// Usage:
struct Model {
    let date: Date
    let text: String
}
let modelArray = [
    Model(date: Date(), text: "Original Date"),
    Model(date: Date().addingTimeInterval(86400), text: "+1 day"),
    Model(date: Date().addingTimeInterval(172800), text: "+2 days"),
    Model(date: Date().addingTimeInterval(86401), text: "+1 day & +1 second"),
    Model(date: Date().addingTimeInterval(172801), text: "+2 days & +1 second"),
    Model(date: Date().addingTimeInterval(86400), text: "+1 day"),
    Model(date: Date().addingTimeInterval(172800), text: "+2 days")
]
let groupSorted = modelArray.groupByDay(dateKey: { $0.date })
print(groupSorted) // [["+2 days & +1 second", "+2 days", "+2 days"], ["+1 day & +1 second", "+1 day", "+1 day"], ["Original Date"]]
let groupSortedDesc = modelArray.groupByDay(dateKey: { $0.date }, order: .orderedDescending)
print(groupSortedDesc) // [["Original Date"], ["+1 day", "+1 day", "+1 day & +1 second"], ["+2 days", "+2 days", "+2 days & +1 second"]]