SectionedArray.swift

struct SectionedArray<GroupKey, Item> where GroupKey: Comparable & Hashable {

    struct Section {
        let key: GroupKey
        var items: [Item]
    }

    var sections: [Section]

    var count: Int {
        return sections.count
    }

    var first: Section? {
        return sections.first
    }

    var last: Section? {
        return sections.last
    }

    public var isEmpty: Bool {
        return sections.isEmpty
    }

    init<S: Sequence>(_ sequence: S, sortingBy isOrderedBefore: (Item, Item) throws -> Bool, groupingBy groupFunction: (Item) throws -> GroupKey) rethrows where S.Element == Item {
        let sortedItems = try sequence.sorted(by: isOrderedBefore)
        let grouped = try Dictionary(grouping: sortedItems, by: groupFunction)
        let sortedGroups = grouped.sorted { $0.key < $1.key }
        sections = sortedGroups.map { Section(key: $0.key, items: $0.value) }
    }

    init(sections: [Section]) {
        self.sections = sections
    }

}

extension SectionedArray: Collection {

    func index(after i: Int) -> Int {
        return i + 1
    }

    subscript(position: Int) -> SectionedArray<GroupKey, Item>.Section {
        return sections[position]
    }

    var startIndex: Int {
        return 0
    }

    var endIndex: Int {
        return count
    }

}