file-navigation.swift

struct File {}

struct Landscape {
    let parents: FileList
    let children: FileList
}

struct FileList {
    fileprivate let all: Array<File>
    fileprivate let before: ArraySlice<File>
    fileprivate let after: ArraySlice<File>
    
    var previous: FileList {
        guard !before.isEmpty else { return self }
        return shift(by: before.count - 1)
    }
    
    var next: FileList {
        guard !after.isEmpty else { return self }
        return shift(by: before.count + 1)
    }

    var current: File? {
        return before.last
    }
    
    var isEmpty: Bool {
        return all.isEmpty
    }
    
    fileprivate func shift(by prefixLength: Int) -> FileList {
        return FileList(
            all: all,
            before: all.prefix(prefixLength), 
            after: all.suffix(all.count - prefixLength)
        )
    }
}

extension FileList {
    fileprivate init(_ elements: File...) {
        self.all = elements
        self.before = elements.prefix(1)
        self.after = elements.suffix(elements.count - 1)
    }
}