Creating a FIFO fixed size array in Swift

Question

I'm attempting to create a FIFO array in swift. I want to create something that acts like this:

var Arr = FixedFIFOArray<Int>(maxSize:3)
Arr.append(1)  //Arr = [1]
Arr.append(2)  //Arr = [1,2]
Arr.append(3)  //Arr = [1,2,3]
Arr.append(4)  //Arr = [2,3,4] <- the max size is fixed to 3, so any
                                  additional values added remove old values

Other than this behavior, it should act like an array: allow slicing, indexing, iterating in for loops, etc.

In any other language, this would be a job for subclassing. We aren't changing much, just adding an initializer and amending a function or two. However, in Swift, we can't subclass array. What is the best way to do this? Do I need to implement every protocol that array implements, and just pass the associated functions off to an array? Something like this:

struct FixedFIFOArray<T> {
    var _maxSize: Int
    var _array: [T] = []

    init(maxSize: Int) {
        self._maxSize = maxSize
    }

}

extension FixedFIFOArray : Collection {
    //...
}

extension FixedFIFOArray : RandomAccessCollection {
    //...
}

extension FixedFIFOArray : Sequence {
    //...
}
// etc...

This seems like a lot of work to do something so simple. What am I missing?

maybe you could write an extension for array where you define a different append function something like appendFIFO(element: T, forSize: Int). It depends on the usecase but maybe thats enough — j0h4nn3s
– j0h4nn3s, Commented Nov 28, 2017 at 20:56
I thought of the same approach. You would also need a function to trim an array to keep just the last n items. — claude31
– claude31, Commented Nov 28, 2017 at 21:22
Most protocol methods have default implementations. It suffices to implement startIndex, endIndex, and subscript. — Martin R
– Martin R, Commented Nov 28, 2017 at 21:23

Martin R · Accepted Answer · 2017-11-28 22:06:31Z

It is not as bad as it seems, because many protocol requirements have default implementations. Unfortunately I do not have the perfect recipe to find a "minimal" implementation.

Some information can be found in the RandomAccessCollection documentation where some methods are marked as "Required. Default implementation provided."
You can also start with an empty implementation extension FixedFIFOArray : RandomAccessCollection {} and study the error messages or try the Fix-its.
With "Jump to Definiton" in the Xcode editor one can inspect the protocol definitions and extension methods.

In your case it turned out that it suffices to implement startIndex, endIndex, and subscript:

extension FixedFIFOArray : RandomAccessCollection {
    var startIndex: Int {
        return _array.startIndex
    }

    var endIndex: Int {
        return _array.endIndex
    }

    subscript(i: Int) -> T {
        return _array[i]
    }
}

Or, if you need a read-write subscript:

    subscript(i: Int) -> T {
        get {
            return _array[i]
        }
        set {
            _array[i] = newValue
        }
    }

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