objectsat.swift

extension Collection where
    Indices.Iterator.Element: Equatable,
    Index == Self.Indices.Iterator.Element
{
    /// Returns optional value for safely indexed value
    /// and nil if index is out of bounds
    public subscript(safe index: Self.Index) -> Self.Iterator.Element? {
        guard indices.contains(index) else { return nil }
        return self[index]
    }
    
    /// Returns objects safely collected at existing desired indices.
    public func objects(collectedAt desiredIndices: Self.Index...) -> [Self.Iterator.Element] {
        return desiredIndices.flatMap({ self[safe: $0] })
    }

    /// Returns objects safely found at existing desired indices.
    public func objects(safelyAt desiredIndices: Self.Index...) -> [Self.Iterator.Element?] {
        return desiredIndices.map({ self[safe: $0] })
    }
}

extension Collection {
    /// Returns objects found at existing desired indices.
    /// No safety guarantees
    public func objects(at desiredIndices: Self.Index...) -> [Self.Iterator.Element] {
        return desiredIndices.map({ self[$0] })
    }
}

extension Collection {
    /// Returns objects found at desired subscript indices.
    /// No safety guarantees
    subscript(_ idx1: Self.Index, _ idx2: Self.Index, _ rest: Self.Index...) -> [Self.Iterator.Element] {
        return [self[idx1], self[idx2]] + rest.map({ self[$0] })
    }
}