mapExceptLast.swift

let xs = [0,1,2,3,4,5]

let double: (Int) -> Int = { $0 * 2 }

let isEven: (Int) -> Bool = { $0 % 2 == 0 }

// Goal: [0,2,4,6,4,10]. Double all but the last even element.
extension Collection {
    func mapExceptLast(matching predicate: (Element) -> Bool, transform: (Element) -> Element) -> [Element] {
        var result: [Element] = []
        var lastMatch: (Element, Int)?
        
        for element in self {
            if predicate(element) {
                lastMatch = (element, result.count)
            }
            result.append(transform(element))
        }
        // If we found a match, replace the last with untransformed element
        if let (element, i) = lastMatch {
            result[i] = element
        }
        return result
    }
}

print(xs.mapExceptLast(matching: isEven, transform: double))