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.

// This impl makes me sad. Lots of state and copies the entire collection twice (once during transforming, and once to unreverse)
extension Collection {
    func mapExceptLast(matching predicate: (Element) -> Bool, transform: (Element) -> Element) -> [Element] {
        var result: [Element] = []
        var found = false
        for element in reversed() {
            if !found && predicate(element) {
                result.append(element)
                found = true
            } else {
                result.append(transform(element))
            }
        }
        return Array(result.reversed())
    }
}

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

extension Collection {
  func mapExceptLast(matching predicate: (Element) -> Bool, transform: (Element) -> Element) -> [Element] {
    var found = false
    return Array(
      reversed().reduce(into: [Element]()) { (inoutResult, next) in
        guard !found && predicate(next) else {
          if predicate(next) { found = true }
          return inoutResult.append(transform(next))
        }
        return inoutResult.append(next)
    }).reversed()
  }
}

My attempt -> still copies twice though