countpredicates.swift

enum CountPredicate {
  case greaterThan(_ n: Int)
  case equalTo(_ n: Int)
}

extension Collection {

  func count(is predicate: CountPredicate) -> Bool {
    switch predicate {
    case .equalTo(let val):
      return _countIsEqualTo(val)
    case .greaterThan(let val):
      return _countIsGreaterThan(val)
    }
  }

  private func _countIsEqualTo(_ n: Int) -> Bool {
    var tmp = 0
    for _ in self {
      tmp += 1
      if tmp > n { return false } // Seen more than `n` elements. Not equal.
    }
    return tmp == n
  }

  private func _countIsGreaterThan(_ n: Int) -> Bool {
    var tmp = 0
    for _ in self {
      tmp += 1
      if tmp > n { return true } // Seen more than `n` elements. count > n.
    }
    return n < 0
  }
}

// RAC.count is O(1) anyway.
extension RandomAccessCollection {
  func count(is predicate: CountPredicate) -> Bool {
    switch predicate {
    case .greaterThan(let val): return self.count > val
    case .equalTo(let val):     return self.count == val
    }
  }
}