LazySplitSequence.swift

struct LazySplitSequence<Base: Sequence> : Sequence, LazySequenceProtocol {
  struct Iterator : IteratorProtocol {
    mutating func next() -> [Base.Element]? {
      var result: [Base.Element] = []

      if splits == 0 {
        while let element = iterator.next() {
          result.append(element)
        }
        return result.isEmpty ? nil : result
      }
      splits -= 1
      
      var iterated = false
      while let element = iterator.next() {
        iterated = true
        if separator(element) {
          if omittingEmptySubsequences && result.isEmpty {
            continue
          }
          break
        }
        result.append(element)
      }
      return iterated
        ? result
        : nil
    }
    
    var iterator: Base.Iterator
    var splits: Int
    let omittingEmptySubsequences: Bool
    let separator: (Base.Element) -> Bool
  }
  
  func makeIterator() -> Iterator {
    return Iterator(iterator: base.makeIterator(), splits: maxSplits, omittingEmptySubsequences: omittingEmptySubsequences, separator: separator)
  }

  var base: Base
  let maxSplits: Int
  let omittingEmptySubsequences: Bool
  let separator: (Base.Element) -> Bool
}

extension LazySequenceProtocol {
  func split(maxSplits: Int = Int.max, omittingEmptySubsequences: Bool = true, whereSeparator isSeparator: @escaping (Element) -> Bool) -> LazySplitSequence<Self> {
    return LazySplitSequence(base: self, maxSplits: maxSplits, omittingEmptySubsequences: omittingEmptySubsequences, separator: isSeparator)
  }
}

extension LazySequenceProtocol where Element: Equatable {
  func split(separator: Element, maxSplits: Int = Int.max, omittingEmptySubsequences: Bool = true) -> LazySplitSequence<Self> {
    return LazySplitSequence(base: self, maxSplits: maxSplits, omittingEmptySubsequences: omittingEmptySubsequences, separator: { $0 == separator })
  }
}

let a = [1, 2, 3, 3, 3, 4, 5, 6, 7, 8]
print(a.split { $0 % 3 == 0 })

let s = a.lazy.split { $0 % 3 == 0 }
print(s)
for b in s {
  print(b)
}
// ambiguous :/
// let s2 = a.lazy.split(separator: 3)