Sequence().unique(by:).swift

extension Sequence {
  
  /**
   Returns a filtered "view" of `self` where only one for all elements that return the same value for `taggingHandler` is kept. In other words: every element in the returned array was the first to return its value from `taggingHandler` when traversing the sequence in order.
   
   ```swift
   [1, 1, 1, 2, 3, 3, 4, 5, 5, 5].unique { $0 }
   // = [1, 2, 3, 4, 5]
   
   [0, 1, -1, -2, 2, -3, -3].unique { abs($0) }
   // = [0, 1, -2, -3]
   ```
   
   - parameter taggingHandler: Returns a hashable value used to identify the unique values in the sequence.
   */
  func unique<T: Hashable>(by taggingHandler: (_ element: Self.Iterator.Element) -> T) -> [Self.Iterator.Element] {
    var knownTags = Set<T>()
    
    return self.filter { element -> Bool in
      let tag = taggingHandler(element)
      
      if !knownTags.contains(tag) {
        knownTags.insert(tag)
        return true
      }
      
      return false
    }
  }
  
}