BidirectionalMap.swift

import Foundation

public class BidirectionalMap<Left: Hashable, Right: Hashable> {
  private var leftToRightMapping = [Left : Right]()
  private var rightToLeftMapping = [Right : Left]()
  
  public subscript(left: Left) -> Right? {
    get { return leftToRightMapping[left] }
    
    set (newValue) {
      let oldRight = leftToRightMapping[left]
      leftToRightMapping[left] = newValue
      
      if let newValue = newValue {
        rightToLeftMapping[newValue] = left
      } else if let oldRight = oldRight {
        rightToLeftMapping[oldRight] = nil
      }
    }
  }
  
  public subscript(right: Right) -> Left? {
    get { return rightToLeftMapping[right] }
    
    set (newValue) {
      let oldLeft = rightToLeftMapping[right]
      rightToLeftMapping[right] = newValue
      
      if let newValue = newValue {
        leftToRightMapping[newValue] = right
      } else if let oldLeft = oldLeft {
        leftToRightMapping[oldLeft] = nil
      }
    }
  }
}

let map = BidirectionalMap = [Int: String]()

map[1] = "hello"

print(map["hello"]) // 1
print(map[1]) // "hello"

Both directions are O(1)