反轉Multimap之以功能性的方式在斯卡拉

Question

在Scala中，如果我們有多重映射它映射字符串設置[字符串]，例如：

val a = Map(
    "Account1" -> Set("Cars", "Trucks"), 
    "Account2" -> Set("Trucks", "Boats") 
) 

什麼是反相/倒車它與最終的一種優雅的方式：

Map(
    "Boats" -> Set("Account2"), 
    "Cars" -> Set("Account1"), 
    "Trucks" -> Set("Account1", "Account2") 
) 

Answer 1

很抱歉的長眼線，你可以打破它，這僅僅是一個快速&骯髒的解決方案，但它確實工作

scala> a.toList.flatMap{ case (k,v:Set[_]) => v map (x => (x,k))}.groupBy(_._1).map{ case (k,v:List[(String,String)]) => (k,v.map(_._2).toSet)} 
res45: scala.collection.immutable.Map[String,scala.collection.immutable.Set[String]] = Map(Boats -> Set(Account2), Trucks -> Set(Account1, Account2), Cars -> Set(Account1)) 

，或者如果你喜歡閱讀的版本（同上述只是擊穿線路）:)：

scala> val aTuples = a.toList.flatMap{ case (k,v:Set[_]) => v map (x => (x,k))} 
aTuples: List[(String, String)] = List((Cars,Account1), (Trucks,Account1), (Trucks,Account2), (Boats,Account2)) 

scala> val grouped = aTuples.groupBy(_._1) 
grouped: scala.collection.immutable.Map[String,List[(String, String)]] = Map(Boats -> List((Boats,Account2)), Trucks -> List((Trucks,Account1), (Trucks,Account2)), Cars -> List((Cars,Account1))) 

scala> val flattenAndToSet = grouped.map{ case (k,v:List[(String,String)]) => (k,v.map(_._2).toSet)} 
flattenAndToSet: scala.collection.immutable.Map[String,scala.collection.immutable.Set[String]] = Map(Boats -> Set(Account2), Trucks -> Set(Account1, Account2), Cars -> Set(Account1))