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scalatypeshashscala-2.8perfect-hash

Perfect hash in Scala

I have some class C:

class C (...) { ... }

I want to use it to index an efficient map. The most efficient map is an Array. So I add a "global" "static" counter in companion object to give each object unique id:

object C {
  var id_counter = 0
}

In primary constructor of C, with each creation of C I want to remember global counter value and increase it.
Question 1: How to do it?

Now I can use id in C objects as perfect hash to index array. But array does not preserve type information like map would, that a given array is indexed by C's id.

Question 2: Is it possible to have it with type safety?

Update:
Type safety in question 2 concerns type of index of map, to avoid mixing two not related ints. The value of course is (type) safe..

Question 1 asks how to increment a variable in default contructor?
Ie: Where to put?

id_counter += 1
Solution

  • Answer to your question 2:

    case class C_Id(val asInt: Int)

object C {
  private var list: ArrayBuffer[C] 
  // resizable array in scala.collection.mutable
  // you can also use ArrayList

  def apply(id: C_Id) = list(id.asInt) // only accepts an id of C
  ...
}

class C (...) {
  // in constructor:
  list += this
}

    To edited question 1: The default constructor is just the body of the type, except the definitions of methods and other constructors.