README.md

Pairing Heap

Based on the Pairing Heap discussed in section 5.5 of the book Purely Functional Data Structures with the following Standard ML implementation:

functor PairingHeap (Element : ORDERED) : HEAP =
struct
  structure Elem = Element

  datatype Heap = E | T of Elem.T * Heap list

  val empty = E
  fun isEmpty E = true | isEmpty _ = false

  fun merge (h, E) = h
    | merge (E, h) = h
    | merge (h1 as T (x, hs1), h2 as T (y, hs2)) =
        if Elem.leq (x, y) then T (x, h2 :: hs1) else T (y, h1 :: hs2)
  fun insert (x, h) = merge (T (x, []), h)

  fun mergePairs [] = E
    | mergePairs [h] = h
    | mergePairs (h1 :: h2 :: hs) = merge (merge (h1, h2), mergePairs hs)

  fun findMin E = raise EMPTY
    | findMin (T (x, hs)) = x
  fun deleteMin E = raise EMPTY
    | deleteMin (T (x, hs)) = mergePairs hs
end