gistfile1.ml

#light

[<AutoOpen>]
module Operators =
  // Untuple forward operator
  let (||>) (x,y) f = f x y

module Map =
  let union m1 m2 =
    (m1, m2) ||> Map.fold_right Map.add
    
  let intersect m1 m2 =
    (m2, Map.empty) 
      ||> Map.fold_right (fun k _ acc ->
                            match Map.tryfind k m1 with
                            | None   -> acc
                            | Some x -> Map.add k x acc) 

// Returns a distance-based similarity score for p1 and p2
let sim_distance (prefs:Map<string, Map<string, float>>) p1 p2  =
  // Get the list of shared items
  let si = Map.intersect prefs.[p1] prefs.[p2] |> Seq.map (fun kvp -> kvp.Key)  
  
  // No ratings in common, return 0
  match Seq.length si with
  | 0 -> 0.
  | _ -> 
    // Add up the squares of all the differences
    let sum_of_squares = 
      seq {for item in si -> pown (prefs.[p1].[item] - prefs.[p2].[item]) 2}
      |> Seq.sum
    1. / (1. + (sqrt sum_of_squares))