#!/usr/bin/env -S dotnet fsi

type HashSet<'a> = System.Collections.Generic.HashSet<'a>

let a = HashSet [1..10]

let b = HashSet [5..10]

let c = HashSet [6;7;11]

printfn "%A" (Set.intersectMany [(Set a); (Set b); (Set c)])

let intersect (x:HashSet<'a>) (y:HashSet<'a>) =

let smaller, greater =

if x.Count < y.Count then x,y else y,x

let newHashSet = HashSet<'a>()

for x in smaller do

if greater.Contains x then

ignore (newHashSet.Add x)

newHashSet

printfn "%A" (List.ofSeq (intersect a b))

printfn "%A" (List.ofSeq (intersect (intersect a b) c))

let clone (set:HashSet<'a>) =

let nh = HashSet()

for x in set do

nh.Add x |> ignore

nh

let intersectMany (sets:seq<HashSet<'a>>) =

if Seq.isEmpty sets then

HashSet()

else

let smallest = clone (sets |> Seq.minBy (fun set -> set.Count))

for set in sets do

smallest.IntersectWith set

smallest

printfn "%A" (List.ofSeq (intersectMany [a;b;c]))

printfn "a: %A" (List.ofSeq a)

printfn "b: %A" (List.ofSeq b)

printfn "c: %A" (List.ofSeq c)