RandomPair.fs

module Utils =
    
    let private rng = new Random()

    let rec PickRandomPair<'TKey when 'TKey : equality> (pool:('TKey * int) list) =

        let decreasePair(needle:'TKey, pool:('TKey * int) list) =
            let decrease (key:'TKey, value:int) = if key = needle then (key, value - 1) else (key, value)
            pool |> Seq.fold (fun acc r -> (decrease(r) :: acc)) []

        let i = rng.Next(0, pool.Count())
        let (key, value) = pool.[i]
            
        match value with
        | 0 -> PickRandomPair(pool)
        | _ -> (key, decreasePair(key, pool))

RandomPairTest.fs

[<TestFixture>]
type Geometry() =

    [<Test>]
    member this.TestRandomPair() =
        let pairs = [
            (1, 2);
            (2, 2)
        ]

        let (r, npairs) = pickRandomPair(pairs)

        let countTwo = query {
            for (n, quantity) in npairs do
            where (quantity = 2)
            count
        }

        Assert.IsTrue(r = 1 || r = 2)
        Assert.AreEqual(1, countTwo)