open System

type Result = Even of int | Odd of int 

let agentLogic (inbox:MailboxProcessor<AsyncReplyChannel<Result>>) =
    let rec odd(counter) = async {
        let! msg = inbox.Receive()
        msg.Reply(Odd counter)
        return! even(counter + 1)} 
        
    and even(counter) = async {
        let! msg = inbox.Receive()
        msg.Reply(Even counter)
        return! odd(counter + 1)}

    odd 1

let agent = MailboxProcessor<_>.Start agentLogic
let getNext () = agent.PostAndReply id

type internal Token = int
type internal ThrottlingAgentMessage<'t> = 
    | GetToken of AsyncReplyChannel<Token>
    | ReturnToken of Token

type ThrottlingAgent(limit) = 
    let agent = MailboxProcessor<ThrottlingAgentMessage<_>>.Start(fun agent -> 
    
        /// Represents a state when the agent is blocked - uses 'Scan' to wait for completion of some work
        let rec blocked() = 
            agent.Scan(function ReturnToken t -> Some( hasCapacity (limit - 1)) | _ -> None) 
        
        /// Represents a state when the agent has capacity
        and hasCapacity inUse = async { 
            let! msg = agent.Receive()
            match msg with 
            | ReturnToken _  -> return! hasCapacity (inUse - 1))
            | GetToken resp  ->
                resp.Reply(1)
                if inUse < limit - 1 then return! hasCapacity (inUse + 1)
                else return! blocked() }
        hasCapacity 0)          // Start in working state with zero running work items
    member x.GetToken() = agent.PostAndReply( GetToken )
    member x.ReturnToken(token) = agent.Post( ReturnToken token )

let (|Even|Odd|) i = 
    if i % 2 = 0 then Even else Odd

let testNumber i =
    match i with
    | Even -> printfn "%d is even" i
    | Odd -> printfn "%d is odd" i

let (|DivisibleBy|_|) by n = 
    if n % by = 0 then Some DivisibleBy else None

let fizzBuzz = function 
    | DivisibleBy 3 & DivisibleBy 5 -> "FizzBuzz" 
    | DivisibleBy 3 -> "Fizz" 
    | DivisibleBy 5 -> "Buzz" 
    | i -> string i