module Env

    open Types

    type Env = System.Collections.Generic.Dictionary<string, Node>
    type EnvChain = Env list

    let errSymbolNotFound s = EvalError(sprintf "'%s' not found" s)
    let errNoEnvironment () = EvalError("no environment")
    let errTooManyValues () = EvalError("too many values")
    let errNotEnoughValues () = EvalError("not enough values")
    let errExpectedSymbol () = EvalError("expected symbol")
    let errOnlyOneSymbol () = EvalError("only one symbol after &")

    let makeEmpty () = Env()

    let ofList lst =
        let env = makeEmpty ()
        let accumulate (e : Env) (k, v) = e.Add(k, v); e
        List.fold accumulate env lst

    let set (env : EnvChain) key node =
        match env with
        | head::_ -> head.[key] <- node
        | _ -> raise <| errNoEnvironment ()

    let rec find (chain : EnvChain) key =
        match chain with
        | [] -> None
        | env::rest ->
            match env.TryGetValue(key) with
            | true, v -> Some(v)
            | false, _ -> find rest key

    let get chain key =
        match find chain key with
        | Some(v) -> v
        | None -> raise <| errSymbolNotFound key

    let makeFunc =
        let counter = ref 0
        let getNext () = System.Threading.Interlocked.Increment(counter)
        fun f -> Func(getNext (), f)

    let makeRootEnv () =
        let wrap name f = name, makeFunc f
        let env =
            [ wrap "+" Core.add;
              wrap "-" Core.subtract;
              wrap "*" Core.multiply;
              wrap "/" Core.divide;
              wrap "list" Core.list;
              wrap "list?" Core.isList;
              wrap "empty?" Core.isEmpty;
              wrap "count" Core.count;
              wrap "=" Core.eq;
              wrap "<" Core.lt;
              wrap "<=" Core.le;
              wrap ">=" Core.ge;
              wrap ">" Core.gt;
              wrap "pr-str" Core.pr_str;
              wrap "str" Core.str;
              wrap "prn" Core.prn;
              wrap "println" Core.println ]
            |> ofList
        [ env ]

    let makeNew outer symbols nodes =
        let env = (makeEmpty ())::outer
        let rec loop symbols nodes =
            match symbols, nodes with
            | [Symbol("&"); Symbol(s)], nodes ->
                set env s (List nodes)
                env
            | Symbol("&")::_, _ -> raise <| errOnlyOneSymbol ()
            | Symbol(s)::symbols, n::nodes -> 
                set env s n
                loop symbols nodes
            | [], [] -> env
            | _, [] -> raise <| errNotEnoughValues ()
            | [], _ -> raise <| errTooManyValues ()
            | _, _ -> raise <| errExpectedSymbol ()
        loop symbols nodes