module Model

open System
open System.IO
open System.Security.Cryptography

type Outcome =
    | Cached
    | Installed

let private checksum path =
    use input = File.OpenRead path
    SHA256.HashData input |> Convert.ToHexString |> _.ToLowerInvariant()

let install expected destination (download: Stream -> unit) =
    if File.Exists destination && checksum destination = expected then
        Cached
    else
        Directory.CreateDirectory(Path.GetDirectoryName destination) |> ignore
        let temporary = destination + ".incoming"

        try
            do
                use output = File.Create temporary
                download output

            if checksum temporary <> expected then
                invalidOp "model checksum differs"

            File.Move(temporary, destination, true)
            Installed
        finally
            if File.Exists temporary then
                File.Delete temporary