Write the first test

NightLight.Core.Tests/Arbitraries.fs

@@ -0,0 +1,22 @@
+namespace NightLight.Core.Tests
+
+open System
+open FsCheck.FSharp
+open NightLight.Core.Models
+
+type Arbitraries =
+    static member Interaction() =
+        let genTimeChangedInteraction =
+            gen {
+                let! time = ArbMap.defaults |> ArbMap.generate<DateTime>
+                return Interaction.TimeChanged time
+            }
+
+        let genHumanInteraction =
+            gen {
+                let! light = Gen.elements lights
+                let! humanInteraction = Gen.elements [ LightTurnedOn light; LightTurnedOff light ]
+                return Interaction.HumanInteraction humanInteraction
+            }
+
+        Gen.oneof [ genTimeChangedInteraction; genHumanInteraction ] |> Arb.fromGen

NightLight.Core.Tests/NightLight.Core.Tests.fsproj

@@ -9,6 +9,8 @@
     <ItemGroup>
         <Compile Include="NightLightStateMachine.fs" />
         <Compile Include="FakeHome.fs" />
+        <Compile Include="Arbitraries.fs" />
+        <Compile Include="NightLightTests.fs" />
     </ItemGroup>
 
     <ItemGroup>

NightLight.Core.Tests/NightLightTests.fs

@@ -0,0 +1,20 @@
+module NightLight.Core.Tests.NightLightTests
+
+open FsCheck.Xunit
+
+let private assertIsOk (result: Result<unit, 'a>) : unit =
+    match result with
+    | Ok() -> ()
+    | Error error -> failwith $"Expected Ok, got Error {error}"
+
+[<Property(Arbitrary = [| typeof<Arbitraries> |])>]
+let ``Brightness should always be under 255`` (fakeHome: FakeHome) (interactions: Interaction list) =
+    interactions
+    |> Seq.iter (fun interaction -> fakeHome.Interact interaction |> assertIsOk)
+
+    fakeHome.LightStates
+    |> Seq.choose (fun (_, state) ->
+        match state with
+        | On brightness -> Some brightness
+        | Off -> None)
+    |> Seq.forall (fun brightness -> brightness < 255uy)