improved integral shrink trees to match behavior of binary search

src/Hedgehog/Gen.fs

@@ -202,7 +202,27 @@ module Gen =
     /// Generates a random number in the given inclusive range.
     [<CompiledName("Integral")>]
     let inline integral (range : Range<'a>) : Gen<'a> =
-        create (Shrink.towards <| Range.origin range) (Random.integral range)
+        let mapFirstDifferently f g = function
+            | [] -> []
+            | x :: xs -> (f x) :: (xs |> List.map g)
+        let rec createTree (destination : ^a) (x : ^a) =
+          let childrenValues =
+              match Shrink.towards destination x |> Seq.toList with
+              | [] -> []
+              | x :: [] -> List.singleton x
+              | _ :: xs -> xs
+          let xs =
+              childrenValues
+              |> Seq.cons destination
+              |> Seq.pairwise
+              |> Seq.toList
+              |> mapFirstDifferently id (fun (d, x) -> (d + LanguagePrimitives.GenericOne, x))
+              |> Seq.map (fun (d, x) -> createTree d x)
+          Node (x, xs)
+        range
+        |> Random.integral
+        |> Random.map (range |> Range.origin |> createTree)
+        |> ofRandom
 
     //
     // Combinators - Choice