don't scroll past top of page

src/Mtlstats/Actions.hs

@@ -38,6 +38,8 @@ module Mtlstats.Actions
   , resetGoalData
   , assignPMins
   , backHome
+  , scrollUp
+  , scrollDown
   ) where
 
 import Control.Monad.Trans.State (modify)
@@ -237,3 +239,11 @@ backHome
   = (progMode     .~ MainMenu)
   . (inputBuffer  .~ "")
   . (scrollOffset .~ 0)
+
+-- | Scrolls the display up
+scrollUp :: ProgState -> ProgState
+scrollUp = scrollOffset %~ max 0 . pred
+
+-- | Scrolls the display down
+scrollDown :: ProgState -> ProgState
+scrollDown = scrollOffset %~ succ

src/Mtlstats/Control.hs

@@ -278,8 +278,8 @@ reportC = Controller
     return C.CursorInvisible
   , handleController = \e -> do
     case e of
-      C.EventSpecialKey C.KeyUpArrow   -> modify $ scrollOffset %~ pred
-      C.EventSpecialKey C.KeyDownArrow -> modify $ scrollOffset %~ succ
+      C.EventSpecialKey C.KeyUpArrow   -> modify scrollUp
+      C.EventSpecialKey C.KeyDownArrow -> modify scrollDown
       C.EventSpecialKey C.KeyHome      -> modify $ scrollOffset .~ 0
       C.EventSpecialKey _              -> modify backHome
       C.EventCharacter _               -> modify backHome

test/ActionsSpec.hs

@@ -61,6 +61,8 @@ spec = describe "Mtlstats.Actions" $ do
   resetGoalDataSpec
   assignPMinsSpec
   backHomeSpec
+  scrollUpSpec
+  scrollDownSpec
 
 startNewSeasonSpec :: Spec
 startNewSeasonSpec = describe "startNewSeason" $ do
@@ -672,3 +674,29 @@ backHomeSpec = describe "backHome" $ do
 
   it "should reset the scroll offset" $
     result^.scrollOffset `shouldBe` 0
+
+scrollUpSpec :: Spec
+scrollUpSpec = describe "scrollUp" $ do
+
+  context "scrolled down" $
+    it "should decrease the scroll offset by one" $ let
+      ps  = newProgState & scrollOffset .~ 10
+      ps' = scrollUp ps
+      in ps'^.scrollOffset `shouldBe` 9
+
+  context "at top" $
+    it "should keep the scroll offset at zero" $ let
+      ps = scrollUp newProgState
+      in ps^.scrollOffset `shouldBe` 0
+
+  context "above top" $
+    it "should return the scroll offset to zero" $ let
+      ps  = newProgState & scrollOffset .~ (-10)
+      ps' = scrollUp ps
+      in ps'^.scrollOffset `shouldBe` 0
+
+scrollDownSpec = describe "scrollDown" $
+  it "should increase the scroll offset" $ let
+    ps  = newProgState & scrollOffset .~ 10
+    ps' = scrollDown ps
+    in ps'^.scrollOffset `shouldBe` 11