1 files changed, 57 insertions, 0 deletions
diff --git a/src/TextViewport/Viewport/Viewport.hs b/src/TextViewport/Viewport/Viewport.hs
new file mode 100644
index 0000000..e6fdaab
--- /dev/null
+++ b/src/TextViewport/Viewport/Viewport.hs
@@ -0,0 +1,57 @@
+module TextViewport.Viewport.Viewport where
+
+import TextViewport.Render.RenderState (RenderState)
+import TextViewport.Render.RenderState qualified as RenderState
+
+
+data Viewport = Viewport
+  { vpWidth  :: !Int
+  , vpHeight :: !Int
+  , vpOffset :: !Int
+  } deriving (Show)
+
+mkViewport :: Int -> Int -> RenderState -> Viewport
+mkViewport width height rs =
+    alignBottom rs Viewport
+      { vpWidth  = width
+      , vpHeight = height
+      , vpOffset = 0
+      }
+
+-- any function that sets vpOffset and can overshoot should use clampViewport
+clampViewport :: RenderState -> Viewport -> Viewport
+clampViewport rs vp =
+  let total  = RenderState.rsLineCount rs
+      maxOff = max 0 (total - vpHeight vp)
+      off    = vpOffset vp
+  in vp { vpOffset = max 0 (min maxOff off) }
+
+scrollBy :: Int -> Viewport -> Viewport
+scrollBy delta vp =
+  vp { vpOffset = vpOffset vp + delta }
+
+scrollUp :: Int -> Viewport -> Viewport
+scrollUp n = scrollBy (-n)
+
+scrollDown :: Int -> Viewport -> Viewport
+scrollDown n = scrollBy n
+
+pageUp :: Viewport -> Viewport
+pageUp vp =
+  scrollBy (-(vpHeight vp)) vp
+
+pageDown :: Viewport -> Viewport
+pageDown vp =
+  scrollBy (vpHeight vp) vp
+
+alignTop :: Viewport -> Viewport
+alignTop vp =
+  vp { vpOffset = 0 }
+
+alignBottom :: RenderState -> Viewport -> Viewport
+alignBottom rs vp =
+  let total = RenderState.rsLineCount rs
+      off   = max 0 (total - vpHeight vp)
+  in vp { vpOffset = off }
+
+