Add 'shf/' from commit 'b6c5a95b664aeb861d7b33ffc9eefe447ba99dd7'

git-subtree-dir: shf
git-subtree-mainline: 401408448f
git-subtree-split: b6c5a95b66

shf/fcore/text-scroll.sml

@@ -0,0 +1,68 @@
+structure TextScroll =
+struct
+  structure TC = TextConstants
+
+  (* calculates new scroll column from integer arguments *)
+  fun calculateScrollColumn
+    (startOfLine, cursorIdx, windowWidth, prevScrollColumn) =
+    let
+      val newColumn = cursorIdx - startOfLine
+      val howManyColumnsCanWeFit =
+        if windowWidth >= TC.textLineWidth then TC.textLineCount
+        else windowWidth div TC.xSpace
+      val howManyColumnsCanWeFit = howManyColumnsCanWeFit - 1
+    in
+      if newColumn < prevScrollColumn then
+        (* we are moving the cursor backwards
+         * so make sure that newColumn is on the left side *)
+        newColumn
+      else if newColumn > prevScrollColumn + howManyColumnsCanWeFit then
+        (* we are scrolling forwards *)
+        newColumn - howManyColumnsCanWeFit
+      else
+        (* we can display the current column without moving the scroll column
+         * so we do that *)
+        prevScrollColumn
+    end
+
+  (* Preqreuisite: move buffer to cursorIdx *)
+  fun getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn) =
+    let
+      val startOfLine = Cursor.vi0 (buffer, cursorIdx)
+    in
+      calculateScrollColumn
+        (startOfLine, cursorIdx, windowWidth, prevScrollColumn)
+    end
+
+  fun getScrollColumnFromString (cursorIdx, windowWidth, prevScrollColumn) =
+    calculateScrollColumn (0, cursorIdx, windowWidth, prevScrollColumn)
+
+  fun getStartLine (prevLineNumber, cursorLine, windowHeight, totalLines) =
+    if cursorLine <= (prevLineNumber + 3) then
+      (* cursorLine is prior to or same as prevLineNumber,
+       * so use cursorLine to calculate the start line we want. *)
+      Int.max (cursorLine - 3, 0)
+    else
+      (* cursorLine > prevLineNumber *)
+      let
+        val howManyLinesWeCanFit = windowHeight div TC.ySpace
+      in
+        if cursorLine > prevLineNumber + (howManyLinesWeCanFit - 3) then
+          (* cursorLine is after the visible part of the screen
+           * so return the mimimum line where cursorLine is visible *)
+          if cursorLine >= totalLines - 3 then
+            Int.max (totalLines - howManyLinesWeCanFit, 0)
+          else
+            cursorLine - howManyLinesWeCanFit + 3
+        else
+          prevLineNumber
+      end
+
+  fun getLineCentre (cursorLine, windowHeight) =
+    let
+      val howManyLinesWeCanFit = windowHeight div TC.ySpace
+      val startLine = cursorLine - (howManyLinesWeCanFit div 2)
+    in
+      Int.max (startLine, 0)
+    end
+end