46 lines
1.5 KiB
Standard ML
46 lines
1.5 KiB
Standard ML
structure TextScroll =
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struct
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structure TC = TextConstants
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(* Preqreuisite: move buffer to cursorIdx *)
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fun getScrollColumn (buffer, cursorIdx, windowWidth, prevScrollColumn) =
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let
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val startOfLine = Cursor.vi0 (buffer, cursorIdx)
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val newColumn = cursorIdx - startOfLine
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val howManyColumnsCanWeFit =
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if windowWidth >= TC.textLineWidth then TC.textLineCount
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else windowWidth div TC.xSpace
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val howManyColumnsCanWeFit = howManyColumnsCanWeFit - 1
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in
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if newColumn < prevScrollColumn then
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(* we are moving the cursor backwards
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* so make sure that newColumn is on the left side *)
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newColumn
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else if newColumn > prevScrollColumn + howManyColumnsCanWeFit then
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(* we are scrolling forwards *)
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newColumn - howManyColumnsCanWeFit
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else
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(* we can display the current column without moving the scroll column
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* so we do that *)
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prevScrollColumn
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end
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fun getStartLine (prevLineNumber, cursorLine, windowHeight) =
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if cursorLine <= prevLineNumber then
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(* if cursorLine is prior or same as prevLineNumber,
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* then use cursorLine as scroll-line-start *)
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cursorLine
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else
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(* cursorLine > prevLineNumber *)
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let
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val howManyLinesWeCanFit = windowHeight div TC.ySpace
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in
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if prevLineNumber + howManyLinesWeCanFit >= cursorLine then
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prevLineNumber
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else
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let val lineDifference = cursorLine - prevLineNumber
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in prevLineNumber + lineDifference
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end
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end
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end
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