/**
   * Internal draw request handler.
   *
   * @param gc Graphics context to update.
   */
  private final void handleDrawRequest(final GC gc) {
    if (mBracketPosition.isDeleted) {
      return;
    }

    int length = mBracketPosition.getLength();
    if (length < 1) {
      return;
    }

    int offset = mBracketPosition.getOffset();
    IRegion region = mSourceViewer.getVisibleRegion();

    if (region.getOffset() <= offset
        && region.getOffset() + region.getLength() >= offset + length) {
      offset -= region.getOffset();
      draw(gc, offset, 1);
    }
  }
 /**
  * Constructor
  *
  * @param sourceViewer Source in which to paint brackets.
  */
 public SchemeParenthesisPainter(final ISourceViewer sourceViewer, SchemeEditor editor) {
   mEditor = editor;
   mSourceViewer = sourceViewer;
   mTextWidget = sourceViewer.getTextWidget();
 }
  /** @see IPainter#paint(int) */
  public final void paint(final int reason) {
    Point selection = mSourceViewer.getSelectedRange();
    if (selection.y > 0) {
      deactivate(true);
      return;
    }

    SexpNavigator explorer = mEditor.getExplorer();

    boolean backward = true;
    boolean closeToParen = false;
    int offset = selection.x;
    IDocument document = mEditor.getDocument();
    try {
      char previousChar = '\0';
      char nextChar = '\0';

      if (selection.x > 0) previousChar = document.getChar(selection.x - 1);

      if (selection.x > 0
          && SchemeScannerUtilities.isClosingParenthesis(previousChar)
          && SchemeTextUtilities.getPartition(document, selection.x - 1).getType()
              == IDocument.DEFAULT_CONTENT_TYPE) {
        closeToParen = true;
      } else {
        nextChar = document.getChar(selection.x);
        if (selection.x < document.getLength() - 1
            && SchemeScannerUtilities.isOpeningParenthesis(nextChar)
            && SchemeTextUtilities.getPartition(document, selection.x).getType()
                == IDocument.DEFAULT_CONTENT_TYPE) {
          closeToParen = true;
          backward = false;
        }
      }

      if (closeToParen && backward && explorer.backwardSexpression(selection.x)) {
        offset = explorer.getListStart();
        char matchingChar = document.getChar(offset);
        mMismatch =
            SchemeScannerUtilities.getParenthesisType(previousChar)
                != SchemeScannerUtilities.getParenthesisType(matchingChar);
      } else {
        if (closeToParen && !backward && explorer.forwardSexpression(selection.x)) {
          offset = explorer.getSexpEnd() - 1;
          char matchingChar = document.getChar(offset);
          mMismatch =
              SchemeScannerUtilities.getParenthesisType(nextChar)
                  != SchemeScannerUtilities.getParenthesisType(matchingChar);
        } else {
          deactivate(true);
          return;
        }
      }

    } catch (BadLocationException exception) {
      deactivate(true);
      return;
    }

    if (mIsActive) {
      // only if different
      if (offset != mBracketPosition.getOffset()) {
        // remove old highlighting
        handleDrawRequest(null);
        // update position
        mBracketPosition.isDeleted = false;
        mBracketPosition.offset = offset;
        mBracketPosition.length = 1;
        // apply new highlighting
        handleDrawRequest(null);
      }
    } else {
      mIsActive = true;

      mBracketPosition.isDeleted = false;
      mBracketPosition.offset = offset;
      mBracketPosition.length = 1;

      mTextWidget.addPaintListener(this);
      mPositionManager.managePosition(mBracketPosition);
      handleDrawRequest(null);
    }
  }