gistfile1.js

// Returns the absolute offset from the top of the container
    function getCharacterOffsetWithin(sel, node) {
      var range = sel.getRangeAt(0);
      var selStr = sel.toString();

      // we sanitize linebreaks and white spaces
      selLen = selStr.replace(/[^A-Za-z0-9.-:\/$ ]/g, "").replace(/(\r\n|\n|\r|\s+)/gm, ' ').length;

      var treeWalker = document.createTreeWalker( node, NodeFilter.SHOW_TEXT, function(node) {

        var filter = NodeFilter.FILTER_REJECT
        ,   nodeRange = rangy.createRange();

        nodeRange.selectNodeContents(node);

        if( nodeRange.compareBoundaryPoints( Range.END_TO_END, range ) < 1 ){
          filter = NodeFilter.FILTER_ACCEPT;
        }

        return filter;
        }, false );
      

      var lefOffset = 0
      ,   begining = 0
      ,   end = 0
      ,   prev = range.startContainer
      ,   prevContent = prev.textContent;
      
      while (treeWalker.nextNode()) {
        lefOffset += treeWalker.currentNode.length;
      }

      if (prev.nodeType == 3) {
        begining += lefOffset + range.startOffset;
        end += lefOffset + range.endOffset;
      }

      // Wen we are selecting already annotated text we need to ignore the text itself in our offset
      if(prevContent === selStr){
        begining -= prevContent.length;
        end -= prevContent.length;
      }

      return {'beg':begining, 'end':end};
    }