IPython/frontend/html/notebook/static/codemirror/lib/util/foldcode.js

CodeMirror.braceRangeFinder = function(cm, line) {
  var lineText = cm.getLine(line);
  var startChar = lineText.lastIndexOf("{");
  if (startChar < 0 || lineText.lastIndexOf("}") > startChar) return;
  var tokenType = cm.getTokenAt({line: line, ch: startChar}).className;
  var count = 1, lastLine = cm.lineCount(), end;
  outer: for (var i = line + 1; i < lastLine; ++i) {
    var text = cm.getLine(i), pos = 0;
    for (;;) {
      var nextOpen = text.indexOf("{", pos), nextClose = text.indexOf("}", pos);
      if (nextOpen < 0) nextOpen = text.length;
      if (nextClose < 0) nextClose = text.length;
      pos = Math.min(nextOpen, nextClose);
      if (pos == text.length) break;
      if (cm.getTokenAt({line: i, ch: pos + 1}).className == tokenType) {
        if (pos == nextOpen) ++count;
        else if (!--count) { end = i; break outer; }
      }
      ++pos;
    }
  }
  if (end == null || end == line + 1) return;
  return end;
};


CodeMirror.newFoldFunction = function(rangeFinder, markText) {
  var folded = [];
  if (markText == null) markText = '<div style="position: absolute; left: 2px; color:#600">&#x25bc;</div>%N%';

  function isFolded(cm, n) {
    for (var i = 0; i < folded.length; ++i) {
      var start = cm.lineInfo(folded[i].start);
      if (!start) folded.splice(i--, 1);
      else if (start.line == n) return {pos: i, region: folded[i]};
    }
  }

  function expand(cm, region) {
    cm.clearMarker(region.start);
    for (var i = 0; i < region.hidden.length; ++i)
      cm.showLine(region.hidden[i]);
  }

  return function(cm, line) {
    cm.operation(function() {
      var known = isFolded(cm, line);
      if (known) {
        folded.splice(known.pos, 1);
        expand(cm, known.region);
      } else {
        var end = rangeFinder(cm, line);
        if (end == null) return;
        var hidden = [];
        for (var i = line + 1; i < end; ++i) {
          var handle = cm.hideLine(i);
          if (handle) hidden.push(handle);
        }
        var first = cm.setMarker(line, markText);
        var region = {start: first, hidden: hidden};
        cm.onDeleteLine(first, function() { expand(cm, region); });
        folded.push(region);
      }
    });
  };
};