[feature request] relative line numbers in vim mode

[ahaas]

Something I miss from using terminal vim is the option to have relative line numbers; here is an illustration: http://jeffkreeftmeijer.com/images/relativenumber.gif

It would be enabled with :set relativenumber and was very useful for quickly jumping up and down to the desired line (e.g. 10j goes down ten lines) Without them, jumping a desired line often involves either guessing the offset, or repeatedly tapping j/k (an antipattern). One could also jump to the absolute line number, e.g. 1234gg, however for large files this is pretty slow.

Relative line numbers as an option would definitely be great to have in vim mode.

[mtaran-google]

FYI, with today's CodeMirror APIs this is actually not that hard. Basic hacky implementation that works on codemirror.net:

function showRelativeLines(cm) {
  const lineNum = cm.getCursor().line + 1;
  if (cm.state.curLineNum === lineNum) {
    return;
  }
  cm.state.curLineNum = lineNum;
  cm.setOption('lineNumberFormatter', l => 
    l === lineNum ? lineNum : Math.abs(lineNum - l));
}
editor.on('cursorActivity', showRelativeLines)

[jsjoeio]

How would you implement this in cm6?