A bug in the kilo code

Step 85 begins dealing with moving the cursor around tab characters, and step 88 gives the implementation of editorRowCxToRx, which calculates where the cursor should be based on how many tabs it's passed over in the line:

int editorRowCxToRx(erow *row, int cx) {
  int rx = 0;
  int j;
  for (j = 0; j < cx; j++) {
    if (row->chars[j] == '\t')
      rx += (KILO_TAB_STOP - 1) - (rx % KILO_TAB_STOP);
    rx++;
  }
  return rx;
}

Here, the parameter cx is the index into the array of characters of the line text, and the return value row offset of where the cursor should be placed on the screen

The problem comes when moving between two lines. Let's say you've got two lines, something like this:

\t
123456789

Now suppose that the cursor is on the first line, to the right of the tab character. Normally, the cursor will be displayed directly above the '9' character on the second line. Now suppose you press the down arrow to move to the next line, where should the cursor be? Virtually every editor I've ever used will place the cursor on the '9' of the second line.

However, the cx value will still be 1, and so for kilo, the cursor will be placed on the 2.

Maybe this will be fixed later in the project. I'm not going to try to deal with it now (honestly, this whole method of dealing with tabs feels like a botch anyway). Perhaps once I finish working through my first pass over the code.

Neal Ford "The Productive Programmer" (2008)

Neal Ford "The Productive Programmer" talk (2008)

An interesting talk that still seems pretty relevant (although do people still use Subversion anymore, or is Git now the undisputed king of version control?). Anyway, something I feel like would be worth watching again at some point, so I'm enshrining a link here. Note that it's divided up into 7 different videos, with transitions at fairly random points which can be a bit distracting.

kilo commentary:

Steps 80-84 of the kilo construction process have to do with rendering tab characters as spaces on the screen. Part of this is converting tab characters to the equivalent number of spaces, which is done by this bit of code:

      row->render[idx++] = ' ';
      while (idx % KILO_TAB_STOP != 0) row->render[idx++] = ' ';

Which... this is equivalent to this, right?

      do
          row->render[idx++] = ' ';
      while (idx % KILO_TAB_STOP != 0);

I mean, I realize that the do-while construct is relatively uncommon to use in C (at least in my experience), but this is a perfect example of where to use it.