Drawing the rows

This function will be expanded later, but for now all it needs to do is to draw the rows without any highlight.

/// Append rows to be drawn to the surface. Handles escape sequences for syntax
/// highlighting.
fn drawRows(e: *Editor) !void {
    // code to come...
}

We can print a number of rows which is equal to the height of our main window, which is e.screen.rows. We use a for loop with a range, but to the index y we must add e.view.rowoff, which is the current row offset. This will be greater than 0 if we scroll down our window and the first row went off-screen.

    const V = &e.view;
    const rows = e.buffer.rows.items;

    for (0 .. e.screen.rows) |y| {
        const ix: usize = y + V.rowoff;

Since we draw by screen rows, and not by Buffer rows, y may be greater than the number of the Buffer rows, which means we are past the end of the file. In this case we draw a ~ to point that out.

        // past buffer content
        if (ix >= rows.len) {
            try e.toSurface('~');
        }

Otherwise, we are within the file content, but it doesn't mean that there is something to print in all cases:

        // within buffer content
        else {
            // length of the rendered line
            const rowlen = rows[ix].render.len;

            // actual length that should be drawn because visible
            var len = if (V.coloff > rowlen) 0 else rowlen - V.coloff;

For example, if we scrolled the window to the right, the leftmost columns would go off-screen, and e.view.coloff would become positive. If the line is shorter than that, nothing will be printed, because it's completely off-screen.

We also limit len to the number of screen columns:

            len = @min(len, e.screen.cols);

If len > 0 there's something to print: which would be the slice of the rendered line that starts at coloff, and is long len characters.

We append this slice to the surface ArrayList.

            // draw the visible part of the row
            if (len > 0) {
                try e.toSurface(rows[ix].render[V.coloff .. V.coloff + len]);
            }
        }

We end the line after that:

        try e.toSurface(ansi.ClearLine);
        try e.toSurface("\r\n"); // end the line
    }

Again: V.coloff is 0 unless a part of the row went off-screen on the left side.

At this point, if you compile and run:

./kilo kilo

you should already be able to visualize the file on the screen! That's big progress. You can't move the cursor, and you can still quit the editor with Ctrl-Q pressed 3 times.