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Before we deal with movements, we must complete our Row type.

The rxToCx() method

This does the opposite of the cxToRx() method, that is, it finds the real column for an index of the rendered row. It must still iterate the real row, not the rendered one, because from the latter we just couldn't know what was a tab and what a real space character. Therefore we iterate the real row like in cxToRx(), we track both the rendered column and the current index in the real row, and when the resulting rendered column is greater than the requested column we return the current index in the real row.

/// Calculate the position of a rendered column in the real row.
pub fn rxToCx(row: *Row, rx: usize) usize {
    var cur_rx: usize = 0;
    var cx: usize = 0;
    while (cx < row.chars.items.len) : (cx += 1) {
        if (row.chars.items[cx] == '\t') {
            cur_rx += (opt.tabstop - 1) - (cur_rx % opt.tabstop);
        }
        cur_rx += 1;

        if (cur_rx > rx) {
            return cx;
        }
    }
    return cx;
}