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elisp: Columns

 
 31.16 Counting Columns
 ======================
 
 The column functions convert between a character position (counting
 characters from the beginning of the buffer) and a column position
 (counting screen characters from the beginning of a line).
 
    These functions count each character according to the number of
 columns it occupies on the screen.  This means control characters count
 as occupying 2 or 4 columns, depending upon the value of ‘ctl-arrow’,
 and tabs count as occupying a number of columns that depends on the
 value of ‘tab-width’ and on the column where the tab begins.  
 Usual Display.
 
    Column number computations ignore the width of the window and the
 amount of horizontal scrolling.  Consequently, a column value can be
 arbitrarily high.  The first (or leftmost) column is numbered 0.  They
 also ignore overlays and text properties, aside from invisibility.
 
  -- Function: current-column
      This function returns the horizontal position of point, measured in
      columns, counting from 0 at the left margin.  The column position
      is the sum of the widths of all the displayed representations of
      the characters between the start of the current line and point.
 
  -- Command: move-to-column column &optional force
      This function moves point to COLUMN in the current line.  The
      calculation of COLUMN takes into account the widths of the
      displayed representations of the characters between the start of
      the line and point.
 
      When called interactively, COLUMN is the value of prefix numeric
      argument.  If COLUMN is not an integer, an error is signaled.
 
      If it is impossible to move to column COLUMN because that is in the
      middle of a multicolumn character such as a tab, point moves to the
      end of that character.  However, if FORCE is non-‘nil’, and COLUMN
      is in the middle of a tab, then ‘move-to-column’ converts the tab
      into spaces so that it can move precisely to column COLUMN.  Other
      multicolumn characters can cause anomalies despite FORCE, since
      there is no way to split them.
 
      The argument FORCE also has an effect if the line isn’t long enough
      to reach column COLUMN; if it is ‘t’, that means to add whitespace
      at the end of the line to reach that column.
 
      The return value is the column number actually moved to.
 

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