Support display of non-ASCII characters with aligned columns

* src/xdisp.c (get_normal_width): New function.
(gui_produce_glyphs): Use 'get_normal_width' to widen on display
characters whose width is not an integral multiple of the
"standard" width.
(syms_of_xdisp) <align-columns-display>: New boolean variable.

1 files changed, 56 insertions, 0 deletions

diff --git a/src/xdisp.c b/src/xdisp.c
index 9f93799783d..fdcf6a611e6 100644
--- a/src/xdisp.c
+++ b/src/xdisp.c
@@ -28812,6 +28812,21 @@ normal_char_height (struct font *font, int c)
   return ascent + descent;
 }
 
+/* Return the "standard" pixel width of a character from FACE's font,
+   if the font is fixed-pitch, zero otherwise.  */
+static int
+get_normal_width (struct face *face)
+{
+  struct font *ascii_font = face->ascii_face->font;
+  /* Heuristics: fixed-pitch fonts have the value of MAX-WIDTH not
+     much larger than AVERAGE-WIDTH. */
+  bool fixed_pitch =
+    ascii_font->average_width == ascii_font->space_width
+    && ascii_font->average_width != 0
+    && ascii_font->max_width < 3 * ascii_font->average_width;
+  return fixed_pitch ? ascii_font->space_width : 0;
+}
+
 /* EXPORT for RIF:
    Set *LEFT and *RIGHT to the left and right overhang of GLYPH on
    frame F.  Overhangs of glyphs other than type CHAR_GLYPH are
@@ -30909,6 +30924,17 @@ gui_produce_glyphs (struct it *it)
               it->phys_ascent = pcm->ascent + boff;
               it->phys_descent = pcm->descent - boff;
               it->pixel_width = pcm->width;
+              if (align_columns_display)
+                {
+                  int unit_width = get_normal_width (face);
+                  if (unit_width > 0)
+                    {
+                      int ncolumns =
+                        (it->pixel_width - 1 + unit_width) / unit_width;
+
+                      it->pixel_width = ncolumns * unit_width;
+                    }
+                }
               /* Don't use font-global values for ascent and descent
                  if they result in an exceedingly large line height.  */
               if (it->override_ascent < 0)
@@ -31486,6 +31512,17 @@ gui_produce_glyphs (struct it *it)
         it->glyph_row->contains_overlapping_glyphs_p = true;
 
       it->pixel_width = cmp->pixel_width;
+      if (align_columns_display)
+        {
+          int unit_width = get_normal_width (face);
+          if (unit_width > 0)
+            {
+              int ncolumns =
+                (it->pixel_width - 1 + unit_width) / unit_width;
+
+              it->pixel_width = ncolumns * unit_width;
+            }
+        }
       it->ascent = it->phys_ascent = cmp->ascent;
       it->descent = it->phys_descent = cmp->descent;
       IT_APPLY_FACE_BOX(it, face);
@@ -31531,6 +31568,17 @@ gui_produce_glyphs (struct it *it)
             it->glyph_row->contains_overlapping_glyphs_p = true;
           it->ascent = it->phys_ascent = metrics.ascent;
           it->descent = it->phys_descent = metrics.descent;
+          if (align_columns_display)
+            {
+              int unit_width = get_normal_width (face);
+              if (unit_width > 0)
+                {
+                  int ncolumns =
+                    (it->pixel_width - 1 + unit_width) / unit_width;
+
+                  it->pixel_width = ncolumns * unit_width;
+                }
+            }
         }
       IT_APPLY_FACE_BOX(it, face);
 
@@ -35607,6 +35655,14 @@ variable are ignored and the default 0.25 is used instead.  */);
 Value is a number or a cons (WIDTH-DPI . HEIGHT-DPI).  */);
   Vdisplay_pixels_per_inch = make_float (72.0);
 
+  DEFVAR_BOOL ("align-columns-display", align_columns_display,
+    doc: /* Whether to align columns on GUI frames.
+If this is non-nil characters displayed on GUI frames will be
+aligned to produce straight columns.  This is achieved by
+enlarging the pixel width of characters to an integral
+multiple of pixels taken by ASCII characters of the same face.  */);
+    align_columns_display = false;
+
 #ifdef GLYPH_DEBUG
   DEFVAR_INT ("debug-end-pos", debug_end_pos, doc: /* Don't ask.  */);
 #endif