add -ruler switch to column highlighter

the -ruler <character> switch on the column highlighter replaces the
column (for empty and whitespace cells) with <character>. this is useful
for drawing rulers. because rulers typically appear over whitespace
regions in other editors, we also do that here.

additionally, the color of the ruler should only be applied to the
ruler, so when -ruler <character> is provided, we do not apply the
<face> parameter to cells which are non-empty, aka where the ruler is
inserted.

Author: enricozb

doc/pages/highlighters.asciidoc

@@ -111,8 +111,13 @@ highlighter is replaced with the new one.
 *fill* <face>::
     fill using the given *face*, mostly useful with regions highlighters
 
-*column* <number> <face>::
-    highlight column *number* with face *face*
+*column* <number> <face> [options]::
+    highlight column *number* with face *face*, with the following *options*:
+
+    *-ruler* <character>:::
+        a single character to display instead of empty or whitespace cells at
+        column *number*. If this option is provided, *face* will also not
+        apply to non-empty cells, and will only apply to *character*.
 
 *line* <number> <face>::
     highlight line *number* with face *face*

src/highlighters.cc

@@ -19,6 +19,7 @@
 #include "utf8.hh"
 #include "utf8_iterator.hh"
 #include "window.hh"
+#include "unicode.hh"
 
 #include <cstdio>
 #include <limits>
@@ -519,16 +520,26 @@ UniquePtr<Highlighter> create_line_highlighter(HighlighterParameters params, Hig
 }
 
 const HighlighterDesc column_desc = {
-    "Parameters: <value string> <face>\n"
-    "Highlight the column given by evaluating <value string> with <face>",
-    {}
+    "Parameters: [-ruler <character>] <column> <face>\n"
+    "Highlight the column <column> with <face>",
+    {
+        { { "ruler", { ArgCompleter{}, "replace empty or whitespace cells with the given character. When provided, <face> is not applied to non-empty cells" } } },
+        ParameterDesc::Flags::SwitchesOnlyAtStart, 2, 2
+    },
 };
 UniquePtr<Highlighter> create_column_highlighter(HighlighterParameters params, Highlighter*)
 {
-    if (params.size() != 2)
-        throw runtime_error("wrong parameter count");
+    ParametersParser parser{params, column_desc.params};
 
-    auto func = [col_expr=params[0], facespec=parse_face(params[1])]
+    auto positionals = parser.positionals_from(0);
+    auto ruler = parser.get_switch("ruler");
+    if (ruler.value_or(" ").char_length() > 1)
+        throw runtime_error("-ruler expects a single character");
+
+    auto ruler_provided = static_cast<bool>(ruler);
+    auto col_char = ruler.value_or(" ").str();
+
+    auto func = [col_expr=positionals[0], facespec=parse_face(positionals[1]), ruler_provided, col_char]
                 (HighlightContext context, DisplayBuffer& display_buffer, BufferRange)
     {
         ColumnCount column = -1;
@@ -563,7 +574,12 @@ UniquePtr<Highlighter> create_column_highlighter(HighlighterParameters params, H
                         atom_it = ++line.split(atom_it, remaining_col);
                     if (atom_it->length() > 1)
                         atom_it = line.split(atom_it, 1_col);
-                    atom_it->face = merge_faces(atom_it->face, face);
+                    if (!ruler_provided)
+                        atom_it->face = merge_faces(atom_it->face, face);
+                    if (ruler_provided && is_blank(atom_it->content()[0])) {
+                        atom_it->replace(col_char);
+                        atom_it->face = merge_faces(atom_it->face, face);
+                    }
                     found = true;
                     break;
                 }
@@ -574,7 +590,7 @@ UniquePtr<Highlighter> create_column_highlighter(HighlighterParameters params, H
 
             if (remaining_col > 0)
                 line.push_back({String{' ', remaining_col}, Face{}});
-            line.push_back({" ", face});
+            line.push_back({col_char, face});
         }
     };