src: Implement <a-m> and <a-M>

Closes #2425

doc/pages/keys.asciidoc

@@ -139,6 +139,18 @@ the Shift modifier and moving will extend each selection instead.
     select to the next sequence enclosed by matching character, see the
     `matching_pairs` option in <<options#,`:doc options`>>
 
+*M*::
+    extend the current selection to the next sequence enclosed by matching
+    character, see the `matching_pairs` option in <<options#,`:doc options`>>
+
+*<a-m>*::
+    select to the previous sequence enclosed by matching character, see the
+    `matching_pairs` option in <<options#,`:doc options`>>
+
+*<a-M>*::
+    extend the current selection to the previous sequence enclosed by matching
+    character, see the `matching_pairs` option in <<options#,`:doc options`>>
+
 *x*::
     select line on which the end of each selection lies (or next line when end lies
     on an end-of-line)

src/normal.cc

@@ -2215,8 +2215,10 @@ static const HashMap<Key, NormalCmd, MemoryDomain::Undefined, KeymapBackend> key
     { {alt('x')}, {"extend selections to whole lines", select<SelectMode::Replace, select_lines>} },
     { {alt('X')}, {"crop selections to whole lines", select<SelectMode::Replace, trim_partial_lines>} },
 
-    { {'m'}, {"select to matching character", select<SelectMode::Replace, select_matching>} },
-    { {'M'}, {"extend to matching character", select<SelectMode::Extend, select_matching>} },
+    { {'m'}, {"select to matching character", select<SelectMode::Replace, select_matching<true>>} },
+    { {alt('m')}, {"backward select to matching character", select<SelectMode::Replace, select_matching<false>>} },
+    { {'M'}, {"extend to matching character", select<SelectMode::Extend, select_matching<true>>} },
+    { {alt('M')}, {"backward extend to matching character", select<SelectMode::Extend, select_matching<false>>} },
 
     { {'/'}, {"select next given regex match", search<SelectMode::Replace, MatchDirection::Forward>} },
     { {'?'}, {"extend with next given regex match", search<SelectMode::Extend, MatchDirection::Forward>} },

src/selectors.cc

@@ -220,6 +220,7 @@ select_to_first_non_blank(const Context& context, const Selection& selection)
     return {it.coord()};
 }
 
+template<bool forward>
 Optional<Selection>
 select_matching(const Context& context, const Selection& selection)
 {
@@ -227,13 +228,23 @@ select_matching(const Context& context, const Selection& selection)
     auto& matching_pairs = context.options()["matching_pairs"].get<Vector<Codepoint, MemoryDomain::Options>>();
     Utf8Iterator it{buffer.iterator_at(selection.cursor()), buffer};
     auto match = matching_pairs.end();
-    while (it != buffer.end())
+
+    if (forward) while (it != buffer.end())
     {
         match = find(matching_pairs, *it);
         if (match != matching_pairs.end())
             break;
         ++it;
     }
+    else while (true)
+    {
+        match = find(matching_pairs, *it);
+        if (match != matching_pairs.end()
+            or it == buffer.begin())
+            break;
+        --it;
+    }
+
     if (match == matching_pairs.end())
         return {};
 
@@ -271,6 +282,10 @@ select_matching(const Context& context, const Selection& selection)
     }
     return {};
 }
+template Optional<Selection>
+select_matching<true>(const Context& context, const Selection& selection);
+template Optional<Selection>
+select_matching<false>(const Context& context, const Selection& selection);
 
 template<typename Iterator, typename Container>
 Optional<std::pair<Iterator, Iterator>>

src/selectors.hh

@@ -31,6 +31,7 @@ select_to_previous_word(const Context& context, const Selection& selection);
 Optional<Selection>
 select_line(const Context& context, const Selection& selection);
 
+template<bool forward>
 Optional<Selection>
 select_matching(const Context& context, const Selection& selection);