Add normal commands to extend selections downwards/upwards.

Since we dropped the we changed the `x` keys behaviors, the `X` and `<a-X>` have been free.

I think it makes sense to make them work mostly like the `/` command, where shift extends the selection and alt changes the direction.

So the implemented behavior is be:
<X>   : select the current and next line (extend downwards)
<a-X> : select the current and previous line (extend upwards)

doc/pages/changelog.asciidoc

@@ -5,6 +5,9 @@ released versions.
 
 == Development version
 
+* `<X>` and `<a-X>` to expand line selections downwards and upwards,
+  respectively. 
+
 * History is now stored linearly instead of in a tree
 
 * `<a-u>` and `<a-U>` now undo selection history

doc/pages/keys.asciidoc

@@ -168,6 +168,12 @@ the Shift modifier and moving will extend each selection instead.
     trim selections to only contain full lines (not including last
     end-of-line)
 
+*X*::
+    expand selections to contain full lines (including end-of-lines) and select the next line
+
+*<a-X>*::
+    expand selections to contain full lines (including end-of-lines) and select the previous line
+
 *%*, *<percent>*::
     select whole buffer
 

src/main.cc

@@ -52,6 +52,8 @@ struct {
         "strings (as double quoted strings)\n"
         "» {+u}show-matching -previous{} switch\n"
         "» {+b}<c-g>{} to cancel current operation\n"
+        "» {+b}<X>{} and {+b}<a-X>{} to expand line selections downwards and "
+        "upwards, respectively\n"
     }, {
         20221031,
         "» {+b}<esc>{} does not end macro recording anymore, use {+b}Q{}\n"

src/normal.cc

@@ -2344,6 +2344,9 @@ static constexpr HashMap<Key, NormalCmd, MemoryDomain::Undefined, KeymapBackend>
     { {'x'}, {"extend selections to whole lines", select<SelectMode::Replace, select_lines>} },
     { {alt('x')}, {"crop selections to whole lines", select<SelectMode::Replace, trim_partial_lines>} },
 
+    { {'X'}, {"extend selections to whole lines downwards", repeated<select<SelectMode::Replace, extend_lines_downwards>>} },
+    { {alt('X')}, {"extend selections to whole lines upwards", repeated<select<SelectMode::Replace, extend_lines_upwards>>} },
+
     { {'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>>} },

src/selectors.cc

@@ -861,6 +861,38 @@ trim_partial_lines(const Context& context, const Selection& selection)
     return Selection{anchor, {cursor, max_column}};
 }
 
+Optional<Selection>
+extend_lines_downwards(const Context& context, const Selection& selection)
+{
+    auto& buffer = context.buffer();
+    BufferCoord anchor = selection.anchor();
+    BufferCoord cursor = selection.cursor();
+    BufferCoord& to_line_start = anchor <= cursor ? anchor : cursor;
+    BufferCoord& to_line_end = anchor <= cursor ? cursor : anchor;
+
+    to_line_start.column = 0;
+    to_line_end.column = buffer[to_line_end.line].length()-1;
+    to_line_end.line = std::min(to_line_end.line + 1, buffer.line_count() - 1);
+
+    return Selection{anchor, {cursor, max_column}};
+}
+
+Optional<Selection>
+extend_lines_upwards(const Context& context, const Selection& selection)
+{
+    auto& buffer = context.buffer();
+    BufferCoord anchor = selection.anchor();
+    BufferCoord cursor = selection.cursor();
+    BufferCoord& to_line_start = anchor <= cursor ? anchor : cursor;
+    BufferCoord& to_line_end = anchor <= cursor ? cursor : anchor;
+
+    to_line_start.column = 0;
+    to_line_start.line = std::max(to_line_start.line - 1, LineCount(0));
+    to_line_end.column = buffer[to_line_end.line].length()-1;
+
+    return Selection{anchor, {cursor, max_column}};
+}
+
 static RegexExecFlags
 match_flags(const Buffer& buf, const BufferIterator& begin, const BufferIterator& end)
 {

src/selectors.hh

@@ -107,6 +107,12 @@ select_lines(const Context& context, const Selection& selection);
 Optional<Selection>
 trim_partial_lines(const Context& context, const Selection& selection);
 
+Optional<Selection>
+extend_lines_downwards(const Context& context, const Selection& selection);
+
+Optional<Selection>
+extend_lines_upwards(const Context& context, const Selection& selection);
+
 enum class RegexMode;
 
 template<RegexMode mode>