Change `+` command not to duplicate identical selections more than once

The current exponential behaviour does not seem that useful, it seems
more predictible that pressing `+` twice would end up with 3 copies
of the original selections instead of 4.

Fixes #4533

doc/pages/changelog.asciidoc

@@ -3,6 +3,11 @@
 This changelog contains major and/or breaking changes to Kakoune between
 released versions.
 
+== Development version
+
+* `+` only duplicates identical selections a single time to avoid surprising
+  and slow exponential growth in the number of selections.
+
 == Kakoune 2023.08.08
 
 * Fix compilation errors on FreeBSD and MacOS using clang

src/main.cc

@@ -45,6 +45,9 @@ struct {
     unsigned int version;
     StringView notes;
 } constexpr version_notes[] = { {
+        0,
+        "» {+b}+{} only duplicates identical selections a single time\n"
+    }, {
         20230805,
         "» Fix FreeBSD/MacOS clang compilation\n"
     }, {

src/normal.cc

@@ -2191,9 +2191,17 @@ void duplicate_selections(Context& context, NormalParams params)
     SelectionList& sels = context.selections();
     Vector<Selection> new_sels;
     const int count = params.count ? params.count : 2;
+    BasicSelection last{BufferCoord{-1,-1}};
+    size_t index = 0;
+    size_t main_index = 0;
     for (const auto& sel : sels)
-        new_sels.insert(new_sels.end(), count, sel);
-    context.selections().set(std::move(new_sels), sels.main_index() * count);
+    {
+        new_sels.insert(new_sels.end(), sel != last ? count : 1, sel);
+        last = sel;
+        if (index++ == sels.main_index())
+            main_index = new_sels.size() - 1;
+    }
+    context.selections().set(std::move(new_sels), main_index);
 }
 
 void force_redraw(Context& context, NormalParams)

src/selection.hh

@@ -13,17 +13,14 @@ using CaptureList = Vector<String, MemoryDomain::Selections>;
 constexpr ColumnCount max_column{std::numeric_limits<int>::max()};
 constexpr ColumnCount max_non_eol_column{max_column-1};
 
-// A selection is a Selection, associated with a CaptureList
-struct Selection
+struct BasicSelection
 {
     static constexpr MemoryDomain Domain = MemoryDomain::Selections;
 
-    Selection() = default;
-    Selection(BufferCoord pos) : Selection(pos,pos) {}
-    Selection(BufferCoord anchor, BufferCoordAndTarget cursor,
-              CaptureList captures = {})
-        : m_anchor{anchor}, m_cursor{cursor},
-          m_captures(std::move(captures)) {}
+    BasicSelection() = default;
+    BasicSelection(BufferCoord pos) : BasicSelection(pos,pos) {}
+    BasicSelection(BufferCoord anchor, BufferCoordAndTarget cursor)
+        : m_anchor{anchor}, m_cursor{cursor} {}
 
     BufferCoord& anchor() { return m_anchor; }
     BufferCoordAndTarget& cursor() { return m_cursor; }
@@ -39,13 +36,7 @@ struct Selection
 
     void set(BufferCoord coord) { set(coord, coord); }
 
-    CaptureList& captures() { return m_captures; }
-    const CaptureList& captures() const { return m_captures; }
-
-    bool operator== (const Selection& other) const
-    {
-        return m_anchor == other.m_anchor and m_cursor == other.m_cursor;
-    }
+    friend bool operator==(const BasicSelection&, const BasicSelection&) = default;
 
     // When selections are single char, we want the anchor to be considered min, and cursor max
     const BufferCoord& min() const { return m_anchor <= m_cursor ? m_anchor : m_cursor; }
@@ -57,11 +48,23 @@ struct Selection
 private:
     BufferCoord m_anchor;
     BufferCoordAndTarget m_cursor;
+};
+;
 
+struct Selection : BasicSelection
+{
+    Selection() = default;
+    Selection(BufferCoord pos) : BasicSelection(pos,pos) {}
+    Selection(BufferCoord anchor, BufferCoordAndTarget cursor, CaptureList captures = {})
+        : BasicSelection{anchor, cursor}, m_captures(std::move(captures)) {}
+    CaptureList& captures() { return m_captures; }
+    const CaptureList& captures() const { return m_captures; }
+
+private:
     CaptureList m_captures;
 };
 
-inline bool overlaps(const Selection& lhs, const Selection& rhs)
+inline bool overlaps(const BasicSelection& lhs, const BasicSelection& rhs)
 {
     return lhs.min() <= rhs.min() ? lhs.max() >= rhs.min()
                                   : lhs.min() <= rhs.max();

test/normal/duplicate-selections/cmd

@@ -0,0 +1 @@
+++ao<esc>

test/normal/duplicate-selections/in

@@ -0,0 +1 @@
+%(f) %(b) %(t)

test/normal/duplicate-selections/out

@@ -0,0 +1 @@
+fooo booo tooo