Implement view swapping

* add Tree::swap_split_in_direction()
* add swap_view_{left,down,up,right} commands, bound to H,J,K,L
  respectively in the Window menu(s)
* add test for view swapping

4 files changed, 191 insertions, 0 deletions

diff --git a/helix-term/src/commands.rs b/helix-term/src/commands.rs
index c3c7d224..2839f495 100644
--- a/helix-term/src/commands.rs
+++ b/helix-term/src/commands.rs
@@ -360,6 +360,10 @@ impl MappableCommand {
         jump_view_left, "Jump to the split to the left",
         jump_view_up, "Jump to the split above",
         jump_view_down, "Jump to the split below",
+        swap_view_right, "Swap with the split to the right",
+        swap_view_left, "Swap with the split to the left",
+        swap_view_up, "Swap with the split above",
+        swap_view_down, "Swap with the split below",
         transpose_view, "Transpose splits",
         rotate_view, "Goto next window",
         hsplit, "Horizontal bottom split",
@@ -3864,6 +3868,22 @@ fn jump_view_down(cx: &mut Context) {
     cx.editor.focus_down()
 }
 
+fn swap_view_right(cx: &mut Context) {
+    cx.editor.swap_right()
+}
+
+fn swap_view_left(cx: &mut Context) {
+    cx.editor.swap_left()
+}
+
+fn swap_view_up(cx: &mut Context) {
+    cx.editor.swap_up()
+}
+
+fn swap_view_down(cx: &mut Context) {
+    cx.editor.swap_down()
+}
+
 fn transpose_view(cx: &mut Context) {
     cx.editor.transpose_view()
 }
diff --git a/helix-term/src/keymap/default.rs b/helix-term/src/keymap/default.rs
index 124517d4..a887f4b3 100644
--- a/helix-term/src/keymap/default.rs
+++ b/helix-term/src/keymap/default.rs
@@ -180,6 +180,10 @@ pub fn default() -> HashMap<Mode, Keymap> {
             "C-j" | "j" | "down" => jump_view_down,
             "C-k" | "k" | "up" => jump_view_up,
             "C-l" | "l" | "right" => jump_view_right,
+            "L" => swap_view_right,
+            "K" => swap_view_up,
+            "H" => swap_view_left,
+            "J" => swap_view_down,
             "n" => { "New split scratch buffer"
                 "C-s" | "s" => hsplit_new,
                 "C-v" | "v" => vsplit_new,
@@ -236,6 +240,10 @@ pub fn default() -> HashMap<Mode, Keymap> {
                 "C-j" | "j" | "down" => jump_view_down,
                 "C-k" | "k" | "up" => jump_view_up,
                 "C-l" | "l" | "right" => jump_view_right,
+                "H" => swap_view_left,
+                "J" => swap_view_down,
+                "K" => swap_view_up,
+                "L" => swap_view_right,
                 "n" => { "New split scratch buffer"
                     "C-s" | "s" => hsplit_new,
                     "C-v" | "v" => vsplit_new,
diff --git a/helix-view/src/editor.rs b/helix-view/src/editor.rs
index 1ad21059..3ba6fea8 100644
--- a/helix-view/src/editor.rs
+++ b/helix-view/src/editor.rs
@@ -885,6 +885,22 @@ impl Editor {
         self.tree.focus_direction(tree::Direction::Down);
     }
 
+    pub fn swap_right(&mut self) {
+        self.tree.swap_split_in_direction(tree::Direction::Right);
+    }
+
+    pub fn swap_left(&mut self) {
+        self.tree.swap_split_in_direction(tree::Direction::Left);
+    }
+
+    pub fn swap_up(&mut self) {
+        self.tree.swap_split_in_direction(tree::Direction::Up);
+    }
+
+    pub fn swap_down(&mut self) {
+        self.tree.swap_split_in_direction(tree::Direction::Down);
+    }
+
     pub fn transpose_view(&mut self) {
         self.tree.transpose();
     }
diff --git a/helix-view/src/tree.rs b/helix-view/src/tree.rs
index 522a79d7..2aa35dee 100644
--- a/helix-view/src/tree.rs
+++ b/helix-view/src/tree.rs
@@ -538,6 +538,24 @@ impl Tree {
         }
     }
 
+    pub fn swap_split_in_direction(&mut self, direction: Direction) {
+        if let Some(id) = self.find_split_in_direction(self.focus, direction) {
+            if let Some([focused, target]) = self.nodes.get_disjoint_mut([self.focus, id]) {
+                match (&mut focused.content, &mut target.content) {
+                    (Content::View(focused), Content::View(target)) => {
+                        std::mem::swap(&mut focused.doc, &mut target.doc);
+                        std::mem::swap(&mut focused.id, &mut target.id);
+                        self.focus = id;
+                    }
+                    // self.focus always points to a view which has a content of Content::View
+                    // and find_split_in_direction() only returns a view which has content of
+                    // Content::View.
+                    _ => unreachable!(),
+                }
+            }
+        }
+    }
+
     pub fn area(&self) -> Rect {
         self.area
     }
@@ -649,4 +667,133 @@ mod test {
         assert_eq!(None, tree.find_split_in_direction(r0, Direction::Right));
         assert_eq!(None, tree.find_split_in_direction(r0, Direction::Up));
     }
+
+    #[test]
+    fn swap_split_in_direction() {
+        let mut tree = Tree::new(Rect {
+            x: 0,
+            y: 0,
+            width: 180,
+            height: 80,
+        });
+
+        let doc_l0 = DocumentId::default();
+        let mut view = View::new(
+            doc_l0,
+            vec![GutterType::Diagnostics, GutterType::LineNumbers],
+        );
+        view.area = Rect::new(0, 0, 180, 80);
+        tree.insert(view);
+
+        let l0 = tree.focus;
+
+        let doc_r0 = DocumentId::default();
+        let view = View::new(
+            doc_r0,
+            vec![GutterType::Diagnostics, GutterType::LineNumbers],
+        );
+        tree.split(view, Layout::Vertical);
+        let r0 = tree.focus;
+
+        tree.focus = l0;
+
+        let doc_l1 = DocumentId::default();
+        let view = View::new(
+            doc_l1,
+            vec![GutterType::Diagnostics, GutterType::LineNumbers],
+        );
+        tree.split(view, Layout::Horizontal);
+        let l1 = tree.focus;
+
+        tree.focus = l0;
+
+        let doc_l2 = DocumentId::default();
+        let view = View::new(
+            doc_l2,
+            vec![GutterType::Diagnostics, GutterType::LineNumbers],
+        );
+        tree.split(view, Layout::Vertical);
+        let l2 = tree.focus;
+
+        // Views in test
+        // | L0  | L2 |    |
+        // |    L1    | R0 |
+
+        // Document IDs in test
+        // | l0  | l2 |    |
+        // |    l1    | r0 |
+
+        fn doc_id(tree: &Tree, view_id: ViewId) -> Option<DocumentId> {
+            if let Content::View(view) = &tree.nodes[view_id].content {
+                Some(view.doc)
+            } else {
+                None
+            }
+        }
+
+        tree.focus = l0;
+        // `*` marks the view in focus from view table (here L0)
+        // | l0*  | l2 |    |
+        // |    l1     | r0 |
+        tree.swap_split_in_direction(Direction::Down);
+        // | l1   | l2 |    |
+        // |    l0*    | r0 |
+        assert_eq!(tree.focus, l1);
+        assert_eq!(doc_id(&tree, l0), Some(doc_l1));
+        assert_eq!(doc_id(&tree, l1), Some(doc_l0));
+        assert_eq!(doc_id(&tree, l2), Some(doc_l2));
+        assert_eq!(doc_id(&tree, r0), Some(doc_r0));
+
+        tree.swap_split_in_direction(Direction::Right);
+
+        // | l1  | l2 |     |
+        // |    r0    | l0* |
+        assert_eq!(tree.focus, r0);
+        assert_eq!(doc_id(&tree, l0), Some(doc_l1));
+        assert_eq!(doc_id(&tree, l1), Some(doc_r0));
+        assert_eq!(doc_id(&tree, l2), Some(doc_l2));
+        assert_eq!(doc_id(&tree, r0), Some(doc_l0));
+
+        // cannot swap, nothing changes
+        tree.swap_split_in_direction(Direction::Up);
+        // | l1  | l2 |     |
+        // |    r0    | l0* |
+        assert_eq!(tree.focus, r0);
+        assert_eq!(doc_id(&tree, l0), Some(doc_l1));
+        assert_eq!(doc_id(&tree, l1), Some(doc_r0));
+        assert_eq!(doc_id(&tree, l2), Some(doc_l2));
+        assert_eq!(doc_id(&tree, r0), Some(doc_l0));
+
+        // cannot swap, nothing changes
+        tree.swap_split_in_direction(Direction::Down);
+        // | l1  | l2 |     |
+        // |    r0    | l0* |
+        assert_eq!(tree.focus, r0);
+        assert_eq!(doc_id(&tree, l0), Some(doc_l1));
+        assert_eq!(doc_id(&tree, l1), Some(doc_r0));
+        assert_eq!(doc_id(&tree, l2), Some(doc_l2));
+        assert_eq!(doc_id(&tree, r0), Some(doc_l0));
+
+        tree.focus = l2;
+        // | l1  | l2* |    |
+        // |    r0     | l0 |
+
+        tree.swap_split_in_direction(Direction::Down);
+        // | l1  | r0  |    |
+        // |    l2*    | l0 |
+        assert_eq!(tree.focus, l1);
+        assert_eq!(doc_id(&tree, l0), Some(doc_l1));
+        assert_eq!(doc_id(&tree, l1), Some(doc_l2));
+        assert_eq!(doc_id(&tree, l2), Some(doc_r0));
+        assert_eq!(doc_id(&tree, r0), Some(doc_l0));
+
+        tree.swap_split_in_direction(Direction::Up);
+        // | l2* | r0 |    |
+        // |    l1    | l0 |
+        assert_eq!(tree.focus, l0);
+        assert_eq!(doc_id(&tree, l0), Some(doc_l2));
+        assert_eq!(doc_id(&tree, l1), Some(doc_l1));
+        assert_eq!(doc_id(&tree, l2), Some(doc_r0));
+        assert_eq!(doc_id(&tree, r0), Some(doc_l0));
+    }
 }