Add a data structure to manage the view stack

build.zig

@@ -28,6 +28,22 @@ pub fn build(b: *Builder) void {
     const run_cmd = exe.run();
     run_cmd.step.dependOn(b.getInstallStep());
 
-    const run_step = b.step("run", "Run the app");
+    const run_step = b.step("run", "Run the compositor");
     run_step.dependOn(&run_cmd.step);
+
+    const test_exe = b.addTest("src/test_main.zig");
+    test_exe.setTarget(target);
+    test_exe.setBuildMode(mode);
+    test_exe.addIncludeDir("protocol");
+    test_exe.linkLibC();
+    test_exe.addIncludeDir("/usr/include/pixman-1");
+    test_exe.addCSourceFile("include/render.c", &[_][]const u8{"-std=c99"});
+    test_exe.addIncludeDir(".");
+    //test_exe.linkSystemLibrary("pixman");
+    test_exe.linkSystemLibrary("wayland-server");
+    test_exe.linkSystemLibrary("wlroots");
+    test_exe.linkSystemLibrary("xkbcommon");
+
+    const test_step = b.step("test", "Run the tests");
+    test_step.dependOn(&test_exe.step);
 }

src/output.zig

@@ -4,6 +4,7 @@ const c = @import("c.zig");
 const Root = @import("root.zig").Root;
 const Server = @import("server.zig").Server;
 const View = @import("view.zig").View;
+const ViewStack = @import("view_stack.zig").ViewStack;
 
 const RenderData = struct {
     output: *c.wlr_output,
@@ -79,22 +80,12 @@ pub const Output = struct {
         const color = [_]f32{ 0.3, 0.3, 0.3, 1.0 };
         c.wlr_renderer_clear(renderer, &color);
 
-        // Each subsequent view is rendered on top of the last.
         // The first view in the list is "on top" so iterate in reverse.
-        var it = output.root.views.last;
-        while (it) |node| : (it = node.prev) {
-            const view = &node.data;
-
-            // Only render currently visible views
-            if (view.current_tags & output.root.current_focused_tags == 0) {
-                continue;
-            }
-
-            // TODO: remove this check and move unmaped views back to unmaped TailQueue
-            if (!view.mapped) {
-                // An unmapped view should not be rendered.
-                continue;
-            }
+        var it = ViewStack.reverseIterator(
+            output.root.views.first,
+            output.root.current_focused_tags,
+        );
+        while (it.next()) |view| {
             output.renderView(view, &now);
         }
 

src/root.zig

@@ -7,6 +7,7 @@ const Output = @import("output.zig").Output;
 const Server = @import("server.zig").Server;
 const Seat = @import("seat.zig").Seat;
 const View = @import("view.zig").View;
+const ViewStack = @import("view_stack.zig").ViewStack;
 
 /// Responsible for all windowing operations
 pub const Root = struct {
@@ -17,10 +18,10 @@ pub const Root = struct {
     wlr_output_layout: *c.wlr_output_layout,
     outputs: std.TailQueue(Output),
 
-    // Must stay ordered, first N views in list are the masters
-    views: std.TailQueue(View),
-    unmapped_views: std.TailQueue(View),
+    /// The top of the stack is the "most important" view.
+    views: ViewStack,
 
+    /// The view that has seat focus, if any.
     focused_view: ?*View,
 
     /// A bit field of focused tags
@@ -33,8 +34,8 @@ pub const Root = struct {
     /// Percentage of the total screen that the master section takes up.
     master_factor: f64,
 
-    // Number of pending configures sent in the current transaction.
-    // A value of 0 means there is no current transaction.
+    /// Number of pending configures sent in the current transaction.
+    /// A value of 0 means there is no current transaction.
     pending_configures: u32,
 
     /// Handles timeout of transactions
@@ -51,8 +52,7 @@ pub const Root = struct {
 
         self.outputs = std.TailQueue(Output).init();
 
-        self.views = std.TailQueue(View).init();
-        self.unmapped_views = std.TailQueue(View).init();
+        self.views.init();
 
         self.focused_view = null;
 
@@ -80,97 +80,97 @@ pub const Root = struct {
     }
 
     pub fn addView(self: *Self, wlr_xdg_surface: *c.wlr_xdg_surface) void {
-        const node = self.views.allocateNode(self.server.allocator) catch unreachable;
-        node.data.init(self, wlr_xdg_surface, self.current_focused_tags);
-        self.unmapped_views.prepend(node);
+        const node = self.server.allocator.create(ViewStack.Node) catch unreachable;
+        node.view.init(self, wlr_xdg_surface, self.current_focused_tags);
+        self.views.push(node);
     }
 
     /// Finds the topmost view under the output layout coordinates lx, ly
     /// returns the view if found, and a pointer to the wlr_surface as well as the surface coordinates
     pub fn viewAt(self: Self, lx: f64, ly: f64, surface: *?*c.wlr_surface, sx: *f64, sy: *f64) ?*View {
-        var it = self.views.last;
-        while (it) |node| : (it = node.prev) {
-            if (node.data.isAt(lx, ly, surface, sx, sy)) {
-                return &node.data;
+        var it = ViewStack.iterator(self.views.first, 0xFFFFFFFF);
+        while (it.next()) |view| {
+            if (view.isAt(lx, ly, surface, sx, sy)) {
+                return view;
             }
         }
         return null;
     }
 
+    /// Clear the current focus.
+    pub fn clearFocus(self: *Self) void {
+        if (self.focused_view) |view| {
+            _ = c.wlr_xdg_toplevel_set_activated(view.wlr_xdg_surface, false);
+        }
+        self.focused_view = null;
+    }
+
     /// Focus the next visible view in the stack, wrapping if needed. Does
     /// nothing if there is only one view in the stack.
-    pub fn focusNextView(self: Self) void {
+    pub fn focusNextView(self: *Self) void {
         if (self.focused_view) |current_focus| {
             // If there is a currently focused view, focus the next visible view in the stack.
-            var it = @fieldParentPtr(std.TailQueue(View).Node, "data", current_focus).next;
-            while (it) |node| : (it = node.next) {
-                const view = &node.data;
-                if (view.current_tags & self.current_focused_tags != 0) {
-                    view.focus(view.wlr_xdg_surface.surface);
-                    return;
-                }
+            const current_node = @fieldParentPtr(ViewStack.Node, "view", current_focus);
+            var it = ViewStack.iterator(current_node, self.current_focused_tags);
+            // Skip past the current node
+            _ = it.next();
+            // Focus the next visible node if there is one
+            if (it.next()) |view| {
+                view.focus(view.wlr_xdg_surface.surface);
+                return;
             }
         }
 
         // There is either no currently focused view or the last visible view in the
         // stack is focused and we need to wrap.
-        var it = self.views.first;
-        while (it) |node| : (it = node.next) {
-            const view = &node.data;
-            if (view.current_tags & self.current_focused_tags != 0) {
-                view.focus(view.wlr_xdg_surface.surface);
-                return;
-            }
+        var it = ViewStack.iterator(self.views.first, self.current_focused_tags);
+        if (it.next()) |view| {
+            view.focus(view.wlr_xdg_surface.surface);
+        } else {
+            // Otherwise clear the focus since there are no visible views
+            self.clearFocus();
         }
     }
 
     /// Focus the previous view in the stack, wrapping if needed. Does nothing
     /// if there is only one view in the stack.
-    pub fn focusPrevView(self: Self) void {
+    pub fn focusPrevView(self: *Self) void {
         if (self.focused_view) |current_focus| {
             // If there is a currently focused view, focus the previous visible view in the stack.
-            var it = @fieldParentPtr(std.TailQueue(View).Node, "data", current_focus).prev;
-            while (it) |node| : (it = node.prev) {
-                const view = &node.data;
-                if (view.current_tags & self.current_focused_tags != 0) {
-                    view.focus(view.wlr_xdg_surface.surface);
-                    return;
-                }
+            const current_node = @fieldParentPtr(ViewStack.Node, "view", current_focus);
+            var it = ViewStack.reverseIterator(current_node, self.current_focused_tags);
+            // Skip past the current node
+            _ = it.next();
+            // Focus the previous visible node if there is one
+            if (it.next()) |view| {
+                view.focus(view.wlr_xdg_surface.surface);
+                return;
             }
         }
 
         // There is either no currently focused view or the first visible view in the
         // stack is focused and we need to wrap.
-        var it = self.views.last;
-        while (it) |node| : (it = node.prev) {
-            const view = &node.data;
-            if (view.current_tags & self.current_focused_tags != 0) {
-                view.focus(view.wlr_xdg_surface.surface);
-                return;
-            }
+        var it = ViewStack.reverseIterator(self.views.last, self.current_focused_tags);
+        if (it.next()) |view| {
+            view.focus(view.wlr_xdg_surface.surface);
+        } else {
+            // Otherwise clear the focus since there are no visible views
+            self.clearFocus();
         }
     }
 
-    // TODO: obsolete this function by using a better data structure
-    fn visibleCount(self: Self, tags: u32) u32 {
-        var count: u32 = 0;
-        var it = self.views.first;
-        while (it) |node| : (it = node.next) {
-            const view = &node.data;
-            if (view.current_tags & tags != 0) {
-                count += 1;
-            }
-        }
-        return count;
-    }
-
     pub fn arrange(self: *Self) void {
         const root_tags = if (self.pending_focused_tags) |tags|
             tags
         else
             self.current_focused_tags;
 
-        const visible_count = self.visibleCount(root_tags);
+        const visible_count = blk: {
+            var count: u32 = 0;
+            var it = ViewStack.pendingIterator(self.views.first, root_tags);
+            while (it.next() != null) count += 1;
+            break :blk count;
+        };
 
         const master_count = util.min(u32, self.master_count, visible_count);
         const slave_count = if (master_count >= visible_count) 0 else visible_count - master_count;
@@ -192,18 +192,8 @@ pub const Root = struct {
         }
 
         var i: u32 = 0;
-        var it = self.views.first;
-        while (it) |node| : (it = node.next) {
-            const view = &node.data;
-
-            if (view.pending_tags) |tags| {
-                if (root_tags & tags == 0) {
-                    continue;
-                }
-            } else if (view.current_tags & root_tags == 0) {
-                continue;
-            }
-
+        var it = ViewStack.pendingIterator(self.views.first, root_tags);
+        while (it.next()) |view| {
             if (i < master_count) {
                 // Add the remainder to the first master to ensure every pixel of height is used
                 const master_height = @divTrunc(@intCast(u32, output_box.height), master_count);
@@ -246,10 +236,8 @@ pub const Root = struct {
         // to reset the pending count to 0 and clear serials from the views
         self.pending_configures = 0;
 
-        var it = self.views.first;
-        while (it) |node| : (it = node.next) {
-            const view = &node.data;
-
+        var it = ViewStack.iterator(self.views.first, 0xFFFFFFFF);
+        while (it.next()) |view| {
             // Clear the serial in case this transaction is interrupting a prior one.
             view.pending_serial = null;
 
@@ -322,10 +310,8 @@ pub const Root = struct {
             self.pending_focused_tags = null;
         }
 
-        var it = self.views.first;
-        while (it) |node| : (it = node.next) {
-            const view = &node.data;
-
+        var it = ViewStack.iterator(self.views.first, 0xFFFFFFFF);
+        while (it.next()) |view| {
             // Ensure that all pending state is cleared
             view.pending_serial = null;
             if (view.pending_box) |state| {

src/server.zig

@@ -7,6 +7,7 @@ const Output = @import("output.zig").Output;
 const Root = @import("root.zig").Root;
 const Seat = @import("seat.zig").Seat;
 const View = @import("view.zig").View;
+const ViewStack = @import("view_stack.zig").ViewStack;
 
 pub const Server = struct {
     const Self = @This();
@@ -193,10 +194,10 @@ pub const Server = struct {
                 },
                 c.XKB_KEY_Return => {
                     if (self.root.focused_view) |current_focus| {
-                        const node = @fieldParentPtr(std.TailQueue(View).Node, "data", current_focus);
+                        const node = @fieldParentPtr(ViewStack.Node, "view", current_focus);
                         if (node != self.root.views.first) {
                             self.root.views.remove(node);
-                            self.root.views.prepend(node);
+                            self.root.views.push(node);
                             self.root.arrange();
                         }
                     }

src/test_main.zig

@@ -0,0 +1,3 @@
+test "river test suite" {
+    _ = @import("view_stack.zig");
+}

src/view.zig

@@ -2,6 +2,7 @@ const std = @import("std");
 const c = @import("c.zig");
 
 const Root = @import("root.zig").Root;
+const ViewStack = @import("view_stack.zig").ViewStack;
 
 pub const View = struct {
     const Self = @This();
@@ -127,13 +128,7 @@ pub const View = struct {
         // Called when the surface is mapped, or ready to display on-screen.
         const view = @fieldParentPtr(View, "listen_map", listener.?);
         view.mapped = true;
-
         view.focus(view.wlr_xdg_surface.surface);
-
-        const node = @fieldParentPtr(std.TailQueue(View).Node, "data", view);
-        view.root.unmapped_views.remove(node);
-        view.root.views.prepend(node);
-
         view.root.arrange();
     }
 
@@ -145,22 +140,11 @@ pub const View = struct {
         if (root.focused_view) |current_focus| {
             // If the view being unmapped is focused
             if (current_focus == view) {
-                // If there are more views
-                if (root.views.len > 1) {
-                    // Focus the previous view.
-                    root.focusPrevView();
-                } else {
-                    // Otherwise clear the focus
-                    root.focused_view = null;
-                    _ = c.wlr_xdg_toplevel_set_activated(view.wlr_xdg_surface, false);
-                }
+                // Focus the previous view. This clears the focus if there are no visible views.
+                root.focusPrevView();
             }
         }
 
-        const node = @fieldParentPtr(std.TailQueue(View).Node, "data", view);
-        root.views.remove(node);
-        root.unmapped_views.append(node);
-
         root.arrange();
     }
 
@@ -168,9 +152,9 @@ pub const View = struct {
         const view = @fieldParentPtr(View, "listen_destroy", listener.?);
         const root = view.root;
 
-        const node = @fieldParentPtr(std.TailQueue(View).Node, "data", view);
-        root.unmapped_views.remove(node);
-        root.unmapped_views.destroyNode(node, root.server.allocator);
+        const node = @fieldParentPtr(ViewStack.Node, "view", view);
+        root.views.remove(node);
+        root.server.allocator.destroy(node);
     }
 
     fn handleCommit(listener: ?*c.wl_listener, data: ?*c_void) callconv(.C) void {

src/view_stack.zig

@@ -0,0 +1,384 @@
+const View = @import("view.zig").View;
+
+/// A specialized doubly-linked stack that allows for filtered iteration
+/// over the nodes
+pub const ViewStack = struct {
+    const Self = @This();
+
+    pub const Node = struct {
+        /// Previous/next nodes in the stack
+        prev: ?*Node,
+        next: ?*Node,
+
+        /// The view stored in this node
+        view: View,
+    };
+
+    /// Top/bottom nodes in the stack
+    first: ?*Node,
+    last: ?*Node,
+
+    /// Total number of views
+    len: u32,
+
+    /// Initialize an undefined stack
+    pub fn init(self: *Self) void {
+        self.first = null;
+        self.last = null;
+        self.len = 0;
+    }
+
+    /// Add a node to the top of the stack.
+    pub fn push(self: *Self, new_node: *Node) void {
+        // Set the prev/next pointers of the new node
+        new_node.prev = null;
+        new_node.next = self.first;
+
+        if (self.first) |first| {
+            // If the list is not empty, set the prev pointer of the current
+            // first node to the new node.
+            first.prev = new_node;
+        } else {
+            // If the list is empty set the last pointer to the new node.
+            self.last = new_node;
+        }
+
+        // Set the first pointer to the new node and increment length
+        self.first = new_node;
+        self.len += 1;
+    }
+
+    /// Remove a node from the view stack. This removes it from the stack of
+    /// all views as well as the stack of visible ones.
+    pub fn remove(self: *Self, target_node: *Node) void {
+        // Set the previous node/list head to the next pointer
+        if (target_node.prev) |prev_node| {
+            prev_node.next = target_node.next;
+        } else {
+            self.first = target_node.next;
+        }
+
+        // Set the next node/list tail to the previous pointer
+        if (target_node.next) |next_node| {
+            next_node.prev = target_node.prev;
+        } else {
+            self.last = target_node.prev;
+        }
+
+        self.len -= 1;
+    }
+
+    const Iterator = struct {
+        it: ?*Node,
+        tags: u32,
+        reverse: bool,
+        pending: bool,
+
+        /// Returns the next node in iteration order, or null if done.
+        /// This function is horribly ugly, but it's well tested below.
+        pub fn next(self: *Iterator) ?*View {
+            while (self.it) |node| : (self.it = if (self.reverse) node.prev else node.next) {
+                if (node.view.mapped and if (self.pending)
+                    if (node.view.pending_tags) |pending_tags|
+                        self.tags & pending_tags != 0
+                    else
+                        self.tags & node.view.current_tags != 0
+                else
+                    self.tags & node.view.current_tags != 0) {
+                    const ret = &node.view;
+                    self.it = if (self.reverse) node.prev else node.next;
+                    return ret;
+                }
+            }
+            return null;
+        }
+    };
+
+    /// Returns an iterator starting at the passed node and filtered by
+    /// checking the passed tags against the current tags of each view.
+    /// Unmapped views are skipped.
+    pub fn iterator(start: ?*Node, tags: u32) Iterator {
+        return Iterator{
+            .it = start,
+            .tags = tags,
+            .reverse = false,
+            .pending = false,
+        };
+    }
+
+    /// Returns a reverse iterator starting at the passed node and filtered by
+    /// checking the passed tags against the current tags of each view.
+    /// Unmapped views are skipped.
+    pub fn reverseIterator(start: ?*Node, tags: u32) Iterator {
+        return Iterator{
+            .it = start,
+            .tags = tags,
+            .reverse = true,
+            .pending = false,
+        };
+    }
+
+    /// Returns an iterator starting at the passed node and filtered by
+    /// checking the passed tags against the pending tags of each view.
+    /// If a view has no pending tags, the current tags are used. Unmapped
+    /// views are skipped.
+    pub fn pendingIterator(start: ?*Node, tags: u32) Iterator {
+        return Iterator{
+            .it = start,
+            .tags = tags,
+            .reverse = false,
+            .pending = true,
+        };
+    }
+};
+
+const testing = @import("std").testing;
+
+test "push/remove" {
+    const allocator = testing.allocator;
+
+    var views: ViewStack = undefined;
+    views.init();
+
+    var one = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(one);
+    var two = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(two);
+    var three = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(three);
+    var four = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(four);
+    var five = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(five);
+
+    testing.expect(views.len == 0);
+    views.push(three); // {3}
+    views.push(one); // {1, 3}
+    views.push(four); // {4, 1, 3}
+    views.push(five); // {5, 4, 1, 3}
+    views.push(two); // {2, 5, 4, 1, 3}
+    testing.expect(views.len == 5);
+
+    // Simple insertion
+    {
+        var it = views.first;
+        testing.expect(it == two);
+        it = it.?.next;
+        testing.expect(it == five);
+        it = it.?.next;
+        testing.expect(it == four);
+        it = it.?.next;
+        testing.expect(it == one);
+        it = it.?.next;
+        testing.expect(it == three);
+        it = it.?.next;
+
+        testing.expect(it == null);
+        testing.expect(views.len == 5);
+
+        testing.expect(views.first == two);
+        testing.expect(views.last == three);
+    }
+
+    // Removal of first
+    views.remove(two);
+    {
+        var it = views.first;
+        testing.expect(it == five);
+        it = it.?.next;
+        testing.expect(it == four);
+        it = it.?.next;
+        testing.expect(it == one);
+        it = it.?.next;
+        testing.expect(it == three);
+        it = it.?.next;
+
+        testing.expect(it == null);
+        testing.expect(views.len == 4);
+
+        testing.expect(views.first == five);
+        testing.expect(views.last == three);
+    }
+
+    // Removal of last
+    views.remove(three);
+    {
+        var it = views.first;
+        testing.expect(it == five);
+        it = it.?.next;
+        testing.expect(it == four);
+        it = it.?.next;
+        testing.expect(it == one);
+        it = it.?.next;
+
+        testing.expect(it == null);
+        testing.expect(views.len == 3);
+
+        testing.expect(views.first == five);
+        testing.expect(views.last == one);
+    }
+
+    // Remove from middle
+    views.remove(four);
+    {
+        var it = views.first;
+        testing.expect(it == five);
+        it = it.?.next;
+        testing.expect(it == one);
+        it = it.?.next;
+
+        testing.expect(it == null);
+        testing.expect(views.len == 2);
+
+        testing.expect(views.first == five);
+        testing.expect(views.last == one);
+    }
+
+    // Reinsertion
+    views.push(two);
+    views.push(three);
+    views.push(four);
+    {
+        var it = views.first;
+        testing.expect(it == four);
+        it = it.?.next;
+        testing.expect(it == three);
+        it = it.?.next;
+        testing.expect(it == two);
+        it = it.?.next;
+        testing.expect(it == five);
+        it = it.?.next;
+        testing.expect(it == one);
+        it = it.?.next;
+
+        testing.expect(it == null);
+        testing.expect(views.len == 5);
+
+        testing.expect(views.first == four);
+        testing.expect(views.last == one);
+    }
+
+    // Clear
+    views.remove(four);
+    views.remove(two);
+    views.remove(three);
+    views.remove(one);
+    views.remove(five);
+
+    testing.expect(views.first == null);
+    testing.expect(views.last == null);
+    testing.expect(views.len == 0);
+}
+
+test "iteration" {
+    const allocator = testing.allocator;
+
+    var views: ViewStack = undefined;
+    views.init();
+
+    var one_a_pb = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(one_a_pb);
+    one_a_pb.view.mapped = true;
+    one_a_pb.view.current_tags = 1 << 0;
+    one_a_pb.view.pending_tags = 1 << 1;
+
+    var two_a = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(two_a);
+    two_a.view.mapped = true;
+    two_a.view.current_tags = 1 << 0;
+
+    var three_b_pa = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(three_b_pa);
+    three_b_pa.view.mapped = true;
+    three_b_pa.view.current_tags = 1 << 1;
+    three_b_pa.view.pending_tags = 1 << 0;
+
+    var four_b = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(four_b);
+    four_b.view.mapped = true;
+    four_b.view.current_tags = 1 << 1;
+
+    var five_b = try allocator.create(ViewStack.Node);
+    defer allocator.destroy(five_b);
+    five_b.view.mapped = true;
+    five_b.view.current_tags = 1 << 1;
+
+    views.push(three_b_pa); // {3}
+    views.push(one_a_pb); // {1, 3}
+    views.push(four_b); // {4, 1, 3}
+    views.push(five_b); // {5, 4, 1, 3}
+    views.push(two_a); // {2, 5, 4, 1, 3}
+
+    // Iteration over all tags
+    {
+        var it = ViewStack.iterator(views.first, 0xFFFFFFFF);
+        testing.expect(it.next() == &two_a.view);
+        testing.expect(it.next() == &five_b.view);
+        testing.expect(it.next() == &four_b.view);
+        testing.expect(it.next() == &one_a_pb.view);
+        testing.expect(it.next() == &three_b_pa.view);
+        testing.expect(it.next() == null);
+    }
+
+    // Iteration over 'a' tags
+    {
+        var it = ViewStack.iterator(views.first, 1 << 0);
+        testing.expect(it.next() == &two_a.view);
+        testing.expect(it.next() == &one_a_pb.view);
+        testing.expect(it.next() == null);
+    }
+
+    // Iteration over 'b' tags
+    {
+        var it = ViewStack.iterator(views.first, 1 << 1);
+        testing.expect(it.next() == &five_b.view);
+        testing.expect(it.next() == &four_b.view);
+        testing.expect(it.next() == &three_b_pa.view);
+        testing.expect(it.next() == null);
+    }
+
+    // Reverse iteration over all tags
+    {
+        var it = ViewStack.reverseIterator(views.last, 0xFFFFFFFF);
+        testing.expect(it.next() == &three_b_pa.view);
+        testing.expect(it.next() == &one_a_pb.view);
+        testing.expect(it.next() == &four_b.view);
+        testing.expect(it.next() == &five_b.view);
+        testing.expect(it.next() == &two_a.view);
+        testing.expect(it.next() == null);
+    }
+
+    // Reverse iteration over 'a' tags
+    {
+        var it = ViewStack.reverseIterator(views.last, 1 << 0);
+        testing.expect(it.next() == &one_a_pb.view);
+        testing.expect(it.next() == &two_a.view);
+        testing.expect(it.next() == null);
+    }
+
+    // Reverse iteration over 'b' tags
+    {
+        var it = ViewStack.reverseIterator(views.last, 1 << 1);
+        testing.expect(it.next() == &three_b_pa.view);
+        testing.expect(it.next() == &four_b.view);
+        testing.expect(it.next() == &five_b.view);
+        testing.expect(it.next() == null);
+    }
+
+    // Iteration over (pending) 'a' tags
+    {
+        var it = ViewStack.pendingIterator(views.first, 1 << 0);
+        testing.expect(it.next() == &two_a.view);
+        testing.expect(it.next() == &three_b_pa.view);
+        testing.expect(it.next() == null);
+    }
+
+    // Iteration over (pending) 'b' tags
+    {
+        var it = ViewStack.pendingIterator(views.first, 1 << 1);
+        testing.expect(it.next() == &five_b.view);
+        testing.expect(it.next() == &four_b.view);
+        testing.expect(it.next() == &one_a_pb.view);
+        testing.expect(it.next() == null);
+    }
+}