feat(rtl): use one sentinel nil node globally

src/kern/init.zig

@@ -1,4 +1,5 @@
 const std = @import("std");
+const rtl = @import("rtl");
 const b = @import("base");
 const pl = b.pl;
 const ke = b.ke;
@@ -10,6 +11,7 @@ var thread0: ke.Thread = undefined;
 var stack: [8192]u8 align(16) = undefined;
 
 pub fn init(boot_info: *pl.BootInfo) linksection(b.init) void {
+    rtl.init();
     std.log.info("hello, world", .{});
     ki.impl.early_init();
     pl.early_init(boot_info);

src/rtl/bst.zig

@@ -7,26 +7,38 @@ pub const Node = struct {
     parent: TaggedPtr(Node),
 };
 
+pub var nil = Node{
+    .left = undefined,
+    .right = undefined,
+    .parent = undefined,
+};
+
+/// Initialize the sentinel nil node. This should be called before using any BSTs.
+pub fn init_nil() void {
+    nil.left = &nil;
+    nil.right = &nil;
+    nil.parent.set_ptr(&nil);
+    nil.parent.set_tag(0);
+}
+
 /// Base struct for binary search trees.
 /// This should be used as a field in other types that are based on binary search trees, such as red-black trees or AVL trees.
 pub fn BST(comptime cmp: fn (*Node, *Node) std.math.Order) type {
     return struct {
         const Self = @This();
 
-        root: *Node = undefined,
-        nil: Node = undefined,
+        root: *Node = &nil,
 
-        /// Initialize the BST. This must be called before using the tree.
-        pub fn init(self: *Self) void {
-            self.nil.left = &self.nil;
-            self.nil.right = &self.nil;
-            self.nil.parent = .init(&self.nil, 0);
-            self.root = &self.nil;
+        pub fn init() Self {
+            return Self{
+                .root = &nil,
+            };
         }
 
         /// Return whether or not the given node is the sentinel nil node.
         pub fn is_nil(self: *Self, node: *Node) bool {
-            return node == &self.nil;
+            _ = self;
+            return node == &nil;
         }
 
         /// Search for a node in the BST that matches `elem`, and return a pointer to it if found.
@@ -79,7 +91,7 @@ pub fn BST(comptime cmp: fn (*Node, *Node) std.math.Order) type {
             }
 
             if (parent == self.root) {
-                return &self.nil;
+                return &nil;
             }
 
             return parent;
@@ -105,9 +117,9 @@ pub fn BST(comptime cmp: fn (*Node, *Node) std.math.Order) type {
 
             if (self.is_nil(current)) {
                 self.root = elem;
-                elem.left = &self.nil;
-                elem.right = &self.nil;
-                elem.parent.set_ptr(&self.nil);
+                elem.left = &nil;
+                elem.right = &nil;
+                elem.parent.set_ptr(&nil);
                 return;
             }
 
@@ -132,8 +144,8 @@ pub fn BST(comptime cmp: fn (*Node, *Node) std.math.Order) type {
                 }
             }
 
-            elem.left = &self.nil;
-            elem.right = &self.nil;
+            elem.left = &nil;
+            elem.right = &nil;
             elem.parent.set_ptr(current);
         }
 
@@ -158,8 +170,9 @@ fn node_cmp(a: *Node, b: *Node) std.math.Order {
 }
 
 test BST {
-    var tree: BST(node_cmp) = undefined;
-    tree.init();
+    init_nil();
+
+    var tree: BST(node_cmp) = .init();
 
     var nodes = [_]MyNode{
         .{ .value = 5, .node = undefined },

src/rtl/rbtree.zig

@@ -68,8 +68,10 @@ pub fn RBTree(comptime cmp: fn (*bst.Node, *bst.Node) std.math.Order) type {
             node.parent.set_ptr(y);
         }
 
-        pub fn init(self: *Self) void {
-            self.tree.init();
+        pub fn init() Self {
+            return Self{
+                .tree = bst.BST(cmp).init(),
+            };
         }
 
         /// Insert the node `elem` into the tree.
@@ -157,7 +159,7 @@ pub fn RBTree(comptime cmp: fn (*bst.Node, *bst.Node) std.math.Order) type {
         /// Delete the node `elem` from the tree.
         pub fn delete(self: *Self, elem: *bst.Node) void {
             var node = elem;
-            var child = &self.tree.nil;
+            var child = &bst.nil;
             var orig_color = get_color(node);
 
             if (self.tree.is_nil(node.left)) {
@@ -304,10 +306,17 @@ fn check_invariants(tree: *RBTree(cmp_test_node)) !void {
     _ = try get_black_height(tree, tree.tree.root);
 }
 
-test "invariants hold after each insertion" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
+fn make_tree() RBTree(cmp_test_node) {
+    bst.init_nil();
+    return .init();
+}
 
+fn insert_all(tree: *RBTree(cmp_test_node), nodes: []TestNode) !void {
+    for (nodes) |*n| try tree.insert(&n.node);
+}
+
+test "invariants hold after each insertion" {
+    var tree = make_tree();
     var nodes = [_]TestNode{
         .{ .value = 10, .node = undefined },
         .{ .value = 5, .node = undefined },
@@ -319,7 +328,6 @@ test "invariants hold after each insertion" {
         .{ .value = 3, .node = undefined },
         .{ .value = 6, .node = undefined },
     };
-
     for (&nodes) |*n| {
         try tree.insert(&n.node);
         try check_invariants(&tree);
@@ -327,33 +335,28 @@ test "invariants hold after each insertion" {
 }
 
 test "ascending insertion stays balanced" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
+    var tree = make_tree();
     var nodes: [10]TestNode = undefined;
     for (&nodes, 0..) |*n, i| {
-        n.value = @intCast(i + 1);
-        n.node = undefined;
+        n.* = .{ .value = @intCast(i + 1), .node = undefined };
         try tree.insert(&n.node);
         try check_invariants(&tree);
     }
 }
 
 test "descending insertion stays balanced" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
+    var tree = make_tree();
     var nodes: [10]TestNode = undefined;
     for (&nodes, 0..) |*n, i| {
-        n.value = @intCast(10 - i);
-        n.node = undefined;
+        n.* = .{ .value = @intCast(10 - i), .node = undefined };
         try tree.insert(&n.node);
         try check_invariants(&tree);
     }
 }
 
 test "single insert and delete" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
-    var n = TestNode{ .value = 69, .node = undefined };
+    var tree = make_tree();
+    var n = TestNode{ .value = 42, .node = undefined };
     try tree.insert(&n.node);
     try check_invariants(&tree);
     tree.delete(&n.node);
@@ -361,37 +364,32 @@ test "single insert and delete" {
 }
 
 test "delete root" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
-
+    var tree = make_tree();
     var nodes = [_]TestNode{
         .{ .value = 5, .node = undefined },
         .{ .value = 3, .node = undefined },
         .{ .value = 7, .node = undefined },
     };
-    for (&nodes) |*n| try tree.insert(&n.node);
+    try insert_all(&tree, &nodes);
     tree.delete(&nodes[0].node);
     try check_invariants(&tree);
     try std.testing.expect(!tree.tree.is_empty());
 }
 
 test "delete leaf" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
+    var tree = make_tree();
     var nodes = [_]TestNode{
         .{ .value = 5, .node = undefined },
         .{ .value = 3, .node = undefined },
         .{ .value = 7, .node = undefined },
     };
-    for (&nodes) |*n| try tree.insert(&n.node);
+    try insert_all(&tree, &nodes);
     tree.delete(&nodes[1].node);
     try check_invariants(&tree);
 }
 
 test "delete node with two children" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
-
+    var tree = make_tree();
     var nodes = [_]TestNode{
         .{ .value = 10, .node = undefined },
         .{ .value = 5, .node = undefined },
@@ -399,15 +397,13 @@ test "delete node with two children" {
         .{ .value = 3, .node = undefined },
         .{ .value = 7, .node = undefined },
     };
-    for (&nodes) |*n| try tree.insert(&n.node);
+    try insert_all(&tree, &nodes);
     tree.delete(&nodes[1].node);
     try check_invariants(&tree);
 }
 
 test "invariants hold after each deletion" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
-
+    var tree = make_tree();
     var nodes = [_]TestNode{
         .{ .value = 5, .node = undefined },
         .{ .value = 3, .node = undefined },
@@ -419,7 +415,7 @@ test "invariants hold after each deletion" {
         .{ .value = 1, .node = undefined },
         .{ .value = 9, .node = undefined },
     };
-    for (&nodes) |*n| try tree.insert(&n.node);
+    try insert_all(&tree, &nodes);
 
     const delete_order = [_]usize{ 3, 0, 7, 1, 5, 2, 6, 4, 8 };
     for (delete_order) |i| {
@@ -430,8 +426,7 @@ test "invariants hold after each deletion" {
 }
 
 test "duplicate insert returns error" {
-    var tree: RBTree(cmp_test_node) = undefined;
-    tree.init();
+    var tree = make_tree();
     var a = TestNode{ .value = 5, .node = undefined };
     var b = TestNode{ .value = 5, .node = undefined };
     try tree.insert(&a.node);

src/rtl/root.zig

@@ -11,6 +11,10 @@ pub const bst = @import("bst.zig");
 pub const BST = bst.BST;
 pub const RBTree = @import("rbtree.zig").RBTree;
 
+pub fn init() void {
+    bst.init_nil();
+}
+
 test {
     std.testing.refAllDecls(@This());
 }