Implement RB-tree as a util (#74)

* Add basic rbtree create + destroy

* Add find functions

* Implement size functoin

* rbtree_node_find should return the node, not element

* Add naive insertion

* Update insertion to be more rigorous + follow rb tree rules

* Make destroy more rigorous

* rbtree_recolor_node is unused

* Increment node count and reject duplicates

* rbtree_size should return int64_t to return -EINVAL

* Make rotation consistent in convention

* Implement rigorous delete function

* Add generic comparator and use that everywhere

* Accept a freer callback

* Add some good docs for everything

* Unnecessary set to 0

Author: ryuukumar

kernel/include/kernel/error.h

@@ -113,3 +113,7 @@
 #define INTERNAL_ECORRQ 2601
 // Empty queue
 #define INTERNAL_EEMPQ 2602
+// Not found
+#define INTERNAL_ENOTFOUND 2603
+// Already exists
+#define INTERNAL_EEXISTS 2604

kernel/include/utils/rbtree.h

@@ -0,0 +1,53 @@
+/*
+ * rbtree.h
+ * Copyright (C) 2026  Aditya Kumar
+ *
+ * This program is free software; you can redistribute it and/or modify it under the terms of the
+ * GNU General Public License as published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without
+ * even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along with this program; if
+ * not, see <https://www.gnu.org/licenses/>.
+ */
+
+#pragma once
+
+#include <stddef.h>
+#include <stdint.h>
+
+typedef struct rbtree_node rbtree_node;
+typedef uint64_t		   rbtree_elem;
+typedef enum { RED, BLACK } rbtree_color;
+
+typedef int (*rbtree_cmp) (rbtree_elem, rbtree_elem);
+typedef void (*rbtree_freer) (rbtree_elem);
+
+struct rbtree_node {
+	rbtree_elem	 value;
+	rbtree_color color;
+	rbtree_node *parent, *left, *right;
+};
+
+typedef struct {
+	rbtree_node* head;
+	rbtree_cmp	 comparator;
+	rbtree_freer freer;
+	size_t		 nodes;
+} rbtree;
+
+rbtree* rbtree_create (rbtree_cmp comparator);
+void	rbtree_destroy (rbtree* rbt);
+
+int rbtree_set_freer (rbtree* rbt, rbtree_freer freer);
+
+int		rbtree_insert (rbtree* rbt, rbtree_elem value);
+int		rbtree_delete (rbtree* rbt, rbtree_elem value);
+int64_t rbtree_size (const rbtree* rbt);
+
+int rbtree_find (rbtree* rbt, rbtree_elem value, rbtree_elem* out);
+int rbtree_find_atleast (rbtree* rbt, rbtree_elem value, rbtree_elem* out);
+int rbtree_find_atmost (rbtree* rbt, rbtree_elem value, rbtree_elem* out);