feat(kernel): Add RB-tree/kfifo/hashtable, wait queues, futex, tiny RCU, page cache and KTest harness

Expands the kernel-and-TTY core with the primitives the higher layers have
been missing, plus an in-kernel harness that proves they work.

Data structures (lib/):
  - rbtree.c: Linux-compatible red-black tree with parent pointers, full
    insert/erase rebalancing, in-order iteration and an invariant validator.
  - kfifo.c: power-of-two byte ring with free-running indices, so the wrap
    path costs one mask instead of a branch per byte.
  - hashtable.c: separately-chained table with FNV-1a hashing, string or
    u64 keys, and rehash-on-growth at a 3/4 load factor.

Kernel subsystems:
  - kernel/wait.c: wait queues and one-shot completion barriers, backed by a
    static sleeper pool so a blocking path never re-enters the allocator.
  - kernel/futex.c: 32-bucket futex hash with value re-check, requeue, and an
    fmutex_t sleeping mutex built on the uncontended-fastpath protocol.
  - kernel/rcu.c: tiny RCU with reader nesting, deferred callbacks, kfree_rcu
    and rcu_assign_pointer/rcu_dereference publish barriers.
  - mm/page_cache.c: write-back block page cache, 4 KB pages indexed by
    (device, page), CLOCK second-chance eviction and dirty write-back.
  - kernel/ktest.c: KUnit-style harness with 6 built-in suites, 21 cases and
    735 assertions covering every subsystem above (all passing under QEMU).

LazyBox userland:
  - coreutils.c: sort, uniq, cut, tr, rev, tac, nl, seq, diff, xxd, du,
    factor, sum and truncate, reading through the VFS and handling the
    zero-length synthetic nodes that procfs and sysfs expose.
  - ktest, rcuinfo, futexinfo, waitinfo and pagecache diagnostics applets.

Co-Authored-By: Claude Opus 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01L5yKJT4Wo2cV3rfPrwY5MP

lib/hashtable.c

@@ -0,0 +1,227 @@
+// Chained hash table with FNV-1a hashing and load-factor driven growth
+#include <lib/hashtable.h>
+#include <lib/string.h>
+#include <mm/kmalloc.h>
+
+#define HT_MAX_LOAD_NUM 3   // Grow once entries > 3/4 of bucket count
+#define HT_MAX_LOAD_DEN 4
+#define HT_MAX_BUCKETS  4096
+
+uint64_t hash_fnv1a(const void* data, size_t len) {
+    const uint8_t* p = (const uint8_t*)data;
+    uint64_t h = 1469598103934665603ULL; // FNV offset basis
+    for (size_t i = 0; i < len; i++) {
+        h ^= p[i];
+        h *= 1099511628211ULL;           // FNV prime
+    }
+    return h;
+}
+
+uint64_t hash_string(const char* s) {
+    if (!s) return 0;
+    return hash_fnv1a(s, strlen(s));
+}
+
+uint64_t hash_u64(uint64_t v) {
+    // Fibonacci-style mixing; cheap and well distributed for dense ids.
+    v ^= v >> 33;
+    v *= 0xFF51AFD7ED558CCDULL;
+    v ^= v >> 33;
+    v *= 0xC4CEB9FE1A85EC53ULL;
+    v ^= v >> 33;
+    return v;
+}
+
+static size_t round_up_pow2(size_t v) {
+    if (v < 8) return 8;
+    size_t p = 1;
+    while (p < v) p <<= 1;
+    return p;
+}
+
+hashtable_t* hashtable_create(size_t initial_buckets, bool string_keys) {
+    hashtable_t* ht = (hashtable_t*)kzalloc(sizeof(hashtable_t));
+    if (!ht) return NULL;
+
+    size_t n = round_up_pow2(initial_buckets);
+    ht->buckets = (hnode_t**)kzalloc(n * sizeof(hnode_t*));
+    if (!ht->buckets) {
+        kfree(ht);
+        return NULL;
+    }
+
+    ht->bucket_count = n;
+    ht->entries      = 0;
+    ht->collisions   = 0;
+    ht->string_keys  = string_keys;
+    return ht;
+}
+
+void hashtable_destroy(hashtable_t* ht) {
+    if (!ht) return;
+
+    for (size_t i = 0; i < ht->bucket_count; i++) {
+        hnode_t* n = ht->buckets[i];
+        while (n) {
+            hnode_t* next = n->next;
+            if (n->skey) kfree(n->skey);
+            kfree(n);
+            n = next;
+        }
+    }
+    kfree(ht->buckets);
+    kfree(ht);
+}
+
+static void hashtable_grow(hashtable_t* ht) {
+    if (ht->bucket_count >= HT_MAX_BUCKETS) return;
+
+    size_t new_count = ht->bucket_count * 2;
+    hnode_t** nb = (hnode_t**)kzalloc(new_count * sizeof(hnode_t*));
+    if (!nb) return; // Growth is best effort; chains simply stay longer.
+
+    size_t collisions = 0;
+    for (size_t i = 0; i < ht->bucket_count; i++) {
+        hnode_t* n = ht->buckets[i];
+        while (n) {
+            hnode_t* next = n->next;
+            size_t idx = (size_t)(n->hash & (new_count - 1));
+            if (nb[idx]) collisions++;
+            n->next = nb[idx];
+            nb[idx] = n;
+            n = next;
+        }
+    }
+
+    kfree(ht->buckets);
+    ht->buckets      = nb;
+    ht->bucket_count = new_count;
+    ht->collisions   = collisions;
+}
+
+static hnode_t* find_node(const hashtable_t* ht, uint64_t hash,
+                          const char* skey, uint64_t ikey) {
+    size_t idx = (size_t)(hash & (ht->bucket_count - 1));
+    for (hnode_t* n = ht->buckets[idx]; n; n = n->next) {
+        if (n->hash != hash) continue;
+        if (ht->string_keys) {
+            if (n->skey && skey && strcmp(n->skey, skey) == 0) return n;
+        } else if (n->ikey == ikey) {
+            return n;
+        }
+    }
+    return NULL;
+}
+
+static int hashtable_insert(hashtable_t* ht, uint64_t hash, const char* skey,
+                            uint64_t ikey, void* value) {
+    hnode_t* existing = find_node(ht, hash, skey, ikey);
+    if (existing) {
+        existing->value = value;
+        return 0;
+    }
+
+    if (ht->entries * HT_MAX_LOAD_DEN > ht->bucket_count * HT_MAX_LOAD_NUM) {
+        hashtable_grow(ht);
+    }
+
+    hnode_t* n = (hnode_t*)kzalloc(sizeof(hnode_t));
+    if (!n) return -1;
+
+    n->hash  = hash;
+    n->ikey  = ikey;
+    n->value = value;
+
+    if (ht->string_keys && skey) {
+        size_t len = strlen(skey);
+        n->skey = (char*)kmalloc(len + 1);
+        if (!n->skey) {
+            kfree(n);
+            return -1;
+        }
+        memcpy(n->skey, skey, len + 1);
+    }
+
+    size_t idx = (size_t)(hash & (ht->bucket_count - 1));
+    if (ht->buckets[idx]) ht->collisions++;
+    n->next = ht->buckets[idx];
+    ht->buckets[idx] = n;
+    ht->entries++;
+    return 0;
+}
+
+static bool hashtable_unlink(hashtable_t* ht, uint64_t hash, const char* skey,
+                             uint64_t ikey) {
+    size_t idx = (size_t)(hash & (ht->bucket_count - 1));
+    hnode_t** link = &ht->buckets[idx];
+
+    while (*link) {
+        hnode_t* n = *link;
+        bool match = (n->hash == hash) &&
+                     (ht->string_keys ? (n->skey && skey && strcmp(n->skey, skey) == 0)
+                                      : (n->ikey == ikey));
+        if (match) {
+            *link = n->next;
+            if (n->skey) kfree(n->skey);
+            kfree(n);
+            ht->entries--;
+            return true;
+        }
+        link = &n->next;
+    }
+    return false;
+}
+
+int hashtable_put(hashtable_t* ht, const char* key, void* value) {
+    if (!ht || !key || !ht->string_keys) return -1;
+    return hashtable_insert(ht, hash_string(key), key, 0, value);
+}
+
+void* hashtable_get(const hashtable_t* ht, const char* key) {
+    if (!ht || !key || !ht->string_keys) return NULL;
+    hnode_t* n = find_node(ht, hash_string(key), key, 0);
+    return n ? n->value : NULL;
+}
+
+bool hashtable_remove(hashtable_t* ht, const char* key) {
+    if (!ht || !key || !ht->string_keys) return false;
+    return hashtable_unlink(ht, hash_string(key), key, 0);
+}
+
+int hashtable_put_u64(hashtable_t* ht, uint64_t key, void* value) {
+    if (!ht || ht->string_keys) return -1;
+    return hashtable_insert(ht, hash_u64(key), NULL, key, value);
+}
+
+void* hashtable_get_u64(const hashtable_t* ht, uint64_t key) {
+    if (!ht || ht->string_keys) return NULL;
+    hnode_t* n = find_node(ht, hash_u64(key), NULL, key);
+    return n ? n->value : NULL;
+}
+
+bool hashtable_remove_u64(hashtable_t* ht, uint64_t key) {
+    if (!ht || ht->string_keys) return false;
+    return hashtable_unlink(ht, hash_u64(key), NULL, key);
+}
+
+size_t hashtable_size(const hashtable_t* ht)       { return ht ? ht->entries : 0; }
+size_t hashtable_buckets(const hashtable_t* ht)    { return ht ? ht->bucket_count : 0; }
+size_t hashtable_collisions(const hashtable_t* ht) { return ht ? ht->collisions : 0; }
+
+size_t hashtable_longest_chain(const hashtable_t* ht) {
+    if (!ht) return 0;
+    size_t longest = 0;
+    for (size_t i = 0; i < ht->bucket_count; i++) {
+        size_t len = 0;
+        for (hnode_t* n = ht->buckets[i]; n; n = n->next) len++;
+        if (len > longest) longest = len;
+    }
+    return longest;
+}
+
+void hashtable_foreach(const hashtable_t* ht, hashtable_iter_fn fn, void* ctx) {
+    if (!ht || !fn) return;
+    for (size_t i = 0; i < ht->bucket_count; i++) {
+        for (hnode_t* n = ht->buckets[i]; n; n = n->next) fn(n, ctx);
+    }
+}

lib/kfifo.c

@@ -0,0 +1,134 @@
+// Power-of-two lock-protected byte ring buffer for SUB-OS
+#include <lib/kfifo.h>
+#include <lib/string.h>
+#include <mm/kmalloc.h>
+
+static size_t round_up_pow2(size_t v) {
+    if (v < 2) return 2;
+    size_t p = 1;
+    while (p < v) p <<= 1;
+    return p;
+}
+
+int kfifo_alloc(kfifo_t* fifo, size_t size) {
+    if (!fifo || size == 0) return -1;
+
+    size = round_up_pow2(size);
+    fifo->buffer = (uint8_t*)kmalloc(size);
+    if (!fifo->buffer) return -1;
+
+    fifo->size        = size;
+    fifo->mask        = size - 1;
+    fifo->in          = 0;
+    fifo->out         = 0;
+    fifo->owns_buffer = true;
+    spinlock_init(&fifo->lock);
+    return 0;
+}
+
+int kfifo_init_static(kfifo_t* fifo, void* buffer, size_t size) {
+    if (!fifo || !buffer || size == 0) return -1;
+    // A caller-supplied buffer must already be a power of two.
+    if (size & (size - 1)) return -1;
+
+    fifo->buffer      = (uint8_t*)buffer;
+    fifo->size        = size;
+    fifo->mask        = size - 1;
+    fifo->in          = 0;
+    fifo->out         = 0;
+    fifo->owns_buffer = false;
+    spinlock_init(&fifo->lock);
+    return 0;
+}
+
+void kfifo_free(kfifo_t* fifo) {
+    if (!fifo) return;
+    if (fifo->owns_buffer && fifo->buffer) kfree(fifo->buffer);
+    fifo->buffer = NULL;
+    fifo->size = fifo->mask = fifo->in = fifo->out = 0;
+    fifo->owns_buffer = false;
+}
+
+void kfifo_reset(kfifo_t* fifo) {
+    if (!fifo) return;
+    spin_lock(&fifo->lock);
+    fifo->in = fifo->out = 0;
+    spin_unlock(&fifo->lock);
+}
+
+size_t kfifo_len(const kfifo_t* fifo) {
+    if (!fifo || !fifo->buffer) return 0;
+    return fifo->in - fifo->out;
+}
+
+size_t kfifo_avail(const kfifo_t* fifo) {
+    if (!fifo || !fifo->buffer) return 0;
+    return fifo->size - (fifo->in - fifo->out);
+}
+
+bool kfifo_is_empty(const kfifo_t* fifo) { return kfifo_len(fifo) == 0; }
+bool kfifo_is_full(const kfifo_t* fifo)  { return kfifo_avail(fifo) == 0; }
+
+// Copy into the ring at a free-running offset, wrapping at most once.
+static void copy_in(kfifo_t* fifo, const uint8_t* src, size_t len, size_t off) {
+    size_t start = off & fifo->mask;
+    size_t first = fifo->size - start;
+    if (first > len) first = len;
+
+    memcpy(fifo->buffer + start, src, first);
+    if (len > first) memcpy(fifo->buffer, src + first, len - first);
+}
+
+static void copy_out(const kfifo_t* fifo, uint8_t* dst, size_t len, size_t off) {
+    size_t start = off & fifo->mask;
+    size_t first = fifo->size - start;
+    if (first > len) first = len;
+
+    memcpy(dst, fifo->buffer + start, first);
+    if (len > first) memcpy(dst + first, fifo->buffer, len - first);
+}
+
+size_t kfifo_in(kfifo_t* fifo, const void* src, size_t len) {
+    if (!fifo || !fifo->buffer || !src || len == 0) return 0;
+
+    spin_lock(&fifo->lock);
+    size_t space = fifo->size - (fifo->in - fifo->out);
+    if (len > space) len = space;
+
+    copy_in(fifo, (const uint8_t*)src, len, fifo->in);
+    fifo->in += len;
+    spin_unlock(&fifo->lock);
+    return len;
+}
+
+size_t kfifo_out(kfifo_t* fifo, void* dst, size_t len) {
+    if (!fifo || !fifo->buffer || !dst || len == 0) return 0;
+
+    spin_lock(&fifo->lock);
+    size_t used = fifo->in - fifo->out;
+    if (len > used) len = used;
+
+    copy_out(fifo, (uint8_t*)dst, len, fifo->out);
+    fifo->out += len;
+    spin_unlock(&fifo->lock);
+    return len;
+}
+
+size_t kfifo_peek(const kfifo_t* fifo, void* dst, size_t len) {
+    if (!fifo || !fifo->buffer || !dst || len == 0) return 0;
+
+    size_t used = fifo->in - fifo->out;
+    if (len > used) len = used;
+
+    copy_out(fifo, (uint8_t*)dst, len, fifo->out);
+    return len;
+}
+
+int kfifo_put_byte(kfifo_t* fifo, uint8_t byte) {
+    return kfifo_in(fifo, &byte, 1) == 1 ? 0 : -1;
+}
+
+int kfifo_get_byte(kfifo_t* fifo, uint8_t* byte_out) {
+    if (!byte_out) return -1;
+    return kfifo_out(fifo, byte_out, 1) == 1 ? 0 : -1;
+}

lib/rbtree.c

@@ -0,0 +1,309 @@
+// Red-Black Tree implementation for SUB-OS (Linux-compatible semantics)
+#include <lib/rbtree.h>
+
+static void rb_rotate_left(struct rb_node* node, struct rb_root* root) {
+    struct rb_node* right  = node->rb_right;
+    struct rb_node* parent = node->rb_parent;
+
+    node->rb_right = right->rb_left;
+    if (right->rb_left) right->rb_left->rb_parent = node;
+
+    right->rb_left   = node;
+    right->rb_parent = parent;
+
+    if (parent) {
+        if (node == parent->rb_left) parent->rb_left = right;
+        else                         parent->rb_right = right;
+    } else {
+        root->rb_node = right;
+    }
+    node->rb_parent = right;
+}
+
+static void rb_rotate_right(struct rb_node* node, struct rb_root* root) {
+    struct rb_node* left   = node->rb_left;
+    struct rb_node* parent = node->rb_parent;
+
+    node->rb_left = left->rb_right;
+    if (left->rb_right) left->rb_right->rb_parent = node;
+
+    left->rb_right  = node;
+    left->rb_parent = parent;
+
+    if (parent) {
+        if (node == parent->rb_right) parent->rb_right = left;
+        else                          parent->rb_left = left;
+    } else {
+        root->rb_node = left;
+    }
+    node->rb_parent = left;
+}
+
+void rb_insert_color(struct rb_node* node, struct rb_root* root) {
+    struct rb_node *parent, *gparent;
+
+    while ((parent = node->rb_parent) && parent->rb_color == RB_RED) {
+        gparent = parent->rb_parent;
+        if (!gparent) break;
+
+        if (parent == gparent->rb_left) {
+            struct rb_node* uncle = gparent->rb_right;
+            if (uncle && uncle->rb_color == RB_RED) {
+                uncle->rb_color   = RB_BLACK;
+                parent->rb_color  = RB_BLACK;
+                gparent->rb_color = RB_RED;
+                node = gparent;
+                continue;
+            }
+            if (parent->rb_right == node) {
+                rb_rotate_left(parent, root);
+                struct rb_node* tmp = parent;
+                parent = node;
+                node = tmp;
+            }
+            parent->rb_color  = RB_BLACK;
+            gparent->rb_color = RB_RED;
+            rb_rotate_right(gparent, root);
+        } else {
+            struct rb_node* uncle = gparent->rb_left;
+            if (uncle && uncle->rb_color == RB_RED) {
+                uncle->rb_color   = RB_BLACK;
+                parent->rb_color  = RB_BLACK;
+                gparent->rb_color = RB_RED;
+                node = gparent;
+                continue;
+            }
+            if (parent->rb_left == node) {
+                rb_rotate_right(parent, root);
+                struct rb_node* tmp = parent;
+                parent = node;
+                node = tmp;
+            }
+            parent->rb_color  = RB_BLACK;
+            gparent->rb_color = RB_RED;
+            rb_rotate_left(gparent, root);
+        }
+    }
+
+    root->rb_node->rb_color = RB_BLACK;
+}
+
+// Rebalance after removing a black node. `node` may be NULL, hence the
+// explicit parent pointer.
+static void rb_erase_color(struct rb_node* node, struct rb_node* parent,
+                           struct rb_root* root) {
+    struct rb_node* other;
+
+    while ((!node || node->rb_color == RB_BLACK) && node != root->rb_node) {
+        if (parent->rb_left == node) {
+            other = parent->rb_right;
+            if (!other) break;
+
+            if (other->rb_color == RB_RED) {
+                other->rb_color  = RB_BLACK;
+                parent->rb_color = RB_RED;
+                rb_rotate_left(parent, root);
+                other = parent->rb_right;
+                if (!other) break;
+            }
+            if ((!other->rb_left  || other->rb_left->rb_color  == RB_BLACK) &&
+                (!other->rb_right || other->rb_right->rb_color == RB_BLACK)) {
+                other->rb_color = RB_RED;
+                node = parent;
+                parent = node->rb_parent;
+            } else {
+                if (!other->rb_right || other->rb_right->rb_color == RB_BLACK) {
+                    if (other->rb_left) other->rb_left->rb_color = RB_BLACK;
+                    other->rb_color = RB_RED;
+                    rb_rotate_right(other, root);
+                    other = parent->rb_right;
+                    if (!other) break;
+                }
+                other->rb_color  = parent->rb_color;
+                parent->rb_color = RB_BLACK;
+                if (other->rb_right) other->rb_right->rb_color = RB_BLACK;
+                rb_rotate_left(parent, root);
+                node = root->rb_node;
+                break;
+            }
+        } else {
+            other = parent->rb_left;
+            if (!other) break;
+
+            if (other->rb_color == RB_RED) {
+                other->rb_color  = RB_BLACK;
+                parent->rb_color = RB_RED;
+                rb_rotate_right(parent, root);
+                other = parent->rb_left;
+                if (!other) break;
+            }
+            if ((!other->rb_left  || other->rb_left->rb_color  == RB_BLACK) &&
+                (!other->rb_right || other->rb_right->rb_color == RB_BLACK)) {
+                other->rb_color = RB_RED;
+                node = parent;
+                parent = node->rb_parent;
+            } else {
+                if (!other->rb_left || other->rb_left->rb_color == RB_BLACK) {
+                    if (other->rb_right) other->rb_right->rb_color = RB_BLACK;
+                    other->rb_color = RB_RED;
+                    rb_rotate_left(other, root);
+                    other = parent->rb_left;
+                    if (!other) break;
+                }
+                other->rb_color  = parent->rb_color;
+                parent->rb_color = RB_BLACK;
+                if (other->rb_left) other->rb_left->rb_color = RB_BLACK;
+                rb_rotate_right(parent, root);
+                node = root->rb_node;
+                break;
+            }
+        }
+    }
+
+    if (node) node->rb_color = RB_BLACK;
+}
+
+void rb_erase(struct rb_node* node, struct rb_root* root) {
+    struct rb_node *child, *parent;
+    int color;
+
+    if (!node->rb_left) {
+        child = node->rb_right;
+    } else if (!node->rb_right) {
+        child = node->rb_left;
+    } else {
+        // Two children: splice in the in-order successor.
+        struct rb_node* old = node;
+        struct rb_node* left;
+
+        node = node->rb_right;
+        while ((left = node->rb_left) != NULL) node = left;
+
+        // Put the successor where `old` used to hang.
+        if (old->rb_parent) {
+            if (old->rb_parent->rb_left == old) old->rb_parent->rb_left = node;
+            else                                old->rb_parent->rb_right = node;
+        } else {
+            root->rb_node = node;
+        }
+
+        child  = node->rb_right;
+        parent = node->rb_parent;
+        color  = node->rb_color;
+
+        if (parent == old) {
+            // The successor is `old`'s direct right child, so it keeps its own
+            // right subtree and becomes the parent of the deficient side.
+            parent = node;
+        } else {
+            if (child) child->rb_parent = parent;
+            parent->rb_left = child;
+
+            node->rb_right = old->rb_right;
+            old->rb_right->rb_parent = node;
+        }
+
+        node->rb_parent = old->rb_parent;
+        node->rb_color  = old->rb_color;
+        node->rb_left   = old->rb_left;
+        old->rb_left->rb_parent = node;
+
+        if (color == RB_BLACK) rb_erase_color(child, parent, root);
+        return;
+    }
+
+    parent = node->rb_parent;
+    color  = node->rb_color;
+
+    if (child) child->rb_parent = parent;
+    if (parent) {
+        if (parent->rb_left == node) parent->rb_left = child;
+        else                         parent->rb_right = child;
+    } else {
+        root->rb_node = child;
+    }
+
+    if (color == RB_BLACK) rb_erase_color(child, parent, root);
+}
+
+struct rb_node* rb_first(const struct rb_root* root) {
+    struct rb_node* n = root->rb_node;
+    if (!n) return NULL;
+    while (n->rb_left) n = n->rb_left;
+    return n;
+}
+
+struct rb_node* rb_last(const struct rb_root* root) {
+    struct rb_node* n = root->rb_node;
+    if (!n) return NULL;
+    while (n->rb_right) n = n->rb_right;
+    return n;
+}
+
+struct rb_node* rb_next(const struct rb_node* node) {
+    if (!node) return NULL;
+
+    if (node->rb_right) {
+        struct rb_node* n = node->rb_right;
+        while (n->rb_left) n = n->rb_left;
+        return n;
+    }
+
+    struct rb_node* parent;
+    while ((parent = node->rb_parent) && node == parent->rb_right) {
+        node = parent;
+    }
+    return parent;
+}
+
+struct rb_node* rb_prev(const struct rb_node* node) {
+    if (!node) return NULL;
+
+    if (node->rb_left) {
+        struct rb_node* n = node->rb_left;
+        while (n->rb_right) n = n->rb_right;
+        return n;
+    }
+
+    struct rb_node* parent;
+    while ((parent = node->rb_parent) && node == parent->rb_left) {
+        node = parent;
+    }
+    return parent;
+}
+
+size_t rb_count(const struct rb_root* root) {
+    size_t count = 0;
+    for (struct rb_node* n = rb_first(root); n; n = rb_next(n)) count++;
+    return count;
+}
+
+// Returns the number of black nodes on any root-to-leaf path, or -1 when the
+// black-height invariant is violated.
+static int rb_black_height_of(const struct rb_node* node) {
+    if (!node) return 1;
+
+    int left  = rb_black_height_of(node->rb_left);
+    int right = rb_black_height_of(node->rb_right);
+    if (left < 0 || right < 0 || left != right) return -1;
+
+    if (node->rb_color == RB_RED) {
+        if ((node->rb_left  && node->rb_left->rb_color  == RB_RED) ||
+            (node->rb_right && node->rb_right->rb_color == RB_RED)) {
+            return -1;
+        }
+        return left;
+    }
+    return left + 1;
+}
+
+int rb_black_height(const struct rb_root* root) {
+    return rb_black_height_of(root->rb_node);
+}
+
+bool rb_validate(const struct rb_root* root) {
+    if (!root->rb_node) return true;
+    if (root->rb_node->rb_color != RB_BLACK) return false;
+    if (root->rb_node->rb_parent != NULL) return false;
+    return rb_black_height_of(root->rb_node) > 0;
+}