// KAT-Coder v2.5 Dev XL

▮ PILLAR BREAKDOWN

▮ CRITICAL BUGS

• FATAL: _put_internal line 305 inserts a NEW key with 'self._freq_map[1].push_front(...)' but never ensures the freq-1 bucket exists — KeyError: 1 on the very first put. The cache cannot store a single key. The _ensure_freq_list(1) helper it should use exists and is used correctly everywhere else (lines 294, 354).
• Transaction.commit() calls _put_internal for buffered writes, so it hits the same KeyError: 1 — committed transactions crash too.
• Test suite cannot execute: crashes at the first cache.put() in main(); the well-built test harness (pass/fail counter, 4 real scenarios) validates nothing.
• No __slots__ on _DLLNode/_CacheNode/_DoublyLinkedList despite the rubric requiring it for memory efficiency.
• _evict_node uses min(self._freq_map) (line 325) when the min-tier empties — a hidden O(F) scan, violating strict O(1).
• No MVCC/version check on transaction commit (lost-update possible if the global key is modified between tx.get and commit); commit is not exception-safe across the writes-vs-deletes loops.

▮ RECOMMENDED USE

▮ REFACTORED PATCH

# FIX 1 (the fatal one-liner): use the helper that already exists.
# line 305, in _put_internal, new-key branch:
- self._freq_map[1].push_front(dll_node)
+ self._ensure_freq_list(1).push_front(dll_node)
# (This single change makes the cache and transactions functional.)

# FIX 2: replace the O(F) min() scan with an incremental bump:
# in _evict_node, when the min-tier bucket empties, min_freq is the
# lowest remaining tier. Since freq only ever increments by 1, the
# next min is almost always min_freq+1; track it incrementally rather
# than scanning. Or, since this only happens on full eviction, accept
# O(F) but only on the empty-cache edge — document it.

# FIX 3: add __slots__ to all internal classes:
class _CacheNode:
    __slots__ = ('key','value','ttl_seconds','expiry_time','freq','dll_node')
    ...

# FIX 4: wrap commit applies in try/except so a mid-commit exception
# does not leave a half-applied global state; consider abort semantics.
# FIX 5: add node.version; in tx commit, abort if the global node for
# a key is not the one seen at tx.get() time.