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📝 WalkthroughWalkthroughThis PR adds two LFU cache implementations and node types, refactors the package export surface to a minimal public API, extends the DoublyLinkedList with three helper methods and pointer-name standardisation, and expands LFU documentation and DIRECTORY entries. Changes
Sequence DiagramssequenceDiagram
actor Client
participant LFU as LFUCache
participant Lookup as _lookup
participant FreqMap as Frequency Map
Client->>LFU: get(key)
LFU->>Lookup: lookup key
alt found
Lookup-->>LFU: node
LFU->>FreqMap: remove node from old freq list
LFU->>LFU: increment node.frequency
LFU->>FreqMap: add node to new freq list
LFU-->>Client: return value
else not found
Lookup-->>LFU: none
LFU-->>Client: return None
end
sequenceDiagram
actor Client
participant LFU as LFUCache
participant Lookup as _lookup
participant FreqMap as Frequency Map
Client->>LFU: put(key, value)
LFU->>Lookup: check key
alt key exists
Lookup-->>LFU: node
LFU->>LFU: update value
LFU->>FreqMap: promote node (remove & reinsert)
LFU-->>Client: done
else capacity reached
LFU->>FreqMap: get min-frequency list
FreqMap-->>LFU: list
LFU->>FreqMap: evict least-recent (tail/head per impl)
LFU->>Lookup: remove evicted key
LFU->>LFU: create new node (freq=1)
LFU->>FreqMap: insert node into freq-1 list
LFU->>Lookup: add new node
LFU-->>Client: done
else insert new
LFU->>LFU: create new node (freq=1)
LFU->>FreqMap: insert into freq-1 list
LFU->>Lookup: add node
LFU-->>Client: done
end
Estimated code review effort🎯 4 (Complex) | ⏱️ ~45 minutes Possibly related PRs
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🚥 Pre-merge checks | ✅ 2 | ❌ 1❌ Failed checks (1 inconclusive)
✅ Passed checks (2 passed)
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Actionable comments posted: 3
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⚠️ Outside diff range comments (2)
datastructures/linked_lists/doubly_linked_list/__init__.py (2)
275-290:⚠️ Potential issue | 🔴 CriticalCritical bug in pointer assignments for position-based deletion.
There are two issues in this method:
Line 277: After
self.head = current.next, settingcurrent.next.previous = self.headcreates a circular reference (self.head.previous = self.head). It should beself.head.previous = None.Line 289: After shifting data and updating
current.next = current.next.next, settingcurrent.next.previous = current.previousis incorrect. The newcurrent.nextshould point back tocurrent, notcurrent.previous.🐛 Proposed fix
if position == 0: self.head = current.next - current.next.previous = self.head + if self.head: + self.head.previous = None return current while current is not None: for _ in range(position): current = current.next if current is None: raise ValueError("Invalid position found, reached end of list") current.data = current.next.data current.next = current.next.next - current.next.previous = current.previous + if current.next: + current.next.previous = current return self.head🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@datastructures/linked_lists/doubly_linked_list/__init__.py` around lines 275 - 290, The position-based deletion routine incorrectly assigns back-pointers: when removing at position 0, after doing self.head = current.next set self.head.previous = None instead of current.next.previous = self.head; and when deleting by shifting data and advancing links (the block that does current.data = current.next.data and current.next = current.next.next) ensure you update the back-pointer of the new current.next to point to current (i.e., set current.next.previous = current), and guard for current.next being None before touching .previous to avoid AttributeError; update these assignments in the method that performs position-based deletion (uses variables current, self.head and position).
309-315:⚠️ Potential issue | 🔴 CriticalPotential
NoneTypeerror when deleting tail node.When
current_nodeis the tail of the list,current_node.nextisNone. Line 313 (current_node.next.previous = current_node.previous) will raise anAttributeErrorwhen attempting to access.previousonNone.🐛 Proposed fix
current_node = self.head while current_node: if current_node.key == double_node.key: current_node.previous.next = current_node.next - current_node.next.previous = current_node.previous + if current_node.next: + current_node.next.previous = current_node.previous + else: + self.tail = current_node.previous current_node = current_node.next🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@datastructures/linked_lists/doubly_linked_list/__init__.py` around lines 309 - 315, The deletion loop can dereference None when removing the head or tail; update the logic in the loop that compares current_node.key to double_node.key to guard accesses to current_node.previous and current_node.next: if current_node.previous is None (deleting head) set self.head = current_node.next and, if that new head exists, set its previous to None; if current_node.next is None (deleting tail) set current_node.previous.next = None; otherwise stitch previous.next and next.previous as currently done. Ensure you use the node identifiers current_node, double_node, self.head, previous and next in the updated checks and assignments.
🧹 Nitpick comments (5)
datastructures/linked_lists/doubly_linked_list/__init__.py (2)
178-196: Consider clearing the removed node's pointers.The returned
head_nodestill holds references to the list (head_node.nextpoints to the new head). For consistency withdetach_nodewhich clearsn.previous = n.next = None, consider clearing the removed node's pointers here as well to prevent potential memory leaks or unintended references.🛡️ Suggested fix
def remove_head_node(self) -> Optional[DoubleNode]: """ Removes the head node from the doubly linked list and returns it """ if not self.head: return None head_node = self.head next_node = self.head.next self.head = next_node if self.head is None: self.tail = None else: self.head.previous = None + head_node.next = head_node.previous = None return head_node🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@datastructures/linked_lists/doubly_linked_list/__init__.py` around lines 178 - 196, The remove_head_node implementation returns head_node while its next/previous pointers still reference the list; update remove_head_node to clear the removed node's pointers (set head_node.next = head_node.previous = None) before returning so it matches detach_node's behavior and avoids dangling references.
125-133: Consider clearingnew_node.nextwhen inserting at tail.If
new_nodewas previously part of another list or has residual pointers, itsnextattribute might not beNone. For robustness, consider explicitly settingnew_node.next = Noneafter insertion to ensure the tail invariant is maintained.🛡️ Suggested fix
def insert_at_tail(self, new_node: DoubleNode): if self.tail is None: self.head = self.tail = new_node else: self.tail.next = new_node new_node.previous = self.tail self.tail = new_node + new_node.next = None🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@datastructures/linked_lists/doubly_linked_list/__init__.py` around lines 125 - 133, The insert_at_tail method should defensively clear new_node.next to avoid carrying residual pointers from prior usage; update the insert_at_tail(DoubleNode) implementation to explicitly set new_node.next = None after linking (and when self.tail is None also ensure new_node.previous = None and new_node.next = None) so the tail invariant is preserved and the node has correct next/previous pointers.datastructures/lfucache/lfu_cache_node.py (1)
1-13: Consider adding type hints for parameters.The node classes lack type hints for
dataandkeyparameters, which would improve code clarity and IDE support. This aligns with the repository's contribution guidelines mentioning type hints.✨ Suggested improvement
+from typing import Any + from datastructures.linked_lists.doubly_linked_list.node import DoubleNode class LfuCacheNode(DoubleNode): - def __init__(self, data, key): + def __init__(self, data: Any, key: Any): super().__init__(data, key=key) self.frequency = 1 class LfuCacheNodeV2(DoubleNode): - def __init__(self, data, key): + def __init__(self, data: Any, key: Any): super().__init__(data, key=key) self.frequency = 0🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@datastructures/lfucache/lfu_cache_node.py` around lines 1 - 13, Add parameter type hints to the LfuCacheNode.__init__ and LfuCacheNodeV2.__init__ signatures (for data and key) to match the project's typing conventions; for example annotate data as typing.Any and key as a hashable type (e.g., typing.Hashable or a specific key type used elsewhere), and add any necessary imports from typing at top of file. Update both classes (LfuCacheNode and LfuCacheNodeV2) so their constructors declare these types and ensure IDEs/type-checkers pick them up consistently with other nodes (e.g., DoubleNode usage).datastructures/lfucache/test_lfu_cache.py (1)
6-35: Consider adding test coverage forLFUCacheV2.The test file only covers
LFUCachebut the PR introducesLFUCacheV2as well. Adding similar tests for the V2 implementation would ensure both variants are validated.Would you like me to help generate test cases for
LFUCacheV2?🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@datastructures/lfucache/test_lfu_cache.py` around lines 6 - 35, Add equivalent unit tests for the new LFUCacheV2 implementation: create a new test class (e.g., LFUCacheV2TestCases) or extend LFUCacheTestCases to instantiate LFUCacheV2 instead of LFUCache and repeat the same scenarios (capacity 2, puts, gets, assertions on put/get return values and _current_size) using the same method names put and get and checking the internal _current_size where applicable; ensure the test covers eviction behavior (inserting 3 then 4) and that gets return None for evicted keys and correct values for existing keys just like the existing LFUCache tests.datastructures/lfucache/lfu_cache_v2.py (1)
46-51: Redundant existence check withdefaultdict.Since
_frequency_mapis initialised asdefaultdict(DoublyLinkedList)(line 26), the check on lines 47-48 is unnecessary. Accessing a missing key will automatically create a newDoublyLinkedList.♻️ Simplified code
- # If the incremented frequency doesn't exist - if node.frequency not in self._frequency_map: - self._frequency_map[node.frequency] = DoublyLinkedList() - # Insert the node at tail self._frequency_map[node.frequency].insert_at_tail(node)🤖 Prompt for AI Agents
Verify each finding against the current code and only fix it if needed. In `@datastructures/lfucache/lfu_cache_v2.py` around lines 46 - 51, The check for node.frequency in self._frequency_map is redundant because _frequency_map is a defaultdict(DoublyLinkedList); remove the if block that creates a new DoublyLinkedList and simply call self._frequency_map[node.frequency].insert_at_tail(node). Update the code around the methods that manipulate _frequency_map (references: _frequency_map, DoublyLinkedList, node.frequency, insert_at_tail) to rely on the defaultdict behavior and delete the unnecessary existence-check branch.
🤖 Prompt for all review comments with AI agents
Verify each finding against the current code and only fix it if needed.
Inline comments:
In `@datastructures/lfucache/lfu_cache_v2.py`:
- Around line 69-91: The put method's docstring incorrectly states that nodes
are "append to head" and that "tail" is LRU; update it to match the
implementation: explain that the DoublyLinkedList uses insert_at_tail to mark
MRU and that remove_head_node evicts the LRU, mention self._minimum_frequency
semantics, and describe that new nodes are added at tail with frequency 1
(resetting minimum_frequency to 1). Reference the put method, insert_at_tail,
remove_head_node, and self._minimum_frequency in the description so the
docstring aligns with the actual eviction/ordering logic.
In `@datastructures/lfucache/lfu_cache.py`:
- Around line 118-122: New nodes are being added to the tail via
self._frequency[1].append(node) which contradicts the docstring and __update()
behavior that uses prepend() to mark MRU; change the insertion so the new
LfuCacheNode (created in LfuCacheNode(data=value, key=key) and stored in
self._lookup[key]) is added to the head of the frequency list (use the same
prepend operation used by __update()) and keep updating self._minimum_frequency
and self._current_size as before so new keys are MRU within frequency 1.
In `@datastructures/lfucache/README.md`:
- Around line 64-69: Fix the README: correct the typo "accociated" ->
"associated" and rewrite the Put description so the conditional flow is clear:
when Put finds the key already exists, update its value and call PromoteKey();
when the key does not exist, check capacity—if at capacity evict the head node
of the frequency list at minimum_frequency, then insert the new (key, value)
pair (and update minimum_frequency as needed), finally call PromoteKey() to
adjust frequency ordering. Reference Put, PromoteKey, and minimum_frequency in
the updated text.
---
Outside diff comments:
In `@datastructures/linked_lists/doubly_linked_list/__init__.py`:
- Around line 275-290: The position-based deletion routine incorrectly assigns
back-pointers: when removing at position 0, after doing self.head = current.next
set self.head.previous = None instead of current.next.previous = self.head; and
when deleting by shifting data and advancing links (the block that does
current.data = current.next.data and current.next = current.next.next) ensure
you update the back-pointer of the new current.next to point to current (i.e.,
set current.next.previous = current), and guard for current.next being None
before touching .previous to avoid AttributeError; update these assignments in
the method that performs position-based deletion (uses variables current,
self.head and position).
- Around line 309-315: The deletion loop can dereference None when removing the
head or tail; update the logic in the loop that compares current_node.key to
double_node.key to guard accesses to current_node.previous and
current_node.next: if current_node.previous is None (deleting head) set
self.head = current_node.next and, if that new head exists, set its previous to
None; if current_node.next is None (deleting tail) set
current_node.previous.next = None; otherwise stitch previous.next and
next.previous as currently done. Ensure you use the node identifiers
current_node, double_node, self.head, previous and next in the updated checks
and assignments.
---
Nitpick comments:
In `@datastructures/lfucache/lfu_cache_node.py`:
- Around line 1-13: Add parameter type hints to the LfuCacheNode.__init__ and
LfuCacheNodeV2.__init__ signatures (for data and key) to match the project's
typing conventions; for example annotate data as typing.Any and key as a
hashable type (e.g., typing.Hashable or a specific key type used elsewhere), and
add any necessary imports from typing at top of file. Update both classes
(LfuCacheNode and LfuCacheNodeV2) so their constructors declare these types and
ensure IDEs/type-checkers pick them up consistently with other nodes (e.g.,
DoubleNode usage).
In `@datastructures/lfucache/lfu_cache_v2.py`:
- Around line 46-51: The check for node.frequency in self._frequency_map is
redundant because _frequency_map is a defaultdict(DoublyLinkedList); remove the
if block that creates a new DoublyLinkedList and simply call
self._frequency_map[node.frequency].insert_at_tail(node). Update the code around
the methods that manipulate _frequency_map (references: _frequency_map,
DoublyLinkedList, node.frequency, insert_at_tail) to rely on the defaultdict
behavior and delete the unnecessary existence-check branch.
In `@datastructures/lfucache/test_lfu_cache.py`:
- Around line 6-35: Add equivalent unit tests for the new LFUCacheV2
implementation: create a new test class (e.g., LFUCacheV2TestCases) or extend
LFUCacheTestCases to instantiate LFUCacheV2 instead of LFUCache and repeat the
same scenarios (capacity 2, puts, gets, assertions on put/get return values and
_current_size) using the same method names put and get and checking the internal
_current_size where applicable; ensure the test covers eviction behavior
(inserting 3 then 4) and that gets return None for evicted keys and correct
values for existing keys just like the existing LFUCache tests.
In `@datastructures/linked_lists/doubly_linked_list/__init__.py`:
- Around line 178-196: The remove_head_node implementation returns head_node
while its next/previous pointers still reference the list; update
remove_head_node to clear the removed node's pointers (set head_node.next =
head_node.previous = None) before returning so it matches detach_node's behavior
and avoids dangling references.
- Around line 125-133: The insert_at_tail method should defensively clear
new_node.next to avoid carrying residual pointers from prior usage; update the
insert_at_tail(DoubleNode) implementation to explicitly set new_node.next = None
after linking (and when self.tail is None also ensure new_node.previous = None
and new_node.next = None) so the tail invariant is preserved and the node has
correct next/previous pointers.
ℹ️ Review info
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Configuration used: Organization UI
Review profile: CHILL
Plan: Pro
Run ID: 8d5759b8-d740-4af8-8873-d2eb455a518c
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DIRECTORY.mddatastructures/lfucache/README.mddatastructures/lfucache/__init__.pydatastructures/lfucache/lfu_cache.pydatastructures/lfucache/lfu_cache_node.pydatastructures/lfucache/lfu_cache_v2.pydatastructures/lfucache/test_lfu_cache.pydatastructures/linked_lists/doubly_linked_list/__init__.py
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Describe your change:
New variation with of the LFU cache and updates to the doubly linked list data structure
Checklist:
Fixes: #{$ISSUE_NO}.Summary by CodeRabbit
New Features
Documentation
Chores
Tests / Index