listach

Listach is a term occasionally used in computer science to describe a hybrid data structure that combines properties of a linked list with an auxiliary indexing mechanism to support efficient lookup while maintaining insertion order. The term is not part of a formal standard and appears mainly in lecture notes, speculative discussions, and some niche libraries. It is not widely adopted in mainstream practice.

Basic design. A typical listach consists of two coordinated components: a doubly linked list that preserves the order of elements, and an index (commonly a hash table or small-scale tree) that maps keys to their corresponding list nodes. Each node stores a key, a value, and a pointer to its location in the list. The index enables fast lookup, while the list ensures stable iteration order over time.

Operations and performance. Insertion appends to the tail of the list and updates the index; deletion removes from both structures. Lookup by key uses the index and then accesses the corresponding list node. Enumeration traverses the linked list in order. Typical time complexities are O(1) for insert, delete, and lookup (average case) and O(n) for full enumeration, with memory overhead from maintaining both structures.

Relations and terminology. Listach is closely related to ordered dictionaries, order-preserving maps, and hybrid data structures used in caches and streaming systems. Because there is no formal standard, implementations vary; some descriptions equate a listach with an order-preserving map, while others treat it as a broader class of ordered, index-backed lists. See also: linked list, hash table, ordered dictionary.