How do Bidirectional Hash Tables Enhance Key-Value Lookup and Retrieval?

How to Construct an Effective Bidirectional Hash Table

Similarly to the Python dict data structure, the bidirectional hash table (hereafter referred to as a bidict) offers a key-value lookup and retrieval mechanism. However, bidicts also enable value-to-key querying, providing a more comprehensive search capability.

An Efficient Bidict Implementation

An efficient implementation of a bidict can be achieved using a class that extends the standard dict data type. This bidict class dynamically maintains an inverse directory that associates values (from the original dict) to a list of corresponding keys.

Key Features

• Auto-updating inverse directory: Changes in the standard dict are automatically reflected in the inverse directory.
• Value-key lists: The inverse directory maps values to lists of keys, allowing for multiple keys to have the same value.
• Custom setters and deleters: Modified setitem and delitem methods ensure proper behavior when setting and deleting items.

Code Breakdown

Implementing the bidict class involves:

• Overriding __init__: Initialize both the standard dict and the inverse directory.
• Overriding __setitem__: Add the new key-value pair to the standard dict and update the inverse directory accordingly.
• Overriding __delitem__: Remove the key from the standard dict and update the inverse directory by removing the key from the value's list.

Usage Example

<code class="python">bd = bidict({'a': 1, 'b': 2}) 
print(bd)                     # {'a': 1, 'b': 2}                 
print(bd.inverse)             # {1: ['a'], 2: ['b']}</code>

By utilizing the inverse directory, you can effortlessly retrieve keys from a given value:

<code class="python">print(bd.inverse[1])             # ['a']</code>

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