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SPDX-FileCopyrightText: 1998-2026 Paulo Moura <pmoura@logtalk.org>
SPDX-License-Identifier: Apache-2.0

Licensed under the Apache License, Version 2.0 (the "License");
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`kmeans_clusterer`
========

k-Means clusterer. It uses Lloyd's algorithm with deterministic
initialization. Supports continuous attributes only.

The library implements the `clusterer_protocol` defined in the
`clustering_protocols` library. It provides predicates for learning a
clusterer from a dataset, assigning new instances to clusters, and
exporting the learned clusterer as a list of predicate clauses or to a
file.

Datasets are represented as objects implementing the
`clustering_dataset_protocol` protocol from the `clustering_protocols`
library.


API documentation
-----------------

Open the [../../apis/library_index.html#kmeans_clusterer](../../apis/library_index.html#kmeans_clusterer)
link in a web browser.


Loading
-------

To load this library, load the `loader.lgt` file:

	| ?- logtalk_load(kmeans_clusterer(loader)).


Testing
-------

To test this library predicates, load the `tester.lgt` file:

	| ?- logtalk_load(kmeans_clusterer(tester)).

To run the performance benchmark suite, load the `tester_performance.lgt`
file:

	| ?- logtalk_load(kmeans_clusterer(tester_performance)).


Features
--------

- **Continuous Datasets**: Accepts datasets containing only continuous attributes.
- **Deterministic Initialization**: Supports `first_k` and deterministic `spread` initialization that repeatedly chooses the farthest example from the centroids selected so far.
- **Optional Feature Scaling**: Continuous attributes can be standardized using z-score scaling.
- **Rich Training Diagnostics**: Learned clusterers report training example count, convergence status, iteration count, and final centroid shift.
- **Portable Export**: Learned clusterers can be exported as clauses or files and reused later.
- **Stable Empty-Cluster Handling**: Empty clusters keep their previous centroids instead of failing.


Options
-------

The following options can be passed to the `learn/3` predicate:

- `k(K)`: Number of clusters to learn. Default is `2`.
- `maximum_iterations(Iterations)`: Maximum number of Lloyd iterations. Default is `100`.
- `tolerance(Tolerance)`: Maximum centroid shift threshold for convergence. Default is `1.0e-6`.
- `initialization(Initialization)`: Centroid initialization strategy. Options: `spread` (default) or `first_k`.
- `feature_scaling(FeatureScaling)`: Whether to standardize continuous attributes before clustering. Options: `on` (default) or `off`.


Diagnostics
-----------

The `diagnostics/2` predicate returns a list containing:

- `model(kmeans_clusterer)`
- `centroid_count(Count)`
- `training_example_count(Count)`
- `convergence(Reason)`
- `iterations(Count)`
- `final_shift(Shift)`
- `options(Options)`


Clusterer representation
------------------------

The learned clusterer is represented as a compound term with the
functor chosen by the user when exporting the clusterer and arity 4.
For example:

	kmeans_clusterer(Encoders, Centroids, Options, Diagnostics)

Where:

- `Encoders`: List of continuous attribute encoders storing attribute name, mean, and scale.
- `Centroids`: List of centroid vectors in cluster-id order.
- `Options`: Effective training options used to learn the clusterer.
- `Diagnostics`: Training diagnostics metadata returned by the `diagnostics/2` predicate.


References
----------

1. MacQueen (1967) - "Some Methods for Classification and Analysis of Multivariate Observations". Proceedings of the Fifth Berkeley Symposium on Mathematical Statistics and Probability.
2. Lloyd (1982) - "Least Squares Quantization in PCM". IEEE Transactions on Information Theory, 28(2), 129-137.
