# HackerNews Announcement

## Title

Generalized K-Means Clustering for Apache Spark with Bregman Divergences

## Body (3,982 characters)

I've built a production-ready K-Means library for Apache Spark that supports multiple distance functions beyond Euclidean.

*Why use this instead of Spark MLlib?*

MLlib's KMeans is hard-coded to Euclidean distance, which is mathematically wrong for many data types:

- *Probability distributions* (topic models, histograms): KL divergence is the natural metric. Euclidean treats [0.5, 0.3, 0.2] and [0.49, 0.31, 0.2] as similar even though they represent different distributions. - *Audio/spectral data*: Itakura-Saito respects multiplicative power spectra. Euclidean incorrectly treats -20dB and -10dB as closer than -10dB and 0dB. - *Count data* (traffic, sales): Generalized-I divergence for Poisson-distributed data. - *Outlier robustness*: L1/Manhattan gives median-based clustering vs mean-based (L2).

Using the wrong divergence yields mathematically valid but semantically meaningless clusters.

*Available divergences:* KL, Itakura-Saito, L1/Manhattan, Generalized-I, Logistic Loss, Squared Euclidean

*What's included:* - 6 algorithms: GeneralizedKMeans, BisectingKMeans, XMeans (auto k), SoftKMeans (fuzzy), StreamingKMeans, KMedoids - Drop-in MLlib replacement (same DataFrame API) - 740 tests, deterministic behavior, cross-version persistence (Spark 3.4↔3.5, Scala 2.12↔2.13) - Automatic optimization (broadcast vs crossJoin based on k×dim to avoid OOM) - Python and Scala APIs

*Example:*

```scala // Clustering topic distributions from LDA val topics: DataFrame = // probability vectors

// WRONG: MLlib with Euclidean new org.apache.spark.ml.clustering.KMeans() .setK(10).fit(topics)

// CORRECT: KL divergence for probabilities new GeneralizedKMeans() .setK(10) .setDivergence("kl") .fit(topics)

// For standard data, drop-in replacement: new GeneralizedKMeans() .setDivergence("squaredEuclidean") .fit(numericData) ```

*Quick comparison:*

| Use Case | MLlib | This Library | |----------|-------|--------------| | General numeric | L2 | L2 (compatible) | | Probability distributions | Wrong | KL divergence | | Outlier-robust | | L1 or KMedoids | | Auto k selection | | XMeans (BIC/AIC) | | Fuzzy clustering | | SoftKMeans |

*Performance:* ~870 pts/sec (SE), ~3,400 pts/sec (KL) on modest hardware. Scales to billions of points with automatic strategy selection.

*Production-ready:* - Cross-version model persistence - Scalability guardrails (chunked assignment) - Determinism tests (same seed → identical results) - Performance regression detection - Executable documentation

GitHub: https://github.com/derrickburns/generalized-kmeans-clusterin...

This started as an experiment to understand Bregman divergences. Surprisingly, KL divergence is often faster than Euclidean for probability data. Open to feedback!