Module hoeffding_classifier 

Available on crate feature alloc only.

Hoeffding Tree Classifier for streaming multi-class classification.

HoeffdingTreeClassifier is a streaming decision tree that grows incrementally using the VFDT algorithm (Domingos & Hulten, 2000). Unlike the existing super::hoeffding::HoeffdingTree (which is a gradient-based regressor), this classifier maintains per-leaf class distributions and splits using information gain.

Algorithm

For each incoming (features, class_label) pair:

1. Route the sample from root to a leaf via threshold comparisons.
2. At the leaf, update class counts and per-feature histogram bins.
3. Once enough samples arrive (grace period), evaluate candidate splits using information gain (entropy reduction).
4. Apply the Hoeffding bound: if the gap between the best and second-best gain exceeds epsilon = sqrt(R^2 * ln(1/delta) / (2n)), commit the split.
5. Split the leaf into two children, partitioning class counts by the chosen threshold.

Key differences from the regressor

• Leaves store class counts, not gradient/hessian sums.
• Split criterion: Information Gain (reduction in entropy).
• Prediction: majority class or class probability distribution.
• No loss function – direct classification.

Examples

use irithyll::tree::hoeffding_classifier::HoeffdingClassifierConfig;
use irithyll::tree::hoeffding_classifier::HoeffdingTreeClassifier;

let config = HoeffdingClassifierConfig::builder()
    .max_depth(6)
    .delta(1e-5)
    .grace_period(50)
    .n_bins(16)
    .build()
    .unwrap();

let mut tree = HoeffdingTreeClassifier::new(config);

// Train: class 0 when x[0] < 5, class 1 otherwise
for i in 0..200 {
    let x = (i as f64) / 20.0;
    let class = if x < 5.0 { 0 } else { 1 };
    tree.train_one(&[x], class);
}

// After training, the tree can predict class labels.
let pred = tree.predict_class(&[2.0]);
assert!(pred == 0 || pred == 1);

Structs

HoeffdingClassifierConfig

Configuration for HoeffdingTreeClassifier.

HoeffdingClassifierConfigBuilder

Builder for HoeffdingClassifierConfig.

HoeffdingTreeClassifier

A streaming decision tree classifier based on the VFDT algorithm.