🔥 Bankai Boruta

The unreleased a7 candidate adds an opt-in exact training session. The materialized path remains the default until AMD runtime/RSS acceptance passes. See validation status and changes.

BankaiBoruta is a fast, scalable feature selection library built for large datasets.

It combines the Boruta all-relevant selection algorithm with the native Rust backend of Bankai Random Forest. Classic Boruta is the default; greedy=True enables the fixed-round Greedy Boruta variant. BankaiRF supplies exact or binned Extra Trees, gain importance and parallel tree training through a scikit-learn-compatible selector. In 0.1.0a6, the Rust selector forwards supported forest parameters from the caller.

⚠️ Bankai Boruta is in Alpha. Its API and fitted objects may change.

Install

Python 3.11 or newer is required. The release installation command is:

python -m pip install bankai-boruta

Quick start

This example generates a binary classification dataset with 1,000 rows and 20 features. It fits selection on the training partition and applies the same feature mask to the held-out partition. A pandas DataFrame supplies feature names.

import pandas as pd
from sklearn.datasets import make_classification
from sklearn.model_selection import train_test_split

from bankai_boruta import BankaiBoruta

values, y = make_classification(
    n_samples=1_000,
    n_features=20,
    n_informative=5,
    n_redundant=3,
    random_state=2077,
)
X = pd.DataFrame(values, columns=[f"feature_{i}" for i in range(values.shape[1])])
X_train, X_test, y_train, y_test = train_test_split(
    X, y, test_size=0.2, stratify=y, random_state=2077,
)

selector = BankaiBoruta(
    n_estimators="auto",
    perc=100,
    alpha=0.05,
    max_iter=100,
    random_state=2077,
)
selector.fit(X_train, y_train)

X_train_selected = selector.transform(X_train)
X_test_selected = selector.transform(X_test)
print(f"Selected features: {selector.n_features_}")
print(selector.get_feature_names_out())
print(f"Train shape: {X_train_selected.shape}; test shape: {X_test_selected.shape}")

selection = pd.DataFrame({
    "feature": selector.feature_names_in_,
    "confirmed": selector.support_,
    "tentative": selector.support_weak_,
    "rank": selector.ranking_,
})
print(selection.sort_values("rank"))

transform returns confirmed features only. fit_transform(X, y) combines the fit and transform steps. NumPy arrays and pandas DataFrames are supported; pandas inputs are converted to NumPy for training.

Selector reference

BankaiBoruta follows scikit-learn's estimator conventions and uses parameter names familiar from BorutaPy. Its constructor defaults are:

from bankai_boruta import BankaiBoruta

selector = BankaiBoruta(
    estimator=None,
    n_estimators="auto",
    perc=100,
    alpha=0.05,
    two_step=True,
    max_iter=100,
    random_state=None,
    verbose=0,
    early_stopping=False,
    n_iter_no_change=20,
    greedy=False,
    bonferroni=True,
    importance_getter="auto",
    store_history=False,
)

The default classifier is configured as follows; remaining parameters follow BankaiRF a15 defaults:

from bankai_random_forest import BankaiRandomForestClassifier

forest = BankaiRandomForestClassifier(
    extra_trees=True,
    extra_trees_backend="binned",
    max_bins=63,
    importance_type="gain",
    max_depth=6,
    n_jobs=-1,
)

Pass a classifier for full forest customization, then use nested parameters such as estimator__max_depth with set_params or parameter search. Weights and original training labels, including text labels, are passed to the Python adapter without re-encoding the labels supplied to the classifier.

forest = BankaiRandomForestClassifier(
    n_estimators=100,
    max_depth=5,
    class_weight="balanced",
    n_jobs=-1,
)
selector = BankaiBoruta(forest, n_estimators=None, random_state=42)
selector.set_params(estimator__max_depth=7)
selector.fit(X_train, y_train)

Native forest controls

The native selector forwards the supplied BankaiRandomForestClassifier settings for the supported native subset. Both exact and binned Extra Trees are available. Exact native selection currently requires float32 input; exact float64 input uses the adapter so BankaiRF can preserve its float64 semantics. This example runs the exact backend with custom bootstrap sampling, criterion, and tree constraints:

forest = BankaiRandomForestClassifier(
    n_estimators=150, max_depth=8, max_bins=None,
    extra_trees=True, extra_trees_backend="exact", importance_type="gain",
    criterion="entropy", min_samples_split=4, min_samples_leaf=2,
    max_leaf_nodes=128, max_samples=0.8, bootstrap=True,
    n_jobs=4, max_features=0.15,
)
selector = BankaiBoruta(forest, n_estimators=150, random_state=42)
selector.fit(X_train.astype("float32"), y_train)
assert selector.diagnostics_["path"] == "rust"

Worker and feature controls use the pinned forest's validation and resolution rules: n_jobs=None means one worker, positive values specify the worker count, and negative values resolve relative to the CPU count. max_features accepts "sqrt", "log2", a positive integer, a fraction in (0, 1], or None. Feature counts resolve against each round's current real-plus-shadow width, including after Classic rejects columns, and fractional counts are floored with a minimum of one. Integer counts must fit that round's width. The resolved worker count controls native preparation and forest training; tree parallelism is capped by the number of trees.

The Rust path also forwards criterion, min_samples_split, min_samples_leaf, min_weight_fraction_leaf, min_impurity_decrease, max_leaf_nodes, bootstrap, max_samples, and the binned backend's binning_strategy, bin_sample_size, and binning_seed. Parameters that are not represented by this native boundary (including sample/class weights, warm_start, OOB scoring, and monotonic constraints) use the Python adapter. To benchmark the materialized Python adapter with the same forest, set importance_getter="feature_importances_" and assert selector.diagnostics_["path"] == "python". Native shadow access avoids the Python augmented matrix; the default exact forest still materializes a float32 real-plus-shadow matrix each round. The opt-in a7 session removes that internal matrix. Native exact fits retain float32; public float64 exact fits preserve the float64 adapter. Native binned inputs retain float32 or float64. Other inputs and adapter inputs use float64. Explicit permutations use uint32 through 2**32 rows, otherwise int64 within platform addressing limits. Binning and training statistics remain unchanged.

fit(X, y, sample_weight=None) accepts non-negative sample weights with positive total weight. Classic selection counts hits and uses binomial tests; its final median comparison marks surviving tentative features as weak. The fitted selector exposes:

The selector supports scikit-learn pipelines and cloning. A fresh clone is used for each round when a supplied classifier enables warm_start, preventing trees from retaining previous shadows. estimator_ contains cloned configuration on the Rust path and the last round's fitted classifier on the adapter path.

BankaiRF backend and performance foundations

The backend combines several sources of computational efficiency:

The scientific papers and author article are linked under References. The LightGBM reference describes binning and histogram principles; the a15 implementation determines the actual forest storage and split behavior. The default binned Extra Trees path uses its own column-bin statistics rather than the global histogram caches of BankaiRF's ordinary Random Forest path.

Rust execution is selected automatically for supported exact or binned Extra Trees gain configurations. Supported forest settings are forwarded into the native fit; unsupported settings, weights, monotonic constraints and custom getters use the Python adapter, preserving estimator parameters and forest validation. Shadow monotonic constraints are always zero. diagnostics_ records the path and its reason.

The a8 release aligns Python and Rust dependencies to BankaiRF 0.1.0a17; the immutable Rust/development Python revision is 9a9a55af107e8cfd9f5aad8dc2c135553ce2531f. RF a17 must be published first. Native fits use a shadow session by default: one canonical bank per fit (float32 values for exact forests, one-byte bin codes for binned Extra Trees) plus shuffled shadow columns, with no augmented float matrix or permutation maps. Exact Extra Trees, exact Random Forests and float32 binned Extra Trees share it. BANKAI_BORUTA_EXACT_REFERENCE=1 selects the a7 materialized path and BANKAI_BORUTA_EXACT_SESSION=1 the a7 virtual session. Independent fits never share sessions. diagnostics_ also reports input_dtype (received dtype), native_input_dtype (validated dtype on the native path, otherwise None), input_conversion_bytes (size of the converted input buffer, zero for a shared NumPy input), permutation_dtype, and permutation_bytes (largest per-round size of permutations plus the reusable row-index buffer during the fit). These are explicit buffer sizes, not RSS or total allocations. The existing copied_bytes remains cumulative augmented float-matrix writes across rounds; it excludes input conversions and permutations. For a 100,000 × 500 float32 array, a5 removes 400,000,000 input-conversion bytes and 200,400,000 permutation bytes versus a4: approximately 572.6 MiB. RSS and timings require measurement. Only local results are covered by the a5 report; the four-profile Linux campaign remains pending.

copied_bytes=0 covers only Python augmented-matrix writes. Internal exact materializations, bin/layout buffers and caches remain part of preparation and measured RSS. The new diagnostics distinguish live buffer capacity from cumulative copy volume; none is a total RSS estimate.

See the reproducible validation report for Rust/adapter importance parity, reference-algorithm comparisons, timing, peak memory, recall and false positives. The measurements describe specific datasets and seeds; actual speed and selection quality depend on the workload and configuration.

Migration from a2

estimator is now the first argument. Configure forest parameters on that classifier instead of the removed selector arguments forest_backend, pruning, importance, importance_scale, input_mode, max_features, max_depth, n_jobs, and max_bins. greedy=True enables hit-once selection; the former aggressive pruning rule is a different algorithm. The bankai-rf, polars, and arrow-test extras were removed. This release supports NumPy and pandas; Polars and Arrow integration is outside its scope.

Greedy mode

selector = BankaiBoruta(
    greedy=True,
    bonferroni=True,
    alpha=0.05,
    random_state=2077,
)
X_selected = selector.fit_transform(X_train, y_train)
print(f"Rounds: {selector.n_iter_}; effective budget: {selector.max_iter_}")

Greedy retains every original feature throughout the campaign. With p original features, it runs K = ceil(log2(p / alpha)) rounds when bonferroni=True, or K = ceil(log2(1 / alpha)) otherwise. The exact floating-point ratio is used to compute the ceiling safely at power-of-two boundaries and very small alpha.

A feature is confirmed after at least one importance above the shadow percentile. Features with no hits are rejected at completion. support_weak_ is always false and ranking_ uses 1 for confirmed and 2 for rejected features. max_iter, two_step, early_stopping, and n_iter_no_change do not control this mode. Greedy relaxes confirmation and can select more noise; the validation report measures that tradeoff without assuming a universal recall advantage.

Development

Local builds need Git, Python 3.11+, stable Rust, and uv. Install Xcode command-line tools on macOS or the Rust MSVC build tools on Windows.

git clone https://github.com/breno-jesus-fernandes/bankai-boruta.git
cd bankai-boruta
git switch perf/exact-session-a7
uv python install 3.11
uv sync --locked --no-install-project --extra test
uv run --no-sync maturin develop --release --locked

Run the Python and Rust checks, then reproduce the benchmark campaign as needed:

uv run --no-sync pytest -q
uv run --no-sync cargo test --workspace --locked
uv run --no-sync python benchmarks/validate_a3.py
uv run --no-sync python benchmarks/summarize_a3.py

Both lockfiles are committed. CI tests Python 3.11–3.14 and builds ABI3 wheels for Linux x86_64/ARM64, macOS x86_64/ARM64 and Windows x86_64, with installed-wheel smoke tests. The scheduled BankaiRF release check reports upstream versions; upgrades must align both dependency pins and pass compatibility/performance checks.

Read the development guide, contribution guide, and Code of Conduct.

References

1. Nicolas Vana (2025), The Greedy Boruta Algorithm: Faster Feature Selection Without Sacrificing Recall, author article in Towards Data Science. The published GreedyBoruta 1.0.0 implementation supplies the fixed-round, hit-once reference used here.
2. Pierre Geurts, Damien Ernst and Louis Wehenkel (2006), Extremely randomized trees, Machine Learning 63, 3–42. Author preprint and publication record.
3. Guolin Ke et al. (2017), LightGBM: A Highly Efficient Gradient Boosting Decision Tree, NeurIPS 30, particularly Section 2.1 on histogram-based learning. Paper PDF and official histogram documentation.
4. Miron B. Kursa and Witold R. Rudnicki (2010), Feature Selection with the Boruta Package, Journal of Statistical Software 36(11), 1–13. BorutaPy implementation.

License

Bankai Boruta is distributed under GPL-3.0-or-later. Its BankaiRF Rust backend includes the maintained GPL-3.0-or-later XRF fork.