Metadata-Version: 2.4
Name: pybiglasso
Version: 0.4.0
Classifier: Intended Audience :: Science/Research
Classifier: Programming Language :: Python :: 3
Classifier: Programming Language :: Python :: 3.11
Classifier: Programming Language :: Python :: 3.12
Classifier: Programming Language :: Python :: 3.13
Classifier: Programming Language :: Python :: 3.14
Classifier: Programming Language :: Python :: 3.15
Classifier: Topic :: Scientific/Engineering :: Mathematics
Requires-Dist: numpy
Requires-Dist: scipy
Requires-Dist: pybiglasso[polars,pandas,sklearn,viz] ; extra == 'all'
Requires-Dist: pandas ; extra == 'pandas'
Requires-Dist: polars ; extra == 'polars'
Requires-Dist: scikit-learn>=1.6 ; extra == 'sklearn'
Requires-Dist: seaborn>=0.13 ; extra == 'viz'
Provides-Extra: all
Provides-Extra: pandas
Provides-Extra: polars
Provides-Extra: sklearn
Provides-Extra: viz
License-File: LICENSE
Summary: Unofficial Python port of R's biglasso: lasso and elastic-net paths for larger-than-memory data.
Keywords: lasso,elastic-net,glmnet,biglasso,out-of-core,larger-than-memory,memmap,logistic-regression,polygenic-risk-score
Author-email: Mike Macpherson <mmacpherson@users.noreply.github.com>
License-Expression: GPL-3.0-or-later
Requires-Python: >=3.11
Description-Content-Type: text/markdown; charset=UTF-8; variant=GFM
Project-URL: Homepage, https://github.com/mmacpherson/pybiglasso
Project-URL: R biglasso, https://github.com/pbreheny/biglasso

# pybiglasso

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**Lasso and elastic-net regression for data larger than memory.**

If your penalized regression won't fit in memory, pybiglasso fits it where the
data already is: on disk. A 12 GB design matrix fits under a 3 GB memory limit,
on an ordinary desktop, in under two minutes (see [Performance](#performance)).

Save the matrix as a numpy memmap and pybiglasso fits the whole regularization
path, with cross-validation to choose the penalty, without loading it into
memory.

To try it without installing anything, open the
[playground](https://mmacpherson.github.io/pybiglasso/). It runs the same Rust
solver, compiled to WebAssembly, in your browser, on example datasets or your
own CSV or Parquet file, which never leaves your machine, and can
cross-validate the fit.

pybiglasso fits least-squares (Gaussian) and logistic (binomial) models, and
it's also fast when the data does fit in memory. For least squares, it is 2.2x
faster than R's glmnet on wide data and 2x slower on tall data. For logistic
regression, on a full 100-step lasso path over 5,000 samples by 50,000
features, it is 2.4x faster than glmnet, 2.5x faster than R biglasso, and 5.2x
faster than skglm, the fastest of the other Python solvers we tested; across
the four benchmark problems, with 1,000 to 50,000 features, it runs 2.1–4.7x
faster than glmnet, 2.5–9x faster than R biglasso, and 5–9x faster than skglm
(see [Performance](#performance)).

It is an unofficial Python port of the R package
[biglasso](https://github.com/pbreheny/biglasso), and reproduces its results to
floating-point precision for both families (see [Validation](#validation)).

## Install

```
pip install pybiglasso
```

The core depends only on numpy and scipy; the solver is compiled Rust, in the
wheel. Extras add optional integrations:

| extra | adds |
|---|---|
| `pybiglasso[polars]` | `BiglassoPath.to_polars()` |
| `pybiglasso[pandas]` | `BiglassoPath.to_pandas()` |
| `pybiglasso[sklearn]` | `BiglassoRegressor`, `BiglassoRegressorCV`, `BiglassoClassifier`, `BiglassoClassifierCV` |
| `pybiglasso[viz]` | `plot_coefficients()`, `plot_deviance()` (seaborn.objects) |
| `pybiglasso[all]` | all of the above |

## Quick start

```python
import numpy as np
import pybiglasso as bl

# open the matrix on disk without loading it (a read-only memmap); store it
# column-major (Fortran order)
X = np.load("X.npy", mmap_mode="r")
y = np.load("y.npy")

path = bl.biglasso(X, y, alpha=1.0, nlambda=100)   # alpha=1: lasso
path
# BiglassoPath(500000 features, 100 lambdas from 0.112 to 0.000112, alpha=1.0)

path.coef                  # scipy.sparse (features x lambdas)
path.n_nonzero             # model size along the path
path.predict(X_new)        # (rows x lambdas); reads only the columns in use
path.score(X_val, y_val, metric="mse")

path.to_polars()           # lambda_, feature, coefficient (nonzero only)
```

To write such a file, open it as a column-major memmap and fill it a block of
columns at a time, so the whole matrix never has to be in memory:

```python
X = np.lib.format.open_memmap(
    "X.npy", mode="w+", dtype=np.float32,
    shape=(n_samples, n_features), fortran_order=True,
)
for start, block in read_blocks():
    X[:, start : start + block.shape[1]] = block
X.flush()
```

X can be float64, float32, any integer type, or bool (a byte per value, read as
0 or 1). Use the narrowest type that holds your values exactly: the fit is
mostly reading X, so a narrower type is faster as well as smaller (see
[storage](#threads-cross-validation-and-storage)).

Unpenalized covariates (say age, sex, and principal components) take a penalty
factor of 0:

```python
pf = np.ones(X.shape[1])
pf[:12] = 0
path = bl.biglasso(X, y, penalty_factor=pf)
```

### Least squares and logistic regression

`family` chooses the model. Least squares (`"gaussian"`) is the default, as in
R biglasso; for a 0/1 response, pass `family="binomial"`:

```python
model = bl.biglasso(X, y_binary, family="binomial")
model.predict_proba(X_new)       # (rows x lambdas)
model.score(X_val, y_val, metric="auc")
```

The two families share one solver, in Python and in the browser playground,
and the same path options. A least-squares path predicts the response,
reports the residual sum of squares as `deviance`, and scores by
`"deviance"`, `"mse"`, or `"r2"`. A logistic path predicts classes, with
log-odds from `decision_function` and probabilities from `predict_proba`,
reports the binomial deviance, and scores by `"deviance"`, `"auc"`, or
`"accuracy"`.

### scikit-learn

The scikit-learn interface has an estimator for each family:
`BiglassoRegressor` and `BiglassoClassifier` fit at one penalty strength, and
`BiglassoRegressorCV` and `BiglassoClassifierCV` choose it by
cross-validation. All four pass scikit-learn's estimator checks and work in
pipelines and grid searches (install `pybiglasso[sklearn]`).

```python
from pybiglasso import BiglassoRegressorCV, BiglassoClassifierCV

reg = BiglassoRegressorCV(l1_ratio=1.0, cv=5).fit(X, y)
clf = BiglassoClassifierCV(l1_ratio=1.0, cv=5, rule="1se").fit(X, y_binary)
clf.alpha_, clf.coef_
```

The estimators use scikit-learn's parameter names, which differ from R's:

| scikit-learn | R / `pybiglasso.biglasso` | meaning |
|---|---|---|
| `alpha` | `lambda` | penalty strength |
| `l1_ratio` | `alpha` | L1/L2 mix; 1 is the lasso |

## Why this exists

At 23andMe, I used R's biglasso package to fit penalized models on large
genotype/phenotype datasets without loading them into memory. Over a holiday
break, I ported it to Python, using numba for speed, partly to gain a better
understanding of how it worked and partly because, assuming I could get it
working, it would be convenient to stay within the Python ecosystem. I validated
it against R biglasso, matching its coefficients, lambda grid, and iteration
counts to floating-point precision, and benchmarked it on data both smaller and
larger than memory.

In 2026 I revived the port for release, working with coding agents (Claude Code
and Codex). That work added least-squares regression, the scikit-learn
estimators and cross-validation, extended the validation (against R across many
more configurations, and with checks independent of R, which turned up the
differences listed below), rewrote the solver in Rust, and carried the
performance work and benchmarks further.

This is an independent port, not affiliated with or endorsed by the biglasso
authors: Yaohui Zeng, Chuyi Wang, Tabitha Peter, and Patrick Breheny. If you use
it, please cite their paper (below).

## Validation

pybiglasso is checked two ways: against R biglasso, which it ports, and
against checks that don't depend on R, which also cover the few places where
the two differ (see [Differences](#differences-from-r-biglasso)).

**Against R.** Every push runs pybiglasso against stored R biglasso 1.7.2
output for 192 configurations (40 least-squares and 152 logistic), comparing
coefficients, deviance, the lambda grid, centering and scaling, strong-rule
rejections, and the iteration count at
every lambda, with `rtol=1e-9`. The grid covers wide and tall problems,
genotype-like dosage data (including int8 and uint8 storage), memmaps, float32,
row subsets, penalty factors (including zeros), user-supplied and linear lambda
grids, `dfmax` truncation, and saturation. The comparison runs pybiglasso with
`r_compatible=True`; for cases with penalty factors it passes R pybiglasso's
lambda grid, which sets aside the zero-penalty-factor case under Differences.
A weekly job reruns the grid live against the current CRAN release, to catch
upstream changes.

**Independent of R.**

- **Optimality.** Converged test paths satisfy the elastic-net optimality
  (KKT) conditions to 1e-6, on wide, correlated, partly unpenalized, and genotype-like
  designs, for both families; duality gaps, some in exact arithmetic, and
  curvature bounds certify how close the coefficients are to the optimum.
- **Reference solutions.** Small problems are solved without pybiglasso, from
  closed forms (orthogonal designs, single features) or by enumerating every
  active set, and the fits are held to those solutions within derived
  numerical bounds.
- **Other solvers.** R's glmnet minimizes the same objective; its paths are
  stored from a pinned release and checked at every lambda, for least squares
  and for logistic fits, including rare-response fits whose log-odds exceed
  R biglasso's cutoff of 10. For logistic regression, scikit-learn's saga
  solver, tightly converged, finds the same coefficients as well: each pair
  passes the optimality conditions, and then their distance is certified, to
  about 1e-7 on tall data and, through the duality gap, to about 1e-5 on wide
  data (all but the wide lasso, whose optimum need not be unique). In the
  benchmarks, glum reaches the same elastic-net objective too.
- **Where R differs.** R's elastic-net update (reproduced by
  `r_compatible=True`) doesn't satisfy the optimality conditions, and its
  coefficients differ from saga's by 2–50% of their size on the test problem.
  This is the basis for the elastic-net difference under Differences.
- **Shifted data.** Adding 1e8 to every column of X leaves the solution
  unchanged and still optimal; this is the case described under Differences
  as data far from zero.

```
uv run pytest               # against stored R output; no R needed
make test-r                 # live, in a container (CONTAINER=podman works too)
make test-r R_VERSION=latest
make soak                   # larger memory-mapped problems; for a big machine
make soak-hypothesis        # property tests, wide random search (part of soak)
```

## Performance

These benchmarks measure speed; correctness is established separately, against
R biglasso and by checks independent of it (see [Validation](#validation)). In
memory, every solver is given the same problem and the same 100-value lambda
path, at its own default tolerance, and its accuracy is reported alongside its
time. All ran on an AMD Ryzen 5 5600X (6 cores, 12 threads) with 64 GB of RAM,
Python 3.14, and R 4.6.1. The scripts and raw results are in
[`benchmarks/`](benchmarks/).

### Least squares

pybiglasso, R biglasso, and glmnet fit the same 100 lambdas, each standardizing
raw X itself. Times are the median of three runs, each in a fresh process, with
R's BLAS limited to one thread, and both thread counts are shown. glmnet's fast
Gaussian path rescales the response by its standard deviation sᵧ, so it was
given α′ = α / (α + (1−α)sᵧ) and λ′ = λ(α + (1−α)sᵧ), which solve the same
standardized objective (for the lasso, α and λ are unchanged); glmnet's generic
`gaussian()` family, which doesn't rescale, serves as a correctness reference
in the tests rather than a timed competitor. pybiglasso's time is `biglasso()`
itself; the logistic table below also includes exporting the coefficients.
Times are in seconds:

| Dataset | Penalty | pybiglasso, 1 thread | pybiglasso, 6 threads | R biglasso, 1 thread | R biglasso, 6 threads | Native glmnet, 1 thread |
|---|---|---|---|---|---|---|
| 5,000 x 1,000 | lasso | 0.52 | 0.47 | 0.90 | 0.70 | 0.25 |
| 5,000 x 1,000 | elastic net (α=0.5) | 0.55 | 0.51 | 0.94 | 0.73 | 0.27 |
| 2,000 x 20,000 | lasso | 0.92 | 0.76 | 2.80 | 0.92 | 1.67 |
| 2,000 x 20,000 | elastic net (α=0.5) | 0.95 | 0.75 | 2.84 | 0.94 | 1.70 |

Dropping generated constant columns leaves 973 and 19,199 features. In each
row's final repetition, every path reached all 100 lambdas without warnings;
the largest optimality (KKT) residual, in response units, is about 1.0e-4 for
pybiglasso and R biglasso and 1.1e-4 for glmnet, and the largest relative
objective difference from glmnet is 1.9e-6 (a pairwise difference, not a
certified suboptimality bound). To regenerate the table:

```bash
make r-image R_VERSION=4.6.1  # once
benchmarks/run-gaussian.sh
python benchmarks/gaussian-table.py benchmarks/results/path-gaussian.jsonl
```

### Logistic regression

Each time is the median of 3 runs, or a single run for a solver slow enough to
take over a third of its time limit. Every multithreaded solver is shown at its
fastest thread count: 1, 6, or 12.

![Time relative to pybiglasso, by dataset and solver](https://raw.githubusercontent.com/mmacpherson/pybiglasso/v0.2.0/docs/benchmarks.png)

**Which solvers.** R biglasso, the original; R's glmnet, whose coordinate
descent biglasso builds on; and every maintained Python package, installable
with pip, that fits L1 or elastic-net logistic regression on a CPU: scikit-learn
(its liblinear and saga solvers), glum, skglm, and statsmodels.

**Other Python solvers.** Each competitor was stopped once it took 10x as long
as pybiglasso, which would otherwise have stretched the run to many hours. (The
cap was set by an earlier, slower pybiglasso; measured against the one timed
here, the caps fall at 11–25x, and glum's one finish, the 50,000 x 2,000
elastic net on 12 threads, is at 18x.) Only skglm finishes within that on every
dataset, at 5–7x for the lasso and 6–9x for the elastic net. statsmodels (lasso
only) ran only on the tall datasets: its L1 fit uses a general-purpose
optimizer with dense matrices over twice the columns, which would need about 12
GB and 76 GB on the wide ones. To show how large the gaps get, the smallest
problem (5,000 x 1,000) also ran with no cap:

| Solver | Lasso | Elastic net (α=0.5) | Worst relative suboptimality |
|---|---|---|---|
| pybiglasso | 0.60 s | 0.60 s | 1e-6 |
| pybiglasso, `eps=1e-10` | 1.37 s | 1.34 s | 3e-10 |
| glmnet 5.1 (R) | 1.4 s (2.3x) | 1.4 s (2.4x) | 2e-6 |
| skglm | 4.1 s (7x) | 4.4 s (7x) | 2e-4 |
| glum, 6 threads | 9.6 s (16x) | 10.9 s (18x) | 6e-9 |
| sklearn liblinear | 20.5 s (34x) | — | 5e-6 |
| sklearn saga | 90.7 s (151x) | 94.8 s (157x) | 1e-5 |
| statsmodels | over 900 s (over 1,000x) | — | — |

**R biglasso 1.7.2.** Times are in seconds; "best" is each package at its
fastest thread count. For the lasso, both packages fit identical paths. For the
elastic net, each ran its own update (see
[Differences](#differences-from-r-biglasso)); pybiglasso with
`r_compatible=True`, which matches R, takes about as long as its default.

| Dataset | Penalty | pybiglasso, 1 thread | R, 1 thread | pybiglasso, best | R, best | Speedup (best) |
|---|---|---|---|---|---|---|
| 5,000 x 1,000 | lasso | 0.62 | 4.06 | 0.60 | 3.92 | 6.5x |
| 5,000 x 1,000 | elastic net | 0.63 | 4.17 | 0.60 | 4.04 | 6.7x |
| 50,000 x 2,000 | lasso | 9.31 | 55.7 | 6.23 | 54.1 | 8.7x |
| 50,000 x 2,000 | elastic net | 9.35 | 56.1 | 6.32 | 52.9 | 8.4x |
| 2,000 x 20,000 | lasso | 1.23 | 4.71 | 1.08 | 2.90 | 2.7x |
| 2,000 x 20,000 | elastic net | 1.31 | 4.80 | 1.08 | 2.99 | 2.8x |
| 5,000 x 50,000 | lasso | 7.45 | 27.2 | 6.29 | 15.5 | 2.5x |
| 5,000 x 50,000 | elastic net | 7.43 | 27.5 | 6.42 | 15.9 | 2.5x |

R's adaptive screening rule is no faster than SSR on these problems.

**glmnet 5.1.** glmnet standardized X itself (`standardize = TRUE`, its
default), as pybiglasso does, so both times include standardizing; the Python
solvers, which don't standardize, were given X already standardized. glmnet fit
with an intercept, along the same lambda grid, at its default tolerance; it runs
on one thread. Its objective is the benchmark's. It fit every path in full, at
2.1–4.7x pybiglasso's best time (1.8–3.1x pybiglasso's on one thread), and about as accurately: a worst relative suboptimality of 2e-8
to 3e-6, against pybiglasso's 6e-8 to 3e-6.

### Out of core

This test fits a 12 GB float64 matrix (20,000 x 75,000) with
memory capped at 3 GB. pybiglasso read a numpy memmap under a cgroup memory
limit; R read a file-backed `big.matrix` in a container with the same limit,
and both were measured from their cgroup's accounting. Before each run, the
matrix was evicted from the page cache. Both fit the same 20-lambda lasso path
and agree to 1e-9, with identical iteration counts. pybiglasso ran twice, and
both runs are shown; R ran once.

| | Fit time | Peak memory | Re-read from disk during the fit |
|---|---|---|---|
| pybiglasso, 1 thread | 111 s, 131 s | 3.2 GB (at the cap) | 7.9 GB, 10.9 GB |
| pybiglasso, 6 threads | 109 s, 151 s | 3.2 GB (at the cap) | 13.3 GB, 15.1 GB |
| R biglasso 1.7.2 | 148 s, plus 41 s to build the `big.matrix` | 3.2 GB (at the cap) | 10.6 GB |

Here the disk sets the pace, and times vary by a third from run to run. Extra
threads don't reliably help: they evict pages each other still need.

### Not compared

Some solvers fall outside the rule above:

- Other languages: Julia's GLMNet.jl, a wrapper of glmnet's earlier Fortran
  solver, and Lasso.jl, written in Julia.
- Other hardware: RAPIDS cuML fits on a GPU, and only data that fits in GPU
  memory.
- Unmaintained: python-glmnet, archived, with its last release in 2020.
- A different problem: celer's logistic regression fits the lasso only, and no
  intercept. skglm, from the same authors, fits an intercept and the elastic
  net, and is compared above.
- Not a solver: benchopt is a benchmarking framework; its logistic-regression
  benchmark fits without an intercept or standardization, which pybiglasso
  doesn't support yet.

## Differences from R biglasso

pybiglasso follows R biglasso 1.7.2 closely, matching it coefficient for
coefficient and down to its iteration and screening counts. It departs in a few
places, mostly where we think R's behavior may not be what was intended; if
we've erred we'd be glad to be corrected. Each departure is marked in the code
with a comment beginning "Differs from R biglasso".

**Both families.**

- **Zero penalty factors.** With a `penalty.factor` of 0 and the default
  lambda grid, R's grid comes out as `lambda = 0` at every step. pybiglasso
  instead fits the unpenalized columns first, and starts the path at the
  smallest lambda that keeps every penalized column at zero.
- **Default `lambda_min`.** R chooses 0.001 or 0.05 by comparing `nrow(X)` with
  `ncol(X)`; pybiglasso compares the number of rows actually fit, which differs
  when `row_indices` is given.
- **Data far from zero.** For columns whose mean is large next to their spread
  (say, values near 1e8 that vary by 1), standardizing from raw sums loses
  precision in floating point. R's scales, and so its fit, can be affected
  without a warning; in our tests some scales came out as NaN. pybiglasso
  computes these columns' moments from centered values, so a constant shift of
  X leaves the solution unchanged.
- **Invalid input.** NaN or infinite values in the rows being fit, and
  non-finite penalty factors or lambdas, raise `ValueError`; R instead drops
  the affected column or returns the null model.
- **Unrepresentable grids.** If an automatic grid overflows (for example,
  with extremely small positive `alpha`), R returns non-finite penalties, and
  pybiglasso raises `ValueError`, for both families and in both modes. Finite
  supplied grids remain supported, including in compatibility mode.

**Least squares.**

- **Constant responses.** R's strict convergence comparison against
  a zero null loss uses the full iteration budget for a constant response.
  pybiglasso stops once the coefficients no longer change;
  `r_compatible=True` retains R's comparison.
- **Zero maximum penalty.** R's logarithmic grid calculation can
  produce non-finite values when the maximum penalty is zero. pybiglasso
  returns a finite all-zero grid, whose fits solve the unpenalized problem.
- **Loss after an iteration limit.** R can leave `loss` at zero when
  the budget ends with outstanding KKT violations. pybiglasso reports the
  returned model's residual sum of squares; `r_compatible=True` retains R's
  bookkeeping.

**Logistic regression.**

- **Elastic-net update.** For `alpha < 1`, R's logistic coordinate update divides by
  `v * (1 + l2)`, where the elastic-net objective in its documentation calls
  for `v + l2`, so the two solve somewhat different problems. pybiglasso uses
  `v + l2`, as glmnet and scikit-learn do; `r_compatible=True` reproduces R
  exactly for this update.
- **Probability clipping.** R sets probabilities
  to 0 or 1 beyond `|eta| > 10`, with weights of `1e-4`. Default mode uses
  glmnet's `pmin = 1e-9` boundary, `|eta| > log(1 / pmin - 1)` (about 20.7).
  This reduces each clipped row's probability error from at most `4.6e-5` to
  `1e-9`. We retain a positive boundary weight of `pmin * (1 - pmin)` so the
  working residual `(y - p) / w` remains defined; glmnet can use zero weights.
  Default-mode coefficients, iterations and screening counts can change,
  for both lasso and elastic net. `r_compatible=True` retains R's cutoff and
  weights. Independent KKT checks against pinned glmnet fits cover the change.
- **Deviance.** `deviance` is the deviance, -2 log-likelihood, of the returned
  model. R reports `loss`, half the deviance, computed before the final
  coordinate sweep and with probabilities clipped at 0 and 1, so it can
  occasionally be infinite. `r_compatible=True` keeps R's version.
- **Saturation.** `stop_on_saturation=False`, an option R doesn't have, fits
  every lambda, even past R's saturation cutoff.

## Threads, cross-validation, and storage

pybiglasso reads the matrix one column at a time, through the memmap, and
never loads or copies it; strong-rule screening skips most columns at most
steps. Cross-validation folds are row indices into the same matrix on disk.

- **Threads help larger fits** (`n_threads=`). Coordinate descent updates
  one coefficient at a time, so the sweep itself is sequential; the screening
  and optimality scans run in parallel across columns, and on tall data the
  logistic fits' per-row IRLS step runs in parallel across rows. Small
  problems gain little, larger ones more: on the 50,000 x 2,000 logistic
  benchmark, the fastest thread count takes two-thirds the time of one (see [Performance](#performance)). Out of core,
  threads can help when memory to spare holds the page cache, by keeping more
  disk reads in flight; under a tight memory limit they hurt, so use
  `n_threads=1` there. Results are bit-identical at any thread count.
- **Cross-validation scales with cores.** `BiglassoRegressorCV(n_jobs=...)`
  and `BiglassoClassifierCV(n_jobs=...)` fit folds in parallel threads that
  share one X (and, for a memmap, one page cache); results don't depend on
  `n_jobs`.
- **Cross-validation can stop each fold early.** With
  `BiglassoClassifierCV(early_stopping=10)`, a fold's path stops once its
  held-out score has gone 10 alphas without improving, as bigstatsr's
  `big_spLogReg` does, and the fit on all the rows stops at the chosen alpha.
  On wide data, where the end of the path costs most, this saves much of the
  work; the cost is that a better score past the stopping point goes unseen.
  It is off by default.
- **Store X compactly.** Since the work is mostly moving bytes, a narrower
  dtype is faster: small integers (counts, categories, 0/1 indicators) as
  `int8` or `uint8` take 8x less memory and disk than float64, and fit
  1.3–1.6x faster in memory; `float32` halves the memory and is about as fast.
  Each value is read as a float64, so values that a narrower dtype holds
  exactly give bit-identical results.
- **Bounds checking is on by default.** Every index into an array is checked,
  which guards against a bug in pybiglasso reading past the end of X (inputs,
  `row_indices` among them, are validated before a fit either way).
  It costs little where X's rows are read in place: a whole Fortran-ordered X,
  or a subset of at least half its rows without repeats, as in
  cross-validation, which is read through a mask. Where every read goes
  through an index or a stride (C-ordered X, or a smaller or repeating
  `row_indices`), it costs up to about 15%. `PYBIGLASSO_BOUNDSCHECK=0` turns it off; the path is the same
  (lambdas, iterations, screening), with identical coefficients.

## How the code is organized

R biglasso implements each family's solver in a few long C++ functions; for
logistic regression, two:
`cdfit_binomial_ssr` standardizes, sets up the lambda grid, and runs the lambda
loop with its strong rule and KKT checks, and `cdfit_binomial_lambda_core` runs
the coordinate descent at each lambda. That keeps the algorithm in a few places,
close to how it's written in the papers.

pybiglasso splits the same algorithm into one small function per step, each
documenting the state it changes, plus a short driver, `fit_lambda`, that reads
like the algorithm's outline. To my eye that's easier to read, test, and modify.
It's a matter of taste, though; some excellent programmers prefer the long
inlined function you can read straight through (John Carmack has
[a good essay](https://web.archive.org/web/20261006034120/http://number-none.com/blow/john_carmack_on_inlined_code.html) on
why). The solver takes the same number of iterations as R's at every lambda; the
R comparison checks this in every case.

The solver is written twice, in the same steps. The one that runs is Rust
(`rust/src/solver.rs`): one call from Python fits the whole path. The readable one,
[`tests/support/reference.py`](tests/support/reference.py), is plain numpy,
step by step, and imports nothing from pybiglasso; the tests hold the Rust
solver to it, and both to R.

| pybiglasso (`rust/src/solver.rs`) | R biglasso (`src/`) |
|---|---|
| `center_scale`, `compute_z` | `standardize_and_get_residual` (utilities.cpp) |
| `update_irls` | probability and weight update in `cdfit_binomial_lambda_core` (binomial.cpp) |
| `update_intercept` | intercept update in `cdfit_binomial_lambda_core` |
| `coordinate_sweep` (`Pass`) | coordinate loop in `cdfit_binomial_lambda_core`; `wcrossprod_resid`, `wsqsum_bm`, `lasso` (utilities.cpp) |
| `check` | `check_strong_set`, `check_rest_set` (utilities.cpp) |
| `fit_lambda` | `cdfit_binomial_lambda_core` |
| `run` | `cdfit_binomial_ssr` |

The least-squares driver and coordinate sweeps, in `rust/src/gaussian.rs`, port
R's Gaussian SSR solver the same way. They share column access,
standardization, and the fixed-order sums with the logistic solver, and since
the curvature is constant they update residuals directly, without the IRLS
step. The numpy reference covers both families.

Every sum over rows runs in an order fixed by the data's shape alone. Threads
split the work by column, and a tall column's rows into 2,048-row chunks whose
sums are added in order. Within a chunk, eight running sums are combined in a
fixed order, so vector instructions of any width (AVX2, AVX-512, ARM's NEON)
give the same result, and no multiply and add are fused into one instruction,
which would round differently. Exponentials and logarithms come from a portable
implementation rather than the operating system's math library, whose last
bits differ between Linux, macOS, and Windows. So results are bit-identical at
any thread count, on any CPU, and on every operating system with a wheel.

## Scope

- Gaussian (least-squares) and binomial (logistic) families; Cox is not ported.
- SSR (sequential strong rule) screening only. R uses its newer Hybrid and
  Adaptive rules only for the plain lasso without penalty factors, and they
  were no faster than SSR on our benchmarks.

## Citation

> Zeng Y, Breheny P (2021). The biglasso Package: A Memory- and
> Computation-Efficient Solver for Lasso Model Fitting with Big Data in R.
> *The R Journal*, 12(2), 6–19.
> [doi:10.32614/RJ-2021-001](https://doi.org/10.32614/RJ-2021-001)

## License

GPL-3.0-or-later, as R biglasso is GPL-3.

