Overview

The stepwise crate is a zero-dependency helper library for writing iterative algorithms.
It supports both numeric techniques—such as gradient descent, fixed-point iteration, Newton–Raphson—and non-numeric stepwise processes, like line-by-line file parsing or tree search.

With stepwise, you can:

• Use the Driver executor in a functional, iterator-like style to configure:
  • iteration limits, timeouts, convergence criteria,
  • logging, and
  • terminal progress bars.
• Control early stopping using metrics that compute moving averages, absolute/relative differences, invocation counts, or detect stalled solvers.
• Use included mini linear algebra functions for computing norms and distances.
• Collect structured data using samplers for ML training, convergence analysis, or plotting.
• Benchmark your own solvers on a suite of problems, and compare them with the built-in algorithms.

Links

driver | algorithms | metrics | linear algebra functions | samplers | problems

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Example 1

use stepwise::{Driver as _, fixed_iters, assert_approx_eq}; 
use stepwise::algos::GradientDescent;

let sphere_gradient = |x: &[f64]| vec![2.0 * x[0], 2.0 * x[1]];
let learning_rate = 0.1;
let initial_guess = vec![5.5, 6.5];

let algo = GradientDescent::new(learning_rate, initial_guess, sphere_gradient);

let (solved, step) = fixed_iters(algo, 1000).solve().expect("solving failed!");

assert_approx_eq!(solved.x(), &[0.0, 0.0]);

Example 2

use std::time::Duration;
use stepwise::prelude::*;

let gd = algos::GradientDescent::new(0.01, vec![1.0, 2.0], problems::sphere_grad);

let mut avg_of_norm = metrics::Ema::with_window(10);

let (solved, step) = fail_after_iters(gd, 10_000)
    .converge_when(|algo, _step| avg_of_norm.observe(algo.gradient.norm_l2()) < 1e-10)
    .show_progress_bar_after(Duration::from_millis(20))
    .on_step(|algo, _step| algo.learning_rate *= 0.9999)
    .solve()
    .expect("solving failed!");

assert_approx_eq!(solved.x(), &[0.0, 0.0]);
assert_eq!(step.iteration(), 1301);

In Example 2 (above):

• Early stopping is used to terminate the algorithm
• An exponential moving average of gradient's l2-norm is used to test it is near zero.
• A progress bar is also printed in the terminal if algorithm runs for more than 20ms.
• The stepwise prelude is used to import both the metrics and the linear algebra trait
• A decaying learning rate is applied after each step.

Creating an Algorithm

Callers will typically interact with your algorithm through the Driver trait.
However, to integrate with stepwise, you must implement the core Algo trait for your algorithm.

This involves defining a single method:

• step(): returns a tuple of:
  • a ControlFlow, indicating whether iteration should continue or stop
  • a Result, capturing any error that may occur during a step

This design separates control flow from failure, supporting fallible solvers with clean early-stopping logic.

Note: accessor methods like x() (for current state) are not part of the trait, but are expected in usable solvers, in order to retrieve the solution after or during iteration.

use std::{error::Error, ops::ControlFlow};

pub trait Algo {
    type Error: Send + Sync + Error + 'static;

    fn step(&mut self) -> (ControlFlow<()>, Result<(), Self::Error>);

    // <default methods omitted>
}

Links

• Changelog
• Motivation
• Related crate: stochy