quad_rs/

lib.rs

//! Adaptive real and complex numerical integration.
//!
//! `quad-rs` provides adaptive Gauss–Kronrod quadrature for real intervals,
//! complex contours, and user-defined parameterised integration paths.
//!
//! The crate is designed around three core ideas:
//!
//! - integrands are ordinary Rust types implementing [`Integrable`],
//! - integration domains are represented by finite contour pieces,
//! - adaptive refinement is driven by local Gauss–Kronrod error estimates.
//!
//! # Features
//!
//! - Real-valued integration over finite intervals.
//! - Complex contour integration.
//! - Piecewise-linear contours.
//! - Circular arcs and closed half-disk contours.
//! - Local contour indentation around poles.
//! - Scalar, complex, vector, matrix, and array-valued outputs via
//!   [`IntegrationOutput`].
//! - Optional storage of quadrature samples for diagnostics and plotting.
//!
//! # Real integration
//!
//! ```
//! use quad_rs::{integrate_real, Integrable, IntegratorConfig};
//!
//! struct Gaussian;
//!
//! impl Integrable for Gaussian {
//!     type Float = f64;
//!     type Input = f64;
//!     type Output = f64;
//!
//!     fn integrand(&self, x: &f64) -> f64 {
//!         (-x * x).exp()
//!     }
//! }
//!
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! let result = integrate_real(
//!     Gaussian,
//!     vec![-4.0, 4.0],
//!     IntegratorConfig::default(),
//! )?;
//!
//! println!("integral = {}", result.integral);
//! println!("error    = {}", result.error);
//! # Ok(())
//! # }
//! ```
//!
//! # Complex contour integration
//!
//! ```
//! use num_complex::Complex;
//! use quad_rs::{integrate_complex, Contour, Integrable, IntegratorConfig};
//!
//! struct InverseZ;
//!
//! impl Integrable for InverseZ {
//!     type Float = f64;
//!     type Input = Complex<f64>;
//!     type Output = Complex<f64>;
//!
//!     fn integrand(&self, z: &Complex<f64>) -> Complex<f64> {
//!         Complex::new(1.0, 0.0) / *z
//!     }
//! }
//!
//! # fn main() -> Result<(), Box<dyn std::error::Error>> {
//! let contour = Contour::upper_half_disk_offset(1.0, 1e-5);
//!
//! let result = integrate_complex(
//!     InverseZ,
//!     contour,
//!     IntegratorConfig::default(),
//! )?;
//!
//! println!("integral = {}", result.integral);
//! # Ok(())
//! # }
//! ```
//!
//! # Contour deformation
//!
//! Known poles can be avoided by locally replacing part of a line segment with
//! a small circular indentation.
//!
//! ```
//! use num_complex::Complex;
//! use quad_rs::{Contour, IndentSide};
//!
//! let contour = Contour::real_axis(5.0)
//!     .indent(
//!         Complex::new(0.0, 0.0),
//!         1e-3,
//!         IndentSide::Left,
//!         1e-10,
//!     );
//! ```
//!
//! This is useful for Cauchy principal values, Green's functions, residue
//! calculations, and `i0⁺`/`i0⁻` prescriptions.
//!
//! # Configuration
//!
//! [`IntegratorConfig`] controls tolerances, quadrature order, error reduction,
//! singularity handling, and whether quadrature samples are stored.
//!
//! ```
//! use quad_rs::{ErrorNorm, IntegratorConfig};
//!
//! let config = IntegratorConfig::default()
//!     .with_absolute_tolerance(1e-10)
//!     .with_relative_tolerance(1e-10)
//!     .with_error_norm(ErrorNorm::Max)
//!     .store_segment_data();
//! ```
//!
//! # Infinite and oscillatory integrals
//!
//! The current algorithms operate on finite contour pieces.
//!
//! Infinite-domain integrals should be handled by truncating the domain,
//! providing a custom parameterised contour piece, or using problem-specific
//! transformations. Highly oscillatory integrals may require manual splitting
//! at known periods or specialized quadrature strategies.
//!
//! # Examples
//!
//! The `examples/` directory includes demonstrations of:
//!
//! - Gaussian quadrature over a real interval,
//! - vector-valued integration,
//! - Fresnel-type oscillatory integrals,
//! - Cauchy's integral formula,
//! - residue-theorem calculations,
//! - indented pole contours,
//! - Sommerfeld-style branch-point integrals,
//! - Bromwich inversion,
//! - half-disk Fourier contours.
//!
//! # Crate structure
//!
//! Most users only need:
//!
//! - [`integrate_real`],
//! - [`integrate_complex`],
//! - [`IntegratorConfig`],
//! - [`Integrable`],
//! - [`Contour`] and related contour constructors.
//!
//! Lower-level types such as segment heaps and Gauss–Kronrod internals are
//! implementation details.

#![allow(dead_code)]
#![allow(clippy::type_complexity)]
#[warn(clippy::all)]
#[warn(missing_docs)]
mod config;
mod contour;
mod core;
mod integrable;
mod output;
mod solve;
mod state;
mod storage;

pub use config::IntegratorConfig;
pub use contour::{CircularArc, Contour, ContourSegment, IndentSide, LineSegment};
pub use core::IntegratorError;
pub use integrable::{ComplexScalar, Integrable, IntegrableFloat};
pub use output::{ErrorNorm, IntegrationOutput};

pub(crate) use state::IntegrationSummary;

pub(crate) use contour::ContourPiece;
use solve::Integrator;
pub(crate) use state::IntegrationState;
pub(crate) use storage::SegmentHeap;

use nalgebra::ComplexField;
use std::ops::Range;
use trellis_runner::{
    AbsoluteTolerancePolicy, EngineFailure, GenerateBuilderFallible, RelativeTolerancePolicy,
    RunSummary, Termination,
};

pub struct IntegrationResult<I, O, F> {
    pub integral: O,
    pub error: F,
    pub evaluations: usize,
    pub refinements: usize,
    pub termination: Termination,
    pub summary: RunSummary<F>,
    pub samples: Option<crate::core::QuadratureSamples<I, O>>,
}

impl<I, O, F> IntegrationResult<I, O, F> {
    fn from_parts(
        result: IntegrationSummary<I, O, F>,
        summary: RunSummary<F>,
        termination: Termination,
    ) -> Self {
        Self {
            integral: result.integral,
            error: result.error,
            evaluations: result.evaluations,
            refinements: result.refinements,
            termination,
            summary,
            samples: result.samples,
        }
    }
}

pub fn integrate_complex<F, P>(
    problem: P,
    contour: Contour<F>,
    config: IntegratorConfig<F>,
) -> Result<IntegrationResult<P::Input, P::Output, F>, IntegratorError<P::Input>>
where
    F: IntegrableFloat + ComplexScalar,
    P: Integrable<Float = F, Input = <F as ComplexScalar>::Complex>,
    <P as Integrable>::Output: IntegrationOutput<P::Input, Float = F>,
{
    let contour = config.deform_contour(contour);

    let integrator = Integrator::complex_contour(contour, &config);

    integrator
        .build_for(problem)
        .with_initial_state(IntegrationState::new())
        .and_policy(AbsoluteTolerancePolicy::new(
            config.absolute_tolerance,
            config.tolerance_window,
        ))
        .and_policy(RelativeTolerancePolicy::new(
            config.relative_tolerance,
            config.tolerance_window,
        ))
        .finalise()
        .run()
        .map(|output| {
            IntegrationResult::from_parts(output.result, output.summary, output.termination)
        })
        .map_err(|EngineFailure::Procedure { error, state: _ }| error)
}

pub fn integrate_interval<F, P>(
    problem: P,
    interval: Range<F>,
    config: IntegratorConfig<F>,
) -> Result<IntegrationResult<F, P::Output, F>, IntegratorError<F>>
where
    F: IntegrableFloat + ComplexField<RealField = F>,
    P: Integrable<Float = F, Input = F>,
    <P as Integrable>::Output: IntegrationOutput<P::Input, Float = F>,
{
    integrate_real(problem, vec![interval.start, interval.end], config)
}

pub fn integrate_real<F, P>(
    problem: P,
    points: Vec<F>,
    config: IntegratorConfig<F>,
) -> Result<IntegrationResult<F, P::Output, F>, IntegratorError<F>>
where
    F: IntegrableFloat + ComplexField<RealField = F>,
    P: Integrable<Float = F, Input = F>,
    <P as Integrable>::Output: IntegrationOutput<P::Input, Float = F>,
{
    let integrator = Integrator::real_piecewise_linear(points, &config);

    integrator
        .build_for(problem)
        .with_initial_state(IntegrationState::new())
        .and_policy(AbsoluteTolerancePolicy::new(
            config.absolute_tolerance,
            config.tolerance_window,
        ))
        .and_policy(RelativeTolerancePolicy::new(
            config.relative_tolerance,
            config.tolerance_window,
        ))
        .finalise()
        .run()
        .map(|output| {
            IntegrationResult::from_parts(output.result, output.summary, output.termination)
        })
        .map_err(|EngineFailure::Procedure { error, state: _ }| error)
}