gauss-quad 0.3.1

// Copyright 2019-2024 Dominique Dresen
// Copyright 2023-2026 Johanna Sörngård
// SPDX-License-Identifier: MIT OR Apache-2.0

//! Numerical integration using a Simpson's rule.
//!
//! A popular quadrature rule (also known as Kepler's barrel rule). It can be derived
//! in the simplest case by replacing the integrand with a parabola that has the same
//! function values at the end points a & b, as well as the Simpson m=(a+b)/2, which
//! results in the integral formula
//! S(f) = (b-a)/6 * [ f(a) + 4f(m) + f(b) ]
//!
//! Dividing the interval \[a, b\] into N neighboring intervals of length h = (b-a)/N and
//! applying the Simpson rule to each subinterval, the integral is given by
//!
//! S(f) = h/6 * [ f(a) + f(b) + 2*Sum_{k=1..N-1} f(x_k) + 4*Sum_{k=1..N} f( (x_{k-1} + x_k)/2 )]
//!
//! with x_k = a + k*h.
//!
//! ```
//! use gauss_quad::simpson::Simpson;
//! use approx::assert_abs_diff_eq;
//!
//! use core::f64::consts::PI;
//!
//! let eps = 0.001;
//!
//! let quad = Simpson::new(10.try_into().unwrap());
//!
//! // integrate some functions
//! let integrate_euler = quad.integrate(0.0, 1.0, |x| x.exp());
//! assert_abs_diff_eq!(integrate_euler, 1.0_f64.exp() - 1.0, epsilon = eps);
//!
//! let integrate_sin = quad.integrate(-PI, PI, |x| x.sin());
//! assert_abs_diff_eq!(integrate_sin, 0.0, epsilon = eps);
//! ```

#[cfg(feature = "rayon")]
use rayon::prelude::{IntoParallelIterator, ParallelIterator};

use crate::__impl_node_rule;

use core::num::NonZeroU32;

/// A Simpson's rule.
///
/// ```
/// # use gauss_quad::simpson::Simpson;
/// // initialize a Simpson rule with 100 subintervals
/// let quad: Simpson = Simpson::new(100.try_into().unwrap());
///
/// // numerically integrate a function from -1.0 to 1.0 using the Simpson rule
/// let approx = quad.integrate(-1.0, 1.0, |x| x * x);
/// ```
#[derive(Debug, Clone, PartialEq)]
#[cfg_attr(feature = "serde", derive(serde::Serialize, serde::Deserialize))]
pub struct Simpson {
    degree: NonZeroU32,
}

impl Simpson {
    /// Initialize a new Simpson rule with `degree` being the number of intervals.
    pub const fn new(degree: NonZeroU32) -> Self {
        Self { degree }
    }

    /// Integrate over the domain [a, b].
    pub fn integrate<F>(&self, a: f64, b: f64, mut integrand: F) -> f64
    where
        F: FnMut(f64) -> f64,
    {
        let n = self.iter().count() as f64;

        let h = (b - a) / n;

        // first sum over the interval edges. Skips first index to sum 1..n-1
        let sum_over_interval_edges: f64 = self
            .iter()
            .skip(1)
            .map(|node| integrand(a + node * h))
            .sum();

        // sum over the midpoints f( (x_{k-1} + x_k)/2 ), as node N is not included,
        // add it in the final result
        let sum_over_midpoints: f64 = self
            .iter()
            .skip(1)
            .map(|node| integrand(a + (2.0 * node - 1.0) * h / 2.0))
            .sum();

        h / 6.0
            * (2.0 * sum_over_interval_edges
                + 4.0 * sum_over_midpoints
                + 4.0 * integrand(a + (2.0 * n - 1.0) * h / 2.0)
                + integrand(a)
                + integrand(b))
    }

    #[cfg(feature = "rayon")]
    /// Same as [`integrate`](Simpson::integrate) but runs in parallel.
    pub fn par_integrate<F>(&self, a: f64, b: f64, integrand: F) -> f64
    where
        F: Fn(f64) -> f64 + Sync,
    {
        let n = f64::from(self.degree().get());

        let h = (b - a) / n;

        let (sum_over_interval_edges, sum_over_midpoints): (f64, f64) = rayon::join(
            || {
                (1..self.degree().get())
                    .into_par_iter()
                    .map(|node| integrand(a + f64::from(node) * h))
                    .sum::<f64>()
            },
            || {
                (1..self.degree().get())
                    .into_par_iter()
                    .map(|node| integrand(a + (2.0 * f64::from(node) - 1.0) * h / 2.0))
                    .sum::<f64>()
            },
        );

        h / 6.0
            * (2.0 * sum_over_interval_edges
                + 4.0 * sum_over_midpoints
                + 4.0 * integrand(a + (2.0 * n - 1.0) * h / 2.0)
                + integrand(a)
                + integrand(b))
    }

    /// Sets the degree of the Simpson rule to a new value.
    pub fn change_degree(&mut self, new_degree: NonZeroU32) {
        self.degree = new_degree;
    }
}

__impl_node_rule! {Simpson, SimpsonIter}

impl From<NonZeroU32> for Simpson {
    #[inline]
    fn from(degree: NonZeroU32) -> Self {
        Self { degree }
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn check_simpson_integration() {
        let quad = Simpson::new(2.try_into().unwrap());
        let integral = quad.integrate(0.0, 1.0, |x| x * x);
        approx::assert_abs_diff_eq!(integral, 1.0 / 3.0, epsilon = 0.0001);
    }

    #[cfg(feature = "rayon")]
    #[test]
    fn par_check_simpson_integration() {
        let quad = Simpson::new(2.try_into().unwrap());
        let integral = quad.par_integrate(0.0, 1.0, |x| x * x);
        approx::assert_abs_diff_eq!(integral, 1.0 / 3.0, epsilon = 0.0001);
    }

    #[test]
    fn check_derives() {
        let quad = Simpson::new(10.try_into().unwrap());
        let quad_clone = quad.clone();
        assert_eq!(quad, quad_clone);
        let other_quad = Simpson::new(3.try_into().unwrap());
        assert_ne!(quad, other_quad);
    }

    #[test]
    fn verify_from_equivalence() {
        let new = Simpson::new(100.try_into().unwrap());
        let from = Simpson::from(NonZeroU32::new(100).unwrap());
        assert_eq!(new, from);
    }

    #[test]
    fn test_change_degree() {
        let mut quad = Simpson::new(1000.try_into().unwrap());
        let new_deg = 2000.try_into().unwrap();
        quad.change_degree(new_deg);
        assert_eq!(quad.degree(), new_deg);
    }
}