sparse_ir/

basis.rs

//! Finite temperature basis for SparseIR
//!
//! This module provides the `FiniteTempBasis` type which represents the
//! intermediate representation (IR) basis for a given temperature.

use std::sync::Arc;

use crate::kernel::{CentrosymmKernel, KernelProperties, LogisticKernel};
use crate::poly::{PiecewiseLegendrePolyVector, default_sampling_points};
use crate::polyfourier::PiecewiseLegendreFTVector;
use crate::sve::{SVEResult, TworkType, compute_sve};
use crate::traits::{Bosonic, Fermionic, StatisticsType};

// Re-export Statistics enum for C-API
#[derive(Debug, Clone, Copy, PartialEq, Eq)]
pub enum Statistics {
    Fermionic,
    Bosonic,
}

/// Finite temperature basis for imaginary time/frequency Green's functions
///
/// For a continuation kernel `K` from real frequencies `ω ∈ [-ωmax, ωmax]` to
/// imaginary time `τ ∈ [0, β]`, this type stores the truncated singular
/// value expansion or IR basis:
///
/// ```text
/// K(τ, ω) ≈ sum(u[l](τ) * s[l] * v[l](ω) for l in 1:L)
/// ```
///
/// This basis is inferred from a reduced form by appropriate scaling of
/// the variables.
///
/// # Type Parameters
///
/// * `K` - Kernel type implementing `KernelProperties + CentrosymmKernel`
/// * `S` - Statistics type (`Fermionic` or `Bosonic`)
#[derive(Clone)]
pub struct FiniteTempBasis<K, S>
where
    K: KernelProperties + CentrosymmKernel + Clone + 'static,
    S: StatisticsType,
{
    /// The kernel used to construct this basis
    kernel: K,

    /// The SVE result (in scaled variables)
    sve_result: Arc<SVEResult>,

    /// Accuracy of the basis (relative error)
    accuracy: f64,

    /// Inverse temperature β
    beta: f64,

    /// Left singular functions on imaginary time axis τ ∈ [0, β]
    /// Arc for efficient sharing (large immutable data)
    u: Arc<PiecewiseLegendrePolyVector>,

    /// Right singular functions on real frequency axis ω ∈ [-ωmax, ωmax]
    /// Arc for efficient sharing (large immutable data)
    v: Arc<PiecewiseLegendrePolyVector>,

    /// Singular values
    s: Vec<f64>,

    /// Left singular functions on Matsubara frequency axis (Fourier transform of u)
    /// Arc for efficient sharing (large immutable data)
    uhat: Arc<PiecewiseLegendreFTVector<S>>,

    /// Full uhat (before truncation to basis size)
    /// Arc for efficient sharing (large immutable data, used for Matsubara sampling)
    uhat_full: Arc<PiecewiseLegendreFTVector<S>>,

    _phantom: std::marker::PhantomData<S>,
}

impl<K, S> FiniteTempBasis<K, S>
where
    K: KernelProperties + CentrosymmKernel + Clone + 'static,
    S: StatisticsType,
{
    // ========== Getters ==========

    /// Get a reference to the kernel
    pub fn kernel(&self) -> &K {
        &self.kernel
    }

    /// Get the SVE result
    pub fn sve_result(&self) -> &Arc<SVEResult> {
        &self.sve_result
    }

    /// Get the accuracy of the basis
    pub fn accuracy(&self) -> f64 {
        self.accuracy
    }

    /// Get the inverse temperature β
    pub fn beta(&self) -> f64 {
        self.beta
    }

    /// Get the left singular functions (u) on imaginary time axis
    pub fn u(&self) -> &Arc<PiecewiseLegendrePolyVector> {
        &self.u
    }

    /// Get the right singular functions (v) on real frequency axis
    pub fn v(&self) -> &Arc<PiecewiseLegendrePolyVector> {
        &self.v
    }

    /// Get the singular values
    pub fn s(&self) -> &[f64] {
        &self.s
    }

    /// Get the left singular functions on Matsubara frequency axis (uhat)
    pub fn uhat(&self) -> &Arc<PiecewiseLegendreFTVector<S>> {
        &self.uhat
    }

    /// Get the full uhat (before truncation)
    pub fn uhat_full(&self) -> &Arc<PiecewiseLegendreFTVector<S>> {
        &self.uhat_full
    }

    // ========== Other methods ==========

    /// Get the frequency cutoff ωmax
    pub fn wmax(&self) -> f64 {
        self.kernel.lambda() / self.beta
    }

    /// Get default Matsubara sampling points as i64 indices (for C-API)
    pub fn default_matsubara_sampling_points_i64(&self, positive_only: bool) -> Vec<i64>
    where
        S: 'static,
    {
        let freqs = self.default_matsubara_sampling_points(positive_only);
        freqs.into_iter().map(|f| f.n()).collect()
    }

    /// Get default Matsubara sampling points as i64 indices with mitigate parameter (for C-API)
    ///
    /// # Panics
    /// Panics if the kernel is not centrosymmetric. This method relies on
    /// centrosymmetry to generate sampling points.
    pub fn default_matsubara_sampling_points_i64_with_mitigate(
        &self,
        positive_only: bool,
        mitigate: bool,
        n_points: usize,
    ) -> Vec<i64>
    where
        S: 'static,
    {
        if !self.kernel().is_centrosymmetric() {
            panic!(
                "default_matsubara_sampling_points_i64_with_mitigate is not supported for non-centrosymmetric kernels. \
                 The current implementation relies on centrosymmetry to generate sampling points."
            );
        }
        let fence = mitigate;
        let freqs = Self::default_matsubara_sampling_points_impl(
            &self.uhat_full,
            n_points,
            fence,
            positive_only,
        );
        freqs.into_iter().map(|f| f.n()).collect()
    }

    /// Create a new FiniteTempBasis
    ///
    /// # Arguments
    ///
    /// * `kernel` - Kernel implementing `KernelProperties + CentrosymmKernel`
    /// * `beta` - Inverse temperature (β > 0)
    /// * `epsilon` - Accuracy parameter (optional, defaults to NaN for auto)
    /// * `max_size` - Maximum number of basis functions (optional)
    ///
    /// # Returns
    ///
    /// A new FiniteTempBasis
    pub fn new(kernel: K, beta: f64, epsilon: Option<f64>, max_size: Option<usize>) -> Self {
        // Validate inputs
        if beta <= 0.0 {
            panic!("Inverse temperature beta must be positive, got {}", beta);
        }

        // Compute SVE
        let epsilon_value = epsilon.unwrap_or(f64::NAN);
        let sve_result = compute_sve(
            kernel.clone(),
            epsilon_value,
            None, // cutoff
            max_size,
            TworkType::Auto,
        );

        Self::from_sve_result(kernel, beta, sve_result, epsilon, max_size)
    }

    /// Create basis from existing SVE result
    ///
    /// This is useful when you want to reuse the same SVE computation
    /// for both fermionic and bosonic bases.
    pub fn from_sve_result(
        kernel: K,
        beta: f64,
        sve_result: SVEResult,
        epsilon: Option<f64>,
        max_size: Option<usize>,
    ) -> Self {
        // Get truncated u, s, v from SVE result
        let (u_sve, s_sve, v_sve) = sve_result.part(epsilon, max_size);

        // Calculate accuracy
        let accuracy = if sve_result.s.len() > s_sve.len() {
            sve_result.s[s_sve.len()] / sve_result.s[0]
        } else {
            sve_result.s[sve_result.s.len() - 1] / sve_result.s[0]
        };

        // Get kernel parameters
        let lambda = kernel.lambda();
        let omega_max = lambda / beta;

        // Scale polynomials to new variables
        // tau = β/2 * (x + 1), w = ωmax * y

        // Transform u: x ∈ [-1, 1] → τ ∈ [0, β]
        let u_knots: Vec<f64> = u_sve.get_polys()[0]
            .knots
            .iter()
            .map(|&x| beta / 2.0 * (x + 1.0))
            .collect();
        let u_delta_x: Vec<f64> = u_sve.get_polys()[0]
            .delta_x
            .iter()
            .map(|&dx| beta / 2.0 * dx)
            .collect();
        let u_symm: Vec<i32> = u_sve.get_polys().iter().map(|p| p.symm).collect();

        let u = u_sve.rescale_domain(u_knots, Some(u_delta_x), Some(u_symm));

        // Transform v: y ∈ [-1, 1] → ω ∈ [-ωmax, ωmax]
        let v_knots: Vec<f64> = v_sve.get_polys()[0]
            .knots
            .iter()
            .map(|&y| omega_max * y)
            .collect();
        let v_delta_x: Vec<f64> = v_sve.get_polys()[0]
            .delta_x
            .iter()
            .map(|&dy| omega_max * dy)
            .collect();
        let v_symm: Vec<i32> = v_sve.get_polys().iter().map(|p| p.symm).collect();

        let v = v_sve.rescale_domain(v_knots, Some(v_delta_x), Some(v_symm));

        // Scale singular values
        // s_scaled = sqrt(β/2 * ωmax) * ωmax^(-ypower) * s_sve
        let ypower = kernel.ypower();
        let scale_factor = (beta / 2.0 * omega_max).sqrt() * omega_max.powi(-ypower);
        let s: Vec<f64> = s_sve.iter().map(|&x| scale_factor * x).collect();

        // Construct uhat (Fourier transform of u)
        // HACK: Fourier transforms only work on unit interval, so we scale the data
        let uhat_base_full = sve_result.u.scale_data(beta.sqrt());
        let conv_rad = kernel.conv_radius();

        // Create statistics marker instance using Default trait
        // S is a zero-sized type (ZST) like Fermionic or Bosonic
        let stat_marker = S::default();

        let uhat_full = PiecewiseLegendreFTVector::<S>::from_poly_vector(
            &uhat_base_full,
            stat_marker,
            Some(conv_rad),
        );

        // Truncate uhat to basis size
        let uhat_polyvec: Vec<_> = uhat_full.polyvec.iter().take(s.len()).cloned().collect();
        let uhat = PiecewiseLegendreFTVector::from_vector(uhat_polyvec);

        Self {
            kernel,
            sve_result: Arc::new(sve_result),
            accuracy,
            beta,
            u: Arc::new(u),
            v: Arc::new(v),
            s,
            uhat: Arc::new(uhat),
            uhat_full: Arc::new(uhat_full),
            _phantom: std::marker::PhantomData,
        }
    }

    /// Get the size of the basis (number of basis functions)
    pub fn size(&self) -> usize {
        self.s.len()
    }

    /// Get the cutoff parameter Λ = β * ωmax
    pub fn lambda(&self) -> f64 {
        self.kernel.lambda()
    }

    /// Get the frequency cutoff ωmax
    pub fn omega_max(&self) -> f64 {
        self.lambda() / self.beta
    }

    /// Get significance of each singular value (s[i] / s[0])
    pub fn significance(&self) -> Vec<f64> {
        let s0 = self.s[0];
        self.s.iter().map(|&s| s / s0).collect()
    }

    /// Get default tau sampling points
    ///
    /// C++ implementation: libsparseir/include/sparseir/basis.hpp:229-270
    ///
    /// Returns sampling points in imaginary time τ ∈ [-β/2, β/2].
    ///
    /// # Panics
    /// Panics if the kernel is not centrosymmetric. This method relies on
    /// centrosymmetry to generate symmetric sampling points.
    pub fn default_tau_sampling_points(&self) -> Vec<f64> {
        if !self.kernel().is_centrosymmetric() {
            panic!(
                "default_tau_sampling_points is not supported for non-centrosymmetric kernels. \
                 The current implementation relies on centrosymmetry to generate symmetric sampling points."
            );
        }
        let points = self.default_tau_sampling_points_size_requested(self.size());
        let basis_size = self.size();
        if points.len() < basis_size {
            eprintln!(
                "Warning: Number of tau sampling points ({}) is less than basis size ({}). \
                 Basis parameters: beta={}, wmax={}, epsilon={:.2e}",
                points.len(),
                basis_size,
                self.beta,
                self.wmax(),
                self.accuracy()
            );
        }
        points
    }

    /// Get default tau sampling points with a requested size
    ///
    /// # Panics
    /// Panics if the kernel is not centrosymmetric. This method relies on
    /// centrosymmetry to generate symmetric sampling points.
    pub fn default_tau_sampling_points_size_requested(&self, size_requested: usize) -> Vec<f64> {
        if !self.kernel().is_centrosymmetric() {
            panic!(
                "default_tau_sampling_points_size_requested is not supported for non-centrosymmetric kernels. \
                 The current implementation relies on centrosymmetry to generate symmetric sampling points."
            );
        }
        // C++: Eigen::VectorXd x = default_sampling_points(*(this->sve_result->u), sz);
        let x = default_sampling_points(&self.sve_result.u, size_requested);
        // C++: Extract unique half of sampling points
        let mut unique_x = Vec::new();
        if x.len() % 2 == 0 {
            // C++: for (auto i = 0; i < x.size() / 2; ++i)
            for i in 0..(x.len() / 2) {
                unique_x.push(x[i]);
            }
        } else {
            // C++: for (auto i = 0; i < x.size() / 2; ++i)
            for i in 0..(x.len() / 2) {
                unique_x.push(x[i]);
            }
            // C++: auto x_new = 0.5 * (unique_x.back() + 0.5);
            let x_new = 0.5 * (unique_x.last().unwrap() + 0.5);
            unique_x.push(x_new);
        }

        // C++: Generate symmetric points
        //      Eigen::VectorXd smpl_taus(2 * unique_x.size());
        //      for (auto i = 0; i < unique_x.size(); ++i) {
        //          smpl_taus(i) = (this->beta / 2.0) * (unique_x[i] + 1.0);
        //          smpl_taus(unique_x.size() + i) = -smpl_taus(i);
        //      }
        let mut smpl_taus = Vec::with_capacity(2 * unique_x.len());
        for &ux in &unique_x {
            smpl_taus.push((self.beta / 2.0) * (ux + 1.0));
        }
        for i in 0..unique_x.len() {
            smpl_taus.push(-smpl_taus[i]);
        }

        // C++: std::sort(smpl_taus.data(), smpl_taus.data() + smpl_taus.size());
        smpl_taus.sort_by(|a, b| a.partial_cmp(b).unwrap());

        // C++: Check if the number of sampling points is even
        if smpl_taus.len() % 2 != 0 {
            panic!("The number of tau sampling points is odd!");
        }

        // C++: Check if tau = 0 is not in the sampling points
        for &tau in &smpl_taus {
            if tau.abs() < 1e-10 {
                eprintln!(
                    "Warning: tau = 0 is in the sampling points (absolute error: {})",
                    tau.abs()
                );
            }
        }

        // C++ implementation returns tau in [-beta/2, beta/2] (does NOT convert to [0, beta])
        // This matches the natural range for centrosymmetric kernels
        smpl_taus
    }

    /// Get default Matsubara frequency sampling points
    ///
    /// Returns sampling points as MatsubaraFreq objects based on extrema
    /// of the Matsubara basis functions (same algorithm as C++/Julia).
    ///
    /// # Arguments
    /// * `positive_only` - If true, returns only non-negative frequencies
    ///
    /// # Returns
    /// Vector of Matsubara frequency sampling points
    ///
    /// # Panics
    /// Panics if the kernel is not centrosymmetric. This method relies on
    /// centrosymmetry to generate sampling points.
    pub fn default_matsubara_sampling_points(
        &self,
        positive_only: bool,
    ) -> Vec<crate::freq::MatsubaraFreq<S>>
    where
        S: 'static,
    {
        if !self.kernel().is_centrosymmetric() {
            panic!(
                "default_matsubara_sampling_points is not supported for non-centrosymmetric kernels. \
                 The current implementation relies on centrosymmetry to generate sampling points."
            );
        }
        let fence = false;
        let points = Self::default_matsubara_sampling_points_impl(
            &self.uhat_full,
            self.size(),
            fence,
            positive_only,
        );
        let basis_size = self.size();
        // For positive_only=true, we need 2*n_sampling_points >= basis_size
        // For positive_only=false, we need n_sampling_points >= basis_size
        let effective_points = if positive_only {
            2 * points.len()
        } else {
            points.len()
        };
        if effective_points < basis_size {
            eprintln!(
                "Warning: Number of Matsubara sampling points ({}{}) is less than basis size ({}). \
                 Basis parameters: beta={}, wmax={}, epsilon={:.2e}",
                points.len(),
                if positive_only { " × 2" } else { "" },
                basis_size,
                self.beta,
                self.wmax(),
                self.accuracy()
            );
        }
        points
    }

    /// Fence Matsubara sampling points to improve conditioning
    ///
    /// This function adds additional sampling points near the outer frequencies
    /// to improve the conditioning of the sampling matrix. This is particularly
    /// important for Matsubara sampling where we cannot freely choose sampling points.
    ///
    /// Implementation matches C++ version in `basis.hpp` (lines 407-452).
    fn fence_matsubara_sampling(
        omega_n: &mut Vec<crate::freq::MatsubaraFreq<S>>,
        positive_only: bool,
    ) where
        S: StatisticsType + 'static,
    {
        use crate::freq::{BosonicFreq, MatsubaraFreq};

        if omega_n.is_empty() {
            return;
        }

        // Collect outer frequencies
        let mut outer_frequencies = Vec::new();
        if positive_only {
            outer_frequencies.push(omega_n[omega_n.len() - 1]);
        } else {
            outer_frequencies.push(omega_n[0]);
            outer_frequencies.push(omega_n[omega_n.len() - 1]);
        }

        for wn_outer in outer_frequencies {
            let outer_val = wn_outer.n();
            // In SparseIR.jl-v1, ωn_diff is always created as BosonicFreq
            // This ensures diff_val is always even (valid for Bosonic)
            let mut diff_val = 2 * (0.025 * outer_val as f64).round() as i64;

            // Handle edge case: if diff_val is 0, set it to 2 (minimum even value for Bosonic)
            if diff_val == 0 {
                diff_val = 2;
            }

            // Get the n value from BosonicFreq (same as diff_val since it's even)
            let wn_diff = BosonicFreq::new(diff_val).unwrap().n();

            // Sign function: returns +1 if n > 0, -1 if n < 0, 0 if n == 0
            // Matches C++ implementation: (a.get_n() > 0) - (a.get_n() < 0)
            let sign_val = if outer_val > 0 {
                1
            } else if outer_val < 0 {
                -1
            } else {
                0
            };

            // Check original size before adding (C++ checks wn.size() before each push)
            let original_size = omega_n.len();
            if original_size >= 20 {
                // For Fermionic: wn_outer.n is odd, wn_diff is even, so wn_outer.n ± wn_diff is odd (valid)
                // For Bosonic: wn_outer.n is even, wn_diff is even, so wn_outer.n ± wn_diff is even (valid)
                let new_n = outer_val - sign_val * wn_diff;
                if let Ok(new_freq) = MatsubaraFreq::<S>::new(new_n) {
                    omega_n.push(new_freq);
                }
            }
            if original_size >= 42 {
                let new_n = outer_val + sign_val * wn_diff;
                if let Ok(new_freq) = MatsubaraFreq::<S>::new(new_n) {
                    omega_n.push(new_freq);
                }
            }
        }

        // Sort and remove duplicates using BTreeSet
        let omega_n_set: std::collections::BTreeSet<MatsubaraFreq<S>> = omega_n.drain(..).collect();
        *omega_n = omega_n_set.into_iter().collect();
    }

    pub fn default_matsubara_sampling_points_impl(
        uhat_full: &PiecewiseLegendreFTVector<S>,
        l: usize,
        fence: bool,
        positive_only: bool,
    ) -> Vec<crate::freq::MatsubaraFreq<S>>
    where
        S: StatisticsType + 'static,
    {
        use crate::freq::MatsubaraFreq;
        use crate::polyfourier::{find_extrema, sign_changes};
        use std::collections::BTreeSet;

        let mut l_requested = l;

        // Adjust l_requested based on statistics (same as C++)
        if S::STATISTICS == crate::traits::Statistics::Fermionic && l_requested % 2 != 0 {
            l_requested += 1;
        } else if S::STATISTICS == crate::traits::Statistics::Bosonic && l_requested % 2 == 0 {
            l_requested += 1;
        }

        // Choose sign_changes or find_extrema based on l_requested
        let mut omega_n = if l_requested < uhat_full.len() {
            sign_changes(&uhat_full[l_requested], positive_only)
        } else {
            find_extrema(&uhat_full[uhat_full.len() - 1], positive_only)
        };

        // For bosons, include zero frequency explicitly to prevent conditioning issues
        if S::STATISTICS == crate::traits::Statistics::Bosonic {
            omega_n.push(MatsubaraFreq::<S>::new(0).unwrap());
        }

        // Sort and remove duplicates using BTreeSet
        let omega_n_set: BTreeSet<MatsubaraFreq<S>> = omega_n.into_iter().collect();
        let mut omega_n: Vec<MatsubaraFreq<S>> = omega_n_set.into_iter().collect();

        // Check expected size
        let expected_size = if positive_only {
            l_requested.div_ceil(2)
        } else {
            l_requested
        };

        if omega_n.len() != expected_size {
            eprintln!(
                "Warning: Requested {} sampling frequencies for basis size L = {}, but got {}.",
                expected_size,
                l,
                omega_n.len()
            );
        }

        // Apply fencing if requested (same as C++ implementation)
        if fence {
            Self::fence_matsubara_sampling(&mut omega_n, positive_only);
        }

        omega_n
    }
    /// Get default omega (real frequency) sampling points
    ///
    /// Returns sampling points on the real-frequency axis ω ∈ [-ωmax, ωmax].
    /// These are used as pole locations for the Discrete Lehmann Representation (DLR).
    ///
    /// The sampling points are chosen as the roots of the L-th basis function
    /// in the spectral domain (v), which provides near-optimal conditioning.
    ///
    /// # Returns
    /// Vector of real-frequency sampling points in [-ωmax, ωmax]
    pub fn default_omega_sampling_points(&self) -> Vec<f64> {
        let sz = self.size();

        // Use UNTRUNCATED sve_result.v (same as C++)
        // C++: default_sampling_points(*(sve_result->v), sz)
        let y = default_sampling_points(&self.sve_result.v, sz);

        // Scale to [-ωmax, ωmax]
        let wmax = self.kernel.lambda() / self.beta;
        let omega_points: Vec<f64> = y.into_iter().map(|yi| wmax * yi).collect();

        omega_points
    }
}

// ============================================================================
// Trait implementations
// ============================================================================

impl<K, S> crate::basis_trait::Basis<S> for FiniteTempBasis<K, S>
where
    K: KernelProperties + CentrosymmKernel + Clone + 'static,
    S: StatisticsType + 'static,
{
    type Kernel = K;

    fn kernel(&self) -> &Self::Kernel {
        &self.kernel
    }

    fn beta(&self) -> f64 {
        self.beta
    }

    fn wmax(&self) -> f64 {
        self.kernel.lambda() / self.beta
    }

    fn lambda(&self) -> f64 {
        self.kernel.lambda()
    }

    fn size(&self) -> usize {
        self.size()
    }

    fn accuracy(&self) -> f64 {
        self.accuracy
    }

    fn significance(&self) -> Vec<f64> {
        if let Some(&first_s) = self.s.first() {
            self.s.iter().map(|&s| s / first_s).collect()
        } else {
            vec![]
        }
    }

    fn svals(&self) -> Vec<f64> {
        self.s.clone()
    }

    fn default_tau_sampling_points(&self) -> Vec<f64> {
        self.default_tau_sampling_points()
    }

    fn default_matsubara_sampling_points(
        &self,
        positive_only: bool,
    ) -> Vec<crate::freq::MatsubaraFreq<S>> {
        self.default_matsubara_sampling_points(positive_only)
    }

    fn evaluate_tau(&self, tau: &[f64]) -> mdarray::DTensor<f64, 2> {
        use crate::taufuncs::normalize_tau;
        use mdarray::DTensor;

        let n_points = tau.len();
        let basis_size = self.size();

        // Evaluate each basis function at all tau points
        // Result: matrix[i, l] = u_l(tau[i])
        // Note: tau can be in [-beta, beta] and will be normalized to [0, beta]
        // self.u polynomials are already scaled to tau ∈ [0, beta] domain
        DTensor::<f64, 2>::from_fn([n_points, basis_size], |idx| {
            let i = idx[0]; // tau index
            let l = idx[1]; // basis function index

            // Normalize tau to [0, beta] with statistics-dependent sign
            let (tau_norm, sign) = normalize_tau::<S>(tau[i], self.beta);

            // Evaluate basis function directly (u polynomials are in tau domain)
            sign * self.u[l].evaluate(tau_norm)
        })
    }

    fn evaluate_matsubara(
        &self,
        freqs: &[crate::freq::MatsubaraFreq<S>],
    ) -> mdarray::DTensor<num_complex::Complex<f64>, 2> {
        use mdarray::DTensor;
        use num_complex::Complex;

        let n_points = freqs.len();
        let basis_size = self.size();

        // Evaluate each basis function at all Matsubara frequencies
        // Result: matrix[i, l] = uhat_l(iωn[i])
        DTensor::<Complex<f64>, 2>::from_fn([n_points, basis_size], |idx| {
            let i = idx[0]; // frequency index
            let l = idx[1]; // basis function index
            self.uhat[l].evaluate(&freqs[i])
        })
    }

    fn evaluate_omega(&self, omega: &[f64]) -> mdarray::DTensor<f64, 2> {
        use mdarray::DTensor;

        let n_points = omega.len();
        let basis_size = self.size();

        // Evaluate each spectral basis function at all omega points
        // Result: matrix[i, l] = V_l(omega[i])
        DTensor::<f64, 2>::from_fn([n_points, basis_size], |idx| {
            let i = idx[0]; // omega index
            let l = idx[1]; // basis function index
            self.v[l].evaluate(omega[i])
        })
    }

    fn default_omega_sampling_points(&self) -> Vec<f64> {
        self.default_omega_sampling_points()
    }
}

// ============================================================================
// Type aliases
// ============================================================================

/// Type alias for fermionic basis with LogisticKernel
pub type FermionicBasis = FiniteTempBasis<LogisticKernel, Fermionic>;

/// Type alias for bosonic basis with LogisticKernel
pub type BosonicBasis = FiniteTempBasis<LogisticKernel, Bosonic>;

#[cfg(test)]
#[path = "basis_tests.rs"]
mod basis_tests;