//! Periodic boundary conditions module
//!
//! This module implements k-point sampling, Coulomb truncation schemes,
//! and head/wing corrections for periodic GW/BSE calculations.
#![allow(clippy::many_single_char_names)] // Mathematical notation

use crate::common::Result;
use ndarray::{Array1, Array2, Array3};
use num_complex::Complex64;

/// k-point mesh for Brillouin zone sampling
#[derive(Debug, Clone)]
pub struct KMesh {
    /// Number of k-points in each direction
    pub mesh_size: [usize; 3],
    /// k-point coordinates
    pub kpoints: Vec<[f64; 3]>,
    /// k-point weights
    pub weights: Vec<f64>,
    /// Time-reversal symmetry
    pub use_symmetry: bool,
}

impl KMesh {
    /// Create a Monkhorst-Pack mesh
    pub fn monkhorst_pack(_nx: usize, _ny: usize, _nz: usize) -> Self {
        todo!("Implement Monkhorst-Pack mesh generation")
    }

    /// Create a Gamma-centered mesh
    pub fn gamma_centered(_nx: usize, _ny: usize, _nz: usize) -> Self {
        todo!("Implement Gamma-centered mesh")
    }

    /// Get number of k-points
    pub fn nkpts(&self) -> usize {
        self.kpoints.len()
    }

    /// Find irreducible k-points using symmetry
    pub fn find_irreducible(&self) -> Result<Vec<usize>> {
        todo!("Implement symmetry reduction")
    }

    /// Get k-point difference q = k - k'
    pub fn get_q_point(&self, _k1: usize, _k2: usize) -> [f64; 3] {
        todo!("Implement q-point calculation")
    }
}

/// Coulomb truncation schemes for 2D/1D systems
#[derive(Debug, Clone)]
pub enum CoulombTruncation {
    /// No truncation (3D periodic)
    None,
    /// Truncation for 2D systems
    TwoDimensional { vacuum_size: f64 },
    /// Truncation for 1D systems
    OneDimensional { vacuum_size: f64 },
    /// Spherical truncation
    Spherical { radius: f64 },
}

/// Truncated Coulomb interaction
pub struct TruncatedCoulomb {
    /// Truncation scheme
    pub scheme: CoulombTruncation,
    /// Reciprocal lattice vectors
    pub gvectors: Vec<[f64; 3]>,
}

impl TruncatedCoulomb {
    /// Create new truncated Coulomb interaction
    #[must_use]
    pub fn new(scheme: CoulombTruncation, gvectors: Vec<[f64; 3]>) -> Self {
        Self { scheme, gvectors }
    }

    /// Compute truncated Coulomb kernel
    pub fn compute_kernel(&self, _q: &[f64; 3]) -> Result<Array1<Complex64>> {
        todo!("Implement truncated Coulomb kernel")
    }

    /// Apply truncation to bare Coulomb
    pub fn apply_truncation(&self, _v_bare: &Array2<f64>) -> Result<Array2<f64>> {
        todo!("Implement Coulomb truncation")
    }
}

/// Head and wing corrections for q→0 limit
pub struct HeadWingCorrection {
    /// Macroscopic dielectric constant
    pub epsilon_macro: f64,
    /// System dimension
    pub dimension: usize,
}

impl HeadWingCorrection {
    /// Create new head/wing correction handler
    #[must_use]
    pub fn new(epsilon_macro: f64, dimension: usize) -> Self {
        Self {
            epsilon_macro,
            dimension,
        }
    }

    /// Compute head correction (G=G'=0)
    pub fn compute_head(&self, _q: &[f64; 3]) -> Result<Complex64> {
        todo!("Implement head correction")
    }

    /// Compute wing corrections (G=0, G'≠0 and vice versa)
    pub fn compute_wings(&self, _q: &[f64; 3], _gvecs: &[[f64; 3]]) -> Result<Array1<Complex64>> {
        todo!("Implement wing corrections")
    }

    /// Apply corrections to dielectric matrix
    pub fn apply_corrections(&self, _epsilon: &mut Array2<Complex64>, _q: &[f64; 3]) -> Result<()> {
        todo!("Implement head/wing application")
    }
}

/// Periodic GW driver
pub struct PeriodicGW {
    /// k-point mesh
    pub kmesh: KMesh,
    /// Coulomb truncation
    pub truncation: Option<TruncatedCoulomb>,
    /// Head/wing corrections
    pub head_wing: Option<HeadWingCorrection>,
}

impl PeriodicGW {
    /// Create new periodic GW calculator
    #[must_use]
    pub fn new(kmesh: KMesh) -> Self {
        Self {
            kmesh,
            truncation: None,
            head_wing: None,
        }
    }

    /// Set Coulomb truncation
    pub fn set_truncation(&mut self, _scheme: CoulombTruncation) {
        todo!("Set up Coulomb truncation")
    }

    /// Compute P0(q,ω) for given q-point
    pub fn compute_p0_q(&self, _q_idx: usize, _omega: Complex64) -> Result<Array2<Complex64>> {
        todo!("Implement P0(q,ω) calculation")
    }

    /// Compute band gap
    pub fn compute_band_gap(&self) -> Result<f64> {
        todo!("Implement band gap calculation")
    }

    /// Compute k-resolved spectral function
    pub fn spectral_function(
        &self,
        _k_idx: usize,
        _omega_range: (f64, f64),
    ) -> Result<Array1<f64>> {
        todo!("Implement spectral function")
    }
}

/// Periodic BSE driver
pub struct PeriodicBSE {
    /// k-point mesh
    pub kmesh: KMesh,
    /// Exciton momentum
    pub q_exciton: [f64; 3],
}

impl PeriodicBSE {
    /// Create new periodic BSE calculator
    #[must_use]
    pub fn new(kmesh: KMesh) -> Self {
        Self {
            kmesh,
            q_exciton: [0.0; 3],
        }
    }

    /// Build BSE kernel in k-space
    pub fn build_kernel_kspace(&self) -> Result<Array3<Complex64>> {
        todo!("Implement k-space BSE kernel")
    }

    /// Compute optical absorption spectrum
    pub fn optical_absorption(&self, _polarization: [f64; 3]) -> Result<Array1<f64>> {
        todo!("Implement optical absorption")
    }

    /// Compute exciton band structure
    pub fn exciton_bands(&self, _q_path: &[[f64; 3]]) -> Result<Array2<f64>> {
        todo!("Implement exciton band structure")
    }
}

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

    #[test]
    #[should_panic(expected = "not yet implemented")]
    fn test_monkhorst_pack_todo() {
        let _ = KMesh::monkhorst_pack(2, 2, 2);
    }

    #[test]
    fn test_truncation_creation() {
        let scheme = CoulombTruncation::TwoDimensional { vacuum_size: 10.0 };
        let coulomb = TruncatedCoulomb::new(scheme, vec![[0.0, 0.0, 0.0]]);
        assert_eq!(coulomb.gvectors.len(), 1);
    }

    #[test]
    fn test_head_wing_creation() {
        let hw = HeadWingCorrection::new(10.0, 3);
        assert_eq!(hw.epsilon_macro, 10.0);
        assert_eq!(hw.dimension, 3);
    }
}
