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use crate::algebra::abstr::Real;
use crate::algebra::linear::vector::Vector;
use crate::analysis::differential_equation::ordinary::ExplicitODE;
#[cfg(feature = "serde")]
use serde::{Deserialize, Serialize};
use std::clone::Clone;

#[cfg_attr(feature = "serde", derive(Serialize, Deserialize))]
#[derive(Clone, Debug)]
pub struct ExplicitRK<T> {
    a: Vec<T>,
    b: Vec<T>,
    b_order: u8,
    c: Vec<T>,
}

impl<T> ExplicitRK<T>
where
    T: Real,
{
    pub fn new(a: Vec<T>, b: Vec<T>, b_order: u8, c: Vec<T>) -> ExplicitRK<T> {
        ExplicitRK { a, b, b_order, c }
    }

    pub fn do_step<O>(&self, ode: &O, t_n: &T, x_n: &Vector<T>, h: &T) -> Vector<T>
    where
        O: ExplicitODE<T>,
    {
        let mut k: Vec<Vector<T>> = Vec::with_capacity(self.b.len());

        k.push(ode.ode(t_n, x_n));

        for j in 1..self.b.len() {
            let i_b = (j - 1) * j / 2;
            let i_e = i_b + j;

            let sum = self.a[i_b..i_e]
                .iter()
                .zip(k.iter())
                .map(|(a_jl, k_l)| k_l * a_jl * *h)
                .fold(x_n.clone(), |a, b| a + b);

            k.push(ode.ode(&(*t_n + self.c[j - 1] * *h), &sum));
        }

        self.b
            .iter()
            .zip(k.iter())
            .map(|(b, k_j)| k_j * &(*b * *h))
            .fold(x_n.clone(), |a, b| a + b)
    }

    pub fn order(&self) -> u8 {
        self.b_order
    }
}