pxdct 0.2.2

/*
 * // Copyright (c) Radzivon Bartoshyk 1/2026. All rights reserved.
 * //
 * // Redistribution and use in source and binary forms, with or without modification,
 * // are permitted provided that the following conditions are met:
 * //
 * // 1.  Redistributions of source code must retain the above copyright notice, this
 * // list of conditions and the following disclaimer.
 * //
 * // 2.  Redistributions in binary form must reproduce the above copyright notice,
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 * // contributors may be used to endorse or promote products derived from
 * // this software without specific prior written permission.
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 */

use crate::mla::fmla;
use crate::util::{DctSample, mixed_radix_inner_twiddle, try_vec};
use crate::{PxdctError, PxdctExecutor};
use num_complex::Complex;
use num_traits::{AsPrimitive, Zero};
use std::sync::Arc;

#[allow(unused)]
pub(crate) struct Dct2MixedRadix6<T> {
    inner_layer: Vec<Complex<T>>,
    sixth_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
    execution_length: usize,
}

impl<T: DctSample> Dct2MixedRadix6<T>
where
    f64: AsPrimitive<T>,
    usize: AsPrimitive<T>,
{
    #[allow(unused)]
    pub(crate) fn new(
        len: usize,
        sixth_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
    ) -> Result<Dct2MixedRadix6<T>, PxdctError> {
        assert_eq!(
            len,
            sixth_dct.length() * 6,
            "Invalid DCT was received, third size is not multiple of full size"
        );
        let inner_layer_groups = sixth_dct.length();
        let mut inner_layer = vec![Complex::<T>::zero(); inner_layer_groups * 4];
        for (i, layer) in inner_layer.chunks_exact_mut(4).enumerate() {
            let angle = (2. * i as f64 + 1.).as_();
            layer[0] = mixed_radix_inner_twiddle(angle, len);
            layer[0].im *= T::SQRT_3;
            layer[1] = mixed_radix_inner_twiddle(2f64.as_() * angle, len);
            layer[1].im *= T::SQRT_3;
            layer[2] = mixed_radix_inner_twiddle(3f64.as_() * angle, len);
            layer[2].im *= T::SQRT_3;
            layer[3] = mixed_radix_inner_twiddle(5f64.as_() * angle, len);
            layer[3].im = -layer[3].im * T::SQRT_3;
        }

        Ok(Dct2MixedRadix6 {
            inner_layer,
            sixth_dct,
            execution_length: len,
        })
    }
}

impl<T: DctSample> PxdctExecutor<T> for Dct2MixedRadix6<T>
where
    f64: AsPrimitive<T>,
{
    fn execute(&self, data: &mut [T]) -> Result<(), PxdctError> {
        if !data.len().is_multiple_of(self.execution_length) {
            return Err(PxdctError::InvalidSizeMultiplier(
                data.len(),
                self.execution_length,
            ));
        }

        let mut scratch = try_vec![T::default(); self.execution_length];

        let len = self.length();
        let sixth_length = self.sixth_dct.length();
        let s_n = len / 3;
        let s_2n = 2 * len / 3;

        for chunk in data.chunks_exact_mut(self.execution_length) {
            let (a_buffer, rem) = scratch.split_at_mut(sixth_length);
            let (b_buffer, rem) = rem.split_at_mut(sixth_length);
            let (c_buffer, rem) = rem.split_at_mut(sixth_length);
            let (d_buffer, rem) = rem.split_at_mut(sixth_length);
            let (e_buffer, rem) = rem.split_at_mut(sixth_length);
            let (f_buffer, _) = rem.split_at_mut(sixth_length);

            for (i, inner_layer) in self.inner_layer.chunks_exact(4).enumerate() {
                let ai = unsafe { *chunk.get_unchecked(i) };
                let bi = unsafe { *chunk.get_unchecked(s_n - i - 1) };
                let ci = unsafe { *chunk.get_unchecked(s_n + i) };
                let di = unsafe { *chunk.get_unchecked(s_2n - i - 1) };
                let ei = unsafe { *chunk.get_unchecked(s_2n + i) };
                let fi = unsafe { *chunk.get_unchecked(len - i - 1) };

                let cos_sin_ai = inner_layer[0];
                let cos_sin_2ai = inner_layer[1];
                let cos_sin_3ai = inner_layer[2];
                let cos_sin_5ai = inner_layer[3];

                let s2 = bi + ei;
                let dcd = ci - di;
                let dbe = bi - ei;

                let ai2 = T::TWO * ai;
                let fi2 = T::TWO * fi;
                let scd = ci + di;

                let sdbedcd = dbe + dcd;
                let ai2dbedcd = ai2 + sdbedcd - fi2;

                let s2scd = s2 + scd;

                let a_comp = ai + s2scd + fi;
                let c_comp = ai2 - s2scd + fi2;
                let d_comp = T::TWO * (ai - sdbedcd - fi);

                let dbedcd = dbe - dcd;

                let c_img = s2 - ci - di;
                let b_zet = dbedcd * cos_sin_ai.im;
                let c_zet = c_img * cos_sin_2ai.im;
                let f_zet = dbedcd * cos_sin_5ai.im;

                let e_comp = fmla(
                    T::TWO * cos_sin_2ai.re,
                    fmla(c_comp, cos_sin_2ai.re, -c_zet),
                    -c_comp,
                );

                unsafe {
                    *a_buffer.get_unchecked_mut(i) = a_comp;
                    *b_buffer.get_unchecked_mut(i) = fmla(ai2dbedcd, cos_sin_ai.re, b_zet);
                    *c_buffer.get_unchecked_mut(i) = fmla(c_comp, cos_sin_2ai.re, c_zet);
                    *d_buffer.get_unchecked_mut(i) = d_comp * cos_sin_3ai.re;
                    *e_buffer.get_unchecked_mut(i) = e_comp;
                    *f_buffer.get_unchecked_mut(i) = fmla(ai2dbedcd, cos_sin_5ai.re, f_zet);
                }
            }

            if a_buffer.len() > 1 {
                self.sixth_dct.execute(&mut scratch)?;
            }

            let (a_buffer, rem) = scratch.split_at_mut(sixth_length);
            let (b_buffer, rem) = rem.split_at_mut(sixth_length);
            let (c_buffer, rem) = rem.split_at_mut(sixth_length);
            let (d_buffer, rem) = rem.split_at_mut(sixth_length);
            let (e_buffer, rem) = rem.split_at_mut(sixth_length);
            let (f_buffer, _) = rem.split_at_mut(sixth_length);

            chunk[0] = a_buffer[0];
            chunk[1] = b_buffer[0] * T::HALF;
            chunk[2] = c_buffer[0] * T::HALF;
            chunk[3] = d_buffer[0] * T::HALF;
            chunk[4] = e_buffer[0] * T::HALF;
            chunk[5] = f_buffer[0] * T::HALF;

            let mut b_diff = chunk[5];
            let mut c_diff = chunk[4];
            let mut e_diff = chunk[3];
            let mut d_diff = chunk[2];
            let mut f_diff = chunk[1];

            for k in 1..sixth_length {
                let deferred_d_diff;
                let deferred_f_diff;
                unsafe {
                    *chunk.get_unchecked_mut(6 * k) = *a_buffer.get_unchecked(k);
                }
                unsafe {
                    deferred_f_diff = *b_buffer.get_unchecked(k) - b_diff;
                    *chunk.get_unchecked_mut(6 * k + 1) = deferred_f_diff;
                }
                unsafe {
                    deferred_d_diff = *c_buffer.get_unchecked(k) - c_diff;
                    *chunk.get_unchecked_mut(6 * k + 2) = deferred_d_diff;
                }
                unsafe {
                    e_diff = *d_buffer.get_unchecked(k) - e_diff;
                    *chunk.get_unchecked_mut(6 * k + 3) = e_diff;
                }
                unsafe {
                    let new_d = *e_buffer.get_unchecked(k) - d_diff;
                    *chunk.get_unchecked_mut(6 * k + 4) = new_d;
                    c_diff = new_d;
                    d_diff = deferred_d_diff;
                }
                unsafe {
                    let new_f = *f_buffer.get_unchecked(k) - f_diff;
                    b_diff = new_f;
                    f_diff = deferred_f_diff;
                    *chunk.get_unchecked_mut(6 * k + 5) = new_f;
                }
            }
        }

        Ok(())
    }

    #[inline]
    fn length(&self) -> usize {
        self.execution_length
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::butterflies::Dct2Butterfly36;
    use crate::tests::naive_dct2;
    use rand::Rng;

    #[test]
    fn test_radix6_dct2() {
        let mut input = vec![0.; 216];
        for z in input.iter_mut() {
            *z = rand::rng().random_range(1.0..2.0);
        }
        let reference_input = input.clone();
        let reference_input = naive_dct2(&reference_input);
        let bf = Dct2MixedRadix6::new(216, Arc::new(Dct2Butterfly36::default())).unwrap();
        bf.execute(&mut input).unwrap();
        input
            .iter()
            .zip(reference_input.iter())
            .enumerate()
            .for_each(|(i, (&src, &r0))| {
                assert!(
                    (src - r0).abs() < 1e-7,
                    "Difference must be < {}, but it was {}, at position {i}",
                    1e-7,
                    (src - r0).abs()
                )
            });
    }
}