pxdct 0.2.2

/*
 * // Copyright (c) Radzivon Bartoshyk 12/2025. 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.
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 * // 2.  Redistributions in binary form must reproduce the above copyright notice,
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 */
use crate::mla::fmla;
use crate::util::{DctSample, try_vec};
use crate::{PxdctError, PxdctExecutor};
use num_complex::Complex;
use num_traits::AsPrimitive;
use std::sync::Arc;

pub(crate) struct SplitRadixDct3<T: DctSample> {
    twiddles: Vec<Complex<T>>,
    half_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
    quarter_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
    execution_length: usize,
}

impl<T: DctSample> SplitRadixDct3<T>
where
    f64: AsPrimitive<T>,
{
    pub(crate) fn new(
        len: usize,
        half_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
        quarter_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
    ) -> Result<SplitRadixDct3<T>, PxdctError> {
        assert_eq!(
            half_dct.length(),
            quarter_dct.length() * 2,
            "Invalid DCT was received, quarter size is not multiple of half for Split-Radix DCT-III"
        );
        use crate::twiddles::compute_twiddle;
        let mut twiddles = try_vec![Complex::<T>::default(); len / 4];
        for (i, twiddle) in twiddles.iter_mut().enumerate() {
            *twiddle = compute_twiddle::<T>(2 * i + 1, len * 4).conj();
        }

        Ok(SplitRadixDct3 {
            twiddles,
            half_dct,
            quarter_dct,
            execution_length: len,
        })
    }
}

impl<T: DctSample> PxdctExecutor<T> for SplitRadixDct3<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 half_len = len / 2;
        let quarter_len = len / 4;

        for chunk in data.chunks_exact_mut(self.execution_length) {
            // divide the output into 3 sub-lists to use for our inner DCTs, one of size N/2 and two of size N/4
            let (recursive_input_evens, recursive_input_odds) = scratch.split_at_mut(half_len);
            let (recursive_input_n1, recursive_input_n3) =
                recursive_input_odds.split_at_mut(quarter_len);

            // do the same pre-loop setup as DCT4ViaDCT3, and since we're skipping the first iteration of the loop we
            // to also set up the corresponding evens cells
            recursive_input_evens[0] = chunk[0];
            recursive_input_evens[1] = chunk[2];
            recursive_input_n1[0] = chunk[1] * T::TWO;
            recursive_input_n3[0] = chunk[len - 1] * T::TWO;

            // populate the recursive input arrays
            for i in 1..quarter_len {
                let k = 4 * i;

                unsafe {
                    // the evens are the easy ones - just copy straight over
                    *recursive_input_evens.get_unchecked_mut(i * 2) = *chunk.get_unchecked(k);
                    *recursive_input_evens.get_unchecked_mut(i * 2 + 1) =
                        *chunk.get_unchecked(k + 2);

                    // for the odd ones we're going to do the same addition/subtraction we do in the setup for DCT4ViaDCT3
                    *recursive_input_n1.get_unchecked_mut(i) =
                        *chunk.get_unchecked(k - 1) + *chunk.get_unchecked(k + 1);
                    *recursive_input_n3.get_unchecked_mut(quarter_len - i) =
                        *chunk.get_unchecked(k - 1) - *chunk.get_unchecked(k + 1);
                }
            }

            //perform our recursive DCTs, using the original buffer as scratch space
            self.half_dct.execute(recursive_input_evens)?;
            self.quarter_dct.execute(recursive_input_n1)?;
            self.quarter_dct.execute(recursive_input_n3)?;

            //merge the results. we're going to combine 2 separate things:
            // - merging the two smaller DCT3 outputs into a DCT4 output
            // - marging the DCT4 outputand the larger DCT3 output into the final output
            for (i, (twiddle, &cosine_value)) in self
                .twiddles
                .iter()
                .zip(recursive_input_n1.iter())
                .enumerate()
            {
                // flip the sign of every other sine value to finish the job of using a DCT3 to compute a DST3
                let sine_value = if i % 2 == 0 {
                    recursive_input_n3[i]
                } else {
                    -recursive_input_n3[i]
                };

                let lower_dct4 = fmla(cosine_value, twiddle.re, sine_value * twiddle.im);
                let upper_dct4 = fmla(cosine_value, twiddle.im, -sine_value * twiddle.re);

                unsafe {
                    let lower_dct3 = *recursive_input_evens.get_unchecked(i);
                    let upper_dct3 = *recursive_input_evens.get_unchecked(half_len - i - 1);

                    *chunk.get_unchecked_mut(i) = lower_dct3 + lower_dct4;
                    *chunk.get_unchecked_mut(len - i - 1) = lower_dct3 - lower_dct4;

                    *chunk.get_unchecked_mut(half_len - i - 1) = upper_dct3 + upper_dct4;
                    *chunk.get_unchecked_mut(half_len + i) = upper_dct3 - upper_dct4;
                }
            }
        }

        Ok(())
    }

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

pub(crate) struct SplitRadixDst3<T: DctSample> {
    split_radix_dct3: SplitRadixDct3<T>,
    execution_length: usize,
}

impl<T: DctSample> SplitRadixDst3<T>
where
    f64: AsPrimitive<T>,
{
    pub(crate) fn new(
        len: usize,
        half_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
        quarter_dct: Arc<dyn PxdctExecutor<T> + Send + Sync>,
    ) -> Result<SplitRadixDst3<T>, PxdctError> {
        assert_eq!(
            half_dct.length(),
            quarter_dct.length() * 2,
            "Invalid DCT was received, quarter size is not multiple of half for Split-Radix DST-III"
        );
        Ok(SplitRadixDst3 {
            split_radix_dct3: SplitRadixDct3::new(len, half_dct, quarter_dct)?,
            execution_length: len,
        })
    }
}

impl<T: DctSample> PxdctExecutor<T> for SplitRadixDst3<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,
            ));
        }

        for chunk in data.chunks_exact_mut(self.execution_length) {
            chunk.reverse();

            self.split_radix_dct3.execute(chunk)?;

            for dst in chunk.chunks_exact_mut(2) {
                dst[1] = dst[1].neg();
            }
        }

        Ok(())
    }

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

#[cfg(test)]
mod tests {
    use super::*;
    use crate::dct3_butterflies::{Dct3Butterfly8, Dct3Butterfly16};
    use crate::tests::{naive_dct3, naive_dst3};
    use rand::Rng;

    #[test]
    fn test_split_dct3() {
        let mut input = vec![0.; 32];
        for z in input.iter_mut() {
            *z = rand::rng().random_range(1.0..2.0);
        }
        let reference_input = input.clone();
        let reference_input = naive_dct3(&reference_input);
        let bf = SplitRadixDct3::new(
            32,
            Arc::new(Dct3Butterfly16::default()),
            Arc::new(Dct3Butterfly8::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()
                )
            });
    }

    #[test]
    fn test_split_dst3() {
        let mut input = vec![0.; 32];
        for z in input.iter_mut() {
            *z = rand::rng().random_range(1.0..2.0);
        }
        let reference_input = input.clone();
        let reference_input = naive_dst3(&reference_input);
        let bf = SplitRadixDst3::new(
            32,
            Arc::new(Dct3Butterfly16::default()),
            Arc::new(Dct3Butterfly8::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()
                )
            });
    }
}