pxdct 0.2.2

Discrete cosine transform
Documentation 
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/*
 * // Copyright (c) Radzivon Bartoshyk 11/2025. 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.
 * //
 * // 2.  Redistributions in binary form must reproduce the above copyright notice,
 * // this list of conditions and the following disclaimer in the documentation
 * // and/or other materials provided with the distribution.
 * //
 * // 3.  Neither the name of the copyright holder nor the names of its
 * // contributors may be used to endorse or promote products derived from
 * // this software without specific prior written permission.
 * //
 * // THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
 * // AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
 * // IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
 * // DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
 * // FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
 * // DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
 * // SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
 * // CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
 * // OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
 * // OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
 */
use crate::spectrum_mul::DctSpectrumMul;
use crate::util::{DctSample, create_dct2_3, try_vec};
use crate::{PxdctError, PxdctExecutor};
use num_complex::Complex;
use num_traits::AsPrimitive;
use std::sync::Arc;
use zaft::{FftDirection, FftExecutor};

pub(crate) struct Dst2Fft<T> {
    twiddles: Vec<Complex<T>>,
    fft_executor: Arc<dyn FftExecutor<T> + Send + Sync>,
    execution_length: usize,
    spectrum_mul: Arc<dyn DctSpectrumMul<T> + Send + Sync>,
}

create_dct2_3!(Dst2Fft);

impl<T: DctSample> PxdctExecutor<T> for Dst2Fft<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![Complex::<T>::default(); self.execution_length];

        let even_end = self.execution_length.div_ceil(2);

        for chunk in data.chunks_exact_mut(self.execution_length) {
            for (dst, src) in scratch
                .iter_mut()
                .zip(chunk.iter().step_by(2))
                .take(even_end)
            {
                *dst = Complex::from(src);
            }

            // the second half is the odd elements, in reverse order
            if self.execution_length > 1 {
                let odd_end = self.execution_length - self.execution_length % 2;
                let buffer = &mut scratch[even_end..even_end + self.execution_length / 2];
                let data_cutoff = &chunk[..odd_end];
                for (dst, src) in buffer
                    .iter_mut()
                    .zip(data_cutoff.iter().rev().step_by(2))
                    .take(self.execution_length / 2)
                {
                    *dst = Complex::from(-*src);
                }
            }

            self.fft_executor
                .execute(&mut scratch)
                .map_err(|x| PxdctError::FftError(x.to_string()))?;

            self.spectrum_mul
                .mul_spectrum_to_real_rev(&scratch, &self.twiddles, chunk);
        }

        Ok(())
    }

    fn length(&self) -> usize {
        self.execution_length
    }
}

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

    #[test]
    fn test_14() {
        let mut array = vec![0., 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13.];
        let dct0 = Dst2Fft::<f32>::new(array.len()).unwrap();
        dct0.execute(&mut array).unwrap();
        static CONTROL: [f32; 14] = [
            58.054127, -31.457716, 19.680326, -16.133354, 12.21731, -11.227128, 9.192388,
            -8.953337, 7.6766434, -7.7694135, 6.8864455, -7.180018, 6.5411296, -7.0,
        ];

        array.iter().zip(CONTROL.iter()).enumerate().for_each(|(i, (&x, &c))| {
            assert!((x - c).abs() < 1e-4, "Difference to control values exceeded 1e-4 when it shouldn't, value {x}, control {c} at {i}");
            });
    }

    #[test]
    fn test_15() {
        let mut array = vec![
            0., 1., 2., 3., 4., 5., 6., 7., 8., 9., 10., 11., 12., 13., 14.,
        ];
        let dct0 = Dst2Fft::<f32>::new(array.len()).unwrap();

        static CONTROL: [f32; 15] = [
            66.96741, -36.073006, 22.652475, -18.439453, 14.000001, -12.759762, 10.461337,
            -10.092246, 8.652474, -8.6602545, 7.662456, -7.885967, 7.1563845, -7.541312, 7.0,
        ];

        dct0.execute(&mut array).unwrap();
        array.iter().zip(CONTROL.iter()).enumerate().for_each(|(i, (&x, &c))| {
            assert!((x - c).abs() < 1e-4, "Difference to control values exceeded 1e-4 when it shouldn't, value {x}, control {c} at {i}");
        });
    }
}