j2k-transcode-metal 0.7.0

Metal acceleration for JPEG-to-HTJ2K coefficient-domain transcode stages
Documentation 
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// SPDX-License-Identifier: MIT OR Apache-2.0

use core::f32::consts::PI;

pub(super) fn idct8_basis_table() -> [f32; 64] {
    let mut table = [0.0; 64];
    for sample_idx in 0..8 {
        for freq in 0..8 {
            table[sample_idx * 8 + freq] = idct8_basis(sample_idx, freq);
        }
    }
    table
}

#[expect(
    clippy::cast_precision_loss,
    reason = "IDCT basis indices are bounded to 0..8 and exactly represented by f32"
)]
fn idct8_basis(sample_idx: usize, freq: usize) -> f32 {
    let scale = if freq == 0 {
        (1.0_f32 / 8.0).sqrt()
    } else {
        (2.0_f32 / 8.0).sqrt()
    };
    scale * (((sample_idx as f32 + 0.5) * freq as f32 * PI) / 8.0).cos()
}