ruda-core 0.1.3

use super::QuantValue;
use alloc::vec::Vec;
use num_traits::PrimInt;

/// Pack signed 8-bit integer values into a sequence of unsigned 32-bit integers.
pub fn pack_i8s_to_u32s(values: Vec<i8>) -> Vec<u32> {
    // Shift and combine groups of four 8-bit values into a u32.
    // Same as doing this:
    //     let result = (d_u8 & 0xFF) << 24 | (c_u8 & 0xFF) << 16 | (b_u8 & 0xFF) << 8 | (a_u8 & 0xFF);
    values
        .chunks(4)
        .map(|x| {
            x.iter()
                .enumerate()
                .fold(0u32, |acc, (i, x)| acc | (*x as u32 & 0xFF) << (i * 8))
        })
        .collect()
}

/// Unpack integer values into a sequence of signed 8-bit integers.
pub(super) fn unpack_q_to_i8s<Q: PrimInt>(
    values: &[Q],
    numel: usize,
    value: &QuantValue,
) -> Vec<i8> {
    let size_store = size_of::<Q>() * 8;
    let size_quant = value.size_bits();
    let num_quants = size_store / size_quant;
    let mask = Q::from((1 << size_quant) - 1).unwrap();
    let sign_shift = 8 - size_quant; // sign extension for sub-byte values
    values
        .iter()
        .enumerate()
        .flat_map(|(i, &packed)| {
            // A single u32 could contain less than four 8-bit values...
            let n = core::cmp::min(num_quants, numel - i * num_quants);
            // Extract each 8-bit segment from u32 and cast back to i8
            // Same as doing this (when 4 values are fully packed):
            //     let a = (packed & 0xFF) as i8;
            //     let b = ((packed >> 8) & 0xFF) as i8;
            //     let c = ((packed >> 16) & 0xFF) as i8;
            //     let d = ((packed >> 24) & 0xFF) as i8;
            (0..n).map(move |i| {
                let raw = (packed >> (i * size_quant) & mask).to_u8().unwrap();
                ((raw << sign_shift) as i8) >> sign_shift
            })
        })
        .collect()
}