//! Original executable ICO encoder: DIB images and transparency masks.
use super::app_icon;

pub fn encode() -> Result<Vec<u8>, Box<dyn std::error::Error>> {
    let sizes = [16usize, 24, 32, 48, 64, 128, 256];
    let mut images = Vec::new();
    for size in sizes {
        let rgba = app_icon::rgba(size)?;
        // ICO DIB: bottom-up BGRA plus a padded one-bit transparency mask.
        let mask_stride = size.div_ceil(32) * 4;
        let mut dib = Vec::new();
        dib.extend_from_slice(&40u32.to_le_bytes());
        dib.extend_from_slice(&(size as i32).to_le_bytes());
        dib.extend_from_slice(&((size * 2) as i32).to_le_bytes());
        dib.extend_from_slice(&1u16.to_le_bytes());
        dib.extend_from_slice(&32u16.to_le_bytes());
        dib.extend_from_slice(&[0; 24]);
        for row in rgba.chunks_exact(size * 4).rev() {
            for p in row.as_chunks::<4>().0.iter() {
                dib.extend_from_slice(&[p[2], p[1], p[0], p[3]]);
            }
        }
        for row in rgba.chunks_exact(size * 4).rev() {
            let mut mask = vec![0u8; mask_stride];
            for (x, p) in row.as_chunks::<4>().0.iter().enumerate() {
                if p[3] == 0 {
                    mask[x / 8] |= 0x80 >> (x % 8);
                }
            }
            dib.extend_from_slice(&mask);
        }
        images.push((size, dib));
    }
    let mut ico = vec![0, 0, 1, 0];
    ico.extend_from_slice(&(images.len() as u16).to_le_bytes());
    let mut offset = 6 + 16 * images.len();
    for (size, data) in &images {
        let dim = if *size == 256 { 0 } else { *size as u8 };
        ico.extend_from_slice(&[dim, dim, 0, 0, 1, 0, 32, 0]);
        ico.extend_from_slice(&(data.len() as u32).to_le_bytes());
        ico.extend_from_slice(&(offset as u32).to_le_bytes());
        offset += data.len();
    }
    for (_, data) in images {
        ico.extend_from_slice(&data);
    }
    Ok(ico)
}