The decomposer, converting the ani file to ico files.

ani2ico.rs

use std::{env, fs, io::Write, path::PathBuf};

const ICON_MAGIC: &[u8] = b"icon";

fn next_partition(buf: &[u8]) -> Option<usize> {
    buf.windows(ICON_MAGIC.len())
        .position(|win| win == ICON_MAGIC)
}

fn hoge() -> Result<(), Box<dyn std::error::Error + 'static>> {
    let path = PathBuf::from(
        env::args()
            .nth(1)
            .expect("Please give .ani file path as an argument"),
    );
    let buf = fs::read(&path)?;

    let mut buf = &buf[..];
    let mut count = 0;

    while let Some(idx) = next_partition(buf) {
        let end = next_partition(&buf[idx + 1..]).unwrap_or(buf.len());

        count += 1;
        let mut out_file_name = path.file_name().unwrap().to_os_string();
        out_file_name.push(format!("{:04}", count));
        let mut out_path = path.clone();
        out_path.set_file_name(out_file_name);
        out_path.set_extension("ico");
        let mut out = fs::OpenOptions::new()
            .write(true)
            .create_new(true)
            .open(out_path)?;

        let mut chunk_idx = 8;
        while idx + chunk_idx + 4 <= end {
            out.write_all(&[if chunk_idx == 10 {
                0x01
            } else {
                buf[idx + chunk_idx]
            }])?;
            chunk_idx += 1;
        }
        if idx + chunk_idx <= end {
            out.write_all(&[buf[idx + chunk_idx]])?;
        }
        if end - idx - chunk_idx <= 3 {
            let i = idx + chunk_idx + 1;
            out.write_all(&buf[i..i + 2])?;
        }
        buf = &buf[idx + 1..];
    }
    Ok(())
}