use {
  pulldown_cmark::{
    Event,
    HeadingLevel::{H2, H3},
    Options, Parser, Tag,
  },
  pulldown_cmark_to_cmark::cmark,
  std::{error::Error, fs},
};

fn main() -> Result<(), Box<dyn Error>> {
  fs::remove_dir_all("book/src").ok();
  fs::create_dir("book/src")?;

  let txt = fs::read_to_string("README.md")?;

  let mut chapters = vec![(1usize, Vec::new())];

  for event in Parser::new_ext(&txt, Options::all()) {
    if let Event::Start(Tag::Heading(level @ (H2 | H3), ..)) = event {
      chapters.push((if level == H2 { 2 } else { 3 }, Vec::new()));
    }
    chapters.last_mut().unwrap().1.push(event);
  }

  let mut summary = String::new();

  for (i, (level, chapter)) in chapters.into_iter().enumerate() {
    let mut txt = String::new();
    cmark(chapter.iter(), &mut txt)?;
    let title = if i == 0 {
      txt = txt.split_inclusive('\n').skip(1).collect::<String>();
      "Introduction"
    } else {
      txt.lines().next().unwrap().split_once(' ').unwrap().1
    };

    let path = format!("book/src/chapter_{}.md", i + 1);
    fs::write(&path, &txt)?;
    summary.push_str(&format!(
      "{}- [{}](chapter_{}.md)\n",
      " ".repeat((level.saturating_sub(1)) * 4),
      title,
      i + 1
    ));
  }

  fs::write("book/src/SUMMARY.md", summary)?;

  Ok(())
}