//! Mime mmagic analogous to [`magic::Cookie`] BUT [`Sync`]

// I don't think I can do better with thread exclusive `libmagic`

use std::cell::OnceCell;

use magic::{Cookie, cookie};

#[derive(Debug, thiserror::Error)]
pub enum NewCookieError {
    #[error(transparent)]
    MimeCookieOpen(#[from] magic::cookie::OpenError),
    #[error(transparent)]
    MimeDBLoad(#[from] magic::cookie::LoadError<magic::cookie::Open>),
    // #[error(transparent)]
    // MimeDBPath(#[from] magic::cookie::InvalidDatabasePathError),
}

pub struct Mime {
    magic_dbs: magic::cookie::DatabasePaths,
}

impl Mime {
    thread_local! {
        static COOKIE_CELL: OnceCell<Cookie<cookie::Load>> = const { OnceCell::new() };
    }

    fn use_cookie<F, T>(&self, f: F) -> Result<T, NewCookieError>
    where
        F: FnOnce(&Cookie<cookie::Load>) -> T,
    {
        Self::COOKIE_CELL.with(|cookie| {
            let may_cookie = cookie.get_or_try_init::<_, NewCookieError>(move || {
                let cookie = magic::Cookie::open(magic::cookie::Flags::MIME)?;
                Ok(cookie.load(&self.magic_dbs)?)
            });

            match may_cookie {
                Ok(c) => Ok(f(c)),
                Err(e) => Err(e),
            }
        })
    }

    #[must_use]
    pub const fn new(magic_dbs: magic::cookie::DatabasePaths) -> Self {
        Self { magic_dbs }
    }

    /// Cookie initialization is delayed, so each call might be the creation of the cookie if its
    /// the first use of the cookie in this thread.
    ///
    /// # Errors
    ///
    /// First layer error fails if new cookie creation failed. Second layer error represents if the
    /// mime search was successful.
    pub fn buffer(
        &self,
        buffer: &[u8],
    ) -> Result<Result<String, magic::cookie::Error>, NewCookieError> {
        self.use_cookie(|c| c.buffer(buffer))
    }
}

// fn __ace(c: Mime) {
//     fn __ace_inner(c: impl Sync + Send) {}
//     __ace_inner(c);
// }