use core::sync::atomic::{AtomicUsize, Ordering};

use std::sync::Once;

static WORKS: AtomicUsize = AtomicUsize::new(0);

static INIT: Once = Once::new();

pub(crate) fn inside_proc_macro() -> bool {

    match WORKS.load(Ordering::Relaxed) {

        1 => return false,

        2 => return true,

        _ => {}

    }

    INIT.call_once(initialize);

    inside_proc_macro()

}

pub(crate) fn force_fallback() {

    WORKS.store(1, Ordering::Relaxed);

}

pub(crate) fn unforce_fallback() {

    initialize();

}

#[cfg(not(no_is_available))]

fn initialize() {

    let available = proc_macro::is_available();

    WORKS.store(available as usize + 1, Ordering::Relaxed);

}

// Swap in a null panic hook to avoid printing "thread panicked" to stderr,

// then use catch_unwind to determine whether the compiler's proc_macro is

// working. When proc-macro2 is used from outside of a procedural macro all

// of the proc_macro crate's APIs currently panic.

//

// The Once is to prevent the possibility of this ordering:

//

//     thread 1 calls take_hook, gets the user's original hook

//     thread 1 calls set_hook with the null hook

//     thread 2 calls take_hook, thinks null hook is the original hook

//     thread 2 calls set_hook with the null hook

//     thread 1 calls set_hook with the actual original hook

//     thread 2 calls set_hook with what it thinks is the original hook

//

// in which the user's hook has been lost.

//

// There is still a race condition where a panic in a different thread can

// happen during the interval that the user's original panic hook is

// unregistered such that their hook is incorrectly not called. This is

// sufficiently unlikely and less bad than printing panic messages to stderr

// on correct use of this crate. Maybe there is a libstd feature request

// here. For now, if a user needs to guarantee that this failure mode does

// not occur, they need to call e.g. `proc_macro2::Span::call_site()` from

// the main thread before launching any other threads.

#[cfg(no_is_available)]

fn initialize() {

    use std::panic::{self, PanicInfo};

    type PanicHook = dyn Fn(&PanicInfo) + Sync + Send + 'static;

    let null_hook: Box<PanicHook> = Box::new(|_panic_info| { /* ignore */ });

    let sanity_check = &*null_hook as *const PanicHook;

    let original_hook = panic::take_hook();

    panic::set_hook(null_hook);

    let works = panic::catch_unwind(proc_macro::Span::call_site).is_ok();

    WORKS.store(works as usize + 1, Ordering::Relaxed);

    let hopefully_null_hook = panic::take_hook();

    panic::set_hook(original_hook);

    if sanity_check != &*hopefully_null_hook {

        panic!("observed race condition in proc_macro2::inside_proc_macro");

    }

}