//! Directory walking
use super::*;
use std::num::NonZeroUsize;
use std::{
    sync::Arc,
    path::{
	Path, PathBuf,
    },
};
use tokio::{
    sync::{
	RwLock,
	Semaphore,
    },
};
use futures::future::{
    Future,
    OptionFuture,
    BoxFuture,
};


lazy_static! {
    pub(super) static ref NUM_CPUS: usize = num_cpus::get();
}

/// Default number of max walkers allowed to work at once on the thread pool.
/// See `Config`.
#[inline] 
pub fn default_max_walkers() -> Option<NonZeroUsize>
{
    NonZeroUsize::new(*NUM_CPUS)
}

#[derive(Debug, Clone, PartialEq, Eq, Hash, PartialOrd, Ord)]
pub struct Config
{
    // None: unlimited. 0: no recursion
    pub recursion_depth: Option<usize>,
    // None: Unlimited tasks.
    pub max_walkers: Option<NonZeroUsize>,
}

impl Default for Config
{
    #[inline]
    fn default() -> Self
    {
	Self {
	    recursion_depth: Some(0),
	    max_walkers: default_max_walkers(),
	}
    }
}


#[derive(Debug, Clone)]
struct UniqueState
{
    // Decrease with each new depth until 0 is hit then do not proceed.
    recursion_depth: Option<usize>,
    output_sender: mpsc::Sender<work::FileInfo>,
}

#[derive(Debug)]
struct SharedState
{
    worker_config: Arc<work::Config>,
    config: Config,
    new_worker_semaphore: Option<Semaphore>,
}

#[derive(Debug, Clone)]
struct State {
    /// Shared state for all iterations
    shared: Arc<SharedState>, //TODO: XXX: *another* Arc slowdown...
    /// State for current iteration
    current: UniqueState,
}

impl State
{
    #[inline] 
    pub fn create_file_info_generator(&self) -> impl Fn(PathBuf, std::fs::Metadata) -> work::FileInfo + '_
    {
	work::FileInfo::factory(Arc::clone(&self.shared.worker_config)) //XXX: ANOTHER unneeded Arc clone..
    }
    #[inline(always)] 
    pub fn shared(&self) -> &SharedState
    {
	&self.shared
    }
    #[inline(always)] 
    pub fn unique(&self) -> &UniqueState
    {
	&self.current
    }
    
    #[inline(always)] 
    pub fn unique_mut(&mut self) -> &mut UniqueState
    {
	&mut self.current
    }

    /// Create a `State` for a recursed child walk. 
    #[inline] 
    pub fn create_child(&self) -> Option<Self>
    {
	let recursion_depth = match self.current.recursion_depth.map(|depth| depth.saturating_sub(1)) {
	    // Prevent child creation if recursion bound hits 0.
	    Some(0) => return None,
	    x => x,
	};
	Some(Self {
	    shared: Arc::clone(&self.shared),
	    current: UniqueState { recursion_depth, output_sender: self.current.output_sender.clone() },
	})
    }
}

/// Walk a single directory in the current task. Dispatch async results to `walk
async fn walk_directory<F>(state: &State, mut push_child: F, whence: impl AsRef<Path>) -> eyre::Result<usize>
where F: FnMut(State, PathBuf)
{
    //use futures::future::FutureExt;
    let mut read = tokio::fs::read_dir(whence).await.wrap_err("Failed to read-dir")?;
    let sender = &state.current.output_sender;
    let file_info = state.create_file_info_generator();
    let mut n = 0;
    while let Some(fso) = read.next_entry().await.wrap_err("Failed to enumerate directory entry")?
    {
	let metadata = match fso.metadata().await {
	    Ok(metadata) => metadata,
	    Err(e) => {
		error!("Failed to stat {:?}: {}", fso.file_name(), e);
		continue;
	    }
	};
	if metadata.is_file() {
	    if let Err(_) = sender.send(file_info(fso.path(), metadata)).await {
		warn!("Worker shut down, stopping iteration");
		break;
	    } else {
		n += 1;
	    }
	} else if metadata.is_dir() {
	    let Some(state) = state.create_child() else { continue };
	    push_child(state, fso.path());
	} // Ignore symlinks.
    }
    Ok(n)
    //todo!("Walk to directory and output its files into `state`'s tx XXX: Does this function need to exist? We could just do this in walk_inner() directly: Explicit boxing doesn't need to be done as we're working with joinhandles and backing tasks")
}

/// This function is called recursively for each subdirectory in `whence` pertaining to the recursion rules.
/// The function should await all its spawned children *after* finishing its own work on files, **and** dropping its semaphore `_permit`. Otherwise deadlock could occour easily.
async fn walk_inner(state: State, whence: PathBuf) -> eyre::Result<usize>
{

    use futures::prelude::*;
    //async move
    {
	let backing_res = tokio::spawn(async move {
	    
	    let _permit = match &state.shared.new_worker_semaphore {
		Some(sem) => Some(sem.acquire().await.expect("Failed to acquire permit")),
		None => None,
	    };
	    // `JoinHandle<eyre::Result<size>>`s to recursion children
	    // XXX: Maybe use mpsc for this instead: We send the JoinHandle's to a rx being looped on in a `join!` in the outer `async move {}` scope at the same time as this one. When the sender is dropped, the channel will close. We can join each child of the stream concurrently with `futures` (probably?) and bubble up panics when they are found.
	    let mut children = Vec::new();
	    // Number of *files* sent to tx from this iteration.
	    
	    let counted = if whence.is_dir() {
		walk_directory(&state, |state, whence| {
		    fn walk_inner2(state: State, whence: PathBuf) -> BoxFuture<'static,  eyre::Result<usize>>
		    {
			walk_inner(state, whence).boxed()
		    }
		    children.push(walk_inner2(state, whence));
		}, &whence).await?
	    } else {
		let metadata = tokio::fs::metadata(&whence).await.wrap_err("Failed to stat top-level file")?;
		if let Err(_) = state.current.output_sender.send(work::FileInfo::new(Arc::clone(&state.shared.worker_config), whence, metadata)).await {
		    return Err(eyre!("Failed to send top-level file to backing sorter: Sorter closed."));
		}
		1
	    };
	    
	    Ok((counted, children))
	}).await.expect("Panic in backing walker thread");

	trace!("Spawning and collecting child workers");
	match backing_res {
	    Ok((counted, children)) => {
		use futures::prelude::*;
		Ok(counted + futures::future::join_all(children.into_iter()).await.into_iter().filter_map(|res| match res {
		    Ok(n) => Some(n),
		    Err(e) => {
			error!("Child failed to walk: {}", e);
			None
		    },
		}).sum::<usize>())
	    },
	    Err(e) => Err(e),
	}
    }
}

#[inline] 
pub fn start_walk(cfg: Config, worker: Arc<work::Config>, whence: impl AsRef<Path>, whereto: mpsc::Sender<work::FileInfo>) -> impl Future<Output = eyre::Result<usize>> + 'static
{
    use futures::prelude::*;
    walk_inner(State {
	current: UniqueState {
	    recursion_depth: cfg.recursion_depth,
	    output_sender: whereto,
	},
	shared: Arc::new(SharedState {
	    new_worker_semaphore: cfg.max_walkers.as_ref().map(|max| Semaphore::new(max.get())),
	    config: cfg,
	    worker_config: worker,
	}),
    }, whence.as_ref().to_owned())
    //.flatten()
}