//! File system helper utilities.

use std::os::unix::io::RawFd;

use failure::Error;
use nix::dir;
use nix::dir::Dir;

use crate::tools::borrow::Tied;

// Since Tied<T, U> implements Deref to U, a Tied<Dir, Iterator> already implements Iterator.
// This is simply a wrapper with a shorter type name mapping nix::Error to failure::Error.
/// Wrapper over a pair of `nix::dir::Dir` and `nix::dir::Iter`, returned by `read_subdir()`.
pub struct ReadDir {
    iter: Tied<Dir, Iterator<Item = nix::Result<dir::Entry>>>,
}

impl Iterator for ReadDir {
    type Item = Result<dir::Entry, Error>;

    fn next(&mut self) -> Option<Self::Item> {
        self.iter.next().map(|res| res.map_err(|e| Error::from(e)))
    }
}

/// Create an iterator over sub directory entries.
/// This uses `openat` on `dirfd`, so `path` can be relative to that or an absolute path.
pub fn read_subdir<P: ?Sized + nix::NixPath>(dirfd: RawFd, path: &P) -> Result<ReadDir, Error> {
    use nix::fcntl::OFlag;
    use nix::sys::stat::Mode;

    let dir = Dir::openat(dirfd, path, OFlag::O_RDONLY, Mode::empty())?;
    let iter = Tied::new(dir, |dir| {
        Box::new(unsafe { (*dir).iter() }) as Box<Iterator<Item = nix::Result<dir::Entry>>>
    });
    Ok(ReadDir { iter })
}

/// Scan through a directory with a regular expression. This is simply a shortcut filtering the
/// results of `read_subdir`. Non-UTF8 comaptible file names are silently ignored.
pub fn scan_subdir<'a, P: ?Sized + nix::NixPath>(
    dirfd: RawFd,
    path: &P,
    regex: &'a regex::Regex,
) -> Result<impl Iterator<Item = Result<nix::dir::Entry, Error>> + 'a, Error> {
    Ok(read_subdir(dirfd, path)?.filter_map(move |entry| {
        match entry {
            Ok(entry) => match entry.file_name().to_str() {
                Ok(name) => {
                    if regex.is_match(name) {
                        Some(Ok(entry))
                    } else {
                        None // Skip values not matching the regex
                    }
                }
                // Skip values which aren't valid utf-8
                Err(_) => None,
            },
            // Propagate errors through
            Err(e) => Some(Err(Error::from(e))),
        }
    }))
}