reverse/src/main.rs

//#[inline] 
fn reverse<T>(slice: &mut [T])
{
    match slice {
	[ref mut a, ref mut rest @ .., ref mut b] => {
	    std::mem::swap(a, b);
	    reverse(rest)
	},
	[] | [_] => (),
    }
}

/// This actually works! And is so easily parallelisable with something like rayon, or asyncing by spawning/creating the tail-call into a new task, then either waiting them concurrently or in parallen (spawn or created future from just calling the function without awaiting it)
#[allow(dead_code)]
fn binsearch<'a, V: ?Sized, T: PartialEq<V> + 'a>(slice: &'a [T], find: &V) -> Option<&'a T>
{
    match slice {
	[ref a, pivot @ .., ref b] => {
	    match (a==find, b==find) {
		(true, _) => Some(a),
		(_, true) => Some(b),
		_ => binsearch(pivot, find),
	    }
	},
	[ref a] if a == find => Some(a),
	_ => None,
    }
}

trait Input: AsRef<[u8]> + std::fmt::Debug{}
impl<T: ?Sized> Input for T
where T: AsRef<[u8]> + std::fmt::Debug{}

fn collect_input() -> Box<dyn Iterator<Item= impl Input + 'static> + 'static>
{
    use std::{
	ffi::{OsStr, OsString},
	os::unix::ffi::*,
    };
    
    #[derive(Debug)]
    enum MaybeUTF8
    {
	UTF8(String),
	Raw(OsString),
    }

    impl AsRef<[u8]> for MaybeUTF8
    {
	#[inline] 
	fn as_ref(&self) -> &[u8]
	{
	    match self {
		Self::UTF8(string) => string.as_bytes(),
		Self::Raw(raw) => raw.as_bytes(),
	    }
	}
    }

    impl From<String> for MaybeUTF8
    {
	#[inline(always)] 
	fn from(from: String) -> Self
	{
	    Self::UTF8(from)
	}
    }

    impl From<OsString> for MaybeUTF8
    {
	#[inline(always)] 
	fn from(from: OsString) -> Self
	{
	    Self::Raw(from)
	}
    }

    #[allow(dead_code)]
    impl MaybeUTF8
    {
	#[inline(always)] 
	pub fn from_raw_bytes(bytes: &[u8]) -> Self
	{
	    Self::Raw(OsStr::from_bytes(bytes).to_os_string())
	}
	#[inline(always)]
	#[deprecated(note="XXX: TODO: Only use this if the read_until() into vec does not add the '\n' into the vec as well. Otherwise, *always* use this.")]
	pub fn from_raw_vec(vec: Vec<u8>) -> Self
	{
	    Self::Raw(OsString::from_vec(vec))
	}
    }
    
    if std::env::args_os().len() <= 1 {
	use std::io::{
	    self,
	    BufRead,
	};
	// No args, collect stdin lines
	if !cfg!(feature="byte-strings") {
	    // Collect utf8 string lines
	    Box::new(io::stdin()
		     .lock()
		     .lines()
		     .filter_map(Result::ok)
		     .map(MaybeUTF8::from))
	} else {
	    // Collect arbitrary byte strings
	    struct OsLineReader<'a>(io::StdinLock<'a>, Vec<u8>);

	    impl<'a> Iterator for OsLineReader<'a>
	    {
		type Item = MaybeUTF8;
		fn next(&mut self) -> Option<Self::Item>
		{
		    Some(match handle_fmt_err_or(self.0.read_until(b'\n', &mut self.1), || 0) {
			0 => return None,
			read_sz => {
			    let line = MaybeUTF8::from_raw_bytes(&self.1[..]); //TODO: XXX: If self.1 here does not have the '\n' added into it by read_until(); use from_raw_vec(self.1.clone()) instead; it'll be more efficient.
			    self.1.clear();
			    //TODO: todo!("Do we need read_sz ({read_sz}) at all here? Will the `\n` be inside the read string?");
			    line
			},
		    })
		}
	    }	   

	    Box::new(OsLineReader(io::stdin().lock(), // Acquire the lock until the iterator is consumed (like all other paths in this function)
				  Vec::with_capacity(4096))) // Buffer of size 4k
	}
    } else {
	// Has arguments, return them
	if cfg!(feature="byte-strings") {
	    Box::new(std::env::args_os().skip(1).map(MaybeUTF8::from))
	} else if cfg!(feature="ignore-invalid-args") {
	    Box::new(std::env::args_os().skip(1).filter_map(|os| os.into_string().ok()).map(MaybeUTF8::from))
	} else {
	    Box::new(std::env::args_os().skip(1).map(|os| os.to_string_lossy().into_owned().into()))	
	}
    }
}

#[cfg_attr(feature="ignore-output-errors", inline)]
fn handle_fmt_err_or<F, T>(res: std::io::Result<T>, or: F) -> T
where F: FnOnce() -> T
{
    #[cfg(not(feature="ignore-output-errors"))]
    if let Err(e) = res {
	eprintln!("[!] failed to write line: {e}");
	or()
    }
    #[cfg(feature="ignore-output-errors")]
    res.unwrap_or_else(|_| or())
}

#[cfg_attr(feature="ignore-output-errors", inline(always))]
fn handle_fmt_err<T>(res: std::io::Result<T>)
{
    #[cfg(not(feature="ignore-output-errors"))]
    if let Err(e) = res {
	eprintln!("[!] failed to write line: {e}");
    }
    let _ = res;
}

fn main() {
    let mut args: Vec<_> = collect_input().collect();
    reverse(&mut args[..]);
    //eprintln!("{:?}", binsearch(&args[..], "1")); // It works!
    #[cfg(feature="output-lines")]
    {
	#[allow(unused_imports)]
	use std::io::{
	    Write, BufWriter,
	};
	#[cfg(feature="buffer-output")] 
	let mut out = BufWriter::new(std::io::stdout().lock());
	#[cfg(not(feature="buffer-output"))] 
	let mut out = std::io::stdout().lock();
	for x in args.iter() {
	    handle_fmt_err({
		if cfg!(feature="output-quoted") {
		    //XXX: This doesn't flush, right? It shouldn't, but maybe we should test it?
		    writeln!(&mut out, "{:?}", x)
		}
		else  {
		    //writeln!(&mut out, "{}", x)
		    out.write(x.as_ref())
			.and_then(|_| out.write(b"\n"))
			.map(|_| {})
		}
	    });
	}
	//#[cfg(feature="buffer-output")] 
	handle_fmt_err(out.flush()); //XXX: Do we need to flush when not buffering output? Does it matter since buffering output will be enabled by default and should almost always be enabled?
    }
    #[cfg(not(feature="output-lines"))]
    println!("{:?}", args);
}