use thiserror::Error;

type Result <T> = std::result::Result <T, TlvError>;

#[derive (Debug, Error)]
pub enum TlvError {
	#[error ("Buffer too big")]
	BufferTooBig,
	
	// Violets are purple,
	// To live is to suffer,
	// The data is too big,
	// For the gosh-darn buffer.
	
	#[error ("Data too big")]
	DataTooBig,
	#[error (transparent)]
	Io (#[from] std::io::Error),
	#[error ("Actual bytes didn't match expected bytes")]
	NotExpected,
	#[error (transparent)]
	TryFromInt (#[from] std::num::TryFromIntError),
}

pub struct Writer <W> {
	_x: std::marker::PhantomData <W>,
}

impl <W: std::io::Write> Writer <W> {
	fn length (w: &mut W, x: u32) -> Result <()> {
		w.write_all (&x.to_le_bytes ())?;
		Ok (())
	}
	
	pub fn lv_bytes (w: &mut W, b: &[u8]) -> Result <()> {
		if b.len () > 2_000_000_000 {
			return Err (TlvError::BufferTooBig);
		}
		
		let l = u32::try_from (b.len ())?;
		
		Self::length (w, l)?;
		w.write_all (b)?;
		
		Ok (())
	}
}

pub struct Reader <R> {
	_x: std::marker::PhantomData <R>,
}

impl <R: std::io::Read> Reader <R> {
	pub fn expect (r: &mut R, expected: &[u8]) -> Result <()> {
		let mut actual = vec! [0u8; expected.len ()];
		r.read_exact (&mut actual)?;
		if actual != expected {
			return Err (TlvError::NotExpected);
		}
		Ok (())
	}
	
	pub fn length (r: &mut R) -> Result <u32> {
		let mut buf = [0; 4];
		r.read_exact (&mut buf)?;
		
		Ok (u32::from_le_bytes (buf))
	}
	
	pub fn lv_bytes_to_vec (r: &mut R, limit: usize) -> Result <Vec <u8>> {
		let l = Self::length (r)?;
		let l = usize::try_from (l)?;
		if l > limit {
			return Err (TlvError::DataTooBig);
		}
		
		let mut v = vec! [0u8; l];
		r.read_exact (&mut v)?;
		
		Ok (v)
	}
	
	pub fn u8 (r: &mut R) -> std::io::Result <u8> {
		let mut buf = [0];
		r.read_exact (&mut buf)?;
		
		Ok (buf [0])
	}
}

#[cfg (test)]
mod test {
	use super::*;
	
	#[test]
	fn test_1 () -> Result <()> {
		use std::io::Cursor;
		
		let b = "hi there".as_bytes ();
		
		let mut w = Cursor::new (Vec::default ());
		
		super::Writer::lv_bytes (&mut w, b)?;
		
		let v = w.into_inner ();
		
		assert_eq! (v, vec! [
			8, 0, 0, 0,
			104, 105, 32,
			116, 104, 101, 114, 101,
		]);
		
		let mut r = Cursor::new (v);
		
		let buf = Reader::lv_bytes_to_vec (&mut r, 1024)?;
		
		assert_eq! (buf.len (), b.len ());
		assert_eq! (b, &buf);
		
		Ok (())
	}
}