//! # nom-varint
//!
//! Parse varints (used by Minecraft, Celeste, and Protocol Buffers) using nom.

#![no_std]

use core::convert::TryInto;
use core::u32;
use nom::bytes::complete::take;
use nom::error::{ErrorKind, ParseError};
use nom::Err::*;
use nom::Needed::Unknown;

/// Parses a varint.
///
/// # Examples
///
/// ```
/// assert_eq!(nom_varint::take_varint::<()>(&[0x0b]), Ok((&[] as &[u8], 0x0b)));
/// ```
pub fn take_varint<'a, E>(i: &'a [u8]) -> nom::IResult<&'a [u8], usize, E>
where
    E: ParseError<&'a [u8]>,
{
    let mut res: usize = 0;
    let mut count: usize = 0;
    let mut remainder = i;
    loop {
        let byte = match take::<usize, &[u8], ()>(1)(remainder) {
            Ok((rest, bytes)) => {
                remainder = rest;
                bytes[0]
            }
            Err(_) => return Err(Incomplete(Unknown)),
        };
        res += ((byte as usize) & 127)
            .checked_shl((count * 7).try_into().unwrap_or(u32::MAX))
            .ok_or_else(|| Error(E::from_error_kind(remainder, ErrorKind::MapOpt)))?;
        count += 1;
        if (byte >> 7) == 0 {
            return Ok((remainder, res));
        }
    }
}

#[cfg(test)]
mod test {
    #[test]
    fn parse_varint_simple() {
        assert_eq!(
            super::take_varint::<()>(&[0x0b, 0x01, 0x02, 0x03]),
            Ok((b"\x01\x02\x03" as &[u8], 11))
        );
    }

    #[test]
    fn parse_varint_twobyte() {
        assert_eq!(
            super::take_varint::<()>(&[0x84, 0x02, 0x04, 0x05, 0x06]),
            Ok((b"\x04\x05\x06" as &[u8], 260))
        );
    }
}