nom/bits/

complete.rs

//! Bit level parsers
//!

use crate::error::{ErrorKind, ParseError};
use crate::internal::{Err, IResult};
use crate::lib::std::ops::{AddAssign, Div, Shl, Shr};
use crate::traits::{Input, ToUsize};

/// Generates a parser taking `count` bits
///
/// # Example
/// ```rust
/// # use nom::bits::complete::take;
/// # use nom::IResult;
/// # use nom::error::{Error, ErrorKind};
/// // Input is a tuple of (input: I, bit_offset: usize)
/// fn parser(input: (&[u8], usize), count: usize)-> IResult<(&[u8], usize), u8> {
///  take(count)(input)
/// }
///
/// // Consumes 0 bits, returns 0
/// assert_eq!(parser(([0b00010010].as_ref(), 0), 0), Ok((([0b00010010].as_ref(), 0), 0)));
///
/// // Consumes 4 bits, returns their values and increase offset to 4
/// assert_eq!(parser(([0b00010010].as_ref(), 0), 4), Ok((([0b00010010].as_ref(), 4), 0b00000001)));
///
/// // Consumes 4 bits, offset is 4, returns their values and increase offset to 0 of next byte
/// assert_eq!(parser(([0b00010010].as_ref(), 4), 4), Ok((([].as_ref(), 0), 0b00000010)));
///
/// // Tries to consume 12 bits but only 8 are available
/// assert_eq!(parser(([0b00010010].as_ref(), 0), 12), Err(nom::Err::Error(Error{input: ([0b00010010].as_ref(), 0), code: ErrorKind::Eof })));
/// ```
pub fn take<I, O, C, E: ParseError<(I, usize)>>(
  count: C,
) -> impl Fn((I, usize)) -> IResult<(I, usize), O, E>
where
  I: Input<Item = u8>,
  C: ToUsize,
  O: From<u8> + AddAssign + Shl<usize, Output = O> + Shr<usize, Output = O>,
{
  let count = count.to_usize();
  move |(input, bit_offset): (I, usize)| {
    if count == 0 {
      Ok(((input, bit_offset), 0u8.into()))
    } else if input.input_len() * 8 < count + bit_offset {
      Err(Err::Error(E::from_error_kind(
        (input, bit_offset),
        ErrorKind::Eof,
      )))
    } else {
      let cnt = (count + bit_offset).div(8);
      let mut acc: O = 0_u8.into();
      let mut offset: usize = bit_offset;
      let mut remaining: usize = count;
      let mut end_offset: usize = 0;

      for byte in input.iter_elements().take(cnt + 1) {
        if remaining == 0 {
          break;
        }
        let val: O = if offset == 0 {
          byte.into()
        } else {
          ((byte << offset) >> offset).into()
        };

        if remaining < 8 - offset {
          acc += val >> (8 - offset - remaining);
          end_offset = remaining + offset;
          break;
        } else {
          acc += val << (remaining - (8 - offset));
          remaining -= 8 - offset;
          offset = 0;
        }
      }
      Ok(((input.take_from(cnt), end_offset), acc))
    }
  }
}

/// Generates a parser taking `count` bits and comparing them to `pattern`
pub fn tag<I, O, C, E: ParseError<(I, usize)>>(
  pattern: O,
  count: C,
) -> impl Fn((I, usize)) -> IResult<(I, usize), O, E>
where
  I: Input<Item = u8> + Clone,
  C: ToUsize,
  O: From<u8> + AddAssign + Shl<usize, Output = O> + Shr<usize, Output = O> + PartialEq,
{
  let count = count.to_usize();
  move |input: (I, usize)| {
    let inp = input.clone();

    take(count)(input).and_then(|(i, o)| {
      if pattern == o {
        Ok((i, o))
      } else {
        Err(Err::Error(error_position!(inp, ErrorKind::TagBits)))
      }
    })
  }
}

/// Parses one specific bit as a bool.
///
/// # Example
/// ```rust
/// # use nom::bits::complete::bool;
/// # use nom::IResult;
/// # use nom::error::{Error, ErrorKind};
///
/// fn parse(input: (&[u8], usize)) -> IResult<(&[u8], usize), bool> {
///     bool(input)
/// }
///
/// assert_eq!(parse(([0b10000000].as_ref(), 0)), Ok((([0b10000000].as_ref(), 1), true)));
/// assert_eq!(parse(([0b10000000].as_ref(), 1)), Ok((([0b10000000].as_ref(), 2), false)));
/// ```
pub fn bool<I, E: ParseError<(I, usize)>>(input: (I, usize)) -> IResult<(I, usize), bool, E>
where
  I: Input<Item = u8>,
{
  let (res, bit): (_, u32) = take(1usize)(input)?;
  Ok((res, bit != 0))
}

#[cfg(test)]
mod test {
  use super::*;

  #[test]
  fn test_take_0() {
    let input = [0b00010010].as_ref();
    let count = 0usize;
    assert_eq!(count, 0usize);
    let offset = 0usize;

    let result: crate::IResult<(&[u8], usize), usize> = take(count)((input, offset));

    assert_eq!(result, Ok(((input, offset), 0)));
  }

  #[test]
  fn test_take_eof() {
    let input = [0b00010010].as_ref();

    let result: crate::IResult<(&[u8], usize), usize> = take(1usize)((input, 8));

    assert_eq!(
      result,
      Err(crate::Err::Error(crate::error::Error {
        input: (input, 8),
        code: ErrorKind::Eof
      }))
    )
  }

  #[test]
  fn test_take_span_over_multiple_bytes() {
    let input = [0b00010010, 0b00110100, 0b11111111, 0b11111111].as_ref();

    let result: crate::IResult<(&[u8], usize), usize> = take(24usize)((input, 4));

    assert_eq!(
      result,
      Ok((([0b11111111].as_ref(), 4), 0b1000110100111111111111))
    );
  }

  #[test]
  fn test_bool_0() {
    let input = [0b10000000].as_ref();

    let result: crate::IResult<(&[u8], usize), bool> = bool((input, 0));

    assert_eq!(result, Ok(((input, 1), true)));
  }

  #[test]
  fn test_bool_eof() {
    let input = [0b10000000].as_ref();

    let result: crate::IResult<(&[u8], usize), bool> = bool((input, 8));

    assert_eq!(
      result,
      Err(crate::Err::Error(crate::error::Error {
        input: (input, 8),
        code: ErrorKind::Eof
      }))
    );
  }
}