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A Less Mad March

Parsing bitstreams with Nom

Programming languages generally only manipulate bytes (groups of 8 bits). It can be pretty tricky to manipulate single bits. But sometimes you need to -- for example, a DNS header has some 4-bit numbers, and encodes some boolean flags into single bits. So we really need a way to parse binary data without chunking it up into bytes of 8 bits.

Luckily, Nom can do this! In the last blog post, we learned how to parse text files with Nom. The trick is to start with simple parsers that parse a few characters at a time. Then, using combinators, combine those simple parsers into more complex parsers that can deserialize an entire structured file. We can reuse this approach for parsing binary data too. Let's see how!

Note that all code examples use Nom 7 -- I'll try to update this if Nom 8 makes breaking changes, but I can't guarantee anything :)

Representing bitstream inputs

In the previous post, we saw that Nom parsers are generic over three types:

  • I, the input
  • O, the output
  • E, the error type

In the last post, we used I = &str, which lets you parse a stream of text. The input type I = &[u8] lets you parse a stream of bytes. But how can we represent a stream of bits? Rust doesn't have any type to represent a bit!

This question is actually really important, and it's going to come up several times in this blog post. How do we represent n < 8 bits?

My first answer was "just use bools" -- you were probably thinking this too. Just represent 0 as false and 1 as true. This definitely works, but it's a bit wasteful. Rust bools actually take up an entire byte. So you could represent a bitstream as &[bool], but it would take 8x as many bytes as necessary! That's fine for many applications, but Nom has a more efficient representation.

The solution to representing a bitstream is pretty clever: we use two values. Firstly, a list of bytes, and secondly, a number representing which exact bit of those bytes is next.

I = (&[u8], usize)

This pair represents a bitstream. The first element is a stream of bits, stored one byte at a time (so, in groups of 8 bits). The second element is an offset, showing which bit (not byte!) should be read next.

For example, let's say we want to parse a sequence of 16 bits like 1111 0000 1100 1100. We'll start by reading from the very first element. This is how I visualize the bitstream and offset:


Note the little arrow pointing at the first bit in the input. To represent this using the I = (&[u8], usize) type, we break the bit input into bytes, and note which bit is currently being pointed to. Like this:

([0b11110000, 0b11001100], 0)

Lets say you parsed 3 bits from there. After that, the bitstream would be

([0b11110000, 0b11001100], 3)

After reading another six bits, the input would have advanced past the first byte:

([0b11110000, 0b11001100], 9)

Because the first byte will never be used again, Nom optimizes by dropping the first byte, adjusting the offset to account for that:

 ([0b11001100], 1)

Because this tuple type appears so often, I usually add a type alias for it:

type BitInput<'a> = (&'a [u8], usize);

Parsing bitstreams with "take"

We just learned how Nom represents inputs that can address individual bits. Now we're going to see how to run parsers on that bitstream input. There's two main bit-level parsers: take and tag.

The nom::bits::complete::take parser is similar to the nom::bytes::complete::take parser from the previous post. It has a parameter, count, and it takes that number of bits from the input, then returns those count bits as the output. If there's not enough bits left in the input (i.e. the input's length in bits is < count) then it panics.

This parser sounds pretty simple, but invoking it takes a little work, because it has a lot of generics 1 and Rust usually can't infer the types. So I've always just used helper functions that provide specific, concrete types. So, let's build a parser that parses four bits. A four-bit number is called a nibble (because it's half a byte... get it... by engineering standards this qualifies as a "joke").

Obviously its I (generic input type) will be the standard BitInput alias. But what will the O (generic output type) be? After all, Rust doesn't have a u4 type. How do we store a 4 bit number?

This question is really just the earlier question of "how can we represent bits in Rust" again. See, I told you it'd come up several times in this post.

Well, Nom's take parser solves this by padding your n bits with leading zeroes, and storing them in some uint type like u8, u16 or whichever one you, the programmer, choose. We'll parse the first 4 bits into a number between 0 and 15, and then just store that number in a u8. This means the 4 most significant bits will always be zero. You can use any uint type, but a u8 will do just fine, and wastes less RAM than using a u16 or any larger type.

use nom::{bits::complete::take, IResult};
type BitInput<'a> = (&'a [u8], usize);

/// Take 4 bits from the BitInput.
/// Store the output in a u8, because there's no u4 type, and u8 is the 
/// closest-available size.
fn take_nibble(i: BitInput) -> IResult<BitInput, u8> {
    // Have to specify some concrete numeric type, otherwise Rust won't know which
    // type of number you're trying to use here. I used usize, but you could use 
    // any uint type.
// Note that Rust number literals let you put underscores wherever you'd like, to
// enhance readability. E.g. you can help separate commas, by writing 1000000 as 
// 1_000_000. 
// I've used them here to visually separate the two u4 values in this u8.
let input = ([0b1010_1111].as_ref(), 0);

let (_input, actual_nibble) = take_nibble(input).unwrap();
let expected_nibble = 0b1010;
assert_eq!(actual_nibble, expected_nibble);

Parsing bitstreams with "tag"

The bitwise tag parser matches a specific pattern of bits, like "0110", from the input. Again, it's a simple idea, but it raises a tricky question: how do we represent a pattern of bits? Yes, this is the third time I've asked "how do we represent bits in Rust". I told you, it's a pretty fundamental question! And it uses a very similar solution.

Nom represents the bit pattern using two parameters:

  • count: how many bits long the pattern is
  • pattern: The pattern itself, padded with leading zeroes to fit into some uint type.

For example:

  • The pattern 101 is represented as (pattern: 00000_101, count: 3).
  • The pattern 111000111 is represented as (pattern: 0000000_111000111, count: 9).

You, the programmer, will choose which uint types to use for pattern and count -- the parser is generic over various uint types. I personally would just use the smallest uint that fits the value. So, example 1's pattern fits in a u8, example 2's fits in a u16, and in both the count fits in a u8. So I'd just use those. I don't really think it really matters that much.

OK, now that we know how to represent a pattern of bits, the tag parser is easy. You supply a pattern of bits, and Nom compares it, bit-by-bit, with the input bits. Like all Nom parsers, tag returns a Result. If the parser matches the input, it returns OK with a pair: (remaining input, matched output). If there's no match, it returns Err. So, for example, parsing the pattern 101 on the bitstream 10100 will return (00, 101).

We're now ready to look at actual code:

use nom::{bits::complete::tag, IResult};

type BitInput<'a> = (&'a [u8], usize);

// This is just a simple wrapper around the `tag` parser, but it makes the 
// parameter types concrete instead of generic, so now Rust knows how to actually
// store the pattern.
fn parser(pattern: u8, count: u8, input: BitInput) -> IResult<BitInput, u8> {
    tag(pattern, num_bits_to_compare)(input)

// The pattern 1111 matches the stream 1111_1111
assert!(parser(0b1111, 4, (&[0b1111_1111], 0)).is_ok());
// The pattern 1 matches the stream too
assert!(parser(0b1, 1, (&[0b1111_1111], 0)).is_ok());
// The pattern 01 does _not_ match the stream
assert!(parser(0b1, 2, (&[0b1111_1111], 0)).is_err());
// The pattern 1111_1110 doesn't match the stream either
assert!(parser(0b1111_1110, 8, (&[0b1111_1111], 0)).is_err());

Bitstreams and combinators

Remember the whole idea of a parser combinator library is: start with a few small parsers, then combine them with combinator functions. We've seen two primitive parsers for bitstreams, tag and take. Here's how to combine them with e.g. the map combinator from my last post:

use nom::{bits::complete::take, combinator::map, IResult};
type BitInput<'a> = (&'a [u8], usize);

/// Takes one bit from the input, returning true for 1 and false for 0.
fn take_bit(i: BitInput) -> IResult<BitInput, bool> {
    map(take(1usize), |bits: u8| bits > 0)(i)

let input = ([0b10101010].as_ref(), 0);
let (input, first_bit) = take_bit(input).unwrap();
assert!(first_bit); // First bit is 1
let (_input, second_bit) = take_bit(input).unwrap();
assert!(!second_bit); // Second bit is 0

Converting bytestreams to bitstreams and back

So far we've learned how to make simple bit parsers, and combine them into complex ones. We've even learned how Nom represents bitstreams. The last question is: where do these bitstreams come from, anyway? After all, most Rust functions represent binary data in bytes (e.g. as Vec<u8> or using the bytes crate). If you're reading binary data from disk, or RAM, or the network, it's almost definitely going to be stored in bytes. So we need a way to turn a bytestream into a bitstream.

Luckily the function nom::bits::bits does exactly that. The docs say it "converts a byte-level input to a bit-level input, for consumption by a parser that uses bits." Perfect!

Again, this function uses a lot of generics which can be confusing, so here's an example showing how it works.

use nom::IResult;
use nom::multi::many0;
use nom::number::complete::be_u16;

type BitInput<'a> = (&'a [u8], usize);

/// Stub example type. Imagine this has to be parsed from individual bits.
struct BitwiseHeader;

/// A bit-level parser
fn parse_header(i: BitInput) -> IResult<BitInput, BitwiseHeader> {

/// Stub example type. 
/// The header has to be parsed from bits, but the body can be parsed from bytes.
struct Message {
    header: BitwiseHeader,
    body: Vec<u16>,

/// A byte-level parser that calls a bit-level parser
fn parse_msg(i: &[u8]) -> IResult<&[u8], Message> {
    /// The header has to be parsed from bits
    let (i, header) = nom::bits::bits(parse_header)(i)?;
    /// But the rest of the message can be parsed from bytes.
    let (i, body) = many0(be_u16)(i)?;
    Ok((i, Message { header, body }))

I got curious about these bitwise parsers because of Advent of Code 2021, day 16. A few weeks later, I wanted to build a DNS client, and realized that parsing the u4s or single-bit flags is easy with Nom. In my next blog post, we'll look at how to parse a real-world example with bitstreams: DNS message headers.



Take a look at its type signature. The generics are, roughly:

  • input: I must be something that works like a binary stream
  • count: C must be some uint type (with some caveats)
  • output: O must be some uint type, but with different caveats
  • And of course the usual generic E for verbose parser errors or the cheaper but less helpful default parser errors.