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title = "Creating a bencode parser with nom"
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description = "The encoding used by the peer-to-peer file sharing system BitTorrent"
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date = 2022-07-15
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date = 2022-09-07
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draft = true
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[taxonomies]
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categories = ["rust"]
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@ -107,6 +107,20 @@ type BenResult<'a> = IResult<&'a [u8], Value<'a>, Error<&'a [u8]>>;
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We use `&'a [u8]` since thats the type of data our parsers will be dealing with.
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# Representing all the possible bencode value types
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To hold all the value types bencode has, an enum like this will do:
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```rust
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#[derive(Debug, Clone)]
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pub enum Value<'a> {
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Bytes(&'a [u8]),
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Integer(i64),
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List(Vec<Self>),
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Dictionary(HashMap<&'a [u8], Self>),
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}
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```
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# Parsing the byte string
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Lets start with the easiest one, the byte strings, as you can recall, made up of an ASCII integer, a colon and the data:
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@ -120,3 +134,184 @@ fn parse_bytes(start_inp: &'a [u8]) -> BenResult<'a> {
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todo!()
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}
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```
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We can use the [digit1](https://docs.rs/nom/7.1.1/nom/character/complete/fn.digit1.html) combinator to get the length of the byte string and then the [char](https://docs.rs/nom/7.1.1/nom/character/complete/fn.char.html) to consume the colon:
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```rust
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let (inp, length) = digit1(start_inp)?;
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// We don't need the colon so we just discard it with '_'.
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let (inp, _) = char(':')(inp)?;
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```
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Now we need to convert the length (which is a number in ASCII) to an integer and check if it's 0, which would be an error:
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```rust
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// SAFETY: digit1 always returns ASCII numbers, which are always valid UTF-8.
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let length = unsafe { std::str::from_utf8_unchecked(length) };
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let length: u64 = length.parse().map_err(Error::ParseIntError)?;
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if length == 0 {
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Err(Error::InvalidBytesLength(start_inp))?
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}
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```
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Then we use the [take](https://docs.rs/nom/7.1.1/nom/bytes/complete/fn.take.html) parser to take an exact amount of elements and finally return it, resulting in the complete function like this:
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```rust
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fn parse_bytes(start_inp: &'a [u8]) -> BenResult<'a> {
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let (inp, length) = digit1(start_inp)?;
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let (inp, _) = char(':')(inp)?;
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// SAFETY: digit1 always returns ASCII numbers, which are always valid UTF-8.
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let length = unsafe { std::str::from_utf8_unchecked(length) };
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let length: u64 = length.parse().map_err(Error::ParseIntError)?;
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if length == 0 {
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Err(Error::InvalidBytesLength(start_inp))?
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}
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let (inp, characters) = take(length)(inp)?;
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Ok((inp, Value::Bytes(characters)))
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}
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```
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# Parsing integers
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Format: `i10e`, `i-30e`
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Integers can come alone or with the positive and negative symbol, we also need to handle the invalid `-0` and they can't have leading `0`s like `002`.
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Here the power of combinators can come to light, we will use the following parsers and combine them:
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- [delimited](https://docs.rs/nom/7.1.1/nom/sequence/fn.delimited.html): Matches an object from the first parser and discards it, then gets an object from the second parser, and finally matches an object from the third parser and discards it.
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- [char](https://docs.rs/nom/7.1.1/nom/character/complete/fn.char.html): Recognizes and consumes a single character.
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- [alt](https://docs.rs/nom/7.1.1/nom/branch/fn.alt.html): Tests a list of parsers one by one until one succeeds.
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- [recognize](https://docs.rs/nom/7.1.1/nom/combinator/fn.recognize.html): If the child parser was successful, return the consumed input as produced value.
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- [pair](https://docs.rs/nom/7.1.1/nom/sequence/fn.pair.html): Gets an object from the first parser, then gets another object from the second parser.
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With this we can handle the following example numbers: `1,+1,-1,10,0,50,-62`
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```rust
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fn parse_integer(start_inp: &'a [u8]) -> BenResult<'a> {
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let (inp, value) = delimited(
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char('i'),
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alt((
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recognize(pair(char('+'), digit1)),
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recognize(pair(char('-'), digit1)),
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digit1,
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)),
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char('e'),
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)(start_inp)?;
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// SAFETY: This will always be a valid UTF-8 sequence.
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let value_str = unsafe { std::str::from_utf8_unchecked(value) };
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if value_str.starts_with("-0") || (value_str.starts_with('0') && value_str.len() > 1) {
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Err(Error::InvalidInteger(start_inp))?
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} else {
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let value_integer: i64 = value_str.parse().map_err(Error::ParseIntError)?;
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Ok((inp, Value::Integer(value_integer)))
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}
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}
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```
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# Parsing lists
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Format: `li2ei3ei4ee`, `l4:spam4:eggsi22eli1ei2eee`
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A list can hold any type of value, including dictionaries and list themselves.
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Now that we can parse numbers and byte strings, parsing lists is just a matter of using those parsers.
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We will use the following new nom parsers:
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- [many_till(f, g)](https://docs.rs/nom/7.1.1/nom/multi/fn.many_till.html): Applies the parser f until the parser g produces a result. Returns a pair consisting of the results of f in a Vec and the result of g.
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We will apply the parser `alt` parser, until the `char` parser recognizes the end character `e`:
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```rust
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// Self here is the enum Value
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fn parse_list(start_inp: &'a [u8]) -> BenResult<'a> {
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let (inp, value) = preceded(
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char('l'),
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many_till(
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alt((
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Self::parse_bytes,
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Self::parse_integer,
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Self::parse_list,
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Self::parse_dict,
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)),
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char('e'),
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),
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)(start_inp)?;
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Ok((inp, Value::List(value.0)))
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}
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```
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# Parsing dictionaries
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Format: `d3:cow3:moo4:spam4:eggse`
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Parsing dictionaries is nearly identical to parsing lists, but we need to parse the keys too, which are byte strings:
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```rust
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fn parse_dict(start_inp: &'a [u8]) -> BenResult<'a> {
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let (inp, value) = preceded(
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char('d'),
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many_till(
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pair(
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Self::parse_bytes,
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alt((
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Self::parse_bytes,
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Self::parse_integer,
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Self::parse_list,
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Self::parse_dict,
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)),
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),
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char('e'),
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),
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)(start_inp)?;
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let data = value.0.into_iter().map(|x| {
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// Keys are always a byte string
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if let Value::Bytes(key) = x.0 {
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(key, x.1)
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} else {
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unreachable!()
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}
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});
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let map = HashMap::from_iter(data);
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Ok((inp, Value::Dictionary(map)))
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}
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```
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# The parser
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And finally, we can make the final parser function which parses all the possible values:
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```rust
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pub fn parse(source: &[u8]) -> Result<Vec<Value>, Error<&[u8]>> {
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let (_, items) = many_till(
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alt((
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Value::parse_bytes,
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Value::parse_integer,
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Value::parse_list,
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Value::parse_dict,
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)),
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eof,
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)(source)?;
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Ok(items.0)
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}
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```
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You can find the full source code here: <https://github.com/edg-l/nom-bencode>
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