use std::convert::TryFrom;
use thiserror::Error;
#[derive(Debug, Error)]
enum Error {
#[error("Invalid varint")]
InvalidVarint,
#[error("Invalid wire-type")]
InvalidWireType,
#[error("Unexpected EOF")]
UnexpectedEOF,
#[error("Invalid length")]
InvalidSize(#[from] std::num::TryFromIntError),
#[error("Unexpected wire-type)")]
UnexpectedWireType,
#[error("Invalid string (not UTF-8)")]
InvalidString,
}
/// A wire type as seen on the wire.
enum WireType {
/// The Varint WireType indicates the value is a single VARINT.
Varint,
//I64, -- not needed for this exercise
/// The Len WireType indicates that the value is a length represented as a VARINT
/// followed by exactly that number of bytes.
Len,
/// The I32 WireType indicates that the value is precisely 4 bytes in little-endian order
/// containing a 32-bit signed integer.
I32,
}
#[derive(Debug)]
/// A field's value, typed based on the wire type.
enum FieldValue<'a> {
Varint(u64),
//I64(i64), -- not needed for this exercise
Len(&'a [u8]),
I32(i32),
}
#[derive(Debug)]
/// A field, containing the field number and its value.
struct Field<'a> {
field_num: u64,
value: FieldValue<'a>,
}
impl TryFrom<u64> for WireType {
type Error = Error;
fn try_from(value: u64) -> Result<WireType, Error> {
Ok(match value {
0 => WireType::Varint,
//1 => WireType::I64, -- not needed for this exercise
2 => WireType::Len,
5 => WireType::I32,
_ => return Err(Error::InvalidWireType),
})
}
}
impl<'a> FieldValue<'a> {
fn as_string(&self) -> Result<&'a str, Error> {
let FieldValue::Len(data) = self else {
return Err(Error::UnexpectedWireType);
};
Ok(std::str::from_utf8(data).map_err(|_| Error::InvalidString)?)
}
fn as_bytes(&self) -> Result<&'a [u8], Error> {
let FieldValue::Len(data) = self else {
return Err(Error::UnexpectedWireType);
};
Ok(data)
}
fn as_u64(&self) -> Result<u64, Error> {
let FieldValue::Varint(value) = self else {
return Err(Error::UnexpectedWireType);
};
Ok(*value)
}
}
/// Parse a VARINT, returning the parsed value and the remaining bytes.
fn parse_varint(data: &[u8]) -> Result<(u64, &[u8]), Error> {
for i in 0..7 {
let Some(b) = data.get(i) else {
return Err(Error::InvalidVarint);
};
if b & 0x80 == 0 {
// This is the last byte of the VARINT, so convert it to
// a u64 and return it.
let mut value = 0u64;
for b in data[..=i].iter().rev() {
value = (value << 7) | (b & 0x7f) as u64;
}
return Ok((value, &data[i + 1..]));
}
}
// More than 7 bytes is invalid.
Err(Error::InvalidVarint)
}
/// Convert a tag into a field number and a WireType.
fn unpack_tag(tag: u64) -> Result<(u64, WireType), Error> {
let field_num = tag >> 3;
let wire_type = WireType::try_from(tag & 0x7)?;
Ok((field_num, wire_type))
}
/// Parse a field, returning the remaining bytes
fn parse_field(data: &[u8]) -> Result<(Field, &[u8]), Error> {
let (tag, remainder) = parse_varint(data)?;
let (field_num, wire_type) = unpack_tag(tag)?;
let (fieldvalue, remainder) = match wire_type {
WireType::Varint => {
let (value, remainder) = parse_varint(remainder)?;
(FieldValue::Varint(value), remainder)
}
WireType::Len => {
let (len, remainder) = parse_varint(remainder)?;
let len: usize = len.try_into()?;
if remainder.len() < len as usize {
return Err(Error::UnexpectedEOF);
}
let (value, remainder) = remainder.split_at(len);
(FieldValue::Len(value), remainder)
}
WireType::I32 => {
if remainder.len() < 4usize {
return Err(Error::UnexpectedEOF);
}
let (value, remainder) = remainder.split_at(4);
// Unwrap error because `value` is definitely 4 bytes long.
let value = i32::from_le_bytes(value.try_into().unwrap());
(FieldValue::I32(value), remainder)
}
};
Ok((
Field {
field_num,
value: fieldvalue,
},
remainder,
))
}
/// Parse a message in the given data, calling `field_callback` for each field in the message.
///
/// The entire input is consumed.
fn parse_message(
mut data: &[u8],
field_callback: impl Fn(Field) -> Result<(), Error>,
) -> Result<(), Error> {
while !data.is_empty() {
let parsed = parse_field(data)?;
field_callback(parsed.0)?;
data = parsed.1;
}
Ok(())
}
fn main() {
/// Handle a field in a Person message.
fn person_field(field: Field) -> Result<(), Error> {
match field.field_num {
1 => println!("name: {}", field.value.as_string()?),
2 => println!("id: {}", field.value.as_u64()?),
3 => {
println!("phone:");
parse_message(field.value.as_bytes()?, phone_number_field)?;
}
_ => {} // skip everything else
}
Ok(())
}
/// Handle a field in a PhoneNumber message.
fn phone_number_field(field: Field) -> Result<(), Error> {
match field.field_num {
1 => println!(" number: {}", field.value.as_string()?),
2 => println!(" type: {}", field.value.as_string()?),
_ => {} // skip everything else
}
Ok(())
}
parse_message(
&[
0x0a, 0x07, 0x6d, 0x61, 0x78, 0x77, 0x65, 0x6c, 0x6c, 0x10, 0x2a, 0x1a, 0x16,
0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x30, 0x32, 0x2d, 0x35, 0x35, 0x35, 0x2d, 0x31,
0x32, 0x31, 0x32, 0x12, 0x04, 0x68, 0x6f, 0x6d, 0x65, 0x1a, 0x18, 0x0a, 0x0e,
0x2b, 0x31, 0x38, 0x30, 0x30, 0x2d, 0x38, 0x36, 0x37, 0x2d, 0x35, 0x33, 0x30,
0x38, 0x12, 0x06, 0x6d, 0x6f, 0x62, 0x69, 0x6c, 0x65,
],
person_field,
)
.unwrap()
}
#[cfg(test)]
mod test {
use super::*;
#[test]
fn as_string() {
assert!(FieldValue::Varint(10).as_string().is_err());
assert!(FieldValue::I32(10).as_string().is_err());
assert_eq!(FieldValue::Len(b"hello").as_string().unwrap(), "hello");
}
#[test]
fn as_bytes() {
assert!(FieldValue::Varint(10).as_bytes().is_err());
assert!(FieldValue::I32(10).as_bytes().is_err());
assert_eq!(FieldValue::Len(b"hello").as_bytes().unwrap(), b"hello");
}
#[test]
fn as_u64() {
assert_eq!(FieldValue::Varint(10).as_u64().unwrap(), 10u64);
assert!(FieldValue::I32(10).as_u64().is_err());
assert!(FieldValue::Len(b"hello").as_u64().is_err());
}
}