# Solution

``````/// A wire type as seen on the wire.
enum WireType {
/// The Varint WireType indicates the value is a single VARINT.
Varint,
/// The I64 WireType indicates that the value is precisely 8 bytes in
/// little-endian order containing a 64-bit signed integer or double type.
//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 or float type.
//I32,  -- not needed for this exercise
}

#[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),  -- not needed for this exercise
}

#[derive(Debug)]
/// A field, containing the field number and its value.
struct Field<'a> {
field_num: u64,
value: FieldValue<'a>,
}

trait ProtoMessage<'a>: Default {
}

impl From<u64> for WireType {
fn from(value: u64) -> Self {
match value {
0 => WireType::Varint,
//1 => WireType::I64,  -- not needed for this exercise
2 => WireType::Len,
//5 => WireType::I32,  -- not needed for this exercise
_ => panic!("Invalid wire type: {value}"),
}
}
}

impl<'a> FieldValue<'a> {
fn as_str(&self) -> &'a str {
let FieldValue::Len(data) = self else {
panic!("Expected string to be a `Len` field");
};
std::str::from_utf8(data).expect("Invalid string")
}

fn as_bytes(&self) -> &'a [u8] {
let FieldValue::Len(data) = self else {
panic!("Expected bytes to be a `Len` field");
};
data
}

fn as_u64(&self) -> u64 {
let FieldValue::Varint(value) = self else {
panic!("Expected `u64` to be a `Varint` field");
};
*value
}
}

/// Parse a VARINT, returning the parsed value and the remaining bytes.
fn parse_varint(data: &[u8]) -> (u64, &[u8]) {
for i in 0..7 {
let Some(b) = data.get(i) else {
panic!("Not enough bytes for varint");
};
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 (value, &data[i + 1..]);
}
}

// More than 7 bytes is invalid.
panic!("Too many bytes for varint");
}

/// Convert a tag into a field number and a WireType.
fn unpack_tag(tag: u64) -> (u64, WireType) {
let field_num = tag >> 3;
let wire_type = WireType::from(tag & 0x7);
(field_num, wire_type)
}

/// Parse a field, returning the remaining bytes
fn parse_field(data: &[u8]) -> (Field, &[u8]) {
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().expect("len not a valid `usize`");
if remainder.len() < len {
panic!("Unexpected EOF");
}
let (value, remainder) = remainder.split_at(len);
(FieldValue::Len(value), remainder)
}
};
(Field { field_num, value: fieldvalue }, remainder)
}

/// Parse a message in the given data, calling `T::add_field` for each field in
/// the message.
///
/// The entire input is consumed.
fn parse_message<'a, T: ProtoMessage<'a>>(mut data: &'a [u8]) -> T {
let mut result = T::default();
while !data.is_empty() {
let parsed = parse_field(data);
data = parsed.1;
}
result
}

#[derive(PartialEq)]
#[derive(Debug, Default)]
struct PhoneNumber<'a> {
number: &'a str,
type_: &'a str,
}

#[derive(PartialEq)]
#[derive(Debug, Default)]
struct Person<'a> {
name: &'a str,
id: u64,
phone: Vec<PhoneNumber<'a>>,
}

impl<'a> ProtoMessage<'a> for Person<'a> {
fn add_field(&mut self, field: Field<'a>) {
match field.field_num {
1 => self.name = field.value.as_str(),
2 => self.id = field.value.as_u64(),
3 => self.phone.push(parse_message(field.value.as_bytes())),
_ => {} // skip everything else
}
}
}

impl<'a> ProtoMessage<'a> for PhoneNumber<'a> {
fn add_field(&mut self, field: Field<'a>) {
match field.field_num {
1 => self.number = field.value.as_str(),
2 => self.type_ = field.value.as_str(),
_ => {} // skip everything else
}
}
}

fn main() {
let person: Person = 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,
]);
println!("{:#?}", person);
}

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

#[test]
fn test_id() {
let person_id: Person = parse_message(&[0x10, 0x2a]);
assert_eq!(person_id, Person { name: "", id: 42, phone: vec![] });
}

#[test]
fn test_name() {
let person_name: Person = parse_message(&[
0x0a, 0x0e, 0x62, 0x65, 0x61, 0x75, 0x74, 0x69, 0x66, 0x75, 0x6c, 0x20,
0x6e, 0x61, 0x6d, 0x65,
]);
assert_eq!(
person_name,
Person { name: "beautiful name", id: 0, phone: vec![] }
);
}

#[test]
fn test_just_person() {
let person_name_id: Person =
parse_message(&[0x0a, 0x04, 0x45, 0x76, 0x61, 0x6e, 0x10, 0x16]);
assert_eq!(person_name_id, Person { name: "Evan", id: 22, phone: vec![] });
}

#[test]
fn test_phone() {
let phone: Person = parse_message(&[
0x0a, 0x00, 0x10, 0x00, 0x1a, 0x16, 0x0a, 0x0e, 0x2b, 0x31, 0x32, 0x33,
0x34, 0x2d, 0x37, 0x37, 0x37, 0x2d, 0x39, 0x30, 0x39, 0x30, 0x12, 0x04,
0x68, 0x6f, 0x6d, 0x65,
]);
assert_eq!(
phone,
Person {
name: "",
id: 0,
phone: vec![PhoneNumber { number: "+1234-777-9090", type_: "home" },],
}
);
}
}``````