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use proc_macro::TokenStream;
use quote::quote;
use syn::{parse_macro_input, Attribute, Data, DeriveInput, Expr, ExprArray, ExprLit, Lit, Meta, Type};
macro_rules! parse_type {
($($tt:tt)*) => {{
let ty: syn::Type = syn::parse_quote! { $($tt)* };
ty
}}
}
pub(super) fn derive_packet_inner(input: TokenStream) -> TokenStream {
let input = parse_macro_input!(input as DeriveInput);
let encode_impl = derive_encode(&input);
let decode_impl = derive_decode(&input);
quote! {
#encode_impl
#decode_impl
}.into()
}
fn derive_encode(input: &DeriveInput) -> proc_macro2::TokenStream {
let key = parse_key(&input);
let target_name = &input.ident;
match &input.data {
Data::Struct(s) => {
let mut encode_fields = Vec::with_capacity(s.fields.len());
for field in &s.fields {
if let Some(ident) = field.ident.as_ref() {
let first_attr = field.attrs.first().and_then(|attr| attr.path().require_ident().ok());
let tokens = match first_attr {
Some(attr) if attr == "u8_enum" => quote! {
(self.#ident as u8).encode(&mut writer)?;
},
Some(attr) if attr == "u16_enum" => quote! {
(self.#ident as u16).encode(&mut writer)?;
},
Some(attr) if attr == "u8_string" => quote! {
// code ripped from unstable str::floor_char_boundary on 2025-04-18
let index = if 255 >= self.#ident.len() {
self.#ident.len()
} else {
let lower_bound = 255 - 3;
let new_index = self.#ident.as_bytes()[lower_bound..=255]
.iter()
.rposition(|b| (*b as i8) >= -0x40);
// SAFETY: we know that the character boundary will be within four bytes
unsafe { lower_bound + new_index.unwrap_unchecked() }
};
let data = &self.#ident[..index];
(data.len() as u8).encode(&mut writer)?;
data.encode(&mut writer)?;
},
_ => quote! {
self.#ident.encode(&mut writer)?;
}
};
encode_fields.push(tokens);
}
}
let output = quote! {
impl Encode for #target_name {
fn encode<W: Write>(&self, writer: &mut W) -> Result<usize, std::io::Error> {
let payload = {
let mut writer = vec![];
#( #encode_fields )*
writer
};
let mut written = 0usize;
written += #key.as_slice().encode(writer)?;
written += PLen(payload.len()).encode(writer)?;
written += payload.encode(writer)?;
Ok(written)
}
}
};
output
},
Data::Enum(_e) => {
let rep = parse_repr(input);
let output = quote! {
impl Encode for #target_name {
fn encode<W: Write>(&self, writer: &mut W) -> Result<usize, std::io::Error> {
let payload = {
let mut writer = vec![];
(*self as #rep).encode(&mut writer)?;
writer
};
let mut written = 0usize;
written += #key.as_slice().encode(writer)?;
written += PLen(payload.len()).encode(writer)?;
written += payload.encode(writer)?;
Ok(written)
}
}
};
output
},
_ => panic!("derive packet doesn't support unions")
}
}
fn derive_decode(input: &DeriveInput) -> proc_macro2::TokenStream {
let key = parse_key(&input);
let target_name = &input.ident;
match &input.data {
Data::Struct(s) => {
let mut decode_fields = Vec::with_capacity(s.fields.len());
for (i, field) in s.fields.iter().enumerate() {
let is_last = i + 1 == s.fields.len();
if let Some(ident) = field.ident.as_ref() {
let ty = &field.ty;
let first_attr = field.attrs.first().and_then(|attr| attr.path().require_ident().ok());
let tokens = match first_attr {
Some(attr) if attr == "u8_enum" => quote! {
#ident: <#ty>::try_from(u8::decode(reader)?)?,
},
Some(attr) if attr == "u16_enum" => quote! {
#ident: <#ty>::try_from(u16::decode(reader)?)?,
},
Some(attr) if attr == "u8_string" => quote! {
#ident: U8String::decode(reader)?.0,
},
_ if ty == &parse_type!{ Vec<u8> } && is_last => quote! {
#ident: {
let offset = reader.stream_position()? - payload_start;
let mut buf = vec![0u8; plen - (offset as usize)];
reader.read_exact(&mut buf)?;
buf
},
},
_ if ty == &parse_type!{ Vec<u64> } && is_last => quote! {
#ident: {
let offset = reader.stream_position()? - payload_start;
let len = plen - (offset as usize);
if len % 8 != 0 {
return Err(DecodeError::WrongLength);
}
let mut buf = vec![0u8; len];
reader.read_exact(&mut buf)?;
buf.chunks_exact(8).map(|x| u64::from_be_bytes(x.try_into().expect("should never fail"))).collect()
},
},
_ if ty == &parse_type!{ String } && is_last => quote! {
#ident: {
let offset = reader.stream_position()? - payload_start;
let mut buf = vec![0u8; plen - (offset as usize)];
reader.read_exact(&mut buf)?;
String::from_utf8(buf)?
},
},
_ if ty == &parse_type!{ Option<Box<Packet>> } && is_last => quote! {
#ident: {
let offset = reader.stream_position()? - payload_start;
let len = plen - (offset as usize);
if len == 0 {
None
} else {
Some(Box::new(Packet::decode(reader)?))
}
}
},
_ => quote! {
#ident: <#ty>::decode(reader)?,
}
};
decode_fields.push(tokens);
}
}
let output = quote! {
impl Decode for #target_name {
fn decode<R: Read + Seek>(reader: &mut R) -> Result<Self, DecodeError> {
let packet_start = reader.stream_position()?;
fn try_decode<R: Read + Seek>(reader: &mut R) -> Result<#target_name, DecodeError> {
let parsed_key = <[u8; 2]>::decode(reader)?;
if #key != parsed_key {
return Err(DecodeError::WrongKey);
}
let plen = PLen::decode(reader)?.0;
let payload_start = reader.stream_position()?;
Ok(#target_name {
#( #decode_fields )*
})
}
let result = try_decode(reader);
if result.is_err() {
reader.seek(std::io::SeekFrom::Start(packet_start))?;
}
result
}
}
};
output
},
Data::Enum(_e) => {
let rep = parse_repr(input);
let output = quote! {
impl Decode for #target_name {
fn decode<R: Read + Seek>(reader: &mut R) -> Result<Self, DecodeError> {
let packet_start = reader.stream_position()?;
fn try_decode<R: Read + Seek>(reader: &mut R) -> Result<#target_name, DecodeError> {
let parsed_key = <[u8; 2]>::decode(reader)?;
if #key != parsed_key {
return Err(DecodeError::WrongKey);
}
let plen = PLen::decode(reader)?.0;
let payload_start = reader.stream_position()?;
Ok(<#target_name>::try_from(<#rep>::decode(reader)?)?)
}
let result = try_decode(reader);
if result.is_err() {
reader.seek(std::io::SeekFrom::Start(packet_start))?;
}
result
}
}
};
output
},
_ => panic!("cannot derive a union")
}
}
fn parse_key(input: &DeriveInput) -> ExprArray {
let Some(key_attr) = input.attrs.iter().find(|attr| attr.path().is_ident("key")) else { panic!("derive requires a `#[key = \"[u8, ...]\"]` attribute") };
parse_key_str(key_attr).expect("key attribute should be the `#[key = \"[u8, ...]\"]` style")
}
fn parse_repr(input: &DeriveInput) -> Type {
let Some(repr_attr) = input.attrs.iter().find(|attr| attr.path().is_ident("repr")) else { panic!("derive enum requires `#[repr(<primitive>)]` attribute") };
repr_attr.parse_args().expect("valid repr argument")
}
fn parse_key_str(attr: &Attribute) -> Result<ExprArray, syn::Error> {
if let Meta::NameValue(meta) = &attr.meta {
if meta.path.is_ident("key") {
if let Expr::Lit(ExprLit { lit: Lit::Str(lit), .. }) = &meta.value {
return lit.parse();
}
}
}
panic!("expected key attribute `#[key = \"[u8, ...]\"]`");
}