packd/packd-proc-macro/src/lib.rs

extern crate proc_macro;

use proc_macro::{Literal, TokenStream, TokenTree};
use proc_macro_hack::proc_macro_hack;
use proc_macro_roids::{DeriveInputExt, DeriveInputStructExt, FieldExt};
use quote::{format_ident, quote};
use std::str::FromStr;

use byteorder::{BigEndian, LittleEndian, NativeEndian};
use proc_macro2::TokenStream as TokenStream2;
use std::iter::FromIterator;
use std::mem::align_of;
use syn::parse::{Parse, ParseBuffer, Parser};
use syn::{parse_macro_input, parse_quote, DeriveInput, Error, Expr, LitStr, Token};

#[derive(Debug)]
struct UnpackInput {
    pattern: String,
    input: Box<Expr>,
}

impl Parse for UnpackInput {
    fn parse<'a>(input: &'a ParseBuffer<'a>) -> Result<Self, syn::Error> {
        let pattern = input.parse::<LitStr>()?.value().into();
        input.parse::<Token![,]>()?;

        Ok(UnpackInput {
            pattern,
            input: input.parse()?,
        })
    }
}

enum Endian {
    NativeEndian,
    LittleEndian,
    BigEndian,
}

#[proc_macro_hack]
pub fn unpack(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as UnpackInput);
    if input.pattern.is_empty() {
        return quote! { () }.into();
    }

    let (order, alignment, indic) = match input
        .pattern
        .chars()
        .nth(0)
        .expect("Can't read first letter of pattern")
    {
        '!' | '>' => (Endian::BigEndian, false, true),

        '<' => (Endian::LittleEndian, false, true),

        '=' => (Endian::NativeEndian, false, true),

        '@' => (Endian::NativeEndian, true, true),

        _ => (Endian::NativeEndian, true, false),
    };

    let rest = if indic {
        input.pattern.chars().skip(1).collect()
    } else {
        input.pattern
    };

    let mut i = 0;

    let x: &[u8] = &[];

    let endian = match order {
        Endian::BigEndian => quote! {  ::packd::parser::Endian::BigEndian },
        Endian::LittleEndian => quote! {  ::packd::parser::Endian::LittleEndian },
        Endian::NativeEndian => quote! { ::packd::parser::Endian::NativeEndian },
    };

    let mut parsers: Vec<_> = vec![];
    let mut marked_values = vec![];
    let mut offset = 0;

    for char in rest.chars() {
        let value = format_ident!("value_{}", i.to_string());
        match char {
            'x' => offset += 1,
            'h' => {
                marked_values.push(i);
                parsers.push(quote! {
                    let #value = ::packd::parser::unpack_h(#endian, &input[offset..offset + 2]);
                    offset += 2;
                });
            }

            'H' => {
                marked_values.push(i);
                parsers.push(quote! {
                    let #value = ::packd::parser::unpack_H(#endian, &input[offset..offset + 2]);
                    offset += 2;
                });

                if alignment {
                    parsers.push(quote! {
                        offset += 2 % ::std::mem::align_of::<u16>();
                    });
                }
            }

            'l' => {
                marked_values.push(i);
                parsers.push(quote! {
                    let #value = ::packd::parser::unpack_l(#endian, &input[offset..offset + 4]);
                    offset += 4;
                });

                if alignment {
                    parsers.push(quote! {
                        offset += 2 % ::std::mem::align_of::<i32>();
                    });
                }
            }

            _ => panic!("Found unrecognized format char '{}'", char),
        }
        i += 1;
    }

    let items = marked_values
        .iter()
        .map(|x| format_ident!("value_{}", x.to_string()))
        .collect::<Vec<_>>();

    let input_val = input.input;

    return quote! {
        {
            let input = #input_val;
            let mut offset = 0;
            #(#parsers)*
            (#(#items,)*)
        }
    }
    .into();
}