engine/scene_update_macros/src/lib.rs

use proc_macro::TokenStream;
use proc_macro2::{Span, TokenStream as TokenStream2};
use quote::{format_ident, quote};
use syn::{
    parse::{Parse, ParseStream},
    parse_macro_input,
    token::Comma,
    Ident, LitInt, Result,
};

struct InputInfo {
    macro_ident: Ident,
    start: usize,
    end: usize,
}

impl Parse for InputInfo {
    fn parse(input: ParseStream) -> Result<Self> {
        let macro_ident = input.parse::<Ident>()?;
        input.parse::<Comma>()?;
        let start = input.parse::<LitInt>()?.base10_parse()?;
        input.parse::<Comma>()?;
        let end = input.parse::<LitInt>()?.base10_parse()?;

        Ok(Self {
            macro_ident,
            start,
            end,
        })
    }
}

struct TupleType {
    little: Ident,
    big: Ident,
}

#[proc_macro]
pub fn implement_pair_update(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as InputInfo);

    let mut generic_count = Vec::new();

    for lhs in input.start..=input.end {
        for rhs in input.start..=input.end {
            generic_count.push((lhs, rhs));
        }
    }

    let generic_tuples: Vec<(Vec<TupleType>, Vec<TupleType>)> = generic_count
        .iter()
        .map(|(lhs_count, rhs_count)| {
            let lhs = (input.start..(input.start + lhs_count))
                .map(|i| TupleType {
                    little: format_ident!("l{}", i),
                    big: format_ident!("L{}", i),
                })
                .collect();

            let rhs = (input.start..(input.start + rhs_count))
                .map(|i| TupleType {
                    little: format_ident!("r{}", i),
                    big: format_ident!("R{}", i),
                })
                .collect();

            (lhs, rhs)
        })
        .collect();

    let invocations: Vec<TokenStream2> = generic_tuples
        .iter()
        .map(|(lhs, rhs)| {
            let lhs_expr = LitInt::new(&format!("{}", lhs.len()), Span::call_site());
            let rhs_expr = LitInt::new(&format!("{}", rhs.len()), Span::call_site());

            let lhs_args: Vec<TokenStream2> = lhs
                .iter()
                .map(|tuple| {
                    let little = &tuple.little;
                    let big = &tuple.big;

                    quote! {
                        [#little: #big],
                    }
                })
                .collect();

            let rhs_args: Vec<TokenStream2> = rhs
                .iter()
                .map(|tuple| {
                    let little = &tuple.little;
                    let big = &tuple.big;

                    quote! {
                        [#little: #big],
                    }
                })
                .collect();

            let macro_ident = &input.macro_ident;

            quote! {
                #macro_ident!(
                    #lhs_expr, (#(#lhs_args)*),
                    #rhs_expr, (#(#rhs_args)*)
                );
            }
        })
        .collect();

    TokenStream::from(quote! {
        #(
            #invocations
        )*
    })
}