/*
Eagle - A library for easy communication in full-stack Rust applications
Copyright (c) 2024 KodiCraft

This program is free software: you can redistribute it and/or modify
it under the terms of the GNU Affero General Public License as
published by the Free Software Foundation, either version 3 of the
License, or (at your option) any later version.

This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
GNU Affero General Public License for more details.

You should have received a copy of the GNU Affero General Public License
along with this program.  If not, see <https://www.gnu.org/licenses/>.
*/
use proc_macro::TokenStream;
use quote::{format_ident, quote};
use syn::{parse_macro_input, spanned::Spanned, DeriveInput, Field, Ident};

#[proc_macro_derive(Protocol)]
pub fn derive_protocol(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    // Must be on an enum
    let enum_ = match &input.data {
        syn::Data::Enum(e) => e,
        _ => {
            return syn::Error::new(input.span(), "Protocol can only be derived on enums")
                .to_compile_error()
                .into()
        }
    };
    let name = &input.ident;

    let error_enum_name = format_ident!("__{}Error", name);
    let answer_enum_name = format_ident!("__{}Answer", name);
    let question_enum_name = format_ident!("__{}Question", name);
    let query_enum_name = format_ident!("__{}Query", name);
    let server_trait_name = format_ident!("{}Server", name);
    let client_struct_name = format_ident!("{}Client", name);

    let vis = &input.vis;

    let mut server_trait = Vec::new();
    let mut server_enum = Vec::new();
    let mut client_impl = Vec::new();
    let mut client_enum = Vec::new();

    let mut query_enum = Vec::new();
    let mut query_from_question_enum = Vec::new();
    let mut query_set_answer = Vec::new();
    let mut query_get_answer = Vec::new();

    for variant in &enum_.variants {
        // Every variant must have 2 fields
        // The first field is the question (serverbound), the second field is the answer (clientbound)
        if variant.fields.len() != 2 {
            return syn::Error::new(
                variant.span(),
                "Every variant on a protocol must have exactly 2 fields",
            )
            .to_compile_error()
            .into();
        }

        let var_name = ident_to_snake_case(&variant.ident);
        let mut variant_fields = variant.fields.iter();
        let question_field = variant_fields.next().unwrap();
        let question_args = field_to_args(question_field);
        let question_tuple_args = field_to_tuple_args(question_field);
        let answer_type = variant_fields.next().unwrap().ty.clone();

        // The variants that either the server or the client will use
        // The "server" enum contains messages the server can send, the "client" enum contains messages the client can send
        server_enum.push(quote! {
            #var_name(#answer_type)
        });
        client_enum.push(quote! {
            #var_name(#question_field)
        });
        // There is a From implementation for the client enum to the query enum
        query_from_question_enum.push(quote! {
            #question_enum_name::#var_name(question) => #query_enum_name::#var_name(question, None),
        });
        // There is a function that must be implemented to set the answer in the query enum
        query_set_answer.push(quote! {
            #query_enum_name::#var_name(question, answer_opt) => match answer {
                #answer_enum_name::#var_name(answer) => {answer_opt.replace(answer);},
                _ => panic!("The answer for this query is not the correct type."),
            },
        });
        // There is a function that must be implemented to get the answer from the query enum
        query_get_answer.push(quote! {
            #query_enum_name::#var_name(_, answer) => match answer {
                Some(answer) => Some(#answer_enum_name::#var_name(answer.clone())),
                None => None
            },
        });
        // The function that the server needs to implement
        server_trait.push(quote! {
            fn #var_name(&mut self, #question_args) -> #answer_type;
        });
        // The function that the client uses to communicate
        client_impl.push(quote! {
            pub async fn #var_name(&mut self, #question_args) -> Result<#answer_type, #error_enum_name> {
                let nonce = self.send(#question_enum_name::#var_name(#question_tuple_args)).await?;
                let answer = self.recv_until(nonce).await?;
                match answer {
                    #answer_enum_name::#var_name(answer) => Ok(answer),
                    _ => panic!("The answer for this query is not the correct type."),
                }
            }
        });
        // The query enum is the same as the source enum, but the second field is always wrapped in a Option<>
        query_enum.push(quote! {
            #var_name(#question_field, Option<#answer_type>)
        });
    }

    // Create an error and result type for sending messages
    let error_enum = quote! {
        #vis enum #error_enum_name {
            SendError(tokio::sync::mpsc::error::SendError<(u64, #question_enum_name)>),
            Closed,
        }
    };
    // Create enums for the types of messages the server and client will use

    let answer_enum = quote! {
        #[derive(serde::Serialize, serde::Deserialize, Clone, Debug)]
        #vis enum #answer_enum_name {
            #(#server_enum), *
        }
    };
    let question_enum = quote! {
        #[derive(serde::Serialize, serde::Deserialize, Clone, Debug)]
        #vis enum #question_enum_name {
            #(#client_enum), *
        }
    };
    // Create an enum to represent the queries the client has sent
    let query_enum = quote! {
        #[derive(Clone, Debug)]
        #vis enum #query_enum_name {
            #(#query_enum), *
        }
        impl #query_enum_name {
            pub fn set_answer(&mut self, answer: #answer_enum_name) {
                match self {
                    #(#query_set_answer)*
                };
            }
            pub fn get_answer(&self) -> Option<#answer_enum_name> {
                match self {
                    #(#query_get_answer)*
                }
            }
        }
        impl From<#question_enum_name> for #query_enum_name {
            fn from(query: #question_enum_name) -> Self {
                match query {
                    #(#query_from_question_enum)*
                }
            }
        }
    };
    // Create a trait which the server will have to implement
    let server_trait = quote! {
        #vis trait #server_trait_name {
            #(#server_trait)*
        }
    };
    // Create a struct which the client will use to communicate
    let client_struct = quote! {
        #vis struct #client_struct_name {
            queries: ::std::collections::HashMap<u64, #query_enum_name>,
            send_queue: tokio::sync::mpsc::Sender<(u64, #question_enum_name)>,
            recv_queue: tokio::sync::mpsc::Receiver<(u64, #answer_enum_name)>,
        } // TODO: This struct will have some fields to handle the actual connection
        impl #client_struct_name {
            pub fn new(send_queue: tokio::sync::mpsc::Sender<(u64, #question_enum_name)>, recv_queue: tokio::sync::mpsc::Receiver<(u64, #answer_enum_name)>) -> Self {
                Self {
                    queries: ::std::collections::HashMap::new(),
                    send_queue,
                    recv_queue,
                }
            }
            async fn send(&mut self, query: #question_enum_name) -> Result<u64, #error_enum_name> {
                let nonce = self.queries.len() as u64;
                let res = self.send_queue.send((nonce, query.clone())).await;
                match res {
                    Ok(_) => {
                        self.queries.insert(nonce, query.into());
                        Ok(nonce)
                    }
                    Err(e) => Err(#error_enum_name::SendError(e)),
                }
            }
            async fn recv_until(&mut self, id: u64) -> Result<#answer_enum_name, #error_enum_name> {
                loop {
                    // Check if we've received the answer for the query we're looking for
                    if let Some(query) = self.queries.get(&id) {
                        if let Some(answer) = query.get_answer() {
                            return Ok(answer);
                        }
                    }
                    match self.recv_queue.recv().await {
                        Some((nonce, answer)) => {
                            // Replace the Option<> in the query with the answer
                            if let Some(query) = self.queries.get_mut(&nonce) {
                                query.set_answer(answer);
                            } else {
                                panic!("Received an answer for a query we did not send");
                            }
                        }
                        None => return Err(#error_enum_name::Closed),
                    };
                }
            }
            #(#client_impl)*
        }
    };

    let expanded = quote! {
        #error_enum
        #answer_enum
        #question_enum
        #query_enum
        #server_trait
        #client_struct
    };
    expanded.into()
}

fn ident_to_snake_case(ident: &Ident) -> Ident {
    let ident = ident.to_string();
    let mut out = String::new();
    for (i, c) in ident.chars().enumerate() {
        if c.is_uppercase() {
            if i != 0 {
                out.push('_');
            }
            out.push(c.to_lowercase().next().unwrap());
        } else {
            out.push(c);
        }
    }
    Ident::new(&out, ident.span())
}

fn field_to_args(field: &Field) -> proc_macro2::TokenStream {
    let type_ = &field.ty;
    if let syn::Type::Tuple(tuple) = type_ {
        let mut args = Vec::new();
        for (i, elem) in tuple.elems.iter().enumerate() {
            let arg = Ident::new(&format!("arg{}", i), elem.span());
            args.push(quote! { #arg: #elem });
        }
        quote! { #( #args ), * }
    } else {
        quote! { arg: #type_ }
    }
}

fn field_to_tuple_args(field: &Field) -> proc_macro2::TokenStream {
    let type_ = &field.ty;
    if let syn::Type::Tuple(tuple) = type_ {
        let mut args = Vec::new();
        for (i, elem) in tuple.elems.iter().enumerate() {
            let arg = Ident::new(&format!("arg{}", i), elem.span());
            args.push(quote! { #arg });
        }
        quote! { ( #( #args ), * ) }
    } else {
        quote! { (arg) }
    }
}