//! Procedural macros backing the public `wip-interface` authoring facade.
//!
//! This is an implementation crate. Applications should depend on
//! `wip-interface`, which re-exports both macros alongside the traits and types
//! used by their generated code.

#![deny(missing_docs)]

use std::collections::HashSet;

use proc_macro::TokenStream;
use proc_macro2::{Span, TokenStream as TokenStream2};
use quote::{format_ident, quote};
use syn::ext::IdentExt;
use syn::spanned::Spanned;
use syn::visit::{self, Visit};
use syn::{
    Attribute, Data, DataEnum, DeriveInput, Error, Fields, FnArg, GenericArgument, Ident,
    ItemTrait, Lit, Meta, Pat, PathArguments, ReturnType, TraitItem, TraitItemFn, Type,
    parse_macro_input,
};

/// Declares a public Rust trait as a WIP interface.
#[proc_macro_attribute]
pub fn interface(attribute: TokenStream, item: TokenStream) -> TokenStream {
    let attribute = TokenStream2::from(attribute);
    let item = parse_macro_input!(item as ItemTrait);

    match expand_interface(attribute, item) {
        Ok(tokens) => tokens.into(),
        Err(error) => error.into_compile_error().into(),
    }
}

/// Derives the WIP type declaration and logical value codec for a DTO.
#[proc_macro_derive(WipType)]
pub fn derive_wip_type(item: TokenStream) -> TokenStream {
    let item = parse_macro_input!(item as DeriveInput);

    match expand_wip_type(item) {
        Ok(tokens) => tokens.into(),
        Err(error) => error.into_compile_error().into(),
    }
}

#[derive(Clone)]
struct Parameter {
    ident: Ident,
    name: String,
    ty: Type,
    optional: bool,
    documentation: Option<Documentation>,
}

struct Operation {
    ident: Ident,
    name: String,
    arguments_ident: Ident,
    parameters: Vec<Parameter>,
    result_ty: Type,
    unit_result: bool,
    documentation: Option<Documentation>,
    return_documentation: Option<Documentation>,
}

#[derive(Clone)]
struct Documentation {
    summary: String,
    details: Option<String>,
}

fn expand_interface(attribute: TokenStream2, item: ItemTrait) -> syn::Result<TokenStream2> {
    if !attribute.is_empty() {
        return Err(Error::new_spanned(
            attribute,
            "the interface attribute does not accept arguments",
        ));
    }

    validate_interface_header(&item)?;

    let trait_ident = &item.ident;
    let definition_ident = format_ident!("{}Definition", trait_ident, span = trait_ident.span());
    let trait_documentation = documentation(&item.attrs);
    let mut operation_names = HashSet::new();
    let mut operations = Vec::new();

    for trait_item in &item.items {
        let method = match trait_item {
            TraitItem::Fn(method) => method,
            TraitItem::Type(item) => {
                return Err(Error::new_spanned(
                    item,
                    "associated types are not supported in WIP interfaces",
                ));
            }
            TraitItem::Const(item) => {
                return Err(Error::new_spanned(
                    item,
                    "associated constants are not supported in WIP interfaces",
                ));
            }
            other => {
                return Err(Error::new_spanned(
                    other,
                    "only operation methods are supported in WIP interfaces",
                ));
            }
        };

        let operation = parse_operation(trait_ident, method)?;
        if !operation_names.insert(operation.name.clone()) {
            return Err(Error::new(
                operation.ident.span(),
                format!("duplicate operation name `{}`", operation.name),
            ));
        }
        operations.push(operation);
    }

    // PascalCase normalization is convenient for the public generated item,
    // but distinct Rust names such as `item` and `item_` normalize identically.
    // Add a stable source-name suffix to every member of a colliding group so
    // otherwise-valid operations always produce distinct public Rust items.
    let mut seen_argument_items = HashSet::new();
    let mut colliding_argument_items = HashSet::new();
    for operation in &operations {
        let name = operation.arguments_ident.to_string();
        if !seen_argument_items.insert(name.clone()) {
            colliding_argument_items.insert(name);
        }
    }
    for operation in &mut operations {
        if colliding_argument_items.contains(&operation.arguments_ident.to_string()) {
            let suffix = operation
                .name
                .as_bytes()
                .iter()
                .map(|byte| format!("{byte:02x}"))
                .collect::<String>();
            operation.arguments_ident = format_ident!(
                "{}{}ArgumentsN{}",
                trait_ident,
                pascal_case(&operation.name),
                suffix,
                span = operation.ident.span()
            );
        }
    }

    let argument_items = operations.iter().map(argument_item);
    let operation_declarations = operations.iter().map(operation_declaration);
    let declaration_collectors = operations.iter().flat_map(operation_collectors);
    let dispatch_arms = operations.iter().map(dispatch_arm);
    let trait_doc = documentation_tokens(trait_documentation.as_ref());
    let mut emitted_item = item.clone();
    strip_parameter_documentation(&mut emitted_item);

    Ok(quote! {
        #emitted_item

        #[doc = concat!("Generated WIP interface definition for [`", stringify!(#trait_ident), "`].")]
        pub struct #definition_ident;

        impl ::wip_interface::InterfaceDefinition for #definition_ident {
            fn descriptor() -> ::wip_interface::wip_protocol::InterfaceDescriptor {
                let mut declarations = ::std::vec::Vec::new();
                #(#declaration_collectors)*

                ::wip_interface::wip_protocol::InterfaceDescriptor {
                    format: ::std::string::String::from(
                        ::wip_interface::wip_protocol::INTERFACE_FORMAT_V1,
                    ),
                    documentation: #trait_doc,
                    types: declarations,
                    operations: ::std::vec![#(#operation_declarations),*],
                }
            }
        }

        impl<T: #trait_ident> ::wip_interface::InterfaceImplementation<T> for #definition_ident {
            fn dispatch(
                implementation: &T,
                operation: &str,
                arguments: ::std::collections::BTreeMap<
                    ::std::string::String,
                    ::wip_interface::wip_protocol::Value,
                >,
            ) -> ::std::result::Result<
                ::wip_interface::wip_protocol::Value,
                ::wip_interface::DispatchError,
            > {
                let descriptor = Self::descriptor();
                match operation {
                    #(#dispatch_arms)*
                    _ => ::std::result::Result::Err(
                        ::wip_interface::DispatchError::UnknownOperation(
                            ::std::string::String::from(operation),
                        ),
                    ),
                }
            }
        }

        impl #definition_ident {
            /// Builds the interface descriptor generated from the trait declaration.
            pub fn descriptor() -> ::wip_interface::wip_protocol::InterfaceDescriptor {
                <Self as ::wip_interface::InterfaceDefinition>::descriptor()
            }

            /// Dispatches one validated logical operation call to a typed implementation.
            pub fn dispatch<T: #trait_ident>(
                implementation: &T,
                operation: &str,
                arguments: ::std::collections::BTreeMap<
                    ::std::string::String,
                    ::wip_interface::wip_protocol::Value,
                >,
            ) -> ::std::result::Result<
                ::wip_interface::wip_protocol::Value,
                ::wip_interface::DispatchError,
            > {
                <Self as ::wip_interface::InterfaceImplementation<T>>::dispatch(
                    implementation,
                    operation,
                    arguments,
                )
            }
        }

        #(#argument_items)*
    })
}

fn strip_parameter_documentation(item: &mut ItemTrait) {
    for trait_item in &mut item.items {
        let TraitItem::Fn(method) = trait_item else {
            continue;
        };
        for input in &mut method.sig.inputs {
            if let FnArg::Typed(typed) = input {
                typed
                    .attrs
                    .retain(|attribute| !attribute.path().is_ident("doc"));
            }
        }
    }
}

fn validate_interface_header(item: &ItemTrait) -> syn::Result<()> {
    if !matches!(item.vis, syn::Visibility::Public(_)) {
        return Err(Error::new_spanned(
            &item.vis,
            "a WIP interface trait must be public",
        ));
    }
    if item.unsafety.is_some() {
        return Err(Error::new_spanned(
            item.unsafety,
            "unsafe interface traits are not supported",
        ));
    }
    if item.auto_token.is_some() {
        return Err(Error::new_spanned(
            item.auto_token,
            "auto traits are not supported as WIP interfaces",
        ));
    }
    if !item.generics.params.is_empty() || item.generics.where_clause.is_some() {
        return Err(Error::new_spanned(
            &item.generics,
            "generic WIP interface traits are not supported",
        ));
    }
    if item.colon_token.is_some() || !item.supertraits.is_empty() {
        return Err(Error::new_spanned(
            &item.supertraits,
            "WIP interface trait inheritance is not supported",
        ));
    }
    Ok(())
}

fn parse_operation(trait_ident: &Ident, method: &TraitItemFn) -> syn::Result<Operation> {
    let signature = &method.sig;

    if signature.constness.is_some() {
        return Err(Error::new_spanned(
            signature.constness,
            "const operations are not supported",
        ));
    }
    if signature.asyncness.is_some() {
        return Err(Error::new_spanned(
            signature.asyncness,
            "async operations are not supported",
        ));
    }
    if signature.unsafety.is_some() {
        return Err(Error::new_spanned(
            signature.unsafety,
            "unsafe operations are not supported",
        ));
    }
    if signature.abi.is_some() {
        return Err(Error::new_spanned(
            &signature.abi,
            "extern operations are not supported",
        ));
    }
    if signature.variadic.is_some() {
        return Err(Error::new_spanned(
            &signature.variadic,
            "variadic operations are not supported",
        ));
    }
    if !signature.generics.params.is_empty() || signature.generics.where_clause.is_some() {
        return Err(Error::new_spanned(
            &signature.generics,
            "generic operations are not supported",
        ));
    }
    if let Some(default) = &method.default {
        return Err(Error::new_spanned(
            default,
            "WIP operation methods must not have a body",
        ));
    }

    let mut inputs = signature.inputs.iter();
    let receiver = inputs.next().ok_or_else(|| {
        Error::new(
            signature.ident.span(),
            "a WIP operation must have an `&self` receiver",
        )
    })?;
    validate_receiver(receiver)?;

    let mut parameters = Vec::new();
    let mut parameter_names = HashSet::new();
    for input in inputs {
        let typed = match input {
            FnArg::Typed(typed) => typed,
            FnArg::Receiver(receiver) => {
                return Err(Error::new_spanned(
                    receiver,
                    "the `&self` receiver must be the first operation argument",
                ));
            }
        };
        let ident = match typed.pat.as_ref() {
            Pat::Ident(pattern)
                if pattern.by_ref.is_none()
                    && pattern.mutability.is_none()
                    && pattern.subpat.is_none() =>
            {
                pattern.ident.clone()
            }
            pattern => {
                return Err(Error::new_spanned(
                    pattern,
                    "operation parameters must be simple named parameters",
                ));
            }
        };
        let name = source_name(&ident);
        if !parameter_names.insert(name.clone()) {
            return Err(Error::new(
                ident.span(),
                format!("duplicate operation parameter `{name}`"),
            ));
        }
        let (optional, ty) = optional_type(typed.ty.as_ref(), "operation parameter")?;
        validate_value_type(&ty, "operation parameter")?;
        parameters.push(Parameter {
            ident,
            name,
            ty,
            optional,
            documentation: documentation(&typed.attrs),
        });
    }

    let result_ty = parse_operation_result(&signature.output)?;
    let unit_result = is_unit(&result_ty);
    if !unit_result {
        validate_value_type(&result_ty, "operation return type")?;
    }

    let name = source_name(&signature.ident);
    let method_docs = doc_lines(&method.attrs);
    let (operation_doc_lines, return_doc_lines) = split_returns_section(method_docs);
    let pascal_name = pascal_case(&name);
    let arguments_ident = format_ident!(
        "{}{}Arguments",
        trait_ident,
        pascal_name,
        span = signature.ident.span()
    );

    Ok(Operation {
        ident: signature.ident.clone(),
        name,
        arguments_ident,
        parameters,
        result_ty,
        unit_result,
        documentation: documentation_from_lines(operation_doc_lines),
        return_documentation: documentation_from_lines(return_doc_lines),
    })
}

fn validate_receiver(input: &FnArg) -> syn::Result<()> {
    let receiver = match input {
        FnArg::Receiver(receiver) => receiver,
        FnArg::Typed(typed) => {
            return Err(Error::new_spanned(
                typed,
                "a WIP operation must start with an `&self` receiver",
            ));
        }
    };

    let reference = receiver.reference.as_ref();
    let is_plain_shared_reference = reference.is_some()
        && receiver.mutability.is_none()
        && receiver.colon_token.is_none()
        && reference
            .and_then(|(_, lifetime)| lifetime.as_ref())
            .is_none();
    if !is_plain_shared_reference {
        return Err(Error::new_spanned(
            receiver,
            "the only supported receiver is exactly `&self`",
        ));
    }
    Ok(())
}

fn parse_operation_result(output: &ReturnType) -> syn::Result<Type> {
    let ty = match output {
        ReturnType::Default => {
            return Err(Error::new_spanned(
                output,
                "an operation must explicitly return `OperationResult<T>`",
            ));
        }
        ReturnType::Type(_, ty) => ty.as_ref(),
    };

    reject_custom_lifetimes(ty, "operation return type")?;
    let path = match ty {
        Type::Path(path) if path.qself.is_none() => &path.path,
        _ => {
            return Err(Error::new_spanned(
                ty,
                "an operation must explicitly return `OperationResult<T>`",
            ));
        }
    };
    let segment = path.segments.last().ok_or_else(|| {
        Error::new_spanned(
            ty,
            "an operation must explicitly return `OperationResult<T>`",
        )
    })?;
    if segment.ident != "OperationResult" {
        return Err(Error::new_spanned(
            ty,
            "an operation must explicitly return `OperationResult<T>`",
        ));
    }
    let arguments = match &segment.arguments {
        PathArguments::AngleBracketed(arguments) => arguments,
        _ => {
            return Err(Error::new_spanned(
                segment,
                "`OperationResult` must have exactly one application result type",
            ));
        }
    };
    let mut types = arguments.args.iter().filter_map(|argument| match argument {
        GenericArgument::Type(ty) => Some(ty),
        _ => None,
    });
    let result = types.next().cloned();
    if arguments.args.len() != 1 || types.next().is_some() {
        return Err(Error::new_spanned(
            arguments,
            "`OperationResult` must have exactly one application result type",
        ));
    }
    let result = result.ok_or_else(|| {
        Error::new_spanned(
            arguments,
            "`OperationResult` must have exactly one application result type",
        )
    })?;
    if direct_option(&result)?.is_some() || contains_option(&result) {
        return Err(Error::new_spanned(
            result,
            "`Option<T>` is only supported directly on DTO fields and operation parameters",
        ));
    }
    Ok(result)
}

fn argument_item(operation: &Operation) -> TokenStream2 {
    let arguments_ident = &operation.arguments_ident;
    let fields = operation.parameters.iter().map(|parameter| {
        let ident = &parameter.ident;
        let name = &parameter.name;
        let ty = parameter_rust_type(parameter);
        let documentation = format!("Decoded `{name}` operation argument.");
        quote!(
            #[doc = #documentation]
            pub #ident: #ty
        )
    });
    let decoders = operation.parameters.iter().map(|parameter| {
        let ident = &parameter.ident;
        let name = &parameter.name;
        let ty = &parameter.ty;
        if parameter.optional {
            quote!(#ident: decoder.optional::<#ty>(#name)?)
        } else {
            quote!(#ident: decoder.required::<#ty>(#name)?)
        }
    });
    let result_ty = &operation.result_ty;
    let encode_result = if operation.unit_result {
        quote! {
            let () = result;
            ::std::result::Result::Ok(::wip_interface::wip_protocol::Value::Unit)
        }
    } else {
        quote! {
            <#result_ty as ::wip_interface::WipType>::encode(result)
        }
    };

    quote! {
        #[doc = concat!("Decoded arguments for `", stringify!(#arguments_ident), "`'s operation.")]
        pub struct #arguments_ident {
            #(#fields,)*
        }

        impl #arguments_ident {
            /// Decodes named logical values into typed operation arguments.
            pub fn decode(
                arguments: ::std::collections::BTreeMap<
                    ::std::string::String,
                    ::wip_interface::wip_protocol::Value,
                >,
            ) -> ::std::result::Result<Self, ::wip_interface::CodecError> {
                let mut decoder = ::wip_interface::__private::RecordDecoder::new(arguments);
                let decoded = Self {
                    #(#decoders,)*
                };
                decoder.finish()?;
                ::std::result::Result::Ok(decoded)
            }

            /// Encodes this operation's application result.
            pub fn encode_result(
                result: #result_ty,
            ) -> ::std::result::Result<
                ::wip_interface::wip_protocol::Value,
                ::wip_interface::CodecError,
            > {
                #encode_result
            }
        }
    }
}

fn operation_declaration(operation: &Operation) -> TokenStream2 {
    let name = &operation.name;
    let documentation = documentation_tokens(operation.documentation.as_ref());
    let parameters = operation.parameters.iter().map(|parameter| {
        let parameter_name = &parameter.name;
        let parameter_doc = documentation_tokens(parameter.documentation.as_ref());
        let ty = &parameter.ty;
        let optional = parameter.optional;
        quote! {
            ::wip_interface::wip_protocol::ParameterDeclaration {
                name: ::std::string::String::from(#parameter_name),
                required: !#optional,
                documentation: #parameter_doc,
                r#type: <#ty as ::wip_interface::WipType>::type_expr(),
            }
        }
    });
    let result_ty = &operation.result_ty;
    let return_doc = documentation_tokens(operation.return_documentation.as_ref());
    let returns = quote! {
        ::wip_interface::wip_protocol::ReturnDeclaration {
            documentation: #return_doc,
            r#type: <#result_ty as ::wip_interface::WipType>::type_expr(),
        }
    };

    quote! {
        ::wip_interface::wip_protocol::OperationDeclaration {
            name: ::std::string::String::from(#name),
            documentation: #documentation,
            parameters: ::std::vec![#(#parameters),*],
            returns: #returns,
        }
    }
}

fn operation_collectors(operation: &Operation) -> Vec<TokenStream2> {
    let mut collectors = operation
        .parameters
        .iter()
        .map(|parameter| {
            let ty = &parameter.ty;
            quote!(<#ty as ::wip_interface::WipType>::collect_declarations(&mut declarations);)
        })
        .collect::<Vec<_>>();
    if !operation.unit_result {
        let ty = &operation.result_ty;
        collectors.push(
            quote!(<#ty as ::wip_interface::WipType>::collect_declarations(&mut declarations);),
        );
    }
    collectors
}

fn dispatch_arm(operation: &Operation) -> TokenStream2 {
    let operation_name = &operation.name;
    let method_ident = &operation.ident;
    let arguments_ident = &operation.arguments_ident;
    let arguments = operation.parameters.iter().map(|parameter| {
        let ident = &parameter.ident;
        quote!(decoded.#ident)
    });

    quote! {
        #operation_name => {
            ::wip_interface::__private::validate_arguments(
                &descriptor,
                #operation_name,
                &arguments,
            )
            .map_err(::wip_interface::DispatchError::InvalidArguments)?;
            let decoded = #arguments_ident::decode(arguments)
                .map_err(::wip_interface::DispatchError::Decode)?;
            ::std::panic::catch_unwind(::std::panic::AssertUnwindSafe(|| {
                let result = implementation.#method_ident(#(#arguments),*)
                    .map_err(::wip_interface::DispatchError::Host)?;
                let encoded = #arguments_ident::encode_result(result)
                    .map_err(::wip_interface::DispatchError::Encode)?;
                descriptor
                    .validate_result(#operation_name, &encoded)
                    .map_err(::wip_interface::DispatchError::InvalidResult)?;
                ::std::result::Result::Ok(encoded)
            }))
            .map_err(|_| ::wip_interface::DispatchError::Panic)?
        }
    }
}

fn parameter_rust_type(parameter: &Parameter) -> TokenStream2 {
    let ty = &parameter.ty;
    if parameter.optional {
        quote!(::std::option::Option<#ty>)
    } else {
        quote!(#ty)
    }
}

fn expand_wip_type(item: DeriveInput) -> syn::Result<TokenStream2> {
    if !item.generics.params.is_empty() || item.generics.where_clause.is_some() {
        return Err(Error::new_spanned(
            &item.generics,
            "generic DTOs are not supported by `WipType`",
        ));
    }

    let ident = &item.ident;
    let type_name = source_name(ident);
    let type_documentation = documentation(&item.attrs);

    let implementation = match &item.data {
        Data::Struct(data) => {
            let fields = match &data.fields {
                Fields::Named(fields) => &fields.named,
                Fields::Unnamed(fields) => {
                    return Err(Error::new_spanned(
                        fields,
                        "`WipType` supports only structs with named fields",
                    ));
                }
                Fields::Unit => {
                    return Err(Error::new_spanned(
                        &item.ident,
                        "unit structs are not supported by `WipType`",
                    ));
                }
            };
            derive_record(ident, &type_name, type_documentation.as_ref(), fields)?
        }
        Data::Enum(data) => derive_enum(ident, &type_name, type_documentation.as_ref(), data)?,
        Data::Union(data) => {
            return Err(Error::new_spanned(
                data.union_token,
                "Rust unions are not supported by `WipType`",
            ));
        }
    };

    Ok(implementation)
}

fn derive_record(
    ident: &Ident,
    type_name: &str,
    type_documentation: Option<&Documentation>,
    fields: &syn::punctuated::Punctuated<syn::Field, syn::token::Comma>,
) -> syn::Result<TokenStream2> {
    let mut parsed_fields = Vec::new();
    let mut names = HashSet::new();
    for field in fields {
        let Some(field_ident) = field.ident.clone() else {
            return Err(Error::new_spanned(
                field,
                "`WipType` record fields must be named",
            ));
        };
        let name = source_name(&field_ident);
        if !names.insert(name.clone()) {
            return Err(Error::new(
                field_ident.span(),
                format!("duplicate record field `{name}`"),
            ));
        }
        let (optional, ty) = optional_type(&field.ty, "DTO field")?;
        validate_value_type(&ty, "DTO field")?;
        parsed_fields.push(Parameter {
            ident: field_ident,
            name,
            ty,
            optional,
            documentation: documentation(&field.attrs),
        });
    }

    let declaration_doc = documentation_tokens(type_documentation);
    let field_declarations = parsed_fields.iter().map(|field| {
        let name = &field.name;
        let documentation = documentation_tokens(field.documentation.as_ref());
        let ty = &field.ty;
        let optional = field.optional;
        quote! {
            ::wip_interface::wip_protocol::FieldDeclaration {
                name: ::std::string::String::from(#name),
                required: !#optional,
                documentation: #documentation,
                r#type: <#ty as ::wip_interface::WipType>::type_expr(),
            }
        }
    });
    let dependency_collectors = parsed_fields.iter().map(|field| {
        let ty = &field.ty;
        quote!(<#ty as ::wip_interface::WipType>::collect_declarations(declarations);)
    });
    let destructured_fields = parsed_fields.iter().map(|field| &field.ident);
    let encoders = parsed_fields.iter().map(|field| {
        let field_ident = &field.ident;
        let field_name = &field.name;
        let ty = &field.ty;
        if field.optional {
            quote! {
                if let ::std::option::Option::Some(value) = #field_ident {
                    fields.insert(
                        ::std::string::String::from(#field_name),
                        <#ty as ::wip_interface::WipType>::encode(value)?,
                    );
                }
            }
        } else {
            quote! {
                fields.insert(
                    ::std::string::String::from(#field_name),
                    <#ty as ::wip_interface::WipType>::encode(#field_ident)?,
                );
            }
        }
    });
    let decoders = parsed_fields.iter().map(|field| {
        let field_ident = &field.ident;
        let field_name = &field.name;
        let ty = &field.ty;
        if field.optional {
            quote!(#field_ident: decoder.optional::<#ty>(#field_name)?)
        } else {
            quote!(#field_ident: decoder.required::<#ty>(#field_name)?)
        }
    });

    Ok(quote! {
        impl ::wip_interface::WipType for #ident {
            fn type_expr() -> ::wip_interface::wip_protocol::TypeExpr {
                ::wip_interface::wip_protocol::TypeExpr::Named {
                    name: ::std::string::String::from(#type_name),
                }
            }

            fn collect_declarations(
                declarations: &mut ::std::vec::Vec<
                    ::wip_interface::wip_protocol::TypeDeclaration,
                >,
            ) {
                let declaration = ::wip_interface::wip_protocol::TypeDeclaration {
                    name: ::std::string::String::from(#type_name),
                    documentation: #declaration_doc,
                    definition: ::wip_interface::wip_protocol::TypeExpr::Record {
                        fields: ::std::vec![#(#field_declarations),*],
                    },
                };
                if ::wip_interface::__private::register_declaration(declarations, declaration) {
                    #(#dependency_collectors)*
                }
            }

            fn encode(
                self,
            ) -> ::std::result::Result<
                ::wip_interface::wip_protocol::Value,
                ::wip_interface::CodecError,
            > {
                let Self { #(#destructured_fields),* } = self;
                let mut fields = ::std::collections::BTreeMap::new();
                #(#encoders)*
                ::std::result::Result::Ok(
                    ::wip_interface::wip_protocol::Value::Record(fields),
                )
            }

            fn decode(
                value: ::wip_interface::wip_protocol::Value,
            ) -> ::std::result::Result<Self, ::wip_interface::CodecError> {
                match value {
                    ::wip_interface::wip_protocol::Value::Record(fields) => {
                        let mut decoder = ::wip_interface::__private::RecordDecoder::new(fields);
                        let decoded = Self {
                            #(#decoders,)*
                        };
                        decoder.finish()?;
                        ::std::result::Result::Ok(decoded)
                    }
                    actual => ::std::result::Result::Err(
                        ::wip_interface::__private::decode_type_mismatch("record", actual),
                    ),
                }
            }
        }
    })
}

fn derive_enum(
    ident: &Ident,
    type_name: &str,
    type_documentation: Option<&Documentation>,
    data: &DataEnum,
) -> syn::Result<TokenStream2> {
    for variant in &data.variants {
        if let Some((_, discriminant)) = &variant.discriminant {
            return Err(Error::new_spanned(
                discriminant,
                "explicit enum discriminants are not supported by `WipType`",
            ));
        }
    }

    let all_unit = data
        .variants
        .iter()
        .all(|variant| matches!(variant.fields, Fields::Unit));
    let valid_union = data.variants.iter().all(|variant| {
        matches!(variant.fields, Fields::Unit)
            || matches!(&variant.fields, Fields::Unnamed(fields) if fields.unnamed.len() == 1)
    });

    if all_unit {
        derive_unit_enum(ident, type_name, type_documentation, data)
    } else if valid_union {
        derive_union_enum(ident, type_name, type_documentation, data)
    } else {
        let span = data
            .variants
            .iter()
            .find(|variant| {
                !matches!(variant.fields, Fields::Unit)
                    && !matches!(&variant.fields, Fields::Unnamed(fields) if fields.unnamed.len() == 1)
            })
            .map_or_else(|| data.enum_token.span, Spanned::span);
        Err(Error::new(
            span,
            "a `WipType` enum must contain unit variants and/or single-field tuple variants; named-field and multi-field variants are unsupported",
        ))
    }
}

fn derive_unit_enum(
    ident: &Ident,
    type_name: &str,
    type_documentation: Option<&Documentation>,
    data: &DataEnum,
) -> syn::Result<TokenStream2> {
    let declaration_doc = documentation_tokens(type_documentation);
    let declarations = data.variants.iter().map(|variant| {
        let name = source_name(&variant.ident);
        let documentation = documentation_tokens(documentation(&variant.attrs).as_ref());
        quote! {
            ::wip_interface::wip_protocol::EnumCase {
                name: ::std::string::String::from(#name),
                documentation: #documentation,
            }
        }
    });
    let encoders = data.variants.iter().map(|variant| {
        let variant_ident = &variant.ident;
        let name = source_name(variant_ident);
        quote!(Self::#variant_ident => #name)
    });
    let decoders = data.variants.iter().map(|variant| {
        let variant_ident = &variant.ident;
        let name = source_name(variant_ident);
        quote!(#name => ::std::result::Result::Ok(Self::#variant_ident))
    });

    Ok(quote! {
        impl ::wip_interface::WipType for #ident {
            fn type_expr() -> ::wip_interface::wip_protocol::TypeExpr {
                ::wip_interface::wip_protocol::TypeExpr::Named {
                    name: ::std::string::String::from(#type_name),
                }
            }

            fn collect_declarations(
                declarations: &mut ::std::vec::Vec<
                    ::wip_interface::wip_protocol::TypeDeclaration,
                >,
            ) {
                let declaration = ::wip_interface::wip_protocol::TypeDeclaration {
                    name: ::std::string::String::from(#type_name),
                    documentation: #declaration_doc,
                    definition: ::wip_interface::wip_protocol::TypeExpr::Enum {
                        cases: ::std::vec![#(#declarations),*],
                    },
                };
                let _ = ::wip_interface::__private::register_declaration(
                    declarations,
                    declaration,
                );
            }

            fn encode(
                self,
            ) -> ::std::result::Result<
                ::wip_interface::wip_protocol::Value,
                ::wip_interface::CodecError,
            > {
                let variant = match self {
                    #(#encoders,)*
                };
                ::std::result::Result::Ok(
                    ::wip_interface::wip_protocol::Value::String(
                        ::std::string::String::from(variant),
                    ),
                )
            }

            fn decode(
                value: ::wip_interface::wip_protocol::Value,
            ) -> ::std::result::Result<Self, ::wip_interface::CodecError> {
                match value {
                    ::wip_interface::wip_protocol::Value::String(variant) => {
                        match variant.as_str() {
                            #(#decoders,)*
                            _ => ::std::result::Result::Err(
                                ::wip_interface::__private::decode_unknown_variant(
                                    #type_name,
                                    variant,
                                ),
                            ),
                        }
                    }
                    actual => ::std::result::Result::Err(
                        ::wip_interface::__private::decode_type_mismatch("enum", actual),
                    ),
                }
            }
        }
    })
}

fn derive_union_enum(
    ident: &Ident,
    type_name: &str,
    type_documentation: Option<&Documentation>,
    data: &DataEnum,
) -> syn::Result<TokenStream2> {
    struct Variant<'a> {
        ident: &'a Ident,
        name: String,
        payload: Option<&'a Type>,
        documentation: Option<Documentation>,
    }

    let mut variants = Vec::new();
    for variant in &data.variants {
        let payload = match &variant.fields {
            Fields::Unit => None,
            Fields::Unnamed(fields) if fields.unnamed.len() == 1 => {
                let field = fields.unnamed.first().expect("length checked");
                if direct_option(&field.ty)?.is_some() || contains_option(&field.ty) {
                    return Err(Error::new_spanned(
                        &field.ty,
                        "`Option<T>` is only supported directly on DTO fields and operation parameters",
                    ));
                }
                validate_value_type(&field.ty, "union case payload")?;
                Some(&field.ty)
            }
            _ => {
                return Err(Error::new_spanned(
                    variant,
                    "a `WipType` union case must be unit-like or contain exactly one unnamed payload",
                ));
            }
        };
        variants.push(Variant {
            ident: &variant.ident,
            name: source_name(&variant.ident),
            payload,
            documentation: documentation(&variant.attrs),
        });
    }

    let declaration_doc = documentation_tokens(type_documentation);
    let declarations = variants.iter().map(|variant| {
        let name = &variant.name;
        let documentation = documentation_tokens(variant.documentation.as_ref());
        let payload = match variant.payload {
            Some(ty) => quote! {
                ::std::option::Option::Some(
                    <#ty as ::wip_interface::WipType>::type_expr(),
                )
            },
            None => quote!(::std::option::Option::None),
        };
        quote! {
            ::wip_interface::wip_protocol::UnionCase {
                name: ::std::string::String::from(#name),
                documentation: #documentation,
                payload: #payload,
            }
        }
    });
    let dependency_collectors = variants.iter().filter_map(|variant| {
        variant.payload.map(
            |ty| quote!(<#ty as ::wip_interface::WipType>::collect_declarations(declarations);),
        )
    });
    let encoders = variants.iter().map(|variant| {
        let variant_ident = variant.ident;
        let name = &variant.name;
        match variant.payload {
            Some(ty) => quote! {
                Self::#variant_ident(value) => {
                    let mut fields = ::std::collections::BTreeMap::new();
                    fields.insert(
                        ::std::string::String::from(
                            ::wip_interface::wip_protocol::UNION_CASE_FIELD,
                        ),
                        ::wip_interface::wip_protocol::Value::String(
                            ::std::string::String::from(#name),
                        ),
                    );
                    fields.insert(
                        ::std::string::String::from(
                            ::wip_interface::wip_protocol::UNION_VALUE_FIELD,
                        ),
                        <#ty as ::wip_interface::WipType>::encode(value)?,
                    );
                    ::wip_interface::wip_protocol::Value::Record(fields)
                }
            },
            None => quote! {
                Self::#variant_ident => {
                    let mut fields = ::std::collections::BTreeMap::new();
                    fields.insert(
                        ::std::string::String::from(
                            ::wip_interface::wip_protocol::UNION_CASE_FIELD,
                        ),
                        ::wip_interface::wip_protocol::Value::String(
                            ::std::string::String::from(#name),
                        ),
                    );
                    ::wip_interface::wip_protocol::Value::Record(fields)
                }
            },
        }
    });
    let decoders = variants.iter().map(|variant| {
        let variant_ident = variant.ident;
        let name = &variant.name;
        match variant.payload {
            Some(ty) => quote! {
                #name => {
                    let value = decoder.required::<#ty>(
                        ::wip_interface::wip_protocol::UNION_VALUE_FIELD,
                    )?;
                    decoder.finish()?;
                    ::std::result::Result::Ok(Self::#variant_ident(value))
                }
            },
            None => quote! {
                #name => {
                    decoder.finish()?;
                    ::std::result::Result::Ok(Self::#variant_ident)
                }
            },
        }
    });

    Ok(quote! {
        impl ::wip_interface::WipType for #ident {
            fn type_expr() -> ::wip_interface::wip_protocol::TypeExpr {
                ::wip_interface::wip_protocol::TypeExpr::Named {
                    name: ::std::string::String::from(#type_name),
                }
            }

            fn collect_declarations(
                declarations: &mut ::std::vec::Vec<
                    ::wip_interface::wip_protocol::TypeDeclaration,
                >,
            ) {
                let declaration = ::wip_interface::wip_protocol::TypeDeclaration {
                    name: ::std::string::String::from(#type_name),
                    documentation: #declaration_doc,
                    definition: ::wip_interface::wip_protocol::TypeExpr::Union {
                        cases: ::std::vec![#(#declarations),*],
                    },
                };
                if ::wip_interface::__private::register_declaration(declarations, declaration) {
                    #(#dependency_collectors)*
                }
            }

            fn encode(
                self,
            ) -> ::std::result::Result<
                ::wip_interface::wip_protocol::Value,
                ::wip_interface::CodecError,
            > {
                let value = match self {
                    #(#encoders,)*
                };
                ::std::result::Result::Ok(value)
            }

            fn decode(
                value: ::wip_interface::wip_protocol::Value,
            ) -> ::std::result::Result<Self, ::wip_interface::CodecError> {
                match value {
                    ::wip_interface::wip_protocol::Value::Record(fields) => {
                        let mut decoder =
                            ::wip_interface::__private::RecordDecoder::new(fields);
                        let case = decoder.required::<::std::string::String>(
                            ::wip_interface::wip_protocol::UNION_CASE_FIELD,
                        )?;
                        match case.as_str() {
                            #(#decoders,)*
                            _ => ::std::result::Result::Err(
                                ::wip_interface::__private::decode_unknown_variant(
                                    #type_name,
                                    case,
                                ),
                            ),
                        }
                    }
                    actual => ::std::result::Result::Err(
                        ::wip_interface::__private::decode_type_mismatch("union record", actual),
                    ),
                }
            }
        }
    })
}
fn optional_type(ty: &Type, context: &str) -> syn::Result<(bool, Type)> {
    if let Some(inner) = direct_option(ty)? {
        if contains_option(&inner) {
            return Err(Error::new_spanned(
                inner,
                format!("nested `Option<T>` is not supported for {context}"),
            ));
        }
        Ok((true, inner))
    } else {
        if contains_option(ty) {
            return Err(Error::new_spanned(
                ty,
                format!("`Option<T>` must appear directly as the {context} type"),
            ));
        }
        Ok((false, ty.clone()))
    }
}

fn direct_option(ty: &Type) -> syn::Result<Option<Type>> {
    let Type::Path(path) = ty else {
        return Ok(None);
    };
    if path.qself.is_some() {
        return Ok(None);
    }
    let Some(segment) = path.path.segments.last() else {
        return Ok(None);
    };
    if segment.ident != "Option" {
        return Ok(None);
    }
    let PathArguments::AngleBracketed(arguments) = &segment.arguments else {
        return Err(Error::new_spanned(
            segment,
            "`Option` must have exactly one type argument",
        ));
    };
    if arguments.args.len() != 1 {
        return Err(Error::new_spanned(
            arguments,
            "`Option` must have exactly one type argument",
        ));
    }
    match arguments.args.first() {
        Some(GenericArgument::Type(inner)) => Ok(Some(inner.clone())),
        _ => Err(Error::new_spanned(
            arguments,
            "`Option` must have exactly one type argument",
        )),
    }
}

fn validate_value_type(ty: &Type, context: &str) -> syn::Result<()> {
    reject_custom_lifetimes(ty, context)?;
    if let Some(span) = first_reference(ty) {
        return Err(Error::new(
            span,
            format!("borrowed types are not supported as a WIP {context}"),
        ));
    }
    if let Some(span) = first_unit(ty) {
        return Err(Error::new(
            span,
            format!(
                "`()` is only supported as an operation application return, not as a {context}"
            ),
        ));
    }
    if contains_option(ty) {
        return Err(Error::new_spanned(
            ty,
            "`Option<T>` is only supported directly on DTO fields and operation parameters",
        ));
    }
    Ok(())
}

fn reject_custom_lifetimes(ty: &Type, context: &str) -> syn::Result<()> {
    struct Finder(Option<Span>);
    impl<'ast> Visit<'ast> for Finder {
        fn visit_lifetime(&mut self, lifetime: &'ast syn::Lifetime) {
            if self.0.is_none() {
                self.0 = Some(lifetime.span());
            }
        }
    }

    let mut finder = Finder(None);
    finder.visit_type(ty);
    if let Some(span) = finder.0 {
        Err(Error::new(
            span,
            format!("custom lifetimes are not supported in a WIP {context}"),
        ))
    } else {
        Ok(())
    }
}

fn contains_option(ty: &Type) -> bool {
    struct Finder(bool);
    impl<'ast> Visit<'ast> for Finder {
        fn visit_type_path(&mut self, path: &'ast syn::TypePath) {
            if path
                .path
                .segments
                .last()
                .is_some_and(|segment| segment.ident == "Option")
            {
                self.0 = true;
            }
            visit::visit_type_path(self, path);
        }
    }

    let mut finder = Finder(false);
    finder.visit_type(ty);
    finder.0
}

fn first_reference(ty: &Type) -> Option<Span> {
    struct Finder(Option<Span>);
    impl<'ast> Visit<'ast> for Finder {
        fn visit_type_reference(&mut self, reference: &'ast syn::TypeReference) {
            if self.0.is_none() {
                self.0 = Some(reference.span());
            }
            visit::visit_type_reference(self, reference);
        }
    }

    let mut finder = Finder(None);
    finder.visit_type(ty);
    finder.0
}

fn first_unit(ty: &Type) -> Option<Span> {
    struct Finder(Option<Span>);
    impl<'ast> Visit<'ast> for Finder {
        fn visit_type_tuple(&mut self, tuple: &'ast syn::TypeTuple) {
            if tuple.elems.is_empty() && self.0.is_none() {
                self.0 = Some(tuple.span());
            }
            visit::visit_type_tuple(self, tuple);
        }
    }

    let mut finder = Finder(None);
    finder.visit_type(ty);
    finder.0
}

fn is_unit(ty: &Type) -> bool {
    matches!(ty, Type::Tuple(tuple) if tuple.elems.is_empty())
}

fn source_name(ident: &Ident) -> String {
    ident.unraw().to_string()
}

fn pascal_case(name: &str) -> String {
    let mut result = String::new();
    let mut uppercase = true;
    for character in name.chars() {
        if character == '_' {
            uppercase = true;
        } else if uppercase {
            result.extend(character.to_uppercase());
            uppercase = false;
        } else {
            result.push(character);
        }
    }
    result
}

fn documentation(attributes: &[Attribute]) -> Option<Documentation> {
    documentation_from_lines(doc_lines(attributes))
}

fn doc_lines(attributes: &[Attribute]) -> Vec<String> {
    attributes
        .iter()
        .filter_map(|attribute| {
            if !attribute.path().is_ident("doc") {
                return None;
            }
            match &attribute.meta {
                Meta::NameValue(name_value) => match &name_value.value {
                    syn::Expr::Lit(expression) => match &expression.lit {
                        Lit::Str(value) => {
                            let value = value.value();
                            Some(value.strip_prefix(' ').unwrap_or(&value).to_owned())
                        }
                        _ => None,
                    },
                    _ => None,
                },
                _ => None,
            }
        })
        .collect()
}

fn split_returns_section(lines: Vec<String>) -> (Vec<String>, Vec<String>) {
    let Some(start) = lines.iter().position(|line| line.trim() == "# Returns") else {
        return (lines, Vec::new());
    };
    let end = lines[start + 1..]
        .iter()
        .position(|line| line.trim_start().starts_with("# "))
        .map_or(lines.len(), |offset| start + 1 + offset);

    let mut operation = lines[..start].to_vec();
    operation.extend_from_slice(&lines[end..]);
    (operation, lines[start + 1..end].to_vec())
}

fn documentation_from_lines(mut lines: Vec<String>) -> Option<Documentation> {
    while lines.first().is_some_and(|line| line.trim().is_empty()) {
        lines.remove(0);
    }
    while lines.last().is_some_and(|line| line.trim().is_empty()) {
        lines.pop();
    }
    if lines.is_empty() {
        return None;
    }

    let split = lines
        .iter()
        .position(|line| line.trim().is_empty())
        .unwrap_or(lines.len());
    let summary = lines[..split]
        .iter()
        .map(|line| line.trim())
        .collect::<Vec<_>>()
        .join(" ");
    let mut details_lines = if split < lines.len() {
        lines[split + 1..].to_vec()
    } else {
        Vec::new()
    };
    while details_lines
        .first()
        .is_some_and(|line| line.trim().is_empty())
    {
        details_lines.remove(0);
    }
    while details_lines
        .last()
        .is_some_and(|line| line.trim().is_empty())
    {
        details_lines.pop();
    }
    let details = (!details_lines.is_empty()).then(|| details_lines.join("\n"));

    Some(Documentation { summary, details })
}

fn documentation_tokens(documentation: Option<&Documentation>) -> TokenStream2 {
    match documentation {
        Some(documentation) => {
            let summary = &documentation.summary;
            match &documentation.details {
                Some(details) => quote! {
                    ::wip_interface::__private::documentation(#summary, ::std::option::Option::Some(#details))
                },
                None => quote! {
                    ::wip_interface::__private::documentation(#summary, ::std::option::Option::None)
                },
            }
        }
        None => quote! {
            ::wip_interface::__private::documentation("", ::std::option::Option::None)
        },
    }
}