//! Procedural macros for dagx
//!
//! This crate provides the `#[task]` attribute macro that automatically implements
//! the `Task` trait by deriving Input and Output types from the `run()` method signature.

use proc_macro::TokenStream;
use proc_macro2::Span;
use quote::quote;
use syn::{parse_macro_input, FnArg, Ident, ImplItem, ItemImpl, Pat, PatType, ReturnType, Type};

/// Attribute macro to automatically implement the `Task` trait.
///
/// Apply this to an `impl` block containing a `run()` method (sync or async). The macro:
/// - Derives `Input` and `Output` types from the `run()` signature
/// - Automatically implements the `Task` trait
/// - **Generates type-specific extraction logic** - works with ANY type (Send + Sync + 'static)!
/// - Supports both sync and async run methods
/// - Supports stateless (no self) and stateful (&self, &mut self) tasks
/// - Handles various input patterns (no inputs, single input, multiple inputs)
///
/// **Key Feature**: Custom types work automatically without implementing any traits!
/// The macro generates inline extraction logic specific to your task's parameter types.
///
/// # Task Patterns
///
/// The `#[task]` macro supports three patterns based on state requirements:
///
/// ## 1. Stateless Tasks (No State)
///
/// Unit structs for pure computations. Use **no `self` parameter**:
///
/// ```ignore
/// use dagx::{task, Task};
///
/// struct Add;
///
/// #[task]
/// impl Add {
///     async fn run(a: &i32, b: &i32) -> i32 {
///         a + b  // Pure function, no state
///     }
/// }
/// ```
///
/// ## 2. Read-Only State Tasks
///
/// Tasks that read configuration or constant data. Use **`&self`**:
///
/// ```ignore
/// use dagx::{task, Task};
///
/// struct Multiplier {
///     factor: i32,
/// }
///
/// #[task]
/// impl Multiplier {
///     async fn run(&self, input: &i32) -> i32 {
///         input * self.factor  // Read-only access
///     }
/// }
/// ```
///
/// ## 3. Mutable State Tasks
///
/// Tasks that accumulate or modify state. Use **`&mut self`**:
///
/// ```ignore
/// use dagx::{task, Task};
///
/// struct Counter {
///     count: i32,
/// }
///
/// #[task]
/// impl Counter {
///     async fn run(&mut self, increment: &i32) -> i32 {
///         self.count += increment;  // Modifies state
///         self.count
///     }
/// }
/// ```
///
/// # Input Patterns
///
/// ## No Inputs (Source Tasks)
///
/// ```ignore
/// use dagx::{task, Task};
///
/// struct LoadData {
///     value: i32,
/// }
///
/// #[task]
/// impl LoadData {
///     async fn run(&mut self) -> i32 {
///         self.value
///     }
/// }
/// ```
///
/// ## Single Input
///
/// ```ignore
/// use dagx::{task, Task};
///
/// struct Double;
///
/// #[task]
/// impl Double {
///     async fn run(&mut self, input: &i32) -> i32 {
///         input * 2
///     }
/// }
/// ```
///
/// ## Multiple Inputs (up to 8)
///
/// ```ignore
/// use dagx::{task, Task};
///
/// struct Combine;
///
/// #[task]
/// impl Combine {
///     async fn run(&mut self, a: &i32, b: &String, c: &bool) -> String {
///         format!("{}: {} ({})", b, a, c)
///     }
/// }
/// ```
///
/// # Requirements
///
/// - The impl block must contain exactly one `async fn run()` method
/// - The `run()` method can be stateless (no self parameter) or stateful (`&mut self`)
/// - All input parameters must be references (e.g., `&i32`, not `i32`)
///
/// # Generated Code
///
/// The macro transforms your implementation into a full `Task` trait implementation.
///
/// For stateless tasks (no self parameter):
///
/// ```ignore
/// // Your code:
/// #[task]
/// impl Add {
///     async fn run(a: &i32, b: &i32) -> i32 {
///         a + b
///     }
/// }
///
/// // Generated:
/// impl Task<(i32, i32)> for Add {
///     type Output = i32;
///
///     async fn run(&mut self, _input: Self::Input) -> Self::Output {
///         let (a, _input) = _input.next();
///         let (b, _input) = _input.next();
///         Self::run_impl(a, b).await
///     }
/// }
///
/// impl Add {
///     #[inline]
///     async fn run_impl(a: &i32, b: &i32) -> i32 {
///         a + b
///     }
/// }
/// ```
#[proc_macro_attribute]
pub fn task(_attr: TokenStream, item: TokenStream) -> TokenStream {
    let impl_block = parse_macro_input!(item as ItemImpl);

    // Extract the struct name
    let struct_name = &impl_block.self_ty;

    // Find the run() method
    let run_method = match impl_block.items.iter().find_map(|item| {
        if let ImplItem::Fn(method) = item {
            if method.sig.ident == "run" {
                return Some(method);
            }
        }
        None
    }) {
        Some(method) => method,
        None => {
            return syn::Error::new_spanned(
                &impl_block,
                "impl block must contain a run() method\n\n\
                 Expected signature: fn run(&mut self, ...) -> OutputType\n\
                              or: async fn run(&mut self, ...) -> OutputType\n\
                 The #[task] macro requires a run() method to implement the Task trait.",
            )
            .to_compile_error()
            .into();
        }
    };

    // Extract parameters (excluding self)
    let params_result: Result<Vec<_>, _> = run_method
        .sig
        .inputs
        .iter()
        .filter_map(|arg| {
            if let FnArg::Typed(PatType { pat, ty, .. }) = arg {
                // Extract the parameter name
                let param_name = if let Pat::Ident(pat_ident) = &**pat {
                    &pat_ident.ident
                } else {
                    return Some(Err(syn::Error::new_spanned(
                        pat,
                        "Unsupported parameter pattern\n\n\
                         Parameters must be simple identifiers like 'input: &T' or 'a: &i32'.",
                    )));
                };

                // Extract the inner type from &Type
                let inner_type = if let Type::Reference(type_ref) = &**ty {
                    &type_ref.elem
                } else {
                    return Some(Err(syn::Error::new_spanned(
                        ty,
                        "All parameters must be references (&T)\n\n\
                         Task inputs must be references to allow sharing data between tasks.\n\
                         Change this parameter from 'T' to '&T'.",
                    )));
                };

                Some(Ok((param_name.clone(), inner_type.clone())))
            } else {
                None // Skip self parameter
            }
        })
        .collect();

    let params = match params_result {
        Ok(p) => p,
        Err(e) => return e.to_compile_error().into(),
    };

    // Extract return type
    let output_type = match &run_method.sig.output {
        ReturnType::Default => syn::parse_quote!(()),
        ReturnType::Type(_, ty) => ty.clone(),
    };

    // Build Input type based on parameter count
    let types: Vec<_> = params.iter().map(|(_, ty)| ty).collect();
    let input_type = quote! { ( #(#types,)* ) };

    // Generate parameter references for the wrapper run() method
    let (param_destructure, param_refs) = {
        let (destructure, refs): (Vec<_>, Vec<_>) = params
            .iter()
            .map(|(ident, _ty)| {
                let ident = Ident::new(&format!("__dagx_extract_{}", ident), Span::mixed_site());
                (
                    quote! {
                        let (#ident, _input) = _input.next();
                    },
                    ident,
                )
            })
            .unzip();
        (quote! { #(#destructure)* }, quote! { #(#refs),* })
    };

    // Clone the run method and rename it to run_impl
    let mut run_impl_method = run_method.clone();
    run_impl_method.sig.ident = syn::Ident::new("run_impl", run_method.sig.ident.span());

    // Check if the method has a self receiver
    let has_self_receiver = run_method
        .sig
        .inputs
        .iter()
        .any(|arg| matches!(arg, FnArg::Receiver(_)));

    let is_async = run_method.sig.asyncness.is_some();

    let run_call = match (has_self_receiver, is_async) {
        (true, true) => quote! { self.run_impl(#param_refs).await },
        (true, false) => quote! { self.run_impl(#param_refs) },
        (false, true) => quote! { Self::run_impl(#param_refs).await },
        (false, false) => quote! { Self::run_impl(#param_refs) },
    };

    // Generate the Task trait implementation based on whether we have self and async/sync
    quote! {
        impl ::dagx::Task<#input_type> for #struct_name {
            type Output = #output_type;

            async fn run(mut self, mut _input: ::dagx::TaskInput<'_, #input_type>) -> Self::Output {
                #param_destructure
                #run_call
            }
        }

        impl #struct_name {
            #run_impl_method
        }
    }
    .into()
}