//! Chain Attribute Macro Implementation
//!
//! This module provides the `#[chain]` attribute macro for automatically
//! generating structs that implement the `Chain` trait from async functions.

use proc_macro::TokenStream;
use quote::quote;
use syn::parse::{Parse, ParseStream};
use syn::spanned::Spanned;
use syn::{
    FnArg, Ident, ItemFn, Pat, PatType, ReturnType, Signature, Type, TypePath, parse_macro_input,
};

/// Parses the chain attribute arguments
struct ChainAttribute {
    struct_name: Ident,
}

impl Parse for ChainAttribute {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let struct_name = input.parse()?;
        Ok(ChainAttribute { struct_name })
    }
}

/// Extracts the previous type and parameter name from function arguments
fn extract_previous_info(sig: &Signature) -> syn::Result<(Pat, TypePath)> {
    // The function should have exactly one parameter
    if sig.inputs.len() != 1 {
        return Err(syn::Error::new(
            sig.inputs.span(),
            "Chain function must have exactly one parameter",
        ));
    }

    let arg = &sig.inputs[0];
    match arg {
        FnArg::Typed(PatType { pat, ty, .. }) => {
            // Extract the pattern (parameter name)
            let param_pat = (**pat).clone();

            // Extract the type
            match &**ty {
                Type::Path(type_path) => Ok((param_pat, type_path.clone())),
                _ => Err(syn::Error::new(
                    ty.span(),
                    "Parameter type must be a type path",
                )),
            }
        }
        FnArg::Receiver(_) => Err(syn::Error::new(
            arg.span(),
            "Chain function cannot have self parameter",
        )),
    }
}

/// Extracts the return type from the function signature
fn extract_return_type(sig: &Signature) -> syn::Result<TypePath> {
    match &sig.output {
        ReturnType::Type(_, ty) => match &**ty {
            Type::Path(type_path) => Ok(type_path.clone()),
            _ => Err(syn::Error::new(
                ty.span(),
                "Return type must be a type path",
            )),
        },
        ReturnType::Default => Err(syn::Error::new(
            sig.span(),
            "Chain function must have a return type",
        )),
    }
}

/// Implementation of the `#[chain]` attribute macro
///
/// This macro transforms an async function into a struct that implements
/// the `Chain` trait. The struct name is specified in the attribute.
///
/// # Examples
///
/// ```ignore
/// use mingling_macros::chain;
///
/// #[chain(InitEntry)]
/// pub async fn process(data: InitBegin) -> mingling::AnyOutput {
///     AnyOutput::new::<InitResult>("初始化成功！".to_string().into())
/// }
/// ```
///
/// This generates:
/// ```ignore
/// pub struct InitEntry;
/// impl Chain for InitEntry {
///     type Previous = InitBegin;
///     async fn proc(data: Self::Previous) -> mingling::AnyOutput {
///         AnyOutput::new::<InitResult>("初始化成功！".to_string().into())
///     }
/// }
/// ```
pub fn chain_attr(attr: TokenStream, item: TokenStream) -> TokenStream {
    // Parse the attribute arguments
    let chain_attr = parse_macro_input!(attr as ChainAttribute);
    let struct_name = chain_attr.struct_name;

    // Parse the function item
    let input_fn = parse_macro_input!(item as ItemFn);

    // Validate the function
    if !input_fn.sig.asyncness.is_some() {
        return syn::Error::new(input_fn.sig.span(), "Chain function must be async")
            .to_compile_error()
            .into();
    }

    // Extract the previous type and parameter name from function arguments
    let (prev_param, previous_type) = match extract_previous_info(&input_fn.sig) {
        Ok(info) => info,
        Err(e) => return e.to_compile_error().into(),
    };

    // Extract the return type
    let return_type = match extract_return_type(&input_fn.sig) {
        Ok(ty) => ty,
        Err(e) => return e.to_compile_error().into(),
    };

    // Get the function body
    let fn_body = &input_fn.block;

    // Get function attributes (excluding the chain attribute)
    let mut fn_attrs = input_fn.attrs.clone();
    // Remove any #[chain(...)] attributes to avoid infinite recursion
    fn_attrs.retain(|attr| !attr.path().is_ident("chain"));

    // Get function visibility
    let vis = &input_fn.vis;

    // Get function name
    let fn_name = &input_fn.sig.ident;

    // Generate the struct and implementation
    let expanded = quote! {
        #(#fn_attrs)*
        #vis struct #struct_name;

        impl ::mingling::Chain for #struct_name {
            type Previous = #previous_type;

            async fn proc(#prev_param: Self::Previous) -> #return_type {
                // Call the original function
                #fn_name(#prev_param).await
            }
        }

        // Keep the original function for internal use
        #(#fn_attrs)*
        #vis async fn #fn_name(#prev_param: #previous_type) -> #return_type {
            #fn_body
        }
    };

    expanded.into()
}