use proc_macro::TokenStream;
use quote::quote;
use syn::{Data, DeriveInput, Fields, parse_macro_input};

/// Derives the `MergeFrom` trait for a struct.
///
/// This macro automatically implements `MergeFrom` by calling `merge_from`
/// on all fields in the struct.
///
/// # Example
///
/// ```ignore
/// #[derive(Clone, MergeFrom)]
/// struct MySettings {
///     field1: Option<String>,
///     field2: SomeOtherSettings,
/// }
/// ```
#[proc_macro_derive(MergeFrom)]
pub fn derive_merge_from(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);

    let name = &input.ident;
    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();

    let merge_body = match &input.data {
        Data::Struct(data_struct) => match &data_struct.fields {
            Fields::Named(fields) => {
                let field_merges = fields.named.iter().map(|field| {
                    let field_name = &field.ident;
                    quote! {
                        self.#field_name.merge_from(&other.#field_name);
                    }
                });

                quote! {
                    #(#field_merges)*
                }
            }
            Fields::Unnamed(fields) => {
                let field_merges = fields.unnamed.iter().enumerate().map(|(i, _)| {
                    let field_index = syn::Index::from(i);
                    quote! {
                        self.#field_index.merge_from(&other.#field_index);
                    }
                });

                quote! {
                    #(#field_merges)*
                }
            }
            Fields::Unit => {
                quote! {
                    // No fields to merge for unit structs
                }
            }
        },
        Data::Enum(_) => {
            quote! {
                *self = other.clone();
            }
        }
        Data::Union(_) => {
            panic!("MergeFrom cannot be derived for unions");
        }
    };

    let expanded = quote! {
        impl #impl_generics crate::merge_from::MergeFrom for #name #ty_generics #where_clause {
            fn merge_from(&mut self, other: &Self) {
                use crate::merge_from::MergeFrom as _;
                #merge_body
            }
        }
    };

    TokenStream::from(expanded)
}