//! Implementation of [`ops::AddAssign`]-like derive macros.

#[cfg(doc)]
use std::ops;

use proc_macro2::TokenStream;
use quote::{format_ident, ToTokens as _};
use syn::spanned::Spanned as _;

use super::{AssignStructuralExpansion, SkippedFields};
use crate::utils::attr::{self, ParseMultiple as _};

/// Expands an [`ops::AddAssign`]-like derive macro.
///
/// Available macros:
/// - [`AddAssign`](ops::AddAssign)
/// - [`BitAndAssign`](ops::BitAndAssign)
/// - [`BitOrAssign`](ops::BitOrAssign)
/// - [`BitXorAssign`](ops::BitXorAssign)
/// - [`SubAssign`](ops::SubAssign)
pub fn expand(input: &syn::DeriveInput, trait_name: &str) -> syn::Result<TokenStream> {
    let trait_name = normalize_trait_name(trait_name);
    let attr_name = format_ident!("{}", trait_name_to_attribute_name(trait_name));

    let mut variants = vec![];
    match &input.data {
        syn::Data::Struct(data) => {
            if let Some(skip) = attr::Skip::parse_attrs(&input.attrs, &attr_name)? {
                return Err(syn::Error::new(
                    skip.span,
                    format!(
                        "`#[{attr_name}({})]` attribute can be placed only on struct fields",
                        skip.item.name(),
                    ),
                ));
            } else if matches!(data.fields, syn::Fields::Unit) {
                return Err(syn::Error::new(
                    data.struct_token.span(),
                    format!("`{trait_name}` cannot be derived for unit structs"),
                ));
            }
            let mut skipped_fields = SkippedFields::default();
            for (n, field) in data.fields.iter().enumerate() {
                if attr::Skip::parse_attrs(&field.attrs, &attr_name)?.is_some() {
                    _ = skipped_fields.insert(n);
                }
            }
            if data.fields.len() == skipped_fields.len() {
                return Err(syn::Error::new(
                    data.struct_token.span(),
                    format!(
                        "`{trait_name}` cannot be derived for structs with all the fields being \
                         skipped",
                    ),
                ));
            }
            variants.push((None, &data.fields, skipped_fields));
        }
        syn::Data::Enum(data) => {
            return Err(syn::Error::new(
                data.enum_token.span(),
                format!("`{trait_name}` cannot be derived for enums"),
            ));
        }
        syn::Data::Union(data) => {
            return Err(syn::Error::new(
                data.union_token.span(),
                format!("`{trait_name}` cannot be derived for unions"),
            ));
        }
    }

    Ok(AssignStructuralExpansion {
        trait_ty: format_ident!("{trait_name}"),
        method_ident: format_ident!("{}", trait_name_to_method_name(trait_name)),
        self_ty: (&input.ident, &input.generics),
        variants,
        is_enum: false,
    }
    .into_token_stream())
}

/// Matches the provided derive macro `name` to appropriate actual trait name.
fn normalize_trait_name(name: &str) -> &'static str {
    match name {
        "AddAssign" => "AddAssign",
        "BitAndAssign" => "BitAndAssign",
        "BitOrAssign" => "BitOrAssign",
        "BitXorAssign" => "BitXorAssign",
        "SubAssign" => "SubAssign",
        _ => unimplemented!(),
    }
}

/// Matches the provided [`ops::AddAssign`]-like trait `name` to its attribute's name.
fn trait_name_to_attribute_name(name: &str) -> &'static str {
    trait_name_to_method_name(name)
}

/// Matches the provided [`ops::AddAssign`]-like trait `name` to its method name.
fn trait_name_to_method_name(name: &str) -> &'static str {
    match name {
        "AddAssign" => "add_assign",
        "BitAndAssign" => "bitand_assign",
        "BitOrAssign" => "bitor_assign",
        "BitXorAssign" => "bitxor_assign",
        "SubAssign" => "sub_assign",
        _ => unimplemented!(),
    }
}