use rustc_ast as ast;
use rustc_ast::{GenericParamKind, ItemKind, MetaItemInner, MetaItemKind, StmtKind};
use rustc_attr_parsing::{AttributeTemplate, validate_attr};
use rustc_expand::base::{
    Annotatable, DeriveResolution, ExpandResult, ExtCtxt, Indeterminate, MultiItemModifier,
};
use rustc_session::Session;
use rustc_span::{ErrorGuaranteed, Ident, Span, sym};

use crate::cfg_eval::cfg_eval;
use crate::diagnostics;

pub(crate) struct Expander {
    pub is_const: bool,
}

impl MultiItemModifier for Expander {
    fn expand(
        &self,
        ecx: &mut ExtCtxt<'_>,
        span: Span,
        meta_item: &ast::MetaItem,
        item: Annotatable,
        _: bool,
    ) -> ExpandResult<Vec<Annotatable>, Annotatable> {
        let sess = ecx.sess;
        if report_bad_target(sess, &item, span).is_err() {
            // We don't want to pass inappropriate targets to derive macros to avoid
            // follow up errors, all other errors below are recoverable.
            return ExpandResult::Ready(vec![item]);
        }

        let (sess, features) = (ecx.sess, ecx.ecfg.features);
        let result =
            ecx.resolver.resolve_derives(ecx.current_expansion.id, ecx.force_mode, &|| {
                let template = AttributeTemplate {
                    list: Some(&["Trait1, Trait2, ..."]),
                    ..Default::default()
                };
                validate_attr::check_builtin_meta_item(
                    &sess.psess,
                    meta_item,
                    ast::AttrStyle::Outer,
                    sym::derive,
                    template,
                    true,
                );

                let mut resolutions = match &meta_item.kind {
                    MetaItemKind::List(list) => {
                        list.iter()
                            .filter_map(|meta_item_inner| match meta_item_inner {
                                MetaItemInner::MetaItem(meta) => Some(meta),
                                MetaItemInner::Lit(lit) => {
                                    // Reject `#[derive("Debug")]`.
                                    report_unexpected_meta_item_lit(sess, lit);
                                    None
                                }
                            })
                            .map(|meta| {
                                // Reject `#[derive(Debug = "value", Debug(abc))]`, but recover the
                                // paths.
                                report_path_args(sess, meta);
                                meta.path.clone()
                            })
                            .map(|path| DeriveResolution {
                                path,
                                item: dummy_annotatable(),
                                exts: None,
                                is_const: self.is_const,
                            })
                            .collect()
                    }
                    _ => vec![],
                };

                // Do not configure or clone items unless necessary.
                match &mut resolutions[..] {
                    [] => {}
                    [first, others @ ..] => {
                        first.item = cfg_eval(
                            sess,
                            features,
                            item.clone(),
                            ecx.current_expansion.lint_node_id,
                        );
                        for other in others {
                            other.item = first.item.clone();
                        }
                    }
                }

                resolutions
            });

        match result {
            Ok(()) => ExpandResult::Ready(vec![item]),
            Err(Indeterminate) => ExpandResult::Retry(item),
        }
    }
}

// The cheapest `Annotatable` to construct.
fn dummy_annotatable() -> Annotatable {
    Annotatable::GenericParam(ast::GenericParam {
        id: ast::DUMMY_NODE_ID,
        ident: Ident::dummy(),
        attrs: Default::default(),
        bounds: Default::default(),
        is_placeholder: false,
        kind: GenericParamKind::Lifetime,
        colon_span: None,
    })
}

fn report_bad_target(
    sess: &Session,
    item: &Annotatable,
    span: Span,
) -> Result<(), ErrorGuaranteed> {
    let item_kind = match item {
        Annotatable::Item(item) => Some(&item.kind),
        Annotatable::Stmt(stmt) => match &stmt.kind {
            StmtKind::Item(item) => Some(&item.kind),
            _ => None,
        },
        _ => None,
    };

    let bad_target =
        !matches!(item_kind, Some(ItemKind::Struct(..) | ItemKind::Enum(..) | ItemKind::Union(..)));
    if bad_target {
        return Err(sess.dcx().emit_err(diagnostics::BadDeriveTarget { span, item: item.span() }));
    }
    Ok(())
}

fn report_unexpected_meta_item_lit(sess: &Session, lit: &ast::MetaItemLit) {
    let help = match lit.kind {
        ast::LitKind::Str(_, ast::StrStyle::Cooked)
            if rustc_lexer::is_ident(lit.symbol.as_str()) =>
        {
            diagnostics::BadDeriveLitHelp::StrLit { sym: lit.symbol }
        }
        _ => diagnostics::BadDeriveLitHelp::Other,
    };
    sess.dcx().emit_err(diagnostics::BadDeriveLit { span: lit.span, help });
}

fn report_path_args(sess: &Session, meta: &ast::MetaItem) {
    let span = meta.span.with_lo(meta.path.span.hi());

    match meta.kind {
        MetaItemKind::Word => {}
        MetaItemKind::List(..) => {
            sess.dcx().emit_err(diagnostics::DerivePathArgsList { span });
        }
        MetaItemKind::NameValue(..) => {
            sess.dcx().emit_err(diagnostics::DerivePathArgsValue { span });
        }
    }
}
