use clippy_utils::diagnostics::span_lint_and_then;
use rustc_data_structures::fx::FxHashMap;
use rustc_hir::def::{DefKind, Res};
use rustc_hir::{Impl, ItemKind, Node, Path, QPath, TraitRef, TyKind};
use rustc_lint::{LateContext, LateLintPass};
use rustc_middle::ty::AssocKind;
use rustc_session::{declare_lint_pass, declare_tool_lint};
use rustc_span::symbol::Symbol;
use rustc_span::Span;
use std::collections::{BTreeMap, BTreeSet};

declare_clippy_lint! {
    /// ### What it does
    /// It lints if a struct has two method with same time:
    /// one from a trait, another not from trait.
    ///
    /// ### Why is this bad?
    /// Confusing.
    ///
    /// ### Example
    /// ```rust
    /// trait T {
    ///     fn foo(&self) {}
    /// }
    ///
    /// struct S;
    ///
    /// impl T for S {
    ///     fn foo(&self) {}
    /// }
    ///
    /// impl S {
    ///     fn foo(&self) {}
    /// }
    /// ```
    pub SAME_NAME_METHOD,
    restriction,
    "two method with same name"
}

declare_lint_pass!(SameNameMethod => [SAME_NAME_METHOD]);

struct ExistingName {
    impl_methods: BTreeMap<Symbol, Span>,
    trait_methods: BTreeMap<Symbol, Vec<Span>>,
}

impl<'tcx> LateLintPass<'tcx> for SameNameMethod {
    fn check_crate_post(&mut self, cx: &LateContext<'tcx>) {
        let mut map = FxHashMap::<Res, ExistingName>::default();

        for item in cx.tcx.hir().items() {
            if let ItemKind::Impl(Impl {
                items,
                of_trait,
                self_ty,
                ..
            }) = &item.kind
            {
                if let TyKind::Path(QPath::Resolved(_, Path { res, .. })) = self_ty.kind {
                    if !map.contains_key(res) {
                        map.insert(
                            *res,
                            ExistingName {
                                impl_methods: BTreeMap::new(),
                                trait_methods: BTreeMap::new(),
                            },
                        );
                    }
                    let existing_name = map.get_mut(res).unwrap();

                    match of_trait {
                        Some(trait_ref) => {
                            let mut methods_in_trait: BTreeSet<Symbol> = if_chain! {
                                if let Some(Node::TraitRef(TraitRef { path, .. })) =
                                    cx.tcx.hir().find(trait_ref.hir_ref_id);
                                if let Res::Def(DefKind::Trait, did) = path.res;
                                then{
                                    // FIXME: if
                                    // `rustc_middle::ty::assoc::AssocItems::items` is public,
                                    // we can iterate its keys instead of `in_definition_order`,
                                    // which's more efficient
                                    cx.tcx
                                        .associated_items(did)
                                        .in_definition_order()
                                        .filter(|assoc_item| {
                                            matches!(assoc_item.kind, AssocKind::Fn)
                                        })
                                        .map(|assoc_item| assoc_item.ident.name)
                                        .collect()
                                }else{
                                    BTreeSet::new()
                                }
                            };

                            let mut check_trait_method = |method_name: Symbol, trait_method_span: Span| {
                                if let Some(impl_span) = existing_name.impl_methods.get(&method_name) {
                                    span_lint_and_then(
                                        cx,
                                        SAME_NAME_METHOD,
                                        *impl_span,
                                        "method's name is same to an existing method in a trait",
                                        |diag| {
                                            diag.span_note(
                                                trait_method_span,
                                                &format!("existing `{}` defined here", method_name),
                                            );
                                        },
                                    );
                                }
                                if let Some(v) = existing_name.trait_methods.get_mut(&method_name) {
                                    v.push(trait_method_span);
                                } else {
                                    existing_name.trait_methods.insert(method_name, vec![trait_method_span]);
                                }
                            };

                            for impl_item_ref in (*items).iter().filter(|impl_item_ref| {
                                matches!(impl_item_ref.kind, rustc_hir::AssocItemKind::Fn { .. })
                            }) {
                                let method_name = impl_item_ref.ident.name;
                                methods_in_trait.remove(&method_name);
                                check_trait_method(method_name, impl_item_ref.span);
                            }

                            for method_name in methods_in_trait {
                                check_trait_method(method_name, item.span);
                            }
                        },
                        None => {
                            for impl_item_ref in (*items).iter().filter(|impl_item_ref| {
                                matches!(impl_item_ref.kind, rustc_hir::AssocItemKind::Fn { .. })
                            }) {
                                let method_name = impl_item_ref.ident.name;
                                let impl_span = impl_item_ref.span;
                                if let Some(trait_spans) = existing_name.trait_methods.get(&method_name) {
                                    span_lint_and_then(
                                        cx,
                                        SAME_NAME_METHOD,
                                        impl_span,
                                        "method's name is same to an existing method in a trait",
                                        |diag| {
                                            // TODO should we `span_note` on every trait?
                                            // iterate on trait_spans?
                                            diag.span_note(
                                                trait_spans[0],
                                                &format!("existing `{}` defined here", method_name),
                                            );
                                        },
                                    );
                                }
                                existing_name.impl_methods.insert(method_name, impl_span);
                            }
                        },
                    }
                }
            }
        }
    }
}