//! This module is concerned with finding methods that a given type provides.
//! For details about how this works in rustc, see the method lookup page in the
//! [rustc guide](https://rust-lang.github.io/rustc-guide/method-lookup.html)
//! and the corresponding code mostly in librustc_typeck/check/method/probe.rs.
use std::sync::Arc;

use rustc_hash::FxHashMap;

use crate::{
    HirDatabase, Module, Crate, Name, Function, Trait,
    ids::TraitId,
    impl_block::{ImplId, ImplBlock, ImplItem},
    ty::{Ty, TypeCtor},
    nameres::CrateModuleId, resolve::Resolver, traits::TraitItem

};

/// This is used as a key for indexing impls.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum TyFingerprint {
    Apply(TypeCtor),
}

impl TyFingerprint {
    /// Creates a TyFingerprint for looking up an impl. Only certain types can
    /// have impls: if we have some `struct S`, we can have an `impl S`, but not
    /// `impl &S`. Hence, this will return `None` for reference types and such.
    fn for_impl(ty: &Ty) -> Option<TyFingerprint> {
        match ty {
            Ty::Apply(a_ty) => Some(TyFingerprint::Apply(a_ty.ctor)),
            _ => None,
        }
    }
}

#[derive(Debug, PartialEq, Eq)]
pub struct CrateImplBlocks {
    /// To make sense of the CrateModuleIds, we need the source root.
    krate: Crate,
    impls: FxHashMap<TyFingerprint, Vec<(CrateModuleId, ImplId)>>,
    impls_by_trait: FxHashMap<TraitId, Vec<(CrateModuleId, ImplId)>>,
}

impl CrateImplBlocks {
    pub fn lookup_impl_blocks<'a>(&'a self, ty: &Ty) -> impl Iterator<Item = ImplBlock> + 'a {
        let fingerprint = TyFingerprint::for_impl(ty);
        fingerprint.and_then(|f| self.impls.get(&f)).into_iter().flat_map(|i| i.iter()).map(
            move |(module_id, impl_id)| {
                let module = Module { krate: self.krate, module_id: *module_id };
                ImplBlock::from_id(module, *impl_id)
            },
        )
    }

    pub fn lookup_impl_blocks_for_trait<'a>(
        &'a self,
        tr: &Trait,
    ) -> impl Iterator<Item = ImplBlock> + 'a {
        let id = tr.id;
        self.impls_by_trait.get(&id).into_iter().flat_map(|i| i.iter()).map(
            move |(module_id, impl_id)| {
                let module = Module { krate: self.krate, module_id: *module_id };
                ImplBlock::from_id(module, *impl_id)
            },
        )
    }

    fn collect_recursive(&mut self, db: &impl HirDatabase, module: &Module) {
        let module_impl_blocks = db.impls_in_module(module.clone());

        for (impl_id, _) in module_impl_blocks.impls.iter() {
            let impl_block = ImplBlock::from_id(module_impl_blocks.module, impl_id);

            let target_ty = impl_block.target_ty(db);

            if let Some(tr) = impl_block.target_trait(db) {
                self.impls_by_trait
                    .entry(tr.id)
                    .or_insert_with(Vec::new)
                    .push((module.module_id, impl_id));
            } else {
                if let Some(target_ty_fp) = TyFingerprint::for_impl(&target_ty) {
                    self.impls
                        .entry(target_ty_fp)
                        .or_insert_with(Vec::new)
                        .push((module.module_id, impl_id));
                }
            }
        }

        for child in module.children(db) {
            self.collect_recursive(db, &child);
        }
    }

    pub(crate) fn impls_in_crate_query(
        db: &impl HirDatabase,
        krate: Crate,
    ) -> Arc<CrateImplBlocks> {
        let mut crate_impl_blocks = CrateImplBlocks {
            krate,
            impls: FxHashMap::default(),
            impls_by_trait: FxHashMap::default(),
        };
        if let Some(module) = krate.root_module(db) {
            crate_impl_blocks.collect_recursive(db, &module);
        }
        Arc::new(crate_impl_blocks)
    }
}

fn def_crate(db: &impl HirDatabase, ty: &Ty) -> Option<Crate> {
    match ty {
        Ty::Apply(a_ty) => match a_ty.ctor {
            TypeCtor::Adt(def_id) => def_id.krate(db),
            _ => None,
        },
        _ => None,
    }
}

impl Ty {
    /// Look up the method with the given name, returning the actual autoderefed
    /// receiver type (but without autoref applied yet).
    pub fn lookup_method(
        self,
        db: &impl HirDatabase,
        name: &Name,
        resolver: &Resolver,
    ) -> Option<(Ty, Function)> {
        // FIXME: what has priority, an inherent method that needs autoderefs or a trait method?
        let inherent_method = self.clone().iterate_methods(db, |ty, f| {
            let sig = f.signature(db);
            if sig.name() == name && sig.has_self_param() {
                Some((ty.clone(), f))
            } else {
                None
            }
        });
        inherent_method.or_else(|| self.lookup_trait_method(db, name, resolver))
    }

    fn lookup_trait_method(
        self,
        db: &impl HirDatabase,
        name: &Name,
        resolver: &Resolver,
    ) -> Option<(Ty, Function)> {
        let mut candidates = Vec::new();
        for t in resolver.traits_in_scope() {
            let data = t.trait_data(db);
            for item in data.items() {
                match item {
                    &TraitItem::Function(m) => {
                        let sig = m.signature(db);
                        if sig.name() == name && sig.has_self_param() {
                            candidates.push((t, m));
                        }
                    }
                    _ => {}
                }
            }
        }
        // FIXME the implements check may result in other obligations or unifying variables?
        candidates.retain(|(_t, _m)| /* self implements t */ true);
        // FIXME what happens if there are still multiple potential candidates?
        let (_chosen_trait, chosen_method) = candidates.first()?;
        Some((self.clone(), *chosen_method))
    }

    // This would be nicer if it just returned an iterator, but that runs into
    // lifetime problems, because we need to borrow temp `CrateImplBlocks`.
    pub fn iterate_methods<T>(
        self,
        db: &impl HirDatabase,
        mut callback: impl FnMut(&Ty, Function) -> Option<T>,
    ) -> Option<T> {
        // For method calls, rust first does any number of autoderef, and then one
        // autoref (i.e. when the method takes &self or &mut self). We just ignore
        // the autoref currently -- when we find a method matching the given name,
        // we assume it fits.

        // Also note that when we've got a receiver like &S, even if the method we
        // find in the end takes &self, we still do the autoderef step (just as
        // rustc does an autoderef and then autoref again).

        for derefed_ty in self.autoderef(db) {
            let krate = match def_crate(db, &derefed_ty) {
                Some(krate) => krate,
                None => continue,
            };
            let impls = db.impls_in_crate(krate);

            for impl_block in impls.lookup_impl_blocks(&derefed_ty) {
                for item in impl_block.items(db) {
                    match item {
                        ImplItem::Method(f) => {
                            if let Some(result) = callback(&derefed_ty, f) {
                                return Some(result);
                            }
                        }
                        _ => {}
                    }
                }
            }
        }
        None
    }

    // This would be nicer if it just returned an iterator, but that runs into
    // lifetime problems, because we need to borrow temp `CrateImplBlocks`.
    pub fn iterate_impl_items<T>(
        self,
        db: &impl HirDatabase,
        mut callback: impl FnMut(ImplItem) -> Option<T>,
    ) -> Option<T> {
        let krate = def_crate(db, &self)?;
        let impls = db.impls_in_crate(krate);

        for impl_block in impls.lookup_impl_blocks(&self) {
            for item in impl_block.items(db) {
                if let Some(result) = callback(item) {
                    return Some(result);
                }
            }
        }
        None
    }
}