//! 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 ra_db::{Cancelable, SourceRootId};

use crate::{HirDatabase, DefId, module_tree::ModuleId, Module, Crate, Name, Function, impl_block::{ImplId, ImplBlock, ImplItem}};
use super::Ty;

/// This is used as a key for indexing impls.
#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub enum TyFingerprint {
    Adt(DefId),
    // we'll also want to index impls for primitive types etc.
}

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::Adt { def_id, .. } => Some(TyFingerprint::Adt(*def_id)),
            _ => None,
        }
    }
}

#[derive(Debug, PartialEq, Eq)]
pub struct CrateImplBlocks {
    /// To make sense of the ModuleIds, we need the source root.
    source_root_id: SourceRootId,
    impls: FxHashMap<TyFingerprint, Vec<(ModuleId, ImplId)>>,
}

impl CrateImplBlocks {
    pub fn lookup_impl_blocks<'a>(
        &'a self,
        db: &'a impl HirDatabase,
        ty: &Ty,
    ) -> impl Iterator<Item = Cancelable<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_impl_blocks = db.impls_in_module(self.source_root_id, *module_id)?;
                Ok(ImplBlock::from_id(module_impl_blocks, *impl_id))
            })
    }

    fn collect_recursive(&mut self, db: &impl HirDatabase, module: Module) -> Cancelable<()> {
        let module_id = module.def_id.loc(db).module_id;
        let module_impl_blocks = db.impls_in_module(self.source_root_id, module_id)?;

        for (impl_id, impl_data) in module_impl_blocks.impls.iter() {
            let impl_block = ImplBlock::from_id(Arc::clone(&module_impl_blocks), impl_id);

            if let Some(_target_trait) = impl_data.target_trait() {
                // ignore for now
            } else {
                let target_ty =
                    Ty::from_hir(db, &module, Some(&impl_block), impl_data.target_type())?;
                if let Some(target_ty_fp) = TyFingerprint::for_impl(&target_ty) {
                    self.impls
                        .entry(target_ty_fp)
                        .or_insert_with(Vec::new)
                        .push((module_id, impl_id));
                }
            }
        }

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

        Ok(())
    }

    pub(crate) fn impls_in_crate_query(
        db: &impl HirDatabase,
        krate: Crate,
    ) -> Cancelable<Arc<CrateImplBlocks>> {
        let crate_graph = db.crate_graph();
        let file_id = crate_graph.crate_root(krate.crate_id);
        let source_root_id = db.file_source_root(file_id);
        let mut crate_impl_blocks = CrateImplBlocks {
            source_root_id,
            impls: FxHashMap::default(),
        };
        if let Some(module) = krate.root_module(db)? {
            crate_impl_blocks.collect_recursive(db, module)?;
        }
        Ok(Arc::new(crate_impl_blocks))
    }
}

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

impl Ty {
    // TODO: cache this as a query?
    // - if so, what signature? (TyFingerprint, Name)?
    // - or maybe cache all names and def_ids of methods per fingerprint?
    pub fn lookup_method(self, db: &impl HirDatabase, name: &Name) -> Cancelable<Option<DefId>> {
        self.iterate_methods(db, |f| {
            let sig = f.signature(db);
            if sig.name() == name && sig.has_self_arg() {
                Ok(Some(f.def_id()))
            } else {
                Ok(None)
            }
        })
    }

    // This would be nicer if it just returned an iterator, but that's really
    // complicated with all the cancelable operations
    pub fn iterate_methods<T>(
        self,
        db: &impl HirDatabase,
        mut callback: impl FnMut(Function) -> Cancelable<Option<T>>,
    ) -> Cancelable<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(db, &derefed_ty) {
                let impl_block = impl_block?;
                for item in impl_block.items() {
                    match item {
                        ImplItem::Method(f) => {
                            if let Some(result) = callback(f.clone())? {
                                return Ok(Some(result));
                            }
                        }
                        _ => {}
                    }
                }
            }
        }
        Ok(None)
    }
}