//! Name resolution algorithm. The end result of the algorithm is an `ItemMap`:
//! a map which maps each module to its scope: the set of items visible in the
//! module. That is, we only resolve imports here, name resolution of item
//! bodies will be done in a separate step.
//!
//! Like Rustc, we use an interactive per-crate algorithm: we start with scopes
//! containing only directly defined items, and then iteratively resolve
//! imports.
//!
//! To make this work nicely in the IDE scenario, we place `InputModuleItems`
//! in between raw syntax and name resolution. `InputModuleItems` are computed
//! using only the module's syntax, and it is all directly defined items plus
//! imports. The plan is to make `InputModuleItems` independent of local
//! modifications (that is, typing inside a function should not change IMIs),
//! so that the results of name resolution can be preserved unless the module
//! structure itself is modified.
pub(crate) mod lower;

use std::{time, sync::Arc};

use ra_arena::map::ArenaMap;
use test_utils::tested_by;
use rustc_hash::{FxHashMap, FxHashSet};

use crate::{
    Module, ModuleDef,
    Path, PathKind, PersistentHirDatabase,
    Crate, Name,
    module_tree::{ModuleId, ModuleTree},
    nameres::lower::{ImportId, LoweredModule, ImportData},
};

/// `ItemMap` is the result of module name resolution. It contains, for each
/// module, the set of visible items.
#[derive(Default, Debug, PartialEq, Eq)]
pub struct ItemMap {
    pub(crate) extern_prelude: FxHashMap<Name, ModuleDef>,
    per_module: ArenaMap<ModuleId, ModuleScope>,
}

impl std::ops::Index<ModuleId> for ItemMap {
    type Output = ModuleScope;
    fn index(&self, id: ModuleId) -> &ModuleScope {
        &self.per_module[id]
    }
}

#[derive(Debug, Default, PartialEq, Eq, Clone)]
pub struct ModuleScope {
    pub(crate) items: FxHashMap<Name, Resolution>,
}

impl ModuleScope {
    pub fn entries<'a>(&'a self) -> impl Iterator<Item = (&'a Name, &'a Resolution)> + 'a {
        self.items.iter()
    }
    pub fn get(&self, name: &Name) -> Option<&Resolution> {
        self.items.get(name)
    }
}

/// `Resolution` is basically `DefId` atm, but it should account for stuff like
/// multiple namespaces, ambiguity and errors.
#[derive(Debug, Clone, PartialEq, Eq)]
pub struct Resolution {
    /// None for unresolved
    pub def: PerNs<ModuleDef>,
    /// ident by which this is imported into local scope.
    pub import: Option<ImportId>,
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, PartialOrd, Ord, Hash)]
pub enum Namespace {
    Types,
    Values,
}

#[derive(Debug, Copy, Clone, PartialEq, Eq, Hash)]
pub struct PerNs<T> {
    pub types: Option<T>,
    pub values: Option<T>,
}

impl<T> Default for PerNs<T> {
    fn default() -> Self {
        PerNs { types: None, values: None }
    }
}

impl<T> PerNs<T> {
    pub fn none() -> PerNs<T> {
        PerNs { types: None, values: None }
    }

    pub fn values(t: T) -> PerNs<T> {
        PerNs { types: None, values: Some(t) }
    }

    pub fn types(t: T) -> PerNs<T> {
        PerNs { types: Some(t), values: None }
    }

    pub fn both(types: T, values: T) -> PerNs<T> {
        PerNs { types: Some(types), values: Some(values) }
    }

    pub fn is_none(&self) -> bool {
        self.types.is_none() && self.values.is_none()
    }

    pub fn is_both(&self) -> bool {
        self.types.is_some() && self.values.is_some()
    }

    pub fn take(self, namespace: Namespace) -> Option<T> {
        match namespace {
            Namespace::Types => self.types,
            Namespace::Values => self.values,
        }
    }

    pub fn take_types(self) -> Option<T> {
        self.take(Namespace::Types)
    }

    pub fn take_values(self) -> Option<T> {
        self.take(Namespace::Values)
    }

    pub fn get(&self, namespace: Namespace) -> Option<&T> {
        self.as_ref().take(namespace)
    }

    pub fn as_ref(&self) -> PerNs<&T> {
        PerNs { types: self.types.as_ref(), values: self.values.as_ref() }
    }

    pub fn combine(self, other: PerNs<T>) -> PerNs<T> {
        PerNs { types: self.types.or(other.types), values: self.values.or(other.values) }
    }

    pub fn and_then<U>(self, f: impl Fn(T) -> Option<U>) -> PerNs<U> {
        PerNs { types: self.types.and_then(&f), values: self.values.and_then(&f) }
    }

    pub fn map<U>(self, f: impl Fn(T) -> U) -> PerNs<U> {
        PerNs { types: self.types.map(&f), values: self.values.map(&f) }
    }
}

struct Resolver<'a, DB> {
    db: &'a DB,
    input: &'a FxHashMap<ModuleId, Arc<LoweredModule>>,
    krate: Crate,
    module_tree: Arc<ModuleTree>,
    processed_imports: FxHashSet<(ModuleId, ImportId)>,
    result: ItemMap,
}

impl<'a, DB> Resolver<'a, DB>
where
    DB: PersistentHirDatabase,
{
    fn new(
        db: &'a DB,
        input: &'a FxHashMap<ModuleId, Arc<LoweredModule>>,
        krate: Crate,
    ) -> Resolver<'a, DB> {
        let module_tree = db.module_tree(krate);
        Resolver {
            db,
            input,
            krate,
            module_tree,
            processed_imports: FxHashSet::default(),
            result: ItemMap::default(),
        }
    }

    pub(crate) fn resolve(mut self) -> ItemMap {
        self.populate_extern_prelude();
        for (&module_id, items) in self.input.iter() {
            self.populate_module(module_id, Arc::clone(items));
        }

        let mut iter = 0;
        loop {
            iter += 1;
            if iter > 1000 {
                panic!("failed to reach fixedpoint after 1000 iters")
            }
            let processed_imports_count = self.processed_imports.len();
            for &module_id in self.input.keys() {
                self.db.check_canceled();
                self.resolve_imports(module_id);
            }
            if processed_imports_count == self.processed_imports.len() {
                // no new imports resolved
                break;
            }
        }
        self.result
    }

    fn populate_extern_prelude(&mut self) {
        for dep in self.krate.dependencies(self.db) {
            log::debug!("crate dep {:?} -> {:?}", dep.name, dep.krate);
            if let Some(module) = dep.krate.root_module(self.db) {
                self.result.extern_prelude.insert(dep.name.clone(), module.into());
            }
        }
    }

    fn populate_module(&mut self, module_id: ModuleId, input: Arc<LoweredModule>) {
        let mut module_items = ModuleScope::default();
        for (import_id, import_data) in input.imports.iter() {
            if let Some(last_segment) = import_data.path.segments.iter().last() {
                if !import_data.is_glob {
                    let name =
                        import_data.alias.clone().unwrap_or_else(|| last_segment.name.clone());
                    module_items
                        .items
                        .insert(name, Resolution { def: PerNs::none(), import: Some(import_id) });
                }
            }
        }
        // Populate explicitly declared items, except modules
        for (name, &def) in input.declarations.iter() {
            let resolution = Resolution { def, import: None };
            module_items.items.insert(name.clone(), resolution);
        }

        // Populate modules
        for (name, module_id) in module_id.children(&self.module_tree) {
            let module = Module { module_id, krate: self.krate };
            self.add_module_item(&mut module_items, name, PerNs::types(module.into()));
        }

        self.result.per_module.insert(module_id, module_items);
    }

    fn add_module_item(&self, module_items: &mut ModuleScope, name: Name, def: PerNs<ModuleDef>) {
        let resolution = Resolution { def, import: None };
        module_items.items.insert(name, resolution);
    }

    fn resolve_imports(&mut self, module_id: ModuleId) {
        for (import_id, import_data) in self.input[&module_id].imports.iter() {
            if self.processed_imports.contains(&(module_id, import_id)) {
                // already done
                continue;
            }
            if self.resolve_import(module_id, import_id, import_data) == ReachedFixedPoint::Yes {
                log::debug!("import {:?} resolved (or definite error)", import_id);
                self.processed_imports.insert((module_id, import_id));
            }
        }
    }

    fn resolve_import(
        &mut self,
        module_id: ModuleId,
        import_id: ImportId,
        import: &ImportData,
    ) -> ReachedFixedPoint {
        log::debug!("resolving import: {:?}", import);
        let original_module = Module { krate: self.krate, module_id };
        let (def, reached_fixedpoint) =
            self.result.resolve_path_fp(self.db, original_module, &import.path);

        if reached_fixedpoint != ReachedFixedPoint::Yes {
            return reached_fixedpoint;
        }

        if import.is_glob {
            log::debug!("glob import: {:?}", import);
            match def.take_types() {
                Some(ModuleDef::Module(m)) => {
                    if m.krate != self.krate {
                        tested_by!(glob_across_crates);
                        // glob import from other crate => we can just import everything once
                        let item_map = self.db.item_map(m.krate);
                        let scope = &item_map[m.module_id];
                        self.update(module_id, |items| {
                            // TODO: handle shadowing and visibility
                            items.items.extend(
                                scope.items.iter().map(|(name, res)| (name.clone(), res.clone())),
                            );
                        });
                    }
                }
                Some(ModuleDef::Enum(e)) => {
                    tested_by!(glob_enum);
                    // glob import from enum => just import all the variants
                    let variants = e.variants(self.db);
                    let resolutions = variants
                        .into_iter()
                        .filter_map(|variant| {
                            let res = Resolution {
                                def: PerNs::both(variant.into(), e.into()),
                                import: Some(import_id),
                            };
                            let name = variant.name(self.db)?;
                            Some((name, res))
                        })
                        .collect::<Vec<_>>();
                    self.update(module_id, |items| {
                        items.items.extend(resolutions);
                    });
                }
                Some(d) => {
                    log::debug!("glob import {:?} from non-module/enum {:?}", import, d);
                }
                None => {
                    log::debug!("glob import {:?} didn't resolve as type", import);
                }
            }
        } else {
            let last_segment = import.path.segments.last().unwrap();
            let name = import.alias.clone().unwrap_or_else(|| last_segment.name.clone());
            log::debug!("resolved import {:?} ({:?}) to {:?}", name, import, def);

            // extern crates in the crate root are special-cased to insert entries into the extern prelude: rust-lang/rust#54658
            if let Some(root_module) = self.krate.root_module(self.db) {
                if import.is_extern_crate && module_id == root_module.module_id {
                    if let Some(def) = def.take_types() {
                        self.result.extern_prelude.insert(name.clone(), def);
                    }
                }
            }
            self.update(module_id, |items| {
                let res = Resolution { def, import: Some(import_id) };
                items.items.insert(name, res);
            });
        }
        reached_fixedpoint
    }

    fn update(&mut self, module_id: ModuleId, f: impl FnOnce(&mut ModuleScope)) {
        let module_items = self.result.per_module.get_mut(module_id).unwrap();
        f(module_items)
    }
}

#[derive(Debug, Clone, Copy, PartialEq, Eq)]
enum ReachedFixedPoint {
    Yes,
    No,
}

impl ItemMap {
    pub(crate) fn item_map_query(db: &impl PersistentHirDatabase, krate: Crate) -> Arc<ItemMap> {
        let start = time::Instant::now();
        let module_tree = db.module_tree(krate);
        let input = module_tree
            .modules()
            .map(|module_id| (module_id, db.lower_module_module(Module { krate, module_id })))
            .collect::<FxHashMap<_, _>>();

        let resolver = Resolver::new(db, &input, krate);
        let res = resolver.resolve();
        let elapsed = start.elapsed();
        log::info!("item_map: {:?}", elapsed);
        Arc::new(res)
    }

    pub(crate) fn resolve_path(
        &self,
        db: &impl PersistentHirDatabase,
        original_module: Module,
        path: &Path,
    ) -> PerNs<ModuleDef> {
        self.resolve_path_fp(db, original_module, path).0
    }

    // Returns Yes if we are sure that additions to `ItemMap` wouldn't change
    // the result.
    fn resolve_path_fp(
        &self,
        db: &impl PersistentHirDatabase,
        original_module: Module,
        path: &Path,
    ) -> (PerNs<ModuleDef>, ReachedFixedPoint) {
        let mut segments = path.segments.iter().enumerate();
        let mut curr_per_ns: PerNs<ModuleDef> = match path.kind {
            PathKind::Crate => PerNs::types(original_module.crate_root(db).into()),
            PathKind::Self_ => PerNs::types(original_module.into()),
            PathKind::Plain => {
                let segment = match segments.next() {
                    Some((_, segment)) => segment,
                    None => return (PerNs::none(), ReachedFixedPoint::Yes),
                };
                // Resolve in:
                //  - current module / scope
                //  - extern prelude
                match self[original_module.module_id].items.get(&segment.name) {
                    Some(res) if !res.def.is_none() => res.def,
                    _ => {
                        if let Some(def) = self.extern_prelude.get(&segment.name) {
                            PerNs::types(*def)
                        } else {
                            return (PerNs::none(), ReachedFixedPoint::No);
                        }
                    }
                }
            }
            PathKind::Super => {
                if let Some(p) = original_module.parent(db) {
                    PerNs::types(p.into())
                } else {
                    log::debug!("super path in root module");
                    return (PerNs::none(), ReachedFixedPoint::Yes);
                }
            }
            PathKind::Abs => {
                // 2018-style absolute path -- only extern prelude
                let segment = match segments.next() {
                    Some((_, segment)) => segment,
                    None => return (PerNs::none(), ReachedFixedPoint::Yes),
                };
                if let Some(def) = self.extern_prelude.get(&segment.name) {
                    log::debug!("absolute path {:?} resolved to crate {:?}", path, def);
                    PerNs::types(*def)
                } else {
                    return (PerNs::none(), ReachedFixedPoint::No); // extern crate declarations can add to the extern prelude
                }
            }
        };

        for (i, segment) in segments {
            let curr = match curr_per_ns.as_ref().take_types() {
                Some(r) => r,
                None => {
                    // we still have path segments left, but the path so far
                    // didn't resolve in the types namespace => no resolution
                    // (don't break here because curr_per_ns might contain
                    // something in the value namespace, and it would be wrong
                    // to return that)
                    return (PerNs::none(), ReachedFixedPoint::No);
                }
            };
            // resolve segment in curr

            curr_per_ns = match curr {
                ModuleDef::Module(module) => {
                    if module.krate != original_module.krate {
                        let path = Path {
                            segments: path.segments[i..].iter().cloned().collect(),
                            kind: PathKind::Self_,
                        };
                        log::debug!("resolving {:?} in other crate", path);
                        let item_map = db.item_map(module.krate);
                        let def = item_map.resolve_path(db, *module, &path);
                        return (def, ReachedFixedPoint::Yes);
                    }

                    match self[module.module_id].items.get(&segment.name) {
                        Some(res) if !res.def.is_none() => res.def,
                        _ => {
                            log::debug!("path segment {:?} not found", segment.name);
                            return (PerNs::none(), ReachedFixedPoint::No);
                        }
                    }
                }
                ModuleDef::Enum(e) => {
                    // enum variant
                    tested_by!(item_map_enum_importing);
                    match e.variant(db, &segment.name) {
                        Some(variant) => PerNs::both(variant.into(), (*e).into()),
                        None => PerNs::none(),
                    }
                }
                _ => {
                    // could be an inherent method call in UFCS form
                    // (`Struct::method`), or some other kind of associated
                    // item... Which we currently don't handle (TODO)
                    log::debug!(
                        "path segment {:?} resolved to non-module {:?}, but is not last",
                        segment.name,
                        curr,
                    );
                    return (PerNs::none(), ReachedFixedPoint::Yes);
                }
            };
        }
        (curr_per_ns, ReachedFixedPoint::Yes)
    }
}

#[cfg(test)]
mod tests;