//! Methods for lowering the HIR to types. There are two main cases here:
//!
//!  - Lowering a type reference like `&usize` or `Option<foo::bar::Baz>` to a
//!    type: The entry point for this is `Ty::from_hir`.
//!  - Building the type for an item: This happens through the `type_for_def` query.
//!
//! This usually involves resolving names, collecting generic arguments etc.
use std::sync::Arc;
use std::iter;

use crate::{
    Function, Struct, Union, StructField, Enum, EnumVariant, Path, ModuleDef, TypeAlias, Const, Static,
    HirDatabase, BuiltinType,
    type_ref::TypeRef,
    nameres::Namespace,
    resolve::{Resolver, Resolution},
    path::{PathSegment, GenericArg},
    generics::{HasGenericParams},
    adt::VariantDef,
    Trait,
    generics::{WherePredicate, GenericDef},
    ty::AdtDef,
};
use super::{Ty, FnSig, Substs, TypeCtor, TraitRef, GenericPredicate};

impl Ty {
    pub(crate) fn from_hir(db: &impl HirDatabase, resolver: &Resolver, type_ref: &TypeRef) -> Self {
        match type_ref {
            TypeRef::Never => Ty::simple(TypeCtor::Never),
            TypeRef::Tuple(inner) => {
                let inner_tys =
                    inner.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect::<Vec<_>>();
                Ty::apply(
                    TypeCtor::Tuple { cardinality: inner_tys.len() as u16 },
                    Substs(inner_tys.into()),
                )
            }
            TypeRef::Path(path) => Ty::from_hir_path(db, resolver, path),
            TypeRef::RawPtr(inner, mutability) => {
                let inner_ty = Ty::from_hir(db, resolver, inner);
                Ty::apply_one(TypeCtor::RawPtr(*mutability), inner_ty)
            }
            TypeRef::Array(inner) => {
                let inner_ty = Ty::from_hir(db, resolver, inner);
                Ty::apply_one(TypeCtor::Array, inner_ty)
            }
            TypeRef::Slice(inner) => {
                let inner_ty = Ty::from_hir(db, resolver, inner);
                Ty::apply_one(TypeCtor::Slice, inner_ty)
            }
            TypeRef::Reference(inner, mutability) => {
                let inner_ty = Ty::from_hir(db, resolver, inner);
                Ty::apply_one(TypeCtor::Ref(*mutability), inner_ty)
            }
            TypeRef::Placeholder => Ty::Unknown,
            TypeRef::Fn(params) => {
                let inner_tys =
                    params.iter().map(|tr| Ty::from_hir(db, resolver, tr)).collect::<Vec<_>>();
                let sig = Substs(inner_tys.into());
                Ty::apply(TypeCtor::FnPtr { num_args: sig.len() as u16 - 1 }, sig)
            }
            TypeRef::Error => Ty::Unknown,
        }
    }

    pub(crate) fn from_hir_path(db: &impl HirDatabase, resolver: &Resolver, path: &Path) -> Self {
        // Resolve the path (in type namespace)
        let resolution = resolver.resolve_path(db, path).take_types();

        let def = match resolution {
            Some(Resolution::Def(def)) => def,
            Some(Resolution::LocalBinding(..)) => {
                // this should never happen
                panic!("path resolved to local binding in type ns");
            }
            Some(Resolution::GenericParam(idx)) => {
                return Ty::Param {
                    idx,
                    // FIXME: maybe return name in resolution?
                    name: path
                        .as_ident()
                        .expect("generic param should be single-segment path")
                        .clone(),
                };
            }
            Some(Resolution::SelfType(impl_block)) => {
                return impl_block.target_ty(db);
            }
            None => return Ty::Unknown,
        };

        let typable: TypableDef = match def.into() {
            None => return Ty::Unknown,
            Some(it) => it,
        };
        let ty = db.type_for_def(typable, Namespace::Types);
        let substs = Ty::substs_from_path(db, resolver, path, typable);
        ty.subst(&substs)
    }

    pub(super) fn substs_from_path_segment(
        db: &impl HirDatabase,
        resolver: &Resolver,
        segment: &PathSegment,
        resolved: TypableDef,
    ) -> Substs {
        let def_generic: Option<GenericDef> = match resolved {
            TypableDef::Function(func) => Some(func.into()),
            TypableDef::Struct(s) => Some(s.into()),
            TypableDef::Union(u) => Some(u.into()),
            TypableDef::Enum(e) => Some(e.into()),
            TypableDef::EnumVariant(var) => Some(var.parent_enum(db).into()),
            TypableDef::TypeAlias(t) => Some(t.into()),
            TypableDef::Const(_) | TypableDef::Static(_) | TypableDef::BuiltinType(_) => None,
        };
        substs_from_path_segment(db, resolver, segment, def_generic, false)
    }

    /// Collect generic arguments from a path into a `Substs`. See also
    /// `create_substs_for_ast_path` and `def_to_ty` in rustc.
    pub(super) fn substs_from_path(
        db: &impl HirDatabase,
        resolver: &Resolver,
        path: &Path,
        resolved: TypableDef,
    ) -> Substs {
        let last = path.segments.last().expect("path should have at least one segment");
        let segment = match resolved {
            TypableDef::Function(_)
            | TypableDef::Struct(_)
            | TypableDef::Union(_)
            | TypableDef::Enum(_)
            | TypableDef::Const(_)
            | TypableDef::Static(_)
            | TypableDef::TypeAlias(_)
            | TypableDef::BuiltinType(_) => last,
            TypableDef::EnumVariant(_) => {
                // the generic args for an enum variant may be either specified
                // on the segment referring to the enum, or on the segment
                // referring to the variant. So `Option::<T>::None` and
                // `Option::None::<T>` are both allowed (though the former is
                // preferred). See also `def_ids_for_path_segments` in rustc.
                let len = path.segments.len();
                let segment = if len >= 2 && path.segments[len - 2].args_and_bindings.is_some() {
                    // Option::<T>::None
                    &path.segments[len - 2]
                } else {
                    // Option::None::<T>
                    last
                };
                segment
            }
        };
        Ty::substs_from_path_segment(db, resolver, segment, resolved)
    }
}

pub(super) fn substs_from_path_segment(
    db: &impl HirDatabase,
    resolver: &Resolver,
    segment: &PathSegment,
    def_generic: Option<GenericDef>,
    add_self_param: bool,
) -> Substs {
    let mut substs = Vec::new();
    let def_generics = def_generic.map(|def| def.generic_params(db)).unwrap_or_default();

    let parent_param_count = def_generics.count_parent_params();
    substs.extend(iter::repeat(Ty::Unknown).take(parent_param_count));
    if add_self_param {
        // FIXME this add_self_param argument is kind of a hack: Traits have the
        // Self type as an implicit first type parameter, but it can't be
        // actually provided in the type arguments
        // (well, actually sometimes it can, in the form of type-relative paths: `<Foo as Default>::default()`)
        substs.push(Ty::Unknown);
    }
    if let Some(generic_args) = &segment.args_and_bindings {
        // if args are provided, it should be all of them, but we can't rely on that
        let self_param_correction = if add_self_param { 1 } else { 0 };
        let param_count = def_generics.params.len() - self_param_correction;
        for arg in generic_args.args.iter().take(param_count) {
            match arg {
                GenericArg::Type(type_ref) => {
                    let ty = Ty::from_hir(db, resolver, type_ref);
                    substs.push(ty);
                }
            }
        }
    }
    // add placeholders for args that were not provided
    let supplied_params = substs.len();
    for _ in supplied_params..def_generics.count_params_including_parent() {
        substs.push(Ty::Unknown);
    }
    assert_eq!(substs.len(), def_generics.count_params_including_parent());

    // handle defaults
    if let Some(def_generic) = def_generic {
        let default_substs = db.generic_defaults(def_generic);
        assert_eq!(substs.len(), default_substs.len());

        for (i, default_ty) in default_substs.iter().enumerate() {
            if substs[i] == Ty::Unknown {
                substs[i] = default_ty.clone();
            }
        }
    }

    Substs(substs.into())
}

impl TraitRef {
    pub(crate) fn from_path(
        db: &impl HirDatabase,
        resolver: &Resolver,
        path: &Path,
        explicit_self_ty: Option<Ty>,
    ) -> Option<Self> {
        let resolved = match resolver.resolve_path(db, &path).take_types()? {
            Resolution::Def(ModuleDef::Trait(tr)) => tr,
            _ => return None,
        };
        let mut substs = Self::substs_from_path(db, resolver, path, resolved);
        if let Some(self_ty) = explicit_self_ty {
            // FIXME this could be nicer
            let mut substs_vec = substs.0.to_vec();
            substs_vec[0] = self_ty;
            substs.0 = substs_vec.into();
        }
        Some(TraitRef { trait_: resolved, substs })
    }

    pub(crate) fn from_hir(
        db: &impl HirDatabase,
        resolver: &Resolver,
        type_ref: &TypeRef,
        explicit_self_ty: Option<Ty>,
    ) -> Option<Self> {
        let path = match type_ref {
            TypeRef::Path(path) => path,
            _ => return None,
        };
        TraitRef::from_path(db, resolver, path, explicit_self_ty)
    }

    fn substs_from_path(
        db: &impl HirDatabase,
        resolver: &Resolver,
        path: &Path,
        resolved: Trait,
    ) -> Substs {
        let segment = path.segments.last().expect("path should have at least one segment");
        substs_from_path_segment(db, resolver, segment, Some(resolved.into()), true)
    }

    pub(crate) fn for_trait(db: &impl HirDatabase, trait_: Trait) -> TraitRef {
        let substs = Substs::identity(&trait_.generic_params(db));
        TraitRef { trait_, substs }
    }

    pub(crate) fn for_where_predicate(
        db: &impl HirDatabase,
        resolver: &Resolver,
        pred: &WherePredicate,
    ) -> Option<TraitRef> {
        let self_ty = Ty::from_hir(db, resolver, &pred.type_ref);
        TraitRef::from_path(db, resolver, &pred.trait_ref, Some(self_ty))
    }
}

/// Build the declared type of an item. This depends on the namespace; e.g. for
/// `struct Foo(usize)`, we have two types: The type of the struct itself, and
/// the constructor function `(usize) -> Foo` which lives in the values
/// namespace.
pub(crate) fn type_for_def(db: &impl HirDatabase, def: TypableDef, ns: Namespace) -> Ty {
    match (def, ns) {
        (TypableDef::Function(f), Namespace::Values) => type_for_fn(db, f),
        (TypableDef::Struct(s), Namespace::Types) => type_for_adt(db, s),
        (TypableDef::Struct(s), Namespace::Values) => type_for_struct_constructor(db, s),
        (TypableDef::Enum(e), Namespace::Types) => type_for_adt(db, e),
        (TypableDef::EnumVariant(v), Namespace::Values) => type_for_enum_variant_constructor(db, v),
        (TypableDef::Union(u), Namespace::Types) => type_for_adt(db, u),
        (TypableDef::TypeAlias(t), Namespace::Types) => type_for_type_alias(db, t),
        (TypableDef::Const(c), Namespace::Values) => type_for_const(db, c),
        (TypableDef::Static(c), Namespace::Values) => type_for_static(db, c),
        (TypableDef::BuiltinType(t), Namespace::Types) => type_for_builtin(t),

        // 'error' cases:
        (TypableDef::Function(_), Namespace::Types) => Ty::Unknown,
        (TypableDef::Union(_), Namespace::Values) => Ty::Unknown,
        (TypableDef::Enum(_), Namespace::Values) => Ty::Unknown,
        (TypableDef::EnumVariant(_), Namespace::Types) => Ty::Unknown,
        (TypableDef::TypeAlias(_), Namespace::Values) => Ty::Unknown,
        (TypableDef::Const(_), Namespace::Types) => Ty::Unknown,
        (TypableDef::Static(_), Namespace::Types) => Ty::Unknown,
        (TypableDef::BuiltinType(_), Namespace::Values) => Ty::Unknown,
    }
}

/// Build the signature of a callable item (function, struct or enum variant).
pub(crate) fn callable_item_sig(db: &impl HirDatabase, def: CallableDef) -> FnSig {
    match def {
        CallableDef::Function(f) => fn_sig_for_fn(db, f),
        CallableDef::Struct(s) => fn_sig_for_struct_constructor(db, s),
        CallableDef::EnumVariant(e) => fn_sig_for_enum_variant_constructor(db, e),
    }
}

/// Build the type of a specific field of a struct or enum variant.
pub(crate) fn type_for_field(db: &impl HirDatabase, field: StructField) -> Ty {
    let parent_def = field.parent_def(db);
    let resolver = match parent_def {
        VariantDef::Struct(it) => it.resolver(db),
        VariantDef::EnumVariant(it) => it.parent_enum(db).resolver(db),
    };
    let var_data = parent_def.variant_data(db);
    let type_ref = &var_data.fields().unwrap()[field.id].type_ref;
    Ty::from_hir(db, &resolver, type_ref)
}

/// Resolve the where clause(s) of an item with generics.
pub(crate) fn generic_predicates(
    db: &impl HirDatabase,
    def: GenericDef,
) -> Arc<[GenericPredicate]> {
    let resolver = def.resolver(db);
    let generic_params = def.generic_params(db);
    let predicates = generic_params
        .where_predicates
        .iter()
        .map(|pred| {
            TraitRef::for_where_predicate(db, &resolver, pred)
                .map_or(GenericPredicate::Error, GenericPredicate::Implemented)
        })
        .collect::<Vec<_>>();
    predicates.into()
}

/// Resolve the default type params from generics
pub(crate) fn generic_defaults(db: &impl HirDatabase, def: GenericDef) -> Substs {
    let resolver = def.resolver(db);
    let generic_params = def.generic_params(db);

    let defaults = generic_params
        .params_including_parent()
        .into_iter()
        .map(|p| {
            p.default.as_ref().map_or(Ty::Unknown, |path| Ty::from_hir_path(db, &resolver, path))
        })
        .collect::<Vec<_>>();

    Substs(defaults.into())
}

fn fn_sig_for_fn(db: &impl HirDatabase, def: Function) -> FnSig {
    let signature = def.signature(db);
    let resolver = def.resolver(db);
    let params =
        signature.params().iter().map(|tr| Ty::from_hir(db, &resolver, tr)).collect::<Vec<_>>();
    let ret = Ty::from_hir(db, &resolver, signature.ret_type());
    FnSig::from_params_and_return(params, ret)
}

/// Build the declared type of a function. This should not need to look at the
/// function body.
fn type_for_fn(db: &impl HirDatabase, def: Function) -> Ty {
    let generics = def.generic_params(db);
    let substs = Substs::identity(&generics);
    Ty::apply(TypeCtor::FnDef(def.into()), substs)
}

/// Build the declared type of a const.
fn type_for_const(db: &impl HirDatabase, def: Const) -> Ty {
    let signature = def.signature(db);
    let resolver = def.resolver(db);

    Ty::from_hir(db, &resolver, signature.type_ref())
}

/// Build the declared type of a static.
fn type_for_static(db: &impl HirDatabase, def: Static) -> Ty {
    let signature = def.signature(db);
    let resolver = def.resolver(db);

    Ty::from_hir(db, &resolver, signature.type_ref())
}

/// Build the declared type of a static.
fn type_for_builtin(def: BuiltinType) -> Ty {
    Ty::simple(match def {
        BuiltinType::Char => TypeCtor::Char,
        BuiltinType::Bool => TypeCtor::Bool,
        BuiltinType::Str => TypeCtor::Str,
        BuiltinType::Int(ty) => TypeCtor::Int(ty.into()),
        BuiltinType::Float(ty) => TypeCtor::Float(ty.into()),
    })
}

fn fn_sig_for_struct_constructor(db: &impl HirDatabase, def: Struct) -> FnSig {
    let var_data = def.variant_data(db);
    let fields = match var_data.fields() {
        Some(fields) => fields,
        None => panic!("fn_sig_for_struct_constructor called on unit struct"),
    };
    let resolver = def.resolver(db);
    let params = fields
        .iter()
        .map(|(_, field)| Ty::from_hir(db, &resolver, &field.type_ref))
        .collect::<Vec<_>>();
    let ret = type_for_adt(db, def);
    FnSig::from_params_and_return(params, ret)
}

/// Build the type of a tuple struct constructor.
fn type_for_struct_constructor(db: &impl HirDatabase, def: Struct) -> Ty {
    let var_data = def.variant_data(db);
    if var_data.fields().is_none() {
        return type_for_adt(db, def); // Unit struct
    }
    let generics = def.generic_params(db);
    let substs = Substs::identity(&generics);
    Ty::apply(TypeCtor::FnDef(def.into()), substs)
}

fn fn_sig_for_enum_variant_constructor(db: &impl HirDatabase, def: EnumVariant) -> FnSig {
    let var_data = def.variant_data(db);
    let fields = match var_data.fields() {
        Some(fields) => fields,
        None => panic!("fn_sig_for_enum_variant_constructor called for unit variant"),
    };
    let resolver = def.parent_enum(db).resolver(db);
    let params = fields
        .iter()
        .map(|(_, field)| Ty::from_hir(db, &resolver, &field.type_ref))
        .collect::<Vec<_>>();
    let generics = def.parent_enum(db).generic_params(db);
    let substs = Substs::identity(&generics);
    let ret = type_for_adt(db, def.parent_enum(db)).subst(&substs);
    FnSig::from_params_and_return(params, ret)
}

/// Build the type of a tuple enum variant constructor.
fn type_for_enum_variant_constructor(db: &impl HirDatabase, def: EnumVariant) -> Ty {
    let var_data = def.variant_data(db);
    if var_data.fields().is_none() {
        return type_for_adt(db, def.parent_enum(db)); // Unit variant
    }
    let generics = def.parent_enum(db).generic_params(db);
    let substs = Substs::identity(&generics);
    Ty::apply(TypeCtor::FnDef(def.into()), substs)
}

fn type_for_adt(db: &impl HirDatabase, adt: impl Into<AdtDef> + HasGenericParams) -> Ty {
    let generics = adt.generic_params(db);
    Ty::apply(TypeCtor::Adt(adt.into()), Substs::identity(&generics))
}

fn type_for_type_alias(db: &impl HirDatabase, t: TypeAlias) -> Ty {
    let generics = t.generic_params(db);
    let resolver = t.resolver(db);
    let type_ref = t.type_ref(db);
    let substs = Substs::identity(&generics);
    let inner = Ty::from_hir(db, &resolver, &type_ref);
    inner.subst(&substs)
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum TypableDef {
    Function(Function),
    Struct(Struct),
    Union(Union),
    Enum(Enum),
    EnumVariant(EnumVariant),
    TypeAlias(TypeAlias),
    Const(Const),
    Static(Static),
    BuiltinType(BuiltinType),
}
impl_froms!(
    TypableDef: Function,
    Struct,
    Union,
    Enum,
    EnumVariant,
    TypeAlias,
    Const,
    Static,
    BuiltinType
);

impl From<ModuleDef> for Option<TypableDef> {
    fn from(def: ModuleDef) -> Option<TypableDef> {
        let res = match def {
            ModuleDef::Function(f) => f.into(),
            ModuleDef::Struct(s) => s.into(),
            ModuleDef::Union(u) => u.into(),
            ModuleDef::Enum(e) => e.into(),
            ModuleDef::EnumVariant(v) => v.into(),
            ModuleDef::TypeAlias(t) => t.into(),
            ModuleDef::Const(v) => v.into(),
            ModuleDef::Static(v) => v.into(),
            ModuleDef::BuiltinType(t) => t.into(),
            ModuleDef::Module(_) | ModuleDef::Trait(_) => return None,
        };
        Some(res)
    }
}

#[derive(Clone, Copy, Debug, PartialEq, Eq, Hash)]
pub enum CallableDef {
    Function(Function),
    Struct(Struct),
    EnumVariant(EnumVariant),
}
impl_froms!(CallableDef: Function, Struct, EnumVariant);