use std::collections::HashSet;

use syn::{self, visit};

use internals::ast::Item;
use internals::attr;

macro_rules! path {
    ($first:ident $(:: $rest:ident)*) => {
        syn::Path {
            global: false,
            segments: vec![
                stringify!($first).into(),
                $(
                    stringify!($rest).into(),
                )*
            ],
        }
    };

    (::$first:ident $(:: $rest:ident)*) => {
        syn::Path {
            global: true,
            segments: vec![
                stringify!($first).into(),
                $(
                    stringify!($rest).into(),
                )*
            ],
        }
    };
}

// Remove the default from every type parameter because in the generated impls
// they look like associated types: "error: associated type bindings are not
// allowed here".
pub fn without_defaults(generics: &syn::Generics) -> syn::Generics {
    syn::Generics {
        ty_params: generics.ty_params
            .iter()
            .map(|ty_param| syn::TyParam { default: None, ..ty_param.clone() })
            .collect(),
        ..generics.clone()
    }
}

pub fn with_where_predicates(generics: &syn::Generics,
                             predicates: &[syn::WherePredicate])
                             -> syn::Generics {
    let mut generics = generics.clone();
    generics.where_clause.predicates.extend_from_slice(predicates);
    generics
}

pub fn with_where_predicates_from_fields<F>(item: &Item,
                                            generics: &syn::Generics,
                                            from_field: F)
                                            -> syn::Generics
    where F: Fn(&attr::Field) -> Option<&[syn::WherePredicate]>
{
    let predicates = item.body
        .all_fields()
        .flat_map(|field| from_field(&field.attrs))
        .flat_map(|predicates| predicates.to_vec());

    let mut generics = generics.clone();
    generics.where_clause.predicates.extend(predicates);
    generics
}

// Puts the given bound on any generic type parameters that are used in fields
// for which filter returns true.
//
// For example, the following struct needs the bound `A: Serialize, B: Serialize`.
//
//     struct S<'b, A, B: 'b, C> {
//         a: A,
//         b: Option<&'b B>
//         #[serde(skip_serializing)]
//         c: C,
//     }
pub fn with_bound<F>(item: &Item,
                     generics: &syn::Generics,
                     filter: F,
                     bound: &syn::Path)
                     -> syn::Generics
    where F: Fn(&attr::Field) -> bool
{
    struct FindTyParams {
        // Set of all generic type parameters on the current struct (A, B, C in
        // the example). Initialized up front.
        all_ty_params: HashSet<syn::Ident>,
        // Set of generic type parameters used in fields for which filter
        // returns true (A and B in the example). Filled in as the visitor sees
        // them.
        relevant_ty_params: HashSet<syn::Ident>,
    }
    impl visit::Visitor for FindTyParams {
        fn visit_path(&mut self, path: &syn::Path) {
            if let Some(seg) = path.segments.last() {
                if seg.ident == "PhantomData" {
                    // Hardcoded exception, because PhantomData<T> implements
                    // Serialize and Deserialize whether or not T implements it.
                    return;
                }
            }
            if !path.global && path.segments.len() == 1 {
                let id = path.segments[0].ident.clone();
                if self.all_ty_params.contains(&id) {
                    self.relevant_ty_params.insert(id);
                }
            }
            visit::walk_path(self, path);
        }
    }

    let all_ty_params: HashSet<_> = generics.ty_params
        .iter()
        .map(|ty_param| ty_param.ident.clone())
        .collect();

    let relevant_tys = item.body
        .all_fields()
        .filter(|&field| filter(&field.attrs))
        .map(|field| &field.ty);

    let mut visitor = FindTyParams {
        all_ty_params: all_ty_params,
        relevant_ty_params: HashSet::new(),
    };
    for ty in relevant_tys {
        visit::walk_ty(&mut visitor, ty);
    }

    let new_predicates = generics.ty_params
        .iter()
        .map(|ty_param| ty_param.ident.clone())
        .filter(|id| visitor.relevant_ty_params.contains(id))
        .map(|id| {
            syn::WherePredicate::BoundPredicate(syn::WhereBoundPredicate {
                bound_lifetimes: Vec::new(),
                // the type parameter that is being bounded e.g. T
                bounded_ty: syn::Ty::Path(None, id.into()),
                // the bound e.g. Serialize
                bounds: vec![syn::TyParamBound::Trait(
                    syn::PolyTraitRef {
                        bound_lifetimes: Vec::new(),
                        trait_ref: bound.clone(),
                    },
                    syn::TraitBoundModifier::None
                )],
            })
        });

    let mut generics = generics.clone();
    generics.where_clause.predicates.extend(new_predicates);
    generics
}

pub fn with_self_bound(item: &Item,
                       generics: &syn::Generics,
                       bound: &syn::Path)
                       -> syn::Generics
{
    let mut generics = generics.clone();
    generics.where_clause.predicates.push(
        syn::WherePredicate::BoundPredicate(syn::WhereBoundPredicate {
            bound_lifetimes: Vec::new(),
            // the type that is being bounded e.g. MyStruct<'a, T>
            bounded_ty: type_of_item(item),
            // the bound e.g. Default
            bounds: vec![syn::TyParamBound::Trait(
                syn::PolyTraitRef {
                    bound_lifetimes: Vec::new(),
                    trait_ref: bound.clone(),
                },
                syn::TraitBoundModifier::None
            )],
        })
    );
    generics
}

pub fn with_lifetime_bound(generics: &syn::Generics,
                           lifetime: &str)
                           -> syn::Generics {
    let mut generics = generics.clone();

    for lifetime_def in &mut generics.lifetimes {
        lifetime_def.bounds.push(syn::Lifetime::new(lifetime));
    }

    for ty_param in &mut generics.ty_params {
        ty_param.bounds.push(syn::TyParamBound::Region(syn::Lifetime::new(lifetime)));
    }

    generics.lifetimes.push(syn::LifetimeDef {
        attrs: Vec::new(),
        lifetime: syn::Lifetime::new(lifetime),
        bounds: Vec::new(),
    });

    generics
}

fn type_of_item(item: &Item) -> syn::Ty {
    syn::Ty::Path(None, syn::Path {
        global: false,
        segments: vec![
            syn::PathSegment {
                ident: item.ident.clone(),
                parameters: syn::PathParameters::AngleBracketed(syn::AngleBracketedParameterData {
                    lifetimes: item.generics
                                    .lifetimes
                                    .iter()
                                    .map(|def| def.lifetime.clone())
                                    .collect(),
                    types: item.generics
                                .ty_params
                                .iter()
                                .map(|param| syn::Ty::Path(None, param.ident.clone().into()))
                                .collect(),
                    bindings: Vec::new(),
                }),
            }
        ]
    })
}