use std::borrow::Cow;

use proc_macro2::TokenStream;
use quote::{quote, ToTokens, TokenStreamExt};
use syn::Ident;

use crate::ast::Fields;
use crate::codegen::error::{ErrorCheck, ErrorDeclaration};
use crate::codegen::{Field, FieldsGen};
use crate::usage::{self, IdentRefSet, IdentSet, UsesTypeParams};

/// A variant of the enum which is deriving `FromMeta`.
#[derive(Debug, Clone)]
pub struct Variant<'a> {
    /// The name which will appear in code passed to the `FromMeta` input.
    pub name_in_attr: Cow<'a, String>,

    /// The name of the variant which will be returned for a given `name_in_attr`.
    pub variant_ident: &'a Ident,

    /// The name of the parent enum type.
    pub ty_ident: &'a Ident,

    pub data: Fields<Field<'a>>,

    /// Whether or not the variant should be skipped in the generated code.
    pub skip: bool,

    /// Whether or not the variant should be used to create an instance for
    /// `FromMeta::from_word`.
    pub word: bool,

    pub allow_unknown_fields: bool,
}

impl<'a> Variant<'a> {
    pub fn as_name(&'a self) -> &'a str {
        &self.name_in_attr
    }

    pub fn as_unit_match_arm(&'a self) -> UnitMatchArm<'a> {
        UnitMatchArm(self)
    }

    pub fn as_data_match_arm(&'a self) -> DataMatchArm<'a> {
        DataMatchArm(self)
    }
}

impl<'a> UsesTypeParams for Variant<'a> {
    fn uses_type_params<'b>(
        &self,
        options: &usage::Options,
        type_set: &'b IdentSet,
    ) -> IdentRefSet<'b> {
        self.data.uses_type_params(options, type_set)
    }
}

impl<'a> ToTokens for Variant<'a> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        if self.data.is_unit() {
            self.as_unit_match_arm().to_tokens(tokens);
        } else {
            self.as_data_match_arm().to_tokens(tokens)
        }
    }
}

/// Code generator for an enum variant in a unit match position.
/// This is placed in generated `from_string` calls for the parent enum.
/// Value-carrying variants wrapped in this type will emit code to produce an "unsupported format" error.
pub struct UnitMatchArm<'a>(&'a Variant<'a>);

impl<'a> ToTokens for UnitMatchArm<'a> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        let val: &Variant<'a> = self.0;

        if val.skip {
            return;
        }

        let name_in_attr = &val.name_in_attr;

        let unsupported_format_error = || {
            quote!(::darling::export::Err(
                ::darling::Error::unsupported_format("literal")
            ))
        };

        if val.data.is_unit() {
            let variant_ident = val.variant_ident;
            let ty_ident = val.ty_ident;

            tokens.append_all(quote!(
                #name_in_attr => ::darling::export::Ok(#ty_ident::#variant_ident),
            ));
        } else if val.data.is_newtype() {
            let field = val
                .data
                .fields
                .first()
                .expect("Newtype should have exactly one field");
            let field_ty = field.ty;
            let ty_ident = val.ty_ident;
            let variant_ident = val.variant_ident;
            let unsupported_format = unsupported_format_error();

            tokens.append_all(quote!{
                #name_in_attr => {
                    match <#field_ty as ::darling::FromMeta>::from_none() {
                        ::darling::export::Some(__value) => ::darling::export::Ok(#ty_ident::#variant_ident(__value)),
                        ::darling::export::None => #unsupported_format,
                    }
                }
            })
        } else {
            let unsupported_format = unsupported_format_error();
            tokens.append_all(quote!(
                #name_in_attr => #unsupported_format,
            ));
        }
    }
}

/// Code generator for an enum variant in a data-carrying match position.
/// This is placed in generated `from_list` calls for the parent enum.
/// Unit variants wrapped in this type will emit code to produce an "unsupported format" error.
pub struct DataMatchArm<'a>(&'a Variant<'a>);

impl<'a> ToTokens for DataMatchArm<'a> {
    fn to_tokens(&self, tokens: &mut TokenStream) {
        let val: &Variant<'a> = self.0;

        if val.skip {
            return;
        }

        let name_in_attr = &val.name_in_attr;
        let variant_ident = val.variant_ident;
        let ty_ident = val.ty_ident;

        if val.data.is_unit() {
            tokens.append_all(quote!(
                #name_in_attr => ::darling::export::Err(::darling::Error::unsupported_format("list")),
            ));

            return;
        }

        let vdg = FieldsGen::new(&val.data, val.allow_unknown_fields);

        if val.data.is_struct() {
            let declare_errors = ErrorDeclaration::default();
            let check_errors = ErrorCheck::with_location(name_in_attr);
            let require_fields = vdg.require_fields();
            let decls = vdg.declarations();
            let core_loop = vdg.core_loop();
            let inits = vdg.initializers();

            tokens.append_all(quote!(
                #name_in_attr => {
                    if let ::darling::export::syn::Meta::List(ref __data) = *__nested {
                        let __items = ::darling::export::NestedMeta::parse_meta_list(__data.tokens.clone())?;
                        let __items = &__items;

                        #declare_errors

                        #decls

                        #core_loop

                        #require_fields

                        #check_errors

                        ::darling::export::Ok(#ty_ident::#variant_ident {
                            #inits
                        })
                    } else {
                        ::darling::export::Err(::darling::Error::unsupported_format("non-list"))
                    }
                }
            ));
        } else if val.data.is_newtype() {
            tokens.append_all(quote!(
                #name_in_attr => {
                    ::darling::export::Ok(
                        #ty_ident::#variant_ident(
                            ::darling::FromMeta::from_meta(__nested)
                                .map_err(|e| e.at(#name_in_attr))?)
                    )
                }
            ));
        } else {
            panic!("Match arms aren't supported for tuple variants yet");
        }
    }
}