use proc_macro::TokenStream;
use quote::{quote, ToTokens};
use syn::spanned::Spanned;
use syn::{parse, Attribute, Ident, Item, ItemEnum, ItemStruct, ItemUnion};

#[proc_macro_derive(Component, attributes(component))]
pub fn component_derive(input: TokenStream) -> TokenStream
{
    let item: TypeItem = parse::<Item>(input).unwrap().try_into().unwrap();

    let item_ident = item.ident();

    let component_attr = item.component_attribute().unwrap_or_default();

    let drop_last = component_attr.drop_last;

    quote! {
        impl ecs::component::Component for #item_ident
        {
            fn drop_last(&self) -> bool {
                #drop_last
            }

            fn as_any_mut(&mut self) -> &mut dyn std::any::Any
            {
                self
            }

            fn as_any(&self) -> &dyn std::any::Any
            {
                self
            }
        }

        impl ecs::system::Input for #item_ident {}

        impl ecs::type_name::TypeName for #item_ident
        {
            fn type_name(&self) -> &'static str
            {
                std::any::type_name::<Self>()
            }
        }
    }
    .into()
}

enum TypeItem
{
    Struct(ItemStruct),
    Enum(ItemEnum),
    Union(ItemUnion),
}

impl TypeItem
{
    fn ident(&self) -> &Ident
    {
        match self {
            Self::Struct(struct_item) => &struct_item.ident,
            Self::Enum(enum_item) => &enum_item.ident,
            Self::Union(union_item) => &union_item.ident,
        }
    }

    fn component_attribute(&self) -> Option<ComponentAttribute>
    {
        let item_attrs = match &self {
            Self::Struct(struct_item) => &struct_item.attrs,
            &Self::Enum(enum_item) => &enum_item.attrs,
            &Self::Union(union_item) => &union_item.attrs,
        };

        let mut component_attr: Option<&Attribute> = None;

        for item_attr in item_attrs {
            if component_attr.is_some() {
                panic!("Expected only one component attribute");
            }

            if item_attr.path().get_ident()? == "component" {
                component_attr = Some(item_attr);
            }
        }

        Some(ComponentAttribute::from_attribute(component_attr?).unwrap())
    }
}

impl TryFrom<Item> for TypeItem
{
    type Error = syn::Error;

    fn try_from(item: Item) -> Result<Self, Self::Error>
    {
        match item {
            Item::Struct(struct_item) => Ok(Self::Struct(struct_item)),
            Item::Enum(enum_item) => Ok(Self::Enum(enum_item)),
            Item::Union(union_item) => Ok(Self::Union(union_item)),
            _ => Err(syn::Error::new(
                item.span(),
                "Expected a struct, a enum or a union",
            )),
        }
    }
}

impl ToTokens for TypeItem
{
    fn to_tokens(&self, tokens: &mut proc_macro2::TokenStream)
    {
        match self {
            Self::Struct(struct_item) => struct_item.to_tokens(tokens),
            Self::Enum(enum_item) => enum_item.to_tokens(tokens),
            Self::Union(union_item) => union_item.to_tokens(tokens),
        }
    }
}

#[derive(Debug, Default)]
struct ComponentAttribute
{
    drop_last: bool,
}

impl ComponentAttribute
{
    fn from_attribute(attribute: &Attribute) -> Result<Self, syn::Error>
    {
        let mut drop_last = false;

        attribute.parse_nested_meta(|meta| {
            if meta
                .path
                .get_ident()
                .is_some_and(|flag| flag == "drop_last")
            {
                drop_last = true;

                return Ok(());
            }

            Err(meta.error("Unrecognized token"))
        })?;

        Ok(Self { drop_last })
    }
}