//! Implementation of the content_enum type macro.

use proc_macro2::TokenStream;
use quote::quote;
use syn::{
    parse::{self, Parse, ParseStream},
    Attribute, Expr, ExprLit, Ident, Lit, LitStr, Token,
};

/// Create a content enum from `ContentEnumInput`.
pub fn expand_content_enum(input: ContentEnumInput) -> syn::Result<TokenStream> {
    let attrs = &input.attrs;
    let ident = &input.name;
    let event_type_str = &input.events;

    let variants = input.events.iter().map(to_camel_case).collect::<Vec<_>>();
    let content = input
        .events
        .iter()
        .map(to_event_content_path)
        .collect::<Vec<_>>();

    let content_enum = quote! {
        #( #attrs )*
        #[derive(Clone, Debug, ::serde::Serialize)]
        #[serde(untagged)]
        #[allow(clippy::large_enum_variant)]
        pub enum #ident {
            #(
                #[doc = #event_type_str]
                #variants(#content)
            ),*
        }
    };

    let event_content_impl = quote! {
        impl ::ruma_events::EventContent for #ident {
            fn event_type(&self) -> &str {
                match self {
                    #( Self::#variants(content) => content.event_type() ),*
                }
            }

            fn from_parts(event_type: &str, input: Box<::serde_json::value::RawValue>) -> Result<Self, String> {
                match event_type {
                    #(
                        #event_type_str => {
                            let content = #content::from_parts(event_type, input)?;
                            Ok(#ident::#variants(content))
                        },
                    )*
                    ev => Err(format!("event not supported {}", ev)),
                }
            }
        }
    };

    let marker_trait_impls = marker_traits(ident);

    Ok(quote! {
        #content_enum

        #event_content_impl

        #marker_trait_impls
    })
}

fn marker_traits(ident: &Ident) -> TokenStream {
    match ident.to_string().as_str() {
        "AnyStateEventContent" => quote! {
            impl ::ruma_events::RoomEventContent for #ident {}
            impl ::ruma_events::StateEventContent for #ident {}
        },
        "AnyMessageEventContent" => quote! {
            impl ::ruma_events::RoomEventContent for #ident {}
            impl ::ruma_events::MessageEventContent for #ident {}
        },
        "AnyEphemeralRoomEventContent" => quote! {
            impl ::ruma_events::EphemeralRoomEventContent for #ident {}
        },
        "AnyBasicEventContent" => quote! {
            impl ::ruma_events::BasicEventContent for #ident {}
        },
        _ => TokenStream::new(),
    }
}

fn to_event_content_path(
    name: &LitStr,
) -> syn::punctuated::Punctuated<syn::Token![::], syn::PathSegment> {
    let span = name.span();
    let name = name.value();

    assert_eq!(&name[..2], "m.");

    let path = name[2..].split('.').collect::<Vec<_>>();

    let event_str = path.last().unwrap();
    let event = event_str
        .split('_')
        .map(|s| s.chars().next().unwrap().to_uppercase().to_string() + &s[1..])
        .collect::<String>();

    let content_str = Ident::new(&format!("{}EventContent", event), span);
    let path = path.iter().map(|s| Ident::new(s, span));
    syn::parse_quote! {
        ::ruma_events::#( #path )::*::#content_str
    }
}

/// Splits the given `event_type` string on `.` and `_` removing the `m.room.` then
/// camel casing to give the `EventContent` struct name.
pub(crate) fn to_camel_case(name: &LitStr) -> Ident {
    let span = name.span();
    let name = name.value();

    if &name[..2] != "m." {
        panic!(
            "well-known matrix events have to start with `m.` found `{}`",
            name,
        )
    }

    let s = name[2..]
        .split(&['.', '_'] as &[char])
        .map(|s| s.chars().next().unwrap().to_uppercase().to_string() + &s[1..])
        .collect::<String>();
    Ident::new(&s, span)
}

/// Custom keywords for the `event_content_content_enum!` macro
mod kw {
    syn::custom_keyword!(name);
    syn::custom_keyword!(events);
}

/// The entire `event_content_content_enum!` macro structure directly as it appears in the source code..
pub struct ContentEnumInput {
    /// Outer attributes on the field, such as a docstring.
    pub attrs: Vec<Attribute>,

    /// The name of the event.
    pub name: Ident,

    /// An array of valid matrix event types. This will generate the variants of the event content type "name".
    /// There needs to be a corresponding variant in `ruma_events::EventType` for
    /// this event (converted to a valid Rust-style type name by stripping `m.`, replacing the
    /// remaining dots by underscores and then converting from snake_case to CamelCase).
    pub events: Vec<LitStr>,
}

impl Parse for ContentEnumInput {
    fn parse(input: ParseStream<'_>) -> parse::Result<Self> {
        let attrs = input.call(Attribute::parse_outer)?;
        // name field
        input.parse::<kw::name>()?;
        input.parse::<Token![:]>()?;
        // the name of our content_enum enum
        let name: Ident = input.parse()?;
        input.parse::<Token![,]>()?;

        // events field
        input.parse::<kw::events>()?;
        input.parse::<Token![:]>()?;

        // an array of event names `["m.room.whatever"]`
        let ev_array = input.parse::<syn::ExprArray>()?;
        let events = ev_array
            .elems
            .into_iter()
            .map(|item| {
                if let Expr::Lit(ExprLit {
                    lit: Lit::Str(lit_str),
                    ..
                }) = item
                {
                    Ok(lit_str)
                } else {
                    let msg = "values of field `events` are required to be a string literal";
                    Err(syn::Error::new_spanned(item, msg))
                }
            })
            .collect::<syn::Result<_>>()?;

        Ok(Self {
            attrs,
            name,
            events,
        })
    }
}