//! Implementation detail of `ilex`.

use proc_macro::Delimiter;
use proc_macro::Group;
use proc_macro::Ident;
use proc_macro::Punct;
use proc_macro::Spacing;
use proc_macro::Span;
use proc_macro::TokenStream;
use proc_macro::TokenTree;

/// Generates a lexer spec struct.
///
/// This macro generates the type of struct described in the
/// [crate documentation][crate]. The syntax is as follows.
///
/// ```ignore
/// use ilex::rule::Keyword;
/// use ilex::Lexeme;
///
/// /// My cool spec.
/// #[ilex::spec]
/// struct MySpec {
///   #[named("...")]
///   #[rule(/* ... */)]
///   dollar: Lexeme<Keyword> = "$",
/// }
/// ```
///
/// The type of each field must be a [`Lexeme`] with a [`Rule`] type as its
/// parameter. There are two special attributes that can follow.
///
/// - `#[named]` makes the rule into a *named* rule. This name can be used by
///   diagnostics, and corresponds to calling `Spec::named_rule()`.
///
/// - `#[rule]` is the value to use to construct the rule, which must be
///   `Into<R>`, where `R` is the type inside `Lexeme` (so, above, the rule
///   value must be `Into<Keyword>`). By default, this value is the name of the
///   rule, to make the common case of declaring a keyword as simple as writing
///   `nullptr: Lexeme<Keyword>`, assuming Rust itself doesn't already use that
///   keyword.
///
/// Note that *order matters* for the fields: when breaking a tie between two
/// potential tokens of the same length, the first one in the struct will win.
/// In practice, this means you should put keywords before identifiers.
///
/// Additionally, the following functions will be defined for the `MySpec` type.
///
/// ```
/// # struct Spec;
/// # struct MySpec;
/// # fn norun(_: i32) {
/// impl MySpec {
///   /// Gets the global instance of this spec.
///   pub fn get() -> &'static Self {
///     // ...
/// #   todo!()
///   }
///
///   /// Gets the actual compiled spec.
///   pub fn spec(&self) -> &Spec {
///     // ...
/// #   todo!()
///   }
/// }
/// # }
/// ```
///
// God cross-trait links suck.
/// [`Lexeme`]: https://docs.rs/ilex/latest/ilex/struct.Lexeme.html
/// [`Rule`]: https://docs.rs/ilex/latest/ilex/rule/trait.Rule.html
/// [crate]: https://docs.rs/ilex
#[proc_macro_attribute]
pub fn spec(_attr: TokenStream, item: TokenStream) -> TokenStream {
  // This is implemented as a decl macro, because that's easier to
  // understand and debug than proc macros. I hate proc macros so much.
  let span = Span::call_site();
  let macro_call: [TokenTree; 8] = [
    Punct::new(':', Spacing::Joint).into(),
    Punct::new(':', Spacing::Alone).into(),
    Ident::new("ilex", span).into(),
    Punct::new(':', Spacing::Joint).into(),
    Punct::new(':', Spacing::Alone).into(),
    Ident::new("__spec__", span).into(),
    Punct::new('!', Spacing::Alone).into(),
    Group::new(Delimiter::Brace, item).into(),
  ];

  macro_call.into_iter().collect()
}

// This helper exists only to make the #[spec] field attributes inert.
#[doc(hidden)]
#[proc_macro_derive(derive_hack, attributes(named, rule))]
pub fn derive(_: TokenStream) -> TokenStream {
  TokenStream::new()
}