//! Utilities for proc macro implementation

use super::prelude::*;
use proc_macro_crate::{crate_name, FoundCrate};

//---------- misc ----------

/// Construct a braced group from a token expansion
///
/// Calls `f` to expand tokens, and returns a braced group containing
/// its output.
pub fn braced_group(
    brace_span: Span,
    f: impl FnOnce(&mut TokenAccumulator) -> syn::Result<()>,
) -> syn::Result<proc_macro2::Group> {
    delimit_token_group(Delimiter::Brace, brace_span, f)
}

pub fn delimit_token_group(
    delim: Delimiter,
    delim_span: Span,
    f: impl FnOnce(&mut TokenAccumulator) -> syn::Result<()>,
) -> syn::Result<proc_macro2::Group> {
    let mut out = TokenAccumulator::default();
    f(&mut out)?;
    let out = out.tokens()?;
    let mut out = proc_macro2::Group::new(delim, out);
    out.set_span(delim_span);
    Ok(out)
}

/// Type which parses as `T`, but then discards it
pub struct Discard<T>(PhantomData<T>);

impl<T: Parse> Parse for Discard<T> {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let _: T = input.parse()?;
        Ok(Discard(PhantomData))
    }
}

/// Type which parses as a concatenated series of `T`
pub struct Concatenated<T>(pub Vec<T>);

impl<T: Parse> Parse for Concatenated<T> {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        let mut out = vec![];
        while !input.is_empty() {
            out.push(input.parse()?);
        }
        Ok(Concatenated(out))
    }
}

/// Add a warning about derive-adhoc version incompatibility
pub fn advise_incompatibility(err_needing_advice: syn::Error) -> syn::Error {
    let mut advice = Span::call_site().error(
        "bad input to derive_adhoc_expand inner template expansion proc macro; might be due to incompatible derive-adhoc versions(s)"
    );
    advice.combine(err_needing_advice);
    advice
}

//---------- MakeError ----------

/// Provides `.error()` on `impl Spanned` and `[`[`ErrorLoc`]`]`
pub trait MakeError {
    /// Convenience method to make an error
    fn error<M: Display>(&self, m: M) -> syn::Error;
}

impl<T: Spanned> MakeError for T {
    fn error<M: Display>(&self, m: M) -> syn::Error {
        syn::Error::new(self.span(), m)
    }
}

/// Error location: span and what role that span plays
///
/// When `MakeError::error` is invoked on a slice of these,
/// the strings are included to indicate to the user
/// what role each `Span` location played.
/// For example, `(tspan, "template")`.
pub type ErrorLoc = (Span, &'static str);

/// Generates multiple copies of the error, for multiple places
///
/// # Panics
///
/// Panics if passed an empty slice.
impl MakeError for [ErrorLoc] {
    fn error<M: Display>(&self, m: M) -> syn::Error {
        let mut locs = self.into_iter().cloned();
        let mk = |(span, frag): (Span, _)| {
            span.error(format_args!("{} ({})", &m, frag))
        };
        let first = locs.next().expect("at least one span needed!");
        let mut build = mk(first);
        for rest in locs {
            build.combine(mk(rest))
        }
        build
    }
}

//---------- ToTokensPunctComposable ----------

/// Convert to a token stream in a way that composes nicely
pub trait ToTokensPunctComposable {
    fn to_tokens_punct_composable(&self, out: &mut TokenStream);
}
/// Ensure that there is a trailing punctuation if needed
impl<T, P> ToTokensPunctComposable for Punctuated<T, P>
where
    T: ToTokens,
    P: ToTokens + Default,
{
    fn to_tokens_punct_composable(&self, out: &mut TokenStream) {
        self.to_tokens(out);
        if !self.empty_or_trailing() {
            P::default().to_tokens(out)
        }
    }
}
/// Ensure that something is output, for punctuation `P`
///
/// Implemented for `Option<&&P>` because that's what you get from
/// `Punctuated::pairs().next().punct()`.
impl<P> ToTokensPunctComposable for Option<&&P>
where
    P: ToTokens,
    P: Default,
{
    fn to_tokens_punct_composable(&self, out: &mut TokenStream) {
        if let Some(self_) = self {
            self_.to_tokens(out)
        } else {
            P::default().to_tokens(out)
        }
    }
}

//---------- ErrorAccumulator ----------

/// Contains zero or more `syn::Error`
///
/// # Panics
///
/// Panics if dropped.
///
/// You must call one of the consuming methods, eg `finish`
#[derive(Debug, Default)]
pub struct ErrorAccumulator {
    bad: Option<syn::Error>,
    defused: bool,
}

impl ErrorAccumulator {
    /// Run `f`, accumulate any error, and return an `Ok`
    pub fn handle_in<T, F>(&mut self, f: F) -> Option<T>
    where
        F: FnOnce() -> syn::Result<T>,
    {
        self.handle(f())
    }

    /// Handle a `Result`: accumulate any error, and returni an `Ok`
    pub fn handle<T>(&mut self, result: syn::Result<T>) -> Option<T> {
        match result {
            Ok(y) => Some(y),
            Err(e) => {
                self.push(e);
                None
            }
        }
    }

    /// Accumulate an error
    pub fn push(&mut self, err: syn::Error) {
        if let Some(bad) = &mut self.bad {
            bad.combine(err)
        } else {
            self.bad = Some(err);
        }
    }

    /// If there were any errors, return a single error that combines them
    #[allow(dead_code)]
    pub fn finish(self) -> syn::Result<()> {
        self.finish_with(())
    }

    /// If there were any errors, return `Err`, otherwise `Ok(success)`
    pub fn finish_with<T>(self, success: T) -> syn::Result<T> {
        match self.into_inner() {
            None => Ok(success),
            Some(bad) => Err(bad),
        }
    }

    /// If there any errors, return a single error that combines them
    pub fn into_inner(mut self) -> Option<syn::Error> {
        self.defused = true;
        self.bad.take()
    }
}

impl Drop for ErrorAccumulator {
    fn drop(&mut self) {
        assert!(panicking() || self.defused);
    }
}

//---------- IdentAny ----------

/// Like `syn::Ident` but parses using `parse_any`, accepting keywords
///
/// Used for derive-adhoc's own keywords, which can be Rust keywords,
/// or identifiers.
///
/// Needs care when used with user data, since it might be a keyword,
/// in which case it's not really an *identifier*.
pub struct IdentAny(pub syn::Ident);
impl Parse for IdentAny {
    fn parse(input: ParseStream) -> syn::Result<Self> {
        Ok(IdentAny(Ident::parse_any(input)?))
    }
}
impl Deref for IdentAny {
    type Target = syn::Ident;
    fn deref(&self) -> &syn::Ident {
        &self.0
    }
}
impl Spanned for IdentAny {
    fn span(&self) -> Span {
        self.0.span()
    }
}
impl<T: AsRef<str> + ?Sized> PartialEq<T> for IdentAny {
    fn eq(&self, rhs: &T) -> bool {
        self.0.eq(rhs)
    }
}

//---------- expand_macro_name ----------

/// Return a full path to the location of `derive_adhoc_expand`.
///
/// (This may not work properly if the user
/// imports the crate under a different name.
/// This is a problem with the way cargo and rustc
/// handle imports and proc-macro crates,
/// which I think we can't properly solve here.)
pub fn expand_macro_name() -> Result<TokenStream, syn::Error> {
    let name = crate_name("derive-adhoc-macros")
        .or_else(|_| crate_name("derive-adhoc"));

    // See `tests/pub-export/pub-b/pub-b.rs`.  (The bizarre version
    // has a different crate name, which we must handle heree.)
    #[cfg(feature = "bizarre")]
    let name = name.or_else(|_| crate_name("bizarre-derive-adhoc"));

    match name {
        Ok(FoundCrate::Itself) => Ok(quote!( crate::derive_adhoc_expand )),
        Ok(FoundCrate::Name(name)) => {
            let ident = Ident::new(&name, Span::call_site());
            Ok(quote!( ::#ident::derive_adhoc_expand ))
        }
        Err(e) => Err(Span::call_site().error(
            format_args!("Expected derive-adhoc or derive-adhoc-macro to be present in Cargo.toml: {}", e)
        )),
    }
}

//---------- general keyword enum parsing ----------

/// General-purpose keyword parser
///
/// ```ignore
/// keyword_general!{
///     KW_VAR FROM_ENUM ENUM;
///     KEYWORD [ {BINDINGS} ] [ CONSTRUCTOR-ARGS ] }
/// ```
/// Expands to:
/// ```ignore
///     if KW_VAR = ... {
///         BINDINGS
///         return FROM_ENUM(ENUM::CONSTRUCTOR CONSTRUCTOR-ARGS)
///     }
/// ```
///
/// `KEYWORD` can be `"KEYWORD_STRING": CONSTRUCTOR`
///
/// `CONSTRUCTOR-ARGS`, if present, should be in the `( )` or `{ }`
/// as required by the variant's CONSTRUCTOR.
macro_rules! keyword_general {
    { $kw_var:ident $from_enum:ident $Enum:ident;
      $kw:ident $( $rest:tt )* } => {
        keyword_general!{ $kw_var $from_enum $Enum;
                          @ 1 stringify!($kw), $kw, $($rest)* }
    };
    { $kw_var:ident $from_enum:ident $Enum:ident;
      $kw:literal: $constr:ident $( $rest:tt )* } => {
        keyword_general!{ $kw_var $from_enum $Enum;
                          @ 1 $kw, $constr, $($rest)* }
    };
    { $kw_var:ident $from_enum:ident $Enum:ident;
      @ 1 $kw:expr, $constr:ident, $( $ca:tt )? } => {
        keyword_general!{ $kw_var $from_enum $Enum;
                          @ 2 $kw, $constr, { } $( $ca )? }
    };
    { $kw_var:ident $from_enum:ident $Enum:ident;
      @ 1 $kw:expr, $constr:ident, { $( $bindings:tt )* } $ca:tt } => {
        keyword_general!{ $kw_var $from_enum $Enum;
                          @ 2 $kw, $constr, { $( $bindings )* } $ca }
    };
    { $kw_var:ident $from_enum:ident $Enum:ident;
      @ 2 $kw:expr, $constr:ident,
      { $( $bindings:tt )* } $( $constr_args:tt )?
    } => {
        let _: &IdentAny = &$kw_var;
        if $kw_var == $kw {
            $( $bindings )*
            return $from_enum($Enum::$constr $( $constr_args )*);
        }
    };
    { $($x:tt)* } => { compile_error!(stringify!($($x)*)) };
}