//! Python-like decorator for Rust
//!
//! Example
//! --------
//!
//! ```
//! use pydeco::deco;
//!
//! fn logging<F>(func: F) -> impl Fn(i32) -> i32
//! where
//!     F: Fn(i32) -> i32,
//! {
//!     move |i| {
//!         println!("Input = {}", i);
//!         let out = func(i);
//!         println!("Output = {}", out);
//!         out
//!     }
//! }
//!
//! #[deco(logging)]
//! fn add2(i: i32) -> i32 {
//!     i + 2
//! }
//!
//! add2(2);
//! ```
//!
//! - Decorator with parameter
//!
//! ```
//! use pydeco::deco;
//! use std::{fs, io::Write};
//!
//! fn logging<InputFunc: 'static>(
//!     log_filename: &'static str,
//! ) -> impl Fn(InputFunc) -> Box<dyn Fn(i32) -> i32>
//! where
//!     InputFunc: Fn(i32) -> i32,
//! {
//!     move |func: InputFunc| {
//!         Box::new(move |i: i32| {
//!             let mut f = fs::File::create(log_filename).unwrap();
//!             writeln!(f, "Input = {}", i).unwrap();
//!             let out = func(i);
//!             writeln!(f, "Output = {}", out).unwrap();
//!             out
//!         })
//!     }
//! }
//!
//! #[deco(logging("test.log"))]
//! fn add2(i: i32) -> i32 {
//!     i + 2
//! }
//!
//! add2(2);
//! ```
//!

use anyhow::{bail, Result};
use proc_macro::TokenStream;
use proc_macro2::TokenTree;
use syn::*;

#[proc_macro_attribute]
pub fn deco(attr: TokenStream, func: TokenStream) -> TokenStream {
    let func = func.into();
    let item_fn: ItemFn = syn::parse(func).expect("Input is not a function");
    let vis = &item_fn.vis;
    let ident = &item_fn.sig.ident;
    let block = &item_fn.block;

    let inputs = item_fn.sig.inputs;
    let output = item_fn.sig.output;

    let input_values: Vec<_> = inputs
        .iter()
        .map(|arg| match arg {
            &FnArg::Typed(ref val) => &val.pat,
            _ => unimplemented!("#[deco] cannot be used with associated function"),
        })
        .collect();

    let attr = DecoratorAttr::parse(attr.into()).expect("Failed to parse attribute");
    let caller = match attr {
        DecoratorAttr::Fixed { name } => {
            quote::quote! {
                #vis fn #ident(#inputs) #output {
                    let f = #name(deco_internal);
                    return f(#(#input_values,) *);

                    fn deco_internal(#inputs) #output #block
                }
            }
        }
        DecoratorAttr::Parametric { name, args } => {
            quote::quote! {
                #vis fn #ident(#inputs) #output {
                    let deco = #name(#(#args,) *);
                    let f = deco(deco_internal);
                    return f(#(#input_values,) *);

                    fn deco_internal(#inputs) #output #block
                }
            }
        }
    };
    caller.into()
}

#[derive(Debug, PartialEq)]
enum DecoratorAttr {
    Fixed { name: Ident },
    Parametric { name: Ident, args: Vec<Expr> },
}

impl DecoratorAttr {
    fn parse(attr: proc_macro2::TokenStream) -> Result<Self> {
        let mut ident = None;
        let mut args = Vec::new();
        for at in attr {
            match at {
                TokenTree::Ident(id) => {
                    ident = Some(id);
                }
                TokenTree::Group(grp) => {
                    if ident.is_none() {
                        bail!("Invalid token stream");
                    }
                    for t in grp.stream() {
                        if let Ok(expr) = syn::parse2(t.into()) {
                            args.push(expr);
                        }
                    }
                }
                _ => bail!("Invalid token stream"),
            }
        }
        if let Some(name) = ident {
            if args.is_empty() {
                Ok(DecoratorAttr::Fixed { name })
            } else {
                Ok(DecoratorAttr::Parametric { name, args })
            }
        } else {
            bail!("Decorator name not found");
        }
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use std::str::FromStr;

    #[test]
    fn parse_attr() -> Result<()> {
        let ts = proc_macro2::TokenStream::from_str("logging").unwrap();
        assert!(matches!(DecoratorAttr::parse(ts)?, DecoratorAttr::Fixed {..}));
        Ok(())
    }

    #[test]
    fn parse_attr_parametric_literal() -> Result<()> {
        let ts = proc_macro2::TokenStream::from_str(r#"logging("test.log", 2)"#).unwrap();
        match DecoratorAttr::parse(ts)? {
            DecoratorAttr::Fixed { .. } => bail!("Failed to parse args"),
            DecoratorAttr::Parametric { args, .. } => {
                assert_eq!(args.len(), 2);
            }
        }
        Ok(())
    }

    #[test]
    fn parse_attr_parametric_variable() -> Result<()> {
        let ts =
            proc_macro2::TokenStream::from_str(r#"logging("test.log", some_variable)"#).unwrap();
        match DecoratorAttr::parse(ts)? {
            DecoratorAttr::Fixed { .. } => bail!("Failed to parse args"),
            DecoratorAttr::Parametric { args, .. } => {
                assert_eq!(args.len(), 2);
            }
        }
        Ok(())
    }

    #[test]
    fn parse_attr_parametric_expr() -> Result<()> {
        let ts = proc_macro2::TokenStream::from_str(r#"logging("test.log", (1 + 2))"#).unwrap();
        match DecoratorAttr::parse(ts)? {
            DecoratorAttr::Fixed { .. } => bail!("Failed to parse args"),
            DecoratorAttr::Parametric { args, .. } => {
                assert_eq!(args.len(), 2);
            }
        }
        Ok(())
    }

    #[test]
    fn parse_attr_empty() -> Result<()> {
        let ts = proc_macro2::TokenStream::from_str("").unwrap();
        assert!(DecoratorAttr::parse(ts).is_err());
        Ok(())
    }

    #[test]
    fn parse_attr_invalid() -> Result<()> {
        // inverse order
        let ts = proc_macro2::TokenStream::from_str(r#"("test.log", 2)logging"#).unwrap();
        assert!(DecoratorAttr::parse(ts).is_err());
        Ok(())
    }
}