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//! Transform an arbitrary structs to a http query params
//!
//! This crate generate a function for serialize the fields of an arbitrary structs
//! into a http query params `String` by the usage of a procedural macro with custom derive.
//! The query params `String` return has for purpose to be use with any rust client http lib.    
//!
//! # Getting Start
//!
//! Add `query_params` as a dependency to you `Cargo.toml`.
//!
//! ## Overview
//!
//! ```ignore,
//! #[macro_use]
//! extern crate query_params;
//! 
//! #[derive(QueryParams)]
//! struct PullRequestsParametersApi {
//!     page: i32,
//!     sort: bool,
//!     direction: String,
//!     state: Vec<String>,
//!     // .. other interesting fields ..
//! }
//! 
//! let pr = PullRequestsParametersApi {
//!     page: 2,
//!     sort: true,
//!     direction: "asc".to_string(),
//!     state: vec!["open".to_string(), "closed".to_string()],
//! };
//! 
//! pr.to_query_params();
//! ```
//!
//! ## What that generate
//!
//!
//! ```ignore,
//! #[derive(QueryParams)]
//! struct PullRequestsParametersApi {
//!     page: i32,
//!     sort: bool,
//!     direction: String,
//!     state: Vec<String>,
//!     // .. other interesting fields ..
//! }
//! 
//! // Code generate
//! impl PullRequestsParametersApi {
//!     fn to_query_params(&self) -> String {
//!         let mut buf = String::from("?");
//!         
//!         // Stuff to fill buf with the struct fields content
//!         
//!         return buf
//!     }
//!     // expect "?page=2&sort=true&direction=asc&state=open,closed" with the example above
//! }  
//! ```

#![crate_type = "proc-macro"]

extern crate proc_macro;
extern crate syn;
#[macro_use]
extern crate quote;

use proc_macro::TokenStream;
use syn::{Body, VariantData};

#[proc_macro_derive(QueryParams)]
pub fn derive_query_params(input: TokenStream) -> TokenStream {
    let input = input.to_string();

    let ast = syn::parse_derive_input(&input).unwrap();

    let name = &ast.ident;
    let (impl_generics, ty_generics, where_clause) = ast.generics.split_for_impl();

    let query_params = gen_serialization_query_params(&ast.body);

    let gen = quote! {
        impl #impl_generics #name #ty_generics #where_clause {
            fn to_query_params(&self) -> String {
                #query_params
            }
        }
    };

    gen.parse().expect(format!("An error occurred when parsing the tokens generate for {} struct",name).as_str())
}

/// yolo
fn gen_serialization_query_params(body: &Body) -> quote::Tokens {
    match *body {
        Body::Struct(VariantData::Struct(ref fs)) => {
            let query_params: Vec<quote::Tokens> = get_print_fields(fs);

            quote! {
                let mut buf = String::from("?");

                (#(#query_params),*);

                let len_query_params = buf.len();
                buf.truncate(len_query_params - 1); // remove trailing ampersand

                return buf;
            }
        }
        Body::Struct(VariantData::Tuple(_)) => panic!("#[derive(QueryParams)] is only defined for structs, not tuple"),
        Body::Struct(VariantData::Unit) => panic!("#[derive(QueryParams)] is only defined for structs, not unit"),
        Body::Enum(_) => panic!("#[derive(QueryParams)] is only defined for structs, not enum"),
    }
}

/// something cool
fn get_print_fields(fields: &Vec<syn::Field>) -> Vec<quote::Tokens> {
    fields.iter()
        .map(|f| (&f.ident, &f.ty))
        .map(|(ident, ty)| match ty {
            &syn::Ty::Path(_, ref path) => (ident, extract_type_name(path)),
            _ => unimplemented!(),
        })
        .map(|(ident, path)| match path {
            "Vec" => vec_to_query_params(ident),
            "Option" => option_to_query_params(ident),
            _ => primitive_to_query_params(ident),
        })
        .collect()
}


#[inline]
fn extract_type_name(path: &syn::Path) -> &str {
    path.segments.last().unwrap().ident.as_ref()
}


fn vec_to_query_params(ident: &Option<syn::Ident>) -> quote::Tokens {
    quote! {
        buf.push_str((format!("{}={}&",
            stringify!(#ident),
            self.#ident
                .iter()
                .fold(String::new(), |acc, ref val| acc + &val.to_string() + ","))
                .as_str()
        )
        .replace(",&", "&") // remove trailing comma insert by fold
        .as_str())
    }
}


fn option_to_query_params(ident: &Option<syn::Ident>) -> quote::Tokens {
    quote! {
        if self.#ident.is_some() {
            buf.push_str(format!("{}={}&", stringify!(#ident), self.#ident.as_ref().unwrap()).as_str())
        }
    }
}


fn primitive_to_query_params(ident: &Option<syn::Ident>) -> quote::Tokens {
    quote!{
        buf.push_str(format!("{}={}&", stringify!(#ident), self.#ident).as_str())
    }
}