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//! # interpolate_name
//!
//! `interpolate_name` consists in a set of procedural macro attributes
//! geared towards reduce the boilerplate while writing repetitive tests.
//!
//! - `interpolate_test`: a quick way to test the same function by passing specific
//! arguments and have a test entry for each of them.
//! - `interpolate_name`: a simple function renamer that can be combined
//! with macros to support more complex patterns.

#![feature(proc_macro)]

extern crate proc_macro;
extern crate proc_macro2;
#[macro_use]
extern crate quote;

use proc_macro2::{Ident, Span, TokenStream, TokenTree};

fn fn_name(item: TokenStream) -> Ident {
    let mut tokens = item.into_iter();
    let found = tokens.find(|tok| {
        if let TokenTree::Ident(word) = tok {
            if word == "fn" {
                true
            } else {
                false
            }
        } else {
            false
        }
    }).is_some();

    if !found {
        panic!("the macro attribute applies only to functions")
    }

    match tokens.next() {
        Some(TokenTree::Ident(word)) => word,
        _ => panic!("failed to find function name"),
    }
}

fn fn_attrs_name(item: TokenStream) -> (TokenStream, Ident) {
    let mut tokens = item.into_iter();

    let attrs = tokens.by_ref().take_while(|tok| {
        if let TokenTree::Ident(word) = tok {
            if word == "fn" {
                false
            } else {
                true
            }
        } else {
            true
        }
    }).collect::<Vec<_>>();

    let name = match tokens.next() {
        Some(TokenTree::Ident(word)) => word,
        _ => panic!("the macro attribute applies only to functions"),
    };

    (TokenStream::from_iter(attrs.into_iter()), name)
}


/// Rename the decorated function by appending  `_` and the provided `specifier`.
///
/// ```
/// #[interpolate_name(spec)]
/// fn foo() {}
/// ```
///
/// produces:
/// ```
/// fn foo_spec()
/// ```
#[proc_macro_attribute]
pub fn interpolate_name(
    attr: proc_macro::TokenStream,
    item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let tokens = TokenStream::from(attr).into_iter().collect::<Vec<_>>();
    if tokens.len() != 1 {
        panic!("expected #[interpolate_name(specifier)]");
    }

    let specifier = match &tokens[0] {
        TokenTree::Literal(tt) => tt.to_string(),
        TokenTree::Ident(tt) => tt.to_string(),
        _ => panic!("expected #[interpolate_name(specifier)]"),
    };

    let item = TokenStream::from(item);
    let (attrs, name) = fn_attrs_name(item.clone());
    let interpolated_name = Ident::new(&format!("{}_{}", name.to_string(), specifier), Span::call_site());

    let ret: TokenStream = quote_spanned! {
        proc_macro2::Span::call_site() =>
        #attrs
        fn #interpolated_name() {
            #item

            return #name ();
        }
    }.into();

    ret.into()
}

use std::iter::FromIterator;

/// Generate a new test that calls the decorated function with the provided arguments.
///
/// The test function name is the same as the called plus `_` and `specifier`.
/// Can decorate the same function multiple times.
///
/// ```
/// #[interpolate_test(some, "some", "arguments", 1)]
/// #[interpolate_test(name, "other", "arguments", 42)]
/// fn foo(a: &str, b: &str, c: usize) {
///     println!("{} {} {}", a, b,c);
/// }
/// ```
///
/// produces:
/// ```
/// #[test]
/// fn foo_some() { foo("some", "arguments", 1); }
/// #[test]
/// fn foo_name() { foo("other", "arguments", 42); }
/// ```
#[proc_macro_attribute]
pub fn interpolate_test(
    attr: proc_macro::TokenStream,
    item: proc_macro::TokenStream,
) -> proc_macro::TokenStream {
    let tokens = TokenStream::from(attr).into_iter().collect::<Vec<_>>();
    if tokens.len() < 3  {
        panic!("expected #[interpolate_test(specifier, arguments)]");
    }

    let specifier = match &tokens[0] {
        TokenTree::Literal(tt) => tt.to_string(),
        TokenTree::Ident(tt) => tt.to_string(),
        _ => panic!("expected #[interpolate_test(specifier, arguments)]"),
    };

    match &tokens[1] {
        TokenTree::Punct(p) if p.as_char() == ',' => {}
        _ => panic!("expected #[interpolate_test(specifier, arguments)]")
    }

    let args = TokenStream::from_iter(tokens.into_iter().skip(2));

    let item = TokenStream::from(item);
    let name = fn_name(item.clone());
    let interpolated_name = Ident::new(&format!("{}_{}", name.to_string(), specifier), Span::call_site());

    let ret: TokenStream = quote_spanned! {
        proc_macro2::Span::call_site() =>
        #item

        #[test]
        fn #interpolated_name() {
            return #name (#args);
        }
    }.into();

    ret.into()
}