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#![recursion_limit = "128"]

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

use std::iter;

use proc_macro::TokenStream;
use syn::{Body, Ident, Variant, VariantData};
use quote::Tokens;

fn generate_enum_code(name: &Ident, variants: &[Variant]) -> Tokens {
    let mut enum_count = 0usize;
    for &Variant {
             ref data,
             ref discriminant,
             ..
         } in variants {
        if data != &VariantData::Unit {
            panic!("#[derive(EnumMap)] requires C style style enum");
        }
        if discriminant.is_some() {
            panic!("#[derive(EnumMap)] doesn't currently support discriminants");
        }
        enum_count += 1;
    }

    let variant_a = variants.iter().map(|variant| &variant.ident);
    let variant_b = variants.iter().map(|variant| &variant.ident);
    let repeat_name_a = iter::repeat(name).take(variants.len());
    let repeat_name_b = repeat_name_a.clone();
    let counter = 0..variants.len();

    quote! {
        impl<V> ::enum_map::Internal<V> for #name {
            type Array = [V; #enum_count];
            fn slice(array: &Self::Array) -> &[V] {
                array
            }
            fn slice_mut(array: &mut Self::Array) -> &mut [V] {
                array
            }
            fn from_usize(value: usize) -> Self {
                match value {
                    #(
                        #counter => #repeat_name_a::#variant_a,
                    )*
                    _ => unreachable!()
                }
            }
            fn to_usize(self) -> usize {
                self as usize
            }
            fn from_function<F: Fn(Self) -> V>(f: F) -> Self::Array {
                [#(
                    f(#repeat_name_b::#variant_b),
                )*]
            }
        }
    }
}

#[proc_macro_derive(EnumMap)]
pub fn derive_enum_map(input: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input(&input.to_string()).unwrap();
    match input.body {
            Body::Enum(ref variants) => generate_enum_code(&input.ident, variants),
            _ => panic!("#[derive(EnumMap)] is only defined for enums"),
        }
        .parse()
        .unwrap()
}