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//! A custom derive implementation for Glium Vertex
//!
//! ## Example
//! ```rust
//! extern crate glium;
//! #[macro_use]
//! extern crate glium_derive;
//!
//! #[derive(Clone, Copy, Vertex)]
//! struct MyVertex {
//!     #[glium(attr = "a_pos")
//!     pos: [f32; 3],
//!     #[glium(attr = "a_uv")
//!     uv: [f32; 2],
//!     #[glium(attr = "a_color", normalize)
//!     color: u32,
//!     a_custom: u32, // the attribute can be omitted
//! }
//! ```

#![recursion_limit = "128"]

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

use proc_macro::TokenStream;
use syn::spanned::Spanned;
use syn::{Data, DataStruct, DeriveInput, Fields, Attribute, Meta, MetaList, MetaNameValue, NestedMeta};

#[proc_macro_derive(Vertex, attributes(glium))]
pub fn glium_vertex_derive(input: TokenStream) -> TokenStream {
    let ast = syn::parse(input).unwrap();

    // Build the impl
    impl_glium_vertex_derive(&ast)
}

fn impl_glium_vertex_derive(ast: &DeriveInput) -> TokenStream {
    let struct_name = &ast.ident;
    let generics = &ast.generics;
    let (impl_generics, ty_generics, where_clause) = generics.split_for_impl();

    let fields = match ast.data {
        Data::Struct(DataStruct {
            fields: Fields::Named(ref fields),
            ..
        }) => {
            &fields.named
        },
        _ => {
            panic!("#[derive(Vertex)] only defined for structs.");
        }
    };

    let bindings = fields.iter().map(|field| {
        let attrs = field.attrs.iter()
            .flat_map(Attribute::interpret_meta)
            .flat_map(|meta| {
                match meta {
                    Meta::List(MetaList { ref ident, ref nested, .. }) if ident == "glium" => {
                        nested.iter().cloned().collect()
                    },
                    _ => {
                        Vec::new()
                    }
                }
            });

        let field_name = &field.ident;
        let vertex_attr_lit = format!("{}", field_name.as_ref().unwrap());
        let mut vertex_attr_name = quote!(#vertex_attr_lit);
        let mut normalize = false;

        for meta in attrs {
            match meta {
                NestedMeta::Meta(Meta::NameValue(MetaNameValue { ref ident, ref lit, .. })) => {
                    if quote!(#ident).to_string() == "attr" {
                        vertex_attr_name = quote!(#lit);
                    } else {
                        panic!("Unknown field attribute {}", ident);
                    }
                },
                NestedMeta::Meta(Meta::Word(ref ident)) => {
                    if quote!(#ident).to_string() == "normalize" {
                        normalize = true;
                    } else {
                        panic!("Unknown field attribute {}", ident);
                    }
                },
                _ => (),
            }
        };

        quote_spanned! {field.span()=>
            (
                Cow::Borrowed(#vertex_attr_name),
                {
                    // calculate the offset of the struct fields
                    let dummy: #struct_name = unsafe { ::std::mem::uninitialized() };
                    let offset: usize = {
                        let dummy_ref = &dummy;
                        let field_ref = &dummy.#field_name;
                        (field_ref as *const _ as usize) - (dummy_ref as *const _ as usize)
                    };
                    // NOTE: `glium::vertex::Vertex` requires `#struct_name` to have `Copy` trait
                    // `Copy` excludes `Drop`, so we don't have to `std::mem::forget(dummy)`
                    offset
                },
                {
                    fn attr_type_of_val<T: ::glium::vertex::Attribute>(_: &T) -> ::glium::vertex::AttributeType {
                        <T as ::glium::vertex::Attribute>::get_type()
                    }
                    let dummy: &#struct_name = unsafe { ::std::mem::transmute(0usize) };
                    attr_type_of_val(&dummy.#field_name)
                },
                #normalize
            )
        }
    });

    let stream = quote! {
        impl #impl_generics ::glium::vertex::Vertex for #struct_name #ty_generics #where_clause {
            #[inline]
            fn build_bindings() -> ::glium::vertex::VertexFormat {
                use std::borrow::Cow;

                // TODO: use a &'static [] if possible
                Cow::Owned(vec![
                    #(
                        #bindings,
                    )*
                ])
            }
        }
    };

    stream.into()
}