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extern crate proc_macro;

use proc_macro::TokenStream;
use proc_macro_error::{abort, proc_macro_error};
use quote::quote;
use syn::{
    AngleBracketedGenericArguments, AttrStyle::Outer, Expr::Lit, ExprLit, Field, Fields::Named,
    GenericArgument, Lit::Str, Meta::NameValue, MetaNameValue, PathArguments, PathSegment, Type,
    TypePath,
};

fn default_value(ty: String) -> String {
    match ty.as_str() {
        "usize" | "u8" | "u16" | "u32" | "u64" | "u128" | "isize" | "i8" | "i16" | "i32"
        | "i64" | "i128" => "0",
        "f32" | "f64" => "0.0",
        _ => "\"\"",
    }
    .to_string()
}

fn parse_type(ty: &Type, default: &mut Option<String>, optional: &mut bool) {
    if let Type::Path(TypePath { path, .. }) = ty {
        if let Some(PathSegment { ident, arguments }) = path.segments.last() {
            let id = ident.to_string();
            if arguments.is_none() {
                *default = Some(default_value(id));
            } else if id == "Option" {
                *optional = true;
                if let PathArguments::AngleBracketed(AngleBracketedGenericArguments {
                    args, ..
                }) = arguments
                {
                    if let Some(GenericArgument::Type(ty)) = args.first() {
                        parse_type(&ty, default, optional);
                    }
                }
            }
            // TODO else Complex struct in else
        }
    }
}

fn get_default_and_doc_from_field(field: &Field) -> (Option<String>, Option<String>, bool) {
    let mut doc = None;
    let mut default = None;
    let mut optional = false;
    for attr in field.attrs.iter() {
        match (attr.style, &attr.meta) {
            (Outer, NameValue(MetaNameValue { path, value, .. })) => {
                for seg in path.segments.iter() {
                    if seg.ident.to_string() == "doc" {
                        if let Lit(ExprLit {
                            lit: Str(lit_str), ..
                        }) = value
                        {
                            doc = Some(lit_str.value());
                        }
                    }
                }
            }
            _ => (),
        }
    }
    parse_type(&field.ty, &mut default, &mut optional);
    (default.map(|s| s.to_string()), doc, optional)
}

#[proc_macro_derive(TomlExample)]
#[proc_macro_error]
pub fn derive_patch(item: TokenStream) -> TokenStream {
    let input = syn::parse_macro_input!(item as syn::DeriveInput);
    let struct_name = &input.ident;
    let mut example = String::new();

    let fields = if let syn::Data::Struct(syn::DataStruct { fields, .. }) = &input.data {
        fields
    } else {
        abort!(&input.ident, "TomlExample derive only use for struct")
    };
    if let Named(fields_named) = fields {
        for f in fields_named.named.iter() {
            if let Some(field_name) = f.ident.as_ref().map(|i| i.to_string()) {
                let (default, doc_str, optional) = get_default_and_doc_from_field(&f);

                if let Some(doc_str) = doc_str {
                    example.push('#');
                    example.push_str(&doc_str);
                    example.push('\n');
                }

                if optional {
                    example.push_str("# ");
                }

                if let Some(default) = default {
                    example.push_str(&field_name);
                    example.push_str(" = ");
                    example.push_str(&default);
                } else {
                    example.push_str(&field_name);
                    example.push_str(" = \"\"");
                }
                example.push('\n');
            }
        }
    }

    let output = quote! {
        impl toml_example::TomlExample for #struct_name {
            fn toml_example() -> &'static str {
                #example
            }
        }
    };
    TokenStream::from(output)
}