build_builder 0.0.1

use proc_macro::TokenStream;
use proc_macro2::{Ident, TokenStream as TokenStream2};
use quote::{format_ident, quote};
use syn::{
    parse_macro_input, AngleBracketedGenericArguments, Data, DataStruct, DeriveInput, FieldsNamed,
    GenericArgument, PathArguments, Type,
};

#[proc_macro_derive(Builder)]
pub fn derive_builder(input: TokenStream) -> TokenStream {
    let input = parse_macro_input!(input as DeriveInput);
    let ts = do_expand(&input);
    ts.into()
}

fn do_expand(st: &DeriveInput) -> TokenStream2 {
    let ident = &st.ident;
    let name = format_ident!("{}Builder", ident);
    let tokens = parse_data(&st.data);
    let data: Vec<_> = tokens.iter().map(|token| &token.field).collect();
    let content: Vec<_> = tokens.iter().map(|token| &token.value).collect();
    let impl_block: Vec<_> = tokens.iter().map(|token| &token.impl_block).collect();
    let build_fields: Vec<_> = tokens.iter().map(|token| &token.build_field).collect();
    quote!(
        pub struct #name {
            #(#data),*
        }

        impl #ident {
            pub fn builder () -> #name {
                #name {
                    #(#content),*
                }
            }
        }

        impl #name {
            #(#impl_block)*
        }

        impl #name {
            pub fn build(self) -> #ident {
                #ident {
                  #(#build_fields),*
                }
            }
        }
    )
}

fn parse_data(data: &Data) -> Vec<Token> {
    match data {
        Data::Struct(data) => parse_struct(data),
        Data::Enum(_) => todo!(),
        Data::Union(_) => todo!(),
    }
}

struct Token {
    field: TokenStream2,
    value: TokenStream2,
    impl_block: TokenStream2,
    build_field: TokenStream2,
}

fn parse_struct(data: &DataStruct) -> Vec<Token> {
    let fields = &data.fields;
    match fields {
        syn::Fields::Named(name) => parse_named(name),
        syn::Fields::Unnamed(_) => todo!(),
        syn::Fields::Unit => todo!(),
    }
}

fn get_inner_option_type(ty: &Type) -> Option<&Type> {
    if let Type::Path(ty) = ty {
        let seg = ty.path.segments.last();
        if let Some(seg) = seg {
            let ident = seg.ident.to_string();
            if ident == "Option" {
                if let PathArguments::AngleBracketed(AngleBracketedGenericArguments {
                    args, ..
                }) = &seg.arguments
                {
                    if let Some(GenericArgument::Type(ty)) = args.last() {
                        return Some(ty);
                    }
                }
            }
        }
        None
    } else {
        None
    }
}

fn get_type_name(ty: &Type) -> Option<String> {
    match ty {
        Type::Path(ty) => ty.path.segments.last().map(|seg| seg.ident.to_string()),
        _ => None,
    }
}

fn generate_impl_block(ident: &Option<Ident>, ty_name: &Option<String>, ty: &Type) -> TokenStream2 {
    match ty_name {
        Some(name) if name == "String" => {
            quote!(
                pub fn #ident<T>(&mut self, #ident: T) ->&mut Self where T:AsRef<str> {
                    self.#ident = Some(#ident.as_ref().to_owned());
                    self
                }
            )
        }
        _ => {
            quote!(
                pub fn #ident(&mut self, #ident: #ty) ->&mut Self {
                    self.#ident = Some(#ident);
                    self
                }
            )
        }
    }
}

fn generate_build_field(inner_ty: &Option<&Type>, ident: &Option<Ident>) -> TokenStream2 {
    if inner_ty.is_some() {
        quote!(
            #ident: self.#ident
        )
    } else {
        quote!(
            #ident: self.#ident.expect("#ident is missing")
        )
    }
}
fn parse_named(name: &FieldsNamed) -> Vec<Token> {
    name.named
        .iter()
        .map(|field| {
            let ident = field.ident.to_owned();
            let old_ty = &field.ty;
            let inner_ty = get_inner_option_type(old_ty);

            let ty = match inner_ty {
                Some(ty) => ty,
                None => old_ty,
            };

            let ty_name = get_type_name(ty);
            let field = quote!(
                #ident: Option<#ty>
            );
            let value = quote!(
                #ident: None
            );
            let impl_block = generate_impl_block(&ident, &ty_name, ty);
            let build_field = generate_build_field(&inner_ty, &ident);

            Token {
                field,
                value,
                impl_block,
                build_field,
            }
        })
        .collect::<Vec<_>>()
}