//! This module contains functions to generate default trait impl function bodies where possible.

use syntax::ast::{self, edit::AstNodeEdit, make, AstNode, NameOwner};
use syntax::ted;

/// Generate custom trait bodies where possible.
///
/// Returns `Option` so that we can use `?` rather than `if let Some`. Returning
/// `None` means that generating a custom trait body failed, and the body will remain
/// as `todo!` instead.
pub(crate) fn gen_trait_body(func: &ast::Fn, trait_path: &ast::Path, adt: &ast::Adt) -> Option<()> {
    match trait_path.segment()?.name_ref()?.text().as_str() {
        "Debug" => gen_debug_impl(adt, func),
        "Default" => gen_default_impl(adt, func),
        _ => None,
    }
}

/// Generate a `Debug` impl based on the fields and members of the target type.
fn gen_debug_impl(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
    let annotated_name = adt.name()?;
    match adt {
        // `Debug` cannot be derived for unions, so no default impl can be provided.
        ast::Adt::Union(_) => None,

        // => match self { Self::Variant => write!(f, "Variant") }
        ast::Adt::Enum(enum_) => {
            let list = enum_.variant_list()?;
            let mut arms = vec![];
            for variant in list.variants() {
                let name = variant.name()?;
                let left = make::ext::ident_path("Self");
                let right = make::ext::ident_path(&format!("{}", name));
                let variant_name = make::path_pat(make::path_concat(left, right));

                let target = make::expr_path(make::ext::ident_path("f").into());
                let fmt_string = make::expr_literal(&(format!("\"{}\"", name))).into();
                let args = make::arg_list(vec![target, fmt_string]);
                let macro_name = make::expr_path(make::ext::ident_path("write"));
                let macro_call = make::expr_macro_call(macro_name, args);

                arms.push(make::match_arm(Some(variant_name.into()), None, macro_call.into()));
            }

            let match_target = make::expr_path(make::ext::ident_path("self"));
            let list = make::match_arm_list(arms).indent(ast::edit::IndentLevel(1));
            let match_expr = make::expr_match(match_target, list);

            let body = make::block_expr(None, Some(match_expr));
            let body = body.indent(ast::edit::IndentLevel(1));
            ted::replace(func.body()?.syntax(), body.clone_for_update().syntax());
            Some(())
        }

        ast::Adt::Struct(strukt) => {
            let name = format!("\"{}\"", annotated_name);
            let args = make::arg_list(Some(make::expr_literal(&name).into()));
            let target = make::expr_path(make::ext::ident_path("f"));

            let expr = match strukt.field_list() {
                // => f.debug_struct("Name").finish()
                None => make::expr_method_call(target, make::name_ref("debug_struct"), args),

                // => f.debug_struct("Name").field("foo", &self.foo).finish()
                Some(ast::FieldList::RecordFieldList(field_list)) => {
                    let method = make::name_ref("debug_struct");
                    let mut expr = make::expr_method_call(target, method, args);
                    for field in field_list.fields() {
                        let name = field.name()?;
                        let f_name = make::expr_literal(&(format!("\"{}\"", name))).into();
                        let f_path = make::expr_path(make::ext::ident_path("self"));
                        let f_path = make::expr_ref(f_path, false);
                        let f_path = make::expr_field(f_path, &format!("{}", name)).into();
                        let args = make::arg_list(vec![f_name, f_path]);
                        expr = make::expr_method_call(expr, make::name_ref("field"), args);
                    }
                    expr
                }

                // => f.debug_tuple("Name").field(self.0).finish()
                Some(ast::FieldList::TupleFieldList(field_list)) => {
                    let method = make::name_ref("debug_tuple");
                    let mut expr = make::expr_method_call(target, method, args);
                    for (idx, _) in field_list.fields().enumerate() {
                        let f_path = make::expr_path(make::ext::ident_path("self"));
                        let f_path = make::expr_ref(f_path, false);
                        let f_path = make::expr_field(f_path, &format!("{}", idx)).into();
                        let method = make::name_ref("field");
                        expr = make::expr_method_call(expr, method, make::arg_list(Some(f_path)));
                    }
                    expr
                }
            };

            let method = make::name_ref("finish");
            let expr = make::expr_method_call(expr, method, make::arg_list(None));
            let body = make::block_expr(None, Some(expr)).indent(ast::edit::IndentLevel(1));
            ted::replace(func.body()?.syntax(), body.clone_for_update().syntax());
            Some(())
        }
    }
}

/// Generate a `Debug` impl based on the fields and members of the target type.
fn gen_default_impl(adt: &ast::Adt, func: &ast::Fn) -> Option<()> {
    fn gen_default_call() -> ast::Expr {
        let trait_name = make::ext::ident_path("Default");
        let method_name = make::ext::ident_path("default");
        let fn_name = make::expr_path(make::path_concat(trait_name, method_name));
        make::expr_call(fn_name, make::arg_list(None))
    }
    match adt {
        // `Debug` cannot be derived for unions, so no default impl can be provided.
        ast::Adt::Union(_) => None,
        // Deriving `Debug` for enums is not stable yet.
        ast::Adt::Enum(_) => None,
        ast::Adt::Struct(strukt) => {
            let expr = match strukt.field_list() {
                Some(ast::FieldList::RecordFieldList(field_list)) => {
                    let mut fields = vec![];
                    for field in field_list.fields() {
                        let method_call = gen_default_call();
                        let name_ref = make::name_ref(&field.name()?.to_string());
                        let field = make::record_expr_field(name_ref, Some(method_call));
                        fields.push(field);
                    }
                    let struct_name = make::ext::ident_path("Self");
                    let fields = make::record_expr_field_list(fields);
                    make::record_expr(struct_name, fields).into()
                }
                Some(ast::FieldList::TupleFieldList(field_list)) => {
                    let struct_name = make::expr_path(make::ext::ident_path("Self"));
                    let fields = field_list.fields().map(|_| gen_default_call());
                    make::expr_call(struct_name, make::arg_list(fields))
                }
                None => {
                    let struct_name = make::ext::ident_path("Self");
                    let fields = make::record_expr_field_list(None);
                    make::record_expr(struct_name, fields).into()
                }
            };
            let body = make::block_expr(None, Some(expr)).indent(ast::edit::IndentLevel(1));
            ted::replace(func.body()?.syntax(), body.clone_for_update().syntax());
            Some(())
        }
    }
}