use proc_macro2::{Span, TokenStream};
use quote::{ToTokens, format_ident, quote};
use syn::ext::IdentExt;

use crate::definition::{ArgKind, DecodeMode, FieldAttrs, Struct, StructBuilder};
use crate::definition::{Child, ChildMode, ExtraKind, Field, NewType};

pub(crate) struct Common<'a> {
    pub object: &'a Struct,
    pub ctx: &'a syn::Ident,
    pub span_type: &'a TokenStream,
}

fn child_can_partial(child: &Child) -> bool {
    use ChildMode::*;

    child.option || matches!(child.mode, Bool | Flatten)
}

pub fn emit_struct(s: &Struct, named: bool) -> syn::Result<TokenStream> {
    let s_name = &s.ident;
    let node = syn::Ident::new("node", Span::mixed_site());
    let ctx = syn::Ident::new("ctx", Span::mixed_site());
    let children = syn::Ident::new("children", Span::mixed_site());

    let (_, type_gen, _) = s.generics.split_for_impl();
    let mut common_generics = s.generics.clone();
    let span_ty;
    if let Some(ty) = s.trait_props.span_type.as_ref() {
        span_ty = quote!(#ty);
    } else {
        if common_generics.params.is_empty() {
            common_generics.lt_token = Some(Default::default());
            common_generics.gt_token = Some(Default::default());
        }
        common_generics.params.push(syn::parse2(quote!(S)).unwrap());
        span_ty = quote!(S);
        common_generics
            .make_where_clause()
            .predicates
            .push(syn::parse2(quote!(S: ::knus::traits::ErrorSpan)).unwrap());
    };
    let trait_gen = quote!(<#span_ty>);
    let (impl_gen, _, bounds) = common_generics.split_for_impl();

    let common = Common {
        object: s,
        ctx: &ctx,
        span_type: &span_ty,
    };

    let decode_specials = decode_specials(&common, &node)?;
    let decode_args = decode_args(&common, &node)?;
    let decode_props = decode_props(&common, &node)?;
    let decode_children_normal = decode_children(&common, &children, Some(quote!(#node.span())))?;
    let assign_extra = assign_extra(&common)?;

    let all_fields = s.all_fields();
    let struct_val = if named {
        let assignments = all_fields.iter().map(|f| f.as_assign_pair().unwrap());
        quote!(#s_name { #(#assignments,)* })
    } else {
        let mut fields = all_fields
            .iter()
            .map(|f| (f.as_index().unwrap(), &f.tmp_name))
            .collect::<Vec<_>>();
        fields.sort_by_key(|(idx, _)| *idx);
        assert_eq!(
            fields.iter().map(|(idx, _)| *idx).collect::<Vec<_>>(),
            (0..fields.len()).collect::<Vec<_>>(),
            "all tuple structure fields should be filled in"
        );
        let assignments = fields.iter().map(|(_, v)| v);
        quote! { #s_name(#(#assignments),*) }
    };
    let mut extra_traits = Vec::new();
    let partial_compatible = s.spans.is_empty()
        && s.node_names.is_empty()
        && s.type_names.is_empty()
        && !s.has_arguments
        && (s.properties.iter().all(|x| x.option || x.flatten) && s.var_props.is_none())
        && (s.children.iter().all(child_can_partial) && s.var_children.is_none());
    if partial_compatible {
        let node = syn::Ident::new("node", Span::mixed_site());
        let name = syn::Ident::new("name", Span::mixed_site());
        let value = syn::Ident::new("value", Span::mixed_site());
        let insert_child = insert_child(&common, &node)?;
        let insert_property = insert_property(&common, &name, &value)?;
        extra_traits.push(quote! {
            impl #impl_gen ::knus::traits::DecodePartial #trait_gen
                for #s_name #type_gen
                #bounds
            {
                fn insert_child(&mut self,
                    #node: &::knus::ast::SpannedNode<#span_ty>,
                    #ctx: &mut ::knus::decode::Context<#span_ty>)
                    -> ::std::result::Result<bool, ::knus::errors::DecodeError<#span_ty>>
                {
                    #insert_child
                }
                fn insert_property(&mut self,
                    #name: &::knus::span::Spanned<Box<str>, #span_ty>,
                    #value: &::knus::ast::Value<#span_ty>,
                    #ctx: &mut ::knus::decode::Context<#span_ty>)
                    -> ::std::result::Result<bool, ::knus::errors::DecodeError<#span_ty>>
                {
                    #insert_property
                }
            }
        });
    }
    if !s.has_arguments
        && !s.has_properties
        && s.spans.is_empty()
        && s.node_names.is_empty()
        && s.type_names.is_empty()
    {
        let decode_children = decode_children(&common, &children, None)?;
        extra_traits.push(quote! {
            impl #impl_gen ::knus::traits::DecodeChildren #trait_gen
                for #s_name #type_gen
                #bounds
            {
                fn decode_children(
                    #children: &[::knus::ast::SpannedNode<#span_ty>],
                    #ctx: &mut ::knus::decode::Context<#span_ty>)
                    -> ::std::result::Result<Self, ::knus::errors::DecodeError<#span_ty>>
                {
                    #decode_children
                    #assign_extra
                    Ok(#struct_val)
                }
            }
        });
    }
    Ok(quote! {
        #(#extra_traits)*
        impl #impl_gen ::knus::Decode #trait_gen for #s_name #type_gen
            #bounds
        {
            fn decode_node(#node: &::knus::ast::SpannedNode<#span_ty>,
                           #ctx: &mut ::knus::decode::Context<#span_ty>)
                -> ::std::result::Result<Self, ::knus::errors::DecodeError<#span_ty>>
            {
                #decode_specials
                #decode_args
                #decode_props
                let #children = #node.children.as_ref()
                    .map(|lst| &lst[..]).unwrap_or(&[]);
                #decode_children_normal
                #assign_extra
                Ok(#struct_val)
            }
        }
    })
}
pub fn emit_new_type(s: &NewType) -> syn::Result<TokenStream> {
    let s_name = &s.ident;
    let node = syn::Ident::new("node", Span::mixed_site());
    let ctx = syn::Ident::new("ctx", Span::mixed_site());
    Ok(quote! {
        impl<S: ::knus::traits::ErrorSpan>
            ::knus::Decode<S> for #s_name
        {
            fn decode_node(#node: &::knus::ast::SpannedNode<S>,
                           #ctx: &mut ::knus::decode::Context<S>)
                -> ::std::result::Result<Self, ::knus::errors::DecodeError<S>>
            {
                if #node.arguments.len() > 0 ||
                    #node.properties.len() > 0 ||
                    #node.children.is_some()
                {
                    ::knus::Decode::decode_node(#node, #ctx)
                        .map(Some)
                        .map(#s_name)
                } else {
                    Ok(#s_name(None))
                }
            }
        }
    })
}

pub(crate) fn decode_enum_item(
    s: &Common,
    s_name: impl ToTokens,
    node: &syn::Ident,
    named: bool,
) -> syn::Result<TokenStream> {
    let children = syn::Ident::new("children", Span::mixed_site());
    let decode_args = decode_args(s, node)?;
    let decode_props = decode_props(s, node)?;
    let decode_children = decode_children(s, &children, Some(quote!(#node.span())))?;
    let assign_extra = assign_extra(s)?;
    let all_fields = s.object.all_fields();
    let struct_val = if named {
        let assignments = all_fields.iter().map(|f| f.as_assign_pair().unwrap());
        quote!(#s_name { #(#assignments,)* })
    } else {
        let mut fields = all_fields
            .iter()
            .map(|f| (f.as_index().unwrap(), &f.tmp_name))
            .collect::<Vec<_>>();
        fields.sort_by_key(|(idx, _)| *idx);
        assert_eq!(
            fields.iter().map(|(idx, _)| *idx).collect::<Vec<_>>(),
            (0..fields.len()).collect::<Vec<_>>(),
            "all tuple structure fields should be filled in"
        );
        let assignments = fields.iter().map(|(_, v)| v);
        quote! { #s_name(#(#assignments),*) }
    };
    Ok(quote! {
        #decode_args
        #decode_props
        let #children = #node.children.as_ref()
            .map(|lst| &lst[..]).unwrap_or(&[]);
        #decode_children
        #assign_extra
        Ok(#struct_val)
    })
}

fn decode_value(
    val: &syn::Ident,
    ctx: &syn::Ident,
    mode: &DecodeMode,
    optional: bool,
) -> syn::Result<TokenStream> {
    match mode {
        DecodeMode::Normal => Ok(quote! {
            ::knus::traits::DecodeScalar::decode(#val, #ctx)
        }),
        DecodeMode::Str if optional => {
            Ok(quote![{
                if let Some(typ) = &#val.type_name {
                    #ctx.emit_error(::knus::errors::DecodeError::TypeName {
                        span: typ.span().clone(),
                        found: Some((**typ).clone()),
                        expected: ::knus::errors::ExpectedType::no_type(),
                        rust_type: "str", // TODO(tailhook) show field type
                    });
                }
                match *#val.literal {
                    ::knus::ast::Literal::String(ref s) => {
                        ::std::str::FromStr::from_str(s).map_err(|e| {
                            ::knus::errors::DecodeError::conversion(
                                &#val.literal, e)
                        })
                        .map(Some)
                    }
                    ::knus::ast::Literal::Null => Ok(None),
                    _ => {
                        #ctx.emit_error(
                            ::knus::errors::DecodeError::scalar_kind(
                                ::knus::decode::Kind::String,
                                &#val.literal,
                            )
                        );
                        Ok(None)
                    }
                }
            }])
        }
        DecodeMode::Str => {
            Ok(quote![{
                if let Some(typ) = &#val.type_name {
                    #ctx.emit_error(::knus::errors::DecodeError::TypeName {
                        span: typ.span().clone(),
                        found: Some((**typ).clone()),
                        expected: ::knus::errors::ExpectedType::no_type(),
                        rust_type: "str", // TODO(tailhook) show field type
                    });
                }
                match *#val.literal {
                    ::knus::ast::Literal::String(ref s) => {
                        ::std::str::FromStr::from_str(s).map_err(|e| {
                            ::knus::errors::DecodeError::conversion(
                                &#val.literal, e)
                        })
                    }
                    _ => Err(::knus::errors::DecodeError::scalar_kind(
                        ::knus::decode::Kind::String,
                        &#val.literal,
                    )),
                }
            }])
        }
        DecodeMode::Bytes if optional => Ok(quote! {
            if matches!(&*#val.literal, ::knus::ast::Literal::Null) {
                Ok(None)
            } else {
                match ::knus::decode::bytes(#val, #ctx).try_into() {
                    Ok(v) => Ok(Some(v)),
                    Err(e) => {
                        #ctx.emit_error(
                            ::knus::errors::DecodeError::conversion(
                                &#val.literal, e));
                        Ok(None)
                    }
                }
            }
        }),
        DecodeMode::Bytes => Ok(quote! {
            ::knus::decode::bytes(#val, #ctx).try_into()
            .map_err(|e| ::knus::errors::DecodeError::conversion(
                    &#val.literal, e))
        }),
    }
}

fn decode_specials(s: &Common, node: &syn::Ident) -> syn::Result<TokenStream> {
    let ctx = s.ctx;
    let spans = s.object.spans.iter().flat_map(|span| {
        let fld = &span.field.tmp_name;
        quote! {
            let #fld = ::knus::traits::DecodeSpan::decode_span(
                #node.span(),
                #ctx,
            );
        }
    });
    let node_names = s.object.node_names.iter().flat_map(|node_name| {
        let fld = &node_name.field.tmp_name;
        quote! {
            let #fld = #node.node_name.parse()
                .map_err(|e| {
                    ::knus::errors::DecodeError::conversion(
                        &#node.node_name, e)
                })?;
        }
    });
    let type_names = s.object.type_names.iter().flat_map(|type_name| {
        let fld = &type_name.field.tmp_name;
        if type_name.option {
            quote! {
                let #fld = #node.type_name.as_ref().map(|tn| {
                    tn.as_str()
                        .parse()
                        .map_err(|e| {
                            ::knus::errors::DecodeError::conversion(tn, e)
                        })
                }).transpose()?;
            }
        } else {
            quote! {
                let #fld = if let Some(tn) = #node.type_name.as_ref() {
                    tn.as_str()
                        .parse()
                        .map_err(|e| {
                            ::knus::errors::DecodeError::conversion(tn, e)
                        })?
                } else {
                    return Err(::knus::errors::DecodeError::missing(
                        #node, "type name required"));
                };
            }
        }
    });
    let validate_type = if s.object.type_names.is_empty() {
        Some(quote! {
            if let Some(type_name) = &#node.type_name {
                #ctx.emit_error(::knus::errors::DecodeError::unexpected(
                            type_name, "type name",
                            "no type name expected for this node"));
            }
        })
    } else {
        None
    };
    Ok(quote! {
        #(#spans)*
        #(#node_names)*
        #(#type_names)*
        #validate_type
    })
}

fn decode_args(s: &Common, node: &syn::Ident) -> syn::Result<TokenStream> {
    let ctx = s.ctx;
    let mut decoder = Vec::new();
    let iter_args = syn::Ident::new("iter_args", Span::mixed_site());
    decoder.push(quote! {
        let mut #iter_args = #node.arguments.iter();
    });
    for arg in &s.object.arguments {
        let fld = &arg.field.tmp_name;
        let val = syn::Ident::new("val", Span::mixed_site());
        let decode_value = decode_value(&val, ctx, &arg.decode, arg.option)?;
        match (&arg.default, &arg.kind) {
            (None, ArgKind::Value { option: true }) => {
                decoder.push(quote! {
                    let #fld = #iter_args.next().map(|#val| {
                        #decode_value
                    }).transpose()?.and_then(|v| v);
                });
            }
            (None, ArgKind::Value { option: false }) => {
                let error = if arg.field.is_indexed() {
                    "additional argument is required".into()
                } else {
                    format!("additional argument `{}` is required", fld.unraw())
                };
                decoder.push(quote! {
                    let #val =
                        #iter_args.next().ok_or_else(|| {
                            ::knus::errors::DecodeError::missing(
                                #node, #error)
                        })?;
                    let #fld = #decode_value?;
                });
            }
            (Some(default_value), ArgKind::Value { .. }) => {
                let default = if let Some(expr) = default_value {
                    quote!(#expr)
                } else {
                    quote!(::std::default::Default::default())
                };
                decoder.push(quote! {
                    let #fld = #iter_args.next().map(|#val| {
                        #decode_value
                    }).transpose()?.unwrap_or_else(|| {
                        #default
                    });
                });
            }
        }
    }
    if let Some(var_args) = &s.object.var_args {
        let fld = &var_args.field.tmp_name;
        let val = syn::Ident::new("val", Span::mixed_site());
        let decode_value = decode_value(&val, ctx, &var_args.decode, false)?;
        decoder.push(quote! {
            let #fld = #iter_args.map(|#val| {
                #decode_value
            }).collect::<std::result::Result<_, _>>()?;
        });
    } else {
        decoder.push(quote! {
            if let Some(val) = #iter_args.next() {
                return Err(::knus::errors::DecodeError::unexpected(
                        &val.literal, "argument",
                        "unexpected argument"));
            }
        });
    }
    Ok(quote! { #(#decoder)* })
}

fn decode_props(s: &Common, node: &syn::Ident) -> syn::Result<TokenStream> {
    let mut declare_empty = Vec::new();
    let mut match_branches = Vec::new();
    let mut postprocess = Vec::new();

    let ctx = s.ctx;
    let val = syn::Ident::new("val", Span::mixed_site());
    let name = syn::Ident::new("name", Span::mixed_site());
    let name_str = syn::Ident::new("name_str", Span::mixed_site());

    for prop in &s.object.properties {
        let fld = &prop.field.tmp_name;
        let prop_name = &prop.name;
        let seen_name = format_ident!("seen_{}", fld, span = Span::mixed_site());
        if prop.flatten {
            declare_empty.push(quote! {
                let mut #fld = ::std::default::Default::default();
            });
            match_branches.push(quote! {
                _ if ::knus::traits::DecodePartial::
                    insert_property(&mut #fld, #name, #val, #ctx)?
                => {}
            });
        } else {
            let decode_value = decode_value(&val, ctx, &prop.decode, prop.option)?;
            declare_empty.push(quote! {
                let mut #fld = None;
                let mut #seen_name = false;
            });
            if prop.option {
                match_branches.push(quote! {
                    #prop_name => {
                        #seen_name = true;
                        #fld = #decode_value?;
                    }
                });
            } else {
                match_branches.push(quote! {
                    #prop_name => {
                        #fld = Some(#decode_value?);
                    }
                });
            }
            let req_msg = format!("property `{}` is required", prop_name);
            if let Some(value) = &prop.default {
                let default = if let Some(expr) = value {
                    quote!(#expr)
                } else {
                    quote!(::std::default::Default::default())
                };
                if prop.option {
                    postprocess.push(quote! {
                        if !#seen_name {
                            #fld = #default;
                        };
                    });
                } else {
                    postprocess.push(quote! {
                        let #fld = #fld.unwrap_or_else(|| #default);
                    });
                }
            } else if !prop.option {
                postprocess.push(quote! {
                    let #fld = #fld.ok_or_else(|| {
                        ::knus::errors::DecodeError::missing(
                            #node, #req_msg)
                    })?;
                });
            }
        }
    }
    if let Some(var_props) = &s.object.var_props {
        let fld = &var_props.field.tmp_name;
        let decode_value = decode_value(&val, ctx, &var_props.decode, false)?;
        declare_empty.push(quote! {
            let mut #fld = Vec::new();
        });
        match_branches.push(quote! {
            #name_str => {
                let converted_name = #name_str.parse()
                    .map_err(|e| {
                        ::knus::errors::DecodeError::conversion(#name, e)
                    })?;
                #fld.push((
                    converted_name,
                    #decode_value?,
                ));
            }
        });
        postprocess.push(quote! {
            let #fld = #fld.into_iter().collect();
        });
    } else {
        match_branches.push(quote! {
            #name_str => {
                return Err(::knus::errors::DecodeError::unexpected(
                    #name, "property",
                    format!("unexpected property `{}`",
                            #name_str.escape_default())));
            }
        });
    };
    Ok(quote! {
        #(#declare_empty)*
        for (#name, #val) in #node.properties.iter() {
            match &***#name {
                #(#match_branches)*
            }
        }
        #(#postprocess)*
    })
}

fn unwrap_fn(
    parent: &Common,
    func: &syn::Ident,
    name: &syn::Ident,
    attrs: &FieldAttrs,
) -> syn::Result<TokenStream> {
    let ctx = parent.ctx;
    let span_ty = parent.span_type;
    let mut bld = StructBuilder::new(
        format_ident!("Wrap_{}", name, span = Span::mixed_site()),
        parent.object.trait_props.clone(),
        parent.object.generics.clone(),
    );
    bld.add_field(Field::new_named(name), false, false, attrs)?;
    let object = bld.build();
    let common = Common {
        object: &object,
        ctx: parent.ctx,
        span_type: parent.span_type,
    };

    let node = syn::Ident::new("node", Span::mixed_site());
    let children = syn::Ident::new("children", Span::mixed_site());
    let decode_args = decode_args(&common, &node)?;
    let decode_props = decode_props(&common, &node)?;
    let decode_children = decode_children(&common, &children, Some(quote!(#node.span())))?;
    Ok(quote! {
        let mut #func = |#node: &::knus::ast::SpannedNode<#span_ty>,
                         #ctx: &mut ::knus::decode::Context<#span_ty>|
        {
            #decode_args
            #decode_props
            let #children = #node.children.as_ref()
                .map(|lst| &lst[..]).unwrap_or(&[]);
            #decode_children

            Ok(#name)
        };
    })
}

fn decode_node(
    common: &Common,
    child_def: &Child,
    in_partial: bool,
    child: &syn::Ident,
) -> syn::Result<TokenStream> {
    let ctx = common.ctx;
    let fld = &child_def.field.tmp_name;
    let dest = if in_partial {
        child_def.field.as_token_stream()
    } else {
        quote!(#fld)
    };
    let (init, func) = if let Some(unwrap) = &child_def.unwrap {
        let func = format_ident!("unwrap_{}", fld, span = Span::mixed_site());
        let unwrap_fn = unwrap_fn(common, &func, fld, unwrap)?;
        (unwrap_fn, quote!(#func))
    } else {
        (quote!(), quote!(::knus::Decode::decode_node))
    };
    let value = syn::Ident::new("value", Span::mixed_site());
    let assign = if matches!(child_def.mode, ChildMode::Multi) {
        quote!(#dest.push(#value))
    } else {
        quote!(#dest = Some(#value))
    };
    if in_partial {
        Ok(quote! {
            {
                #init
                let #value = #func(#child, #ctx)?;
                #assign;
                Ok(true)
            }
        })
    } else {
        Ok(quote! {
            {
                #init
                match #func(#child, #ctx) {
                    Ok(#value) => {
                        #assign;
                        None
                    }
                    Err(e) => Some(Err(e)),
                }
            }
        })
    }
}

fn insert_child(s: &Common, node: &syn::Ident) -> syn::Result<TokenStream> {
    let ctx = s.ctx;
    let mut match_branches = Vec::with_capacity(s.object.children.len());
    for child_def in &s.object.children {
        let dest = &child_def.field.as_token_stream();
        let child_name = &child_def.name;
        if matches!(child_def.mode, ChildMode::Flatten) {
            match_branches.push(quote! {
                _ if ::knus::traits::DecodePartial
                    ::insert_child(&mut #dest, #node, #ctx)?
                => Ok(true),
            })
        } else if matches!(child_def.mode, ChildMode::Bool) {
            let dup_err = format!(
                "duplicate node `{}`, single node expected",
                child_name.escape_default()
            );
            match_branches.push(quote! {
                #child_name => {
                    ::knus::decode::check_flag_node(#node, #ctx);
                    if #dest {
                        #ctx.emit_error(
                            ::knus::errors::DecodeError::unexpected(
                                &#node.node_name, "node", #dup_err));
                    } else {
                        #dest = true;
                    }
                    Ok(true)
                }
            });
        } else {
            let dup_err = format!(
                "duplicate node `{}`, single node expected",
                child_name.escape_default()
            );
            let decode = decode_node(s, child_def, true, node)?;
            match_branches.push(quote! {
                #child_name => {
                    if #dest.is_some() {
                        #ctx.emit_error(
                            ::knus::errors::DecodeError::unexpected(
                                &#node.node_name, "node", #dup_err));
                    }
                    #decode
                }
            });
        }
    }
    Ok(quote! {
        match &**#node.node_name {
            #(#match_branches)*
            _ => Ok(false),
        }
    })
}

fn insert_property(s: &Common, name: &syn::Ident, value: &syn::Ident) -> syn::Result<TokenStream> {
    let ctx = s.ctx;
    let mut match_branches = Vec::with_capacity(s.object.children.len());
    for prop in &s.object.properties {
        let dest = &prop.field.as_token_stream();
        let prop_name = &prop.name;
        if prop.flatten {
            match_branches.push(quote! {
                _ if ::knus::traits::DecodePartial
                    ::insert_property(&mut #dest, #name, #value, #ctx)?
                => Ok(true),
            });
        } else {
            let decode_value = decode_value(value, ctx, &prop.decode, prop.option)?;
            if prop.option {
                match_branches.push(quote! {
                    #prop_name => {
                        #dest = #decode_value?;
                        Ok(true)
                    }
                });
            } else {
                match_branches.push(quote! {
                    #prop_name => {
                        #dest = Some(#decode_value?);
                        Ok(true)
                    }
                });
            }
        }
    }
    Ok(quote! {
        match &***#name {
            #(#match_branches)*
            _ => Ok(false),
        }
    })
}

fn decode_children(
    s: &Common,
    children: &syn::Ident,
    err_span: Option<TokenStream>,
) -> syn::Result<TokenStream> {
    let mut declare_empty = Vec::new();
    let mut match_branches = Vec::new();
    let mut postprocess = Vec::new();

    let ctx = s.ctx;
    let child = syn::Ident::new("child", Span::mixed_site());
    let name_str = syn::Ident::new("name_str", Span::mixed_site());
    for child_def in &s.object.children {
        let fld = &child_def.field.tmp_name;
        let child_name = &child_def.name;
        match child_def.mode {
            ChildMode::Flatten => {
                declare_empty.push(quote! {
                    let mut #fld = ::std::default::Default::default();
                });
                match_branches.push(quote! {
                    _ if (
                        match ::knus::traits::DecodePartial
                            ::insert_child(&mut #fld, #child, #ctx)
                        {
                            Ok(true) => return None,
                            Ok(false) => false,
                            Err(e) => return Some(Err(e)),
                        }
                    ) => None,
                })
            }
            ChildMode::Multi => {
                declare_empty.push(quote! {
                    let mut #fld = Vec::new();
                });
                let decode = decode_node(s, child_def, false, &child)?;
                match_branches.push(quote! {
                    #child_name => #decode,
                });
                if let Some(default_value) = &child_def.default {
                    let default = if let Some(expr) = default_value {
                        quote!(#expr)
                    } else {
                        quote!(::std::default::Default::default())
                    };
                    if child_def.option {
                        postprocess.push(quote! {
                            let #fld = if #fld.is_empty() {
                                #default
                            } else {
                                Some(#fld.into_iter().collect())
                            };
                        });
                    } else {
                        postprocess.push(quote! {
                            let #fld = if #fld.is_empty() {
                                #default
                            } else {
                                #fld.into_iter().collect()
                            };
                        });
                    }
                } else if child_def.option {
                    postprocess.push(quote! {
                        let #fld = if #fld.is_empty() {
                            None
                        } else {
                            Some(#fld.into_iter().collect())
                        };
                    });
                } else {
                    postprocess.push(quote! {
                        let #fld = #fld.into_iter().collect();
                    });
                }
            }
            ChildMode::Normal => {
                declare_empty.push(quote! {
                    let mut #fld = None;
                });
                let dup_err = format!(
                    "duplicate node `{}`, single node expected",
                    child_name.escape_default()
                );
                let decode = decode_node(s, child_def, false, &child)?;
                match_branches.push(quote! {
                    #child_name => {
                        if #fld.is_some() {
                            Some(Err(
                                ::knus::errors::DecodeError::unexpected(
                                &#child.node_name, "node", #dup_err)))
                        } else {
                            #decode
                        }
                    }
                });
                let req_msg = format!("child node `{}` is required", child_name);
                if let Some(default_value) = &child_def.default {
                    let default = if let Some(expr) = default_value {
                        quote!(#expr)
                    } else {
                        quote!(::std::default::Default::default())
                    };
                    postprocess.push(quote! {
                        let #fld = #fld.unwrap_or_else(|| #default);
                    });
                } else if !child_def.option {
                    if let Some(span) = &err_span {
                        postprocess.push(quote! {
                            let #fld = #fld.ok_or_else(|| {
                                ::knus::errors::DecodeError::Missing {
                                    span: #span.clone(),
                                    message: #req_msg.into(),
                                }
                            })?;
                        });
                    } else {
                        postprocess.push(quote! {
                            let #fld = #fld.ok_or_else(|| {
                                ::knus::errors::DecodeError::MissingNode {
                                    message: #req_msg.into(),
                                }
                            })?;
                        });
                    }
                }
            }
            ChildMode::Bool => {
                let dup_err = format!(
                    "duplicate node `{}`, single node expected",
                    child_name.escape_default()
                );
                declare_empty.push(quote! {
                    let mut #fld = false;
                });
                match_branches.push(quote! {
                    #child_name => {
                        ::knus::decode::check_flag_node(#child, #ctx);
                        if #fld {
                            #ctx.emit_error(
                                ::knus::errors::DecodeError::unexpected(
                                    &#child.node_name, "node", #dup_err));
                        } else {
                            #fld = true;
                        }
                        None
                    }
                });
            }
        }
    }
    if let Some(var_children) = &s.object.var_children {
        let fld = &var_children.field.tmp_name;

        let (init, func) = if let Some(unwrap) = &var_children.unwrap {
            let func = format_ident!("unwrap_{}", fld, span = Span::mixed_site());
            let unwrap_fn = unwrap_fn(s, &func, fld, unwrap)?;
            (unwrap_fn, quote!(#func))
        } else {
            (quote!(), quote!(::knus::Decode::decode_node))
        };

        match_branches.push(quote! {
            _ => {
                #init
                match #func(#child, #ctx) {
                    Ok(#child) => Some(Ok(#child)),
                    Err(e) => Some(Err(e)),
                }
            }
        });
        Ok(quote! {
            #(#declare_empty)*
            let #fld = #children.iter().flat_map(|#child| {
                match &**#child.node_name {
                    #(#match_branches)*
                }
            }).collect::<::std::result::Result<_, ::knus::errors::DecodeError<_>>>()?;
            #(#postprocess)*
        })
    } else {
        match_branches.push(quote! {
            #name_str => {
                #ctx.emit_error(::knus::errors::DecodeError::unexpected(
                    #child, "node",
                    format!("unexpected node `{}`",
                            #name_str.escape_default())));
                None
            }
        });

        Ok(quote! {
            #(#declare_empty)*
            #children.iter().flat_map(|#child| {
                match &**#child.node_name {
                    #(#match_branches)*
                }
            }).collect::<::std::result::Result<(), ::knus::errors::DecodeError<_>>>()?;
            #(#postprocess)*
        })
    }
}

fn assign_extra(s: &Common) -> syn::Result<TokenStream> {
    let items = s.object.extra_fields.iter().map(|fld| match fld.kind {
        ExtraKind::Auto => {
            let name = &fld.field.tmp_name;
            quote!(let #name = ::std::default::Default::default();)
        }
    });
    Ok(quote!(#(#items)*))
}
