use crate::*;

#[derive(Debug, Clone)]
pub(crate) struct ViewDef(pub(crate) ViewMeta, pub(crate) ViewImpl);

#[derive(Debug, Clone)]
pub(crate) struct ViewMeta { pub(crate) output: Ident }

#[derive(Debug, Clone)]
pub(crate) struct ViewImpl {
    block: ItemImpl,
    exposed: BTreeMap<String, Ident>,
}

impl Parse for ViewMeta {
    fn parse (input: ParseStream) -> Result<Self> {
        Ok(Self {
            output: input.parse::<Ident>()?,
        })
    }
}

impl Parse for ViewImpl {
    fn parse (input: ParseStream) -> Result<Self> {
        let block = input.parse::<ItemImpl>()?;
        let mut exposed: BTreeMap<String, Ident> = Default::default();
        for item in block.items.iter() {
            if let ImplItem::Fn(ImplItemFn { sig: Signature { ident, .. }, .. }) = item {
                let key = format!(":{}", AsKebabCase(format!("{}", &ident)));
                if exposed.contains_key(&key) {
                    return Err(input.error(format!("already defined: {ident}")));
                }
                exposed.insert(key, ident.clone());
            }
        }
        Ok(Self { block, exposed })
    }
}

impl ToTokens for ViewDef {
    fn to_tokens (&self, out: &mut TokenStream2) {
        let Self(_, ViewImpl { block, .. }) = self;
        let generated = self.generated();
        write_quote_to(out, quote! {
            #block
            #generated
        })
    }
}

impl ViewDef {
    fn generated (&self) -> impl ToTokens {
        let Self(ViewMeta { output }, ViewImpl { block, .. }) = self;
        let self_ty  = &block.self_ty;
        let builtins = self.builtins();
        let exposed  = self.exposed();
        quote! {
            /// Generated by [tengri_proc].
            ///
            /// Makes [#self_ty] able to construct the [Render]able
            /// which might correspond to a given [TokenStream],
            /// while taking [#self_ty]'s state into consideration.
            impl<'state> ::tengri::dsl::DslInto<
                Box<dyn ::tengri::output::Render<#output> + 'state>
            > for #self_ty {
                fn try_dsl_into (&self, dsl: &impl ::tengri::dsl::Dsl)
                    -> Perhaps<Box<dyn ::tengri::output::Render<#output> + 'state>>
                {
                    Ok(match dsl.val() { #builtins #exposed _ => return Ok(None) })
                }
            }
        }
    }
    /// Expressions are handled by built-in functions
    /// that operate over constants and symbols.
    fn builtins (&self) -> impl ToTokens {
        let Self(ViewMeta { output }, ViewImpl { .. }) = self;
        let builtins = builtins_with_boxes_output(quote! { #output }).map(|builtin|quote! {
            ::tengri::dsl::Val::Exp(_, expr) => return Ok(Some(
                #builtin::from_dsl(self, expr, ||Box::new("failed to load builtin".into()))?
                    .boxed()
            )),
        });
        quote! { #(#builtins)* }
    }
    /// Symbols are handled by user-taked functions that take no parameters but `&self`.
    fn exposed (&self) -> impl ToTokens {
        let Self(ViewMeta { .. }, ViewImpl { exposed, .. }) = self;
        let exposed = exposed.iter().map(|(key, value)|write_quote(quote! {
            #key => return Ok(Some(self.#value().boxed())),
        }));
        quote! { ::tengri::dsl::Val::Sym(key) => match key.as_ref() { #(#exposed)* } }
    }
}

fn _builtins_with_holes () -> impl Iterator<Item=TokenStream2> {
    builtins_with(quote! { _ }, quote! { _ })
}

fn _builtins_with_boxes () -> impl Iterator<Item=TokenStream2> {
    builtins_with(quote! { _ }, quote! { Box<dyn Render<_>> })
}

fn builtins_with_boxes_output (o: TokenStream2) -> impl Iterator<Item=TokenStream2> {
    builtins_with(quote! { _ }, quote! { Box<dyn Render<#o>> })
}

fn builtins_with (n: TokenStream2, c: TokenStream2) -> impl Iterator<Item=TokenStream2> {
    [
        quote! { When::<       #c    > },
        quote! { Either::<     #c, #c> },
        quote! { Align::<      #c    > },
        quote! { Bsp::<        #c, #c> },
        quote! { Fill::<       #c    > },
        quote! { Fixed::<#n,   #c    > },
        quote! { Min::<#n,     #c    > },
        quote! { Max::<#n,     #c    > },
        quote! { Shrink::<#n,  #c    > },
        quote! { Expand::<#n,  #c    > },
        quote! { Push::<#n,    #c    > },
        quote! { Pull::<#n,    #c    > },
        quote! { Margin::<#n,  #c    > },
        quote! { Padding::<#n, #c    > },
    ].into_iter()
}