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//! # derive_into_owned
//!
//! This crate supports deriving two different methods (not traits):
//!
//!  * `IntoOwned`
//!  * `Borrowed`
//!
//! These were first created to help out with types generated by [`quick_protobuf`] which generates
//! structs with [`Cow`] fields. It is entirely possible that this crate is not needed at all and
//! that there exists better alternatives to what this crate aims to support.
//!
//! ## Definitions, naming
//!
//! "Cow-alike" is used to mean a type that:
//!
//!  * has a lifetime specifier, like `Foo<'a>`
//!  * has an implementation for `fn into_owned(self) -> Foo<'static>`
//!
//! This is a bit of a bad name as [`Cow`] itself has a different kind of `into_owned` which
//! returns the owned version of the generic type parameter. I am open to suggestions for both
//! "Cow-alike" and `into_owned`.
//!
//! ## `IntoOwned`
//!
//! `#[derive(IntoOwned)]` implements a method `fn into_owned(self) -> Foo<'static>` for type
//! `Foo<'a>` which contains [`Cow`] or "Cow-alike" fields. The method returns a version with
//! `'static` lifetime which means the value owns all of it's data. This is useful if you are
//! for example, working with [`tokio-rs`] which currently requires types to be `'static`.
//!
//! ## `Borrowed`
//!
//! `#[derive(Borrowed)]` implements a method `fn borrowed<'b>(&'b self) -> Foo<'b>` for type
//! `Foo<'a>`. This is useful in case you need to transform the value into another type using
//! std conversions like [`From`], but you don't want to clone the data in the process. Note that
//! the all the fields that are not [`Cow`] or "Cow-alike" are just cloned, and new vectors are
//! collected, so this yields savings only when you manage to save big chunks of memory.
//!
//! ## Limitations
//!
//! Currently only the types I needed are supported and this might be a rather limited set of
//! functionality. If you find that this does not work in your case please file an issue at [project
//! repository](https://github.com/koivunej/derive-into-owned/issues).
//!
//! [`quick_protobuf`]: https://github.com/tafia/quick-protobuf/
//! [`tokio-rs`]: https://tokio.rs
//! [`Cow`]: https://doc.rust-lang.org/std/borrow/enum.Cow.html
//! [`From`]: https://doc.rust-lang.org/std/convert/trait.From.html

extern crate proc_macro;
extern crate syn;
#[macro_use]
extern crate quote;

use proc_macro::TokenStream;

#[proc_macro_derive(IntoOwned)]
#[doc(hidden)]
pub fn into_owned(input: TokenStream) -> TokenStream {
    let source = input.to_string();

    let ast = syn::parse_derive_input(&source).unwrap();

    let expanded = impl_with_generator(&ast, IntoOwnedGen);

    expanded.parse().unwrap()
}

#[proc_macro_derive(Borrowed)]
#[doc(hidden)]
pub fn borrowed(input: TokenStream) -> TokenStream {
    let source = input.to_string();

    let ast = syn::parse_derive_input(&source).unwrap();

    let expanded = impl_with_generator(&ast, BorrowedGen);

    expanded.parse().unwrap()
}

fn impl_with_generator<G: BodyGenerator>(ast: &syn::DeriveInput, gen: G) -> quote::Tokens {
    // this is based heavily on https://github.com/asajeffrey/deep-clone/blob/master/deep-clone-derive/lib.rs
    let name = &ast.ident;

    let borrowed_params = gen.quote_borrowed_params(ast);
    let borrowed = if borrowed_params.is_empty() {
        quote! { }
    } else {
        quote! { < #(#borrowed_params),* > }
    };

    let params = gen.quote_type_params(ast);
    let params = if params.is_empty() {
        quote! {}
    } else {
        quote! { < #(#params),* > }
    };


    let owned_params = gen.quote_rhs_params(ast);
    let owned = if owned_params.is_empty() {
        quote! { }
    } else {
        quote! { < #(#owned_params),* > }
    };

    let body = match ast.body {
        syn::Body::Struct(ref variant) => {
            let inner = gen.visit_struct(variant);
            quote! { #name #inner }
        },
        syn::Body::Enum(ref body) => {
            let cases = body.iter()
                .map(|case| {
                    let unqualified_ident = &case.ident;
                    let ident = quote! { #name::#unqualified_ident };

                    gen.visit_enum_data(ident, &case.data)
                })
                .collect::<Vec<_>>();
            quote! { match self { #(#cases),* } }
        },
    };

    gen.combine_impl(borrowed, name, params, owned, body)
}

/// Probably not the best abstraction
trait BodyGenerator {

    fn quote_borrowed_params(&self, ast: &syn::DeriveInput) -> Vec<quote::Tokens> {
        let borrowed_lifetime_params = ast.generics.lifetimes.iter().map(|alpha| quote! { #alpha });
        let borrowed_type_params = ast.generics.ty_params.iter().map(|ty| quote! { #ty });
        borrowed_lifetime_params.chain(borrowed_type_params).collect::<Vec<_>>()
    }

    fn quote_type_params(&self, ast: &syn::DeriveInput) -> Vec<quote::Tokens> {
        ast.generics.lifetimes.iter().map(|alpha| quote! { #alpha }).chain(
            ast.generics.ty_params.iter().map(|ty| { let ref ident = &ty.ident; quote! { #ident } })
        ).collect::<Vec<_>>()
    }

    fn quote_rhs_params(&self, ast: &syn::DeriveInput) -> Vec<quote::Tokens> {
        let owned_lifetime_params = ast.generics.lifetimes.iter().map(|_| quote! { 'static });
        let owned_type_params = ast.generics.ty_params.iter().map(|ty| { let ref ident = &ty.ident; quote! { #ident } });
        owned_lifetime_params.chain(owned_type_params).collect::<Vec<_>>()
    }

    fn visit_struct(&self, data: &syn::VariantData) -> quote::Tokens;
    fn visit_enum_data(&self, variant: quote::Tokens, data: &syn::VariantData) -> quote::Tokens;
    fn combine_impl(&self, borrows: quote::Tokens, name: &syn::Ident, rhs_params: quote::Tokens, owned: quote::Tokens, body: quote::Tokens) -> quote::Tokens;
}

struct IntoOwnedGen;

impl BodyGenerator for IntoOwnedGen {

    fn visit_struct(&self, data: &syn::VariantData) -> quote::Tokens {
        match *data {
            syn::VariantData::Struct(ref body) => {
                let fields = body.iter()
                    .map(|field| {
                        let ident = field.ident.as_ref().unwrap();
                        let field_ref = quote! { self.#ident };
                        let code = FieldKind::resolve(field).move_or_clone_field(&field_ref);
                        quote! { #ident: #code }
                    })
                    .collect::<Vec<_>>();
                quote! { { #(#fields),* } }
            },
            syn::VariantData::Tuple(ref body) => {
                let fields = body.iter()
                    .enumerate()
                    .map(|(index, field)| {
                        let index = syn::Ident::from(index);
                        let index = quote! { self.#index };
                        FieldKind::resolve(field).move_or_clone_field(&index)
                    })
                    .collect::<Vec<_>>();
                quote! { ( #(#fields),* ) }
            },
            syn::VariantData::Unit => {
                quote! {}
            }
        }
    }

    fn visit_enum_data(&self, ident: quote::Tokens, data: &syn::VariantData) -> quote::Tokens {
        match *data {
            syn::VariantData::Struct(ref body) => {
                let idents = body.iter()
                    .map(|field| field.ident.as_ref().unwrap())
                    .collect::<Vec<_>>();
                let cloned = body.iter()
                    .map(|field| {
                        let ref ident = field.ident.as_ref().unwrap();
                        let ident = quote! { #ident };
                        let code = FieldKind::resolve(field).move_or_clone_field(&ident);
                        quote! { #ident: #code }
                    })
                    .collect::<Vec<_>>();
                quote! { #ident { #(#idents),* } => #ident { #(#cloned),* } }
            },
            syn::VariantData::Tuple(ref body) => {
                let idents = (0..body.len())
                    .map(|index| syn::Ident::from(format!("x{}", index)))
                    .collect::<Vec<_>>();
                let cloned = idents.iter().zip(body.iter())
                    .map(|(ident, field)| {
                        let ident = quote! { #ident };
                        FieldKind::resolve(field).move_or_clone_field(&ident)
                    })
                    .collect::<Vec<_>>();
                quote! { #ident ( #(#idents),* ) => #ident ( #(#cloned),* ) }
            },
            syn::VariantData::Unit => {
                quote! { #ident => #ident }
            },
        }
    }

    fn combine_impl(&self, borrowed: quote::Tokens, name: &syn::Ident, params: quote::Tokens, owned: quote::Tokens, body: quote::Tokens) -> quote::Tokens {
        quote! {
            impl #borrowed #name #params {
                /// Returns a version of `self` with all fields converted to owning versions.
                pub fn into_owned(self) -> #name #owned { #body }
            }
        }
    }
}

struct BorrowedGen;

impl BodyGenerator for BorrowedGen {

    fn quote_rhs_params(&self, ast: &syn::DeriveInput) -> Vec<quote::Tokens> {
        let owned_lifetime_params = ast.generics.lifetimes.iter().map(|_| quote! { '__borrowedgen });
        let owned_type_params = ast.generics.ty_params.iter().map(|ty| { let ref ident = &ty.ident; quote! { #ident } });
        owned_lifetime_params.chain(owned_type_params).collect::<Vec<_>>()
    }

    fn visit_struct(&self, data: &syn::VariantData) -> quote::Tokens {
        match *data {
            syn::VariantData::Struct(ref body) => {
                let fields = body.iter()
                    .map(|field| {
                        let ident = field.ident.as_ref().unwrap();
                        let field_ref = quote! { self.#ident };
                        let code = FieldKind::resolve(field).borrow_or_clone(&field_ref);
                        quote! { #ident: #code }
                    })
                    .collect::<Vec<_>>();
                quote! { { #(#fields),* } }
            },
            syn::VariantData::Tuple(ref body) => {
                let fields = body.iter()
                    .enumerate()
                    .map(|(index, field)| {
                        let index = syn::Ident::from(index);
                        let index = quote! { self.#index };
                        FieldKind::resolve(field).borrow_or_clone(&index)
                    })
                    .collect::<Vec<_>>();
                quote! { ( #(#fields),* ) }
            },
            syn::VariantData::Unit => {
                quote! {}
            }
        }
    }

    fn visit_enum_data(&self, ident: quote::Tokens, data: &syn::VariantData) -> quote::Tokens {
        match *data {
            syn::VariantData::Struct(ref body) => {
                let idents = body.iter()
                    .map(|field| field.ident.as_ref().unwrap())
                    .collect::<Vec<_>>();
                let cloned = body.iter()
                    .map(|field| {
                        let ref ident = field.ident.as_ref().unwrap();
                        let ident = quote! { #ident };
                        let code = FieldKind::resolve(field).borrow_or_clone(&ident);
                        quote! { #ident: #code }
                    })
                    .collect::<Vec<_>>();
                quote! { #ident { #(ref #idents),* } => #ident { #(#cloned),* } }
            },
            syn::VariantData::Tuple(ref body) => {
                let idents = (0..body.len())
                    .map(|index| syn::Ident::from(format!("x{}", index)))
                    .collect::<Vec<_>>();
                let cloned = idents.iter().zip(body.iter())
                    .map(|(ident, field)| {
                        let ident = quote! { #ident };
                        FieldKind::resolve(field).borrow_or_clone(&ident)
                    })
                    .collect::<Vec<_>>();
                quote! { #ident ( #(ref #idents),* ) => #ident ( #(#cloned),* ) }
            },
            syn::VariantData::Unit => {
                quote! { #ident => #ident }
            },
        }
    }

    fn combine_impl(&self, borrowed: quote::Tokens, name: &syn::Ident, params: quote::Tokens, owned: quote::Tokens, body: quote::Tokens) -> quote::Tokens {
        quote! {
            impl #borrowed #name #params {
                /// Returns a clone of `self` that shares all the "Cow-alike" data with `self`.
                pub fn borrowed<'__borrowedgen>(&'__borrowedgen self) -> #name #owned { #body }
            }
        }
    }
}

enum FieldKind {
    PlainCow,
    AssumedCow,
    /// Option fields with either PlainCow or AssumedCow
    OptField(usize, Box<FieldKind>),
    IterableField(Box<FieldKind>),
    JustMoved
}

impl FieldKind {

    fn resolve(field: &syn::Field) -> Self {
        match &field.ty {
            &syn::Ty::Path(None, syn::Path { ref segments, .. }) => {
                if is_cow(segments) {
                    FieldKind::PlainCow
                } else if is_cow_alike(segments) {
                    FieldKind::AssumedCow
                } else if let Some(kind) = is_opt_cow(segments) {
                    kind
                } else if let Some(kind) = is_iter_field(segments) {
                    kind
                } else {
                    FieldKind::JustMoved
                }
            },
            _ => FieldKind::JustMoved,
        }
    }

    fn move_or_clone_field(&self, var: &quote::Tokens) -> quote::Tokens {
        use self::FieldKind::*;

        match self {
            &PlainCow => quote! { ::std::borrow::Cow::Owned(#var.into_owned()) },
            &AssumedCow => quote! { #var.into_owned() },
            &OptField(levels, ref inner) => {
                let next = syn::Ident::from("val");
                let next = quote! { #next };

                let mut tokens = inner.move_or_clone_field(&next);

                for _ in 0..(levels - 1) {
                    tokens = quote! { #next.map(|#next| #tokens) };
                }

                quote! { #var.map(|#next| #tokens) }
            }
            &IterableField(ref inner) => {
                let next = syn::Ident::from("x");
                let next = quote! { #next };

                let tokens = inner.move_or_clone_field(&next);

                quote! { #var.into_iter().map(|x| #tokens).collect() }
            }
            &JustMoved => quote! { #var },
        }
    }

    fn borrow_or_clone(&self, var: &quote::Tokens) -> quote::Tokens {
        use self::FieldKind::*;

        match self {
            &PlainCow => quote! { ::std::borrow::Cow::Borrowed(#var.as_ref()) },
            &AssumedCow => quote! { #var.borrowed() },
            &OptField(levels, ref inner) => {
                let next = syn::Ident::from("val");
                let next = quote! { #next };

                let mut tokens = inner.borrow_or_clone(&next);

                for _ in 0..(levels - 1) {
                    tokens = quote! { #next.as_ref().map(|#next| #tokens) };
                }

                quote! { #var.as_ref().map(|#next| #tokens) }
            },
            &IterableField(ref inner) => {
                let next = syn::Ident::from("x");
                let next = quote! { #next };

                let tokens = inner.borrow_or_clone(&next);

                quote! { #var.iter().map(|x| #tokens).collect() }
            }
            &JustMoved => quote! { #var.clone() },
        }
    }
}

fn type_hopefully_is(segments: &Vec<syn::PathSegment>, expected: &str) -> bool {
    let expected = expected.split("::").map(syn::Ident::from).collect::<Vec<_>>();
    if segments.len() > expected.len() {
        return false
    }

    let expected = expected.iter().map(|x| x).collect::<Vec<_>>();
    let segments = segments.iter().map(|x| &x.ident).collect::<Vec<_>>();

    for len in 0..expected.len() {
        if &segments[..] == &expected[expected.len() - len - 1..] {
            return true;
        }
    }

    false
}

fn is_cow(segments: &Vec<syn::PathSegment>) -> bool {
    type_hopefully_is(segments, "std::borrow::Cow")
}

fn is_cow_alike(segments: &Vec<syn::PathSegment>) -> bool {
    if let Some(&syn::PathParameters::AngleBracketed(ref data)) = segments.last().map(|x| &x.parameters) {
        !data.lifetimes.is_empty()
    } else {
        false
    }
}

fn is_opt_cow(mut segments: &Vec<syn::PathSegment>) -> Option<FieldKind> {
    let mut levels = 0;
    loop {
        if type_hopefully_is(segments, "std::option::Option") {
            match *segments.last().unwrap() {
                syn::PathSegment { parameters: syn::PathParameters::AngleBracketed(ref data), .. } => {
                    if !data.lifetimes.is_empty() || !data.bindings.is_empty() {
                        // Option<&'a ?> cannot be moved but let the compiler complain
                        // don't know about data bindings
                        break;
                    }

                    if data.types.len() != 1 {
                        // Option<A, B> probably means some other, movable option
                        break;
                    }

                    match *data.types.first().unwrap() {
                        syn::Ty::Path(None, syn::Path { segments: ref next_segments, ..}) => {
                            levels += 1;
                            segments = next_segments;
                            continue;
                        }
                        _ => break,
                    }
                },
                _ => {}
            }
        } else if is_cow(segments) {
            return Some(FieldKind::OptField(levels, Box::new(FieldKind::PlainCow)));
        } else if is_cow_alike(segments) {
            return Some(FieldKind::OptField(levels, Box::new(FieldKind::AssumedCow)));
        }

        break;
    }

    None
}

fn is_iter_field(mut segments: &Vec<syn::PathSegment>) -> Option<FieldKind> {
    loop {
        // this should be easy to do for arrays as well..
        if type_hopefully_is(segments, "std::vec::Vec") {
            match *segments.last().unwrap() {
                syn::PathSegment { parameters: syn::PathParameters::AngleBracketed(ref data), .. } => {
                    if !data.lifetimes.is_empty() || !data.bindings.is_empty() {
                        break;
                    }

                    if data.types.len() != 1 {
                        // TODO: this could be something like Vec<(u32, Bar<'a>)>?
                        break;
                    }

                    match *data.types.first().unwrap() {
                        syn::Ty::Path(None, syn::Path { segments: ref next_segments, ..}) => {
                            segments = next_segments;
                            continue;
                        }
                        _ => break,
                    }
                },
                _ => {}
            }
        } else if is_cow(segments) {
            return Some(FieldKind::IterableField(Box::new(FieldKind::PlainCow)));
        } else if is_cow_alike(segments) {
            return Some(FieldKind::IterableField(Box::new(FieldKind::AssumedCow)));
        }

        break;
    }

    None
}