Derive ComputeSquaredDistance

1 files changed, 117 insertions, 0 deletions

diff --git a/components/style_derive/compute_squared_distance.rs b/components/style_derive/compute_squared_distance.rs
new file mode 100644
index 00000000000..46aa376319d
--- /dev/null
+++ b/components/style_derive/compute_squared_distance.rs
@@ -0,0 +1,117 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+use quote;
+use std::borrow::Cow;
+use syn;
+use synstructure;
+
+pub fn derive(input: syn::DeriveInput) -> quote::Tokens {
+    let name = &input.ident;
+    let (impl_generics, ty_generics, where_clause) = input.generics.split_for_impl();
+    let mut where_clause = where_clause.clone();
+    for param in &input.generics.ty_params {
+        where_clause.predicates.push(where_predicate(syn::Ty::Path(None, param.ident.clone().into())))
+    }
+
+    let variants = variants(&input);
+    let mut match_body = quote!();
+    match_body.append_all(variants.iter().map(|variant| {
+        let name = match input.body {
+            syn::Body::Struct(_) => Cow::Borrowed(&input.ident),
+            syn::Body::Enum(_) => {
+                Cow::Owned(syn::Ident::from(format!("{}::{}", input.ident, variant.ident)))
+            },
+        };
+        let (this_pattern, this_info) = synstructure::match_pattern(
+            &name,
+            &variant.data,
+            &synstructure::BindOpts::with_prefix(
+                synstructure::BindStyle::Ref,
+                "this".to_owned(),
+            ),
+        );
+        let (other_pattern, other_info) = synstructure::match_pattern(
+            &name,
+            &variant.data,
+            &synstructure::BindOpts::with_prefix(
+                synstructure::BindStyle::Ref,
+                "other".to_owned(),
+            ),
+        );
+        let sum = if this_info.is_empty() {
+            quote! { ::values::distance::SquaredDistance::Value(0.) }
+        } else {
+            let mut sum = quote!();
+            sum.append_separated(this_info.iter().zip(&other_info).map(|(this, other)| {
+                where_clause.predicates.push(where_predicate(this.field.ty.clone()));
+                quote! {
+                    ::values::distance::ComputeSquaredDistance::compute_squared_distance(#this, #other)?
+                }
+            }), "+");
+            sum
+        };
+        quote! {
+            (&#this_pattern, &#other_pattern) => {
+                Ok(#sum)
+            }
+        }
+    }));
+
+    if variants.len() > 1 {
+        match_body = quote! { #match_body, _ => Err(()), };
+    }
+
+    quote! {
+        impl #impl_generics ::values::distance::ComputeSquaredDistance for #name #ty_generics #where_clause {
+            #[allow(unused_variables, unused_imports)]
+            #[inline]
+            fn compute_squared_distance(
+                &self,
+                other: &Self,
+            ) -> Result<::values::distance::SquaredDistance, ()> {
+                match (self, other) {
+                    #match_body
+                }
+            }
+        }
+    }
+}
+
+fn variants(input: &syn::DeriveInput) -> Cow<[syn::Variant]> {
+    match input.body {
+        syn::Body::Enum(ref variants) => (&**variants).into(),
+        syn::Body::Struct(ref data) => {
+            vec![syn::Variant {
+                ident: input.ident.clone(),
+                attrs: input.attrs.clone(),
+                data: data.clone(),
+                discriminant: None,
+            }].into()
+        },
+    }
+}
+
+fn where_predicate(ty: syn::Ty) -> syn::WherePredicate {
+    syn::WherePredicate::BoundPredicate(
+        syn::WhereBoundPredicate {
+            bound_lifetimes: vec![],
+            bounded_ty: ty,
+            bounds: vec![syn::TyParamBound::Trait(
+                syn::PolyTraitRef {
+                    bound_lifetimes: vec![],
+                    trait_ref: syn::Path {
+                        global: true,
+                        segments: vec![
+                            "values".into(),
+                            "distance".into(),
+                            "ComputeSquaredDistance".into(),
+                        ],
+                    },
+                },
+                syn::TraitBoundModifier::None,
+            )],
+        },
+    )
+}