3 files changed, 116 insertions, 0 deletions
diff --git a/derive-macro/src/handle_enum.rs b/derive-macro/src/handle_enum.rs
new file mode 100644
index 0000000..b2b14c0
--- /dev/null
+++ b/derive-macro/src/handle_enum.rs
@@ -0,0 +1,55 @@
+use proc_macro2::{Ident, Literal, TokenStream};
+use quote::quote;
+use syn::{DataEnum, Variant};
+use super::generate_fields;
+
+fn variant_weight(variant: &Variant) -> Literal {
+    for attr in variant.attrs.iter() {
+        if attr.path.is_ident("weight") {
+            return attr.parse_args::<Literal>().expect("expected literal for `#[weight(...)]`")
+        }
+    }
+    Literal::u64_suffixed(1)
+}
+
+pub fn generate(name: &Ident, ty: DataEnum) -> TokenStream {
+    let mut variant_weights = ty.variants.into_iter()
+        .map(|variant| (variant_weight(&variant), variant));
+
+    let mut arms = TokenStream::new();
+    let mut total_weight = quote! { 0 };
+    if let Some((weight, variant)) = variant_weights.next() {
+        let variant_name = variant.ident;
+        let fields = generate_fields(variant.fields);
+        arms.extend(quote! {
+            let end = #weight;
+            if 0 <= value && value < end {
+                return Self::#variant_name #fields
+            }
+        });
+        total_weight = quote! { #weight };
+        for (weight, variant) in variant_weights {
+            let variant_name = variant.ident;
+            let fields = generate_fields(variant.fields);
+            arms.extend(quote! {
+                let start = end;
+                let end = start + #weight;
+                if start <= value && value < end {
+                    return Self::#variant_name #fields
+                }
+            });
+            total_weight = quote! { #total_weight + #weight };
+        }
+    }
+
+    quote! {
+        impl generate_random::GenerateRandom for #name {
+            fn generate_random<R: rand::Rng + ?Sized>(rng: &mut R) -> Self {
+                let total_weight = #total_weight;
+                let value = rng.gen_range(0..total_weight);
+                #arms
+                unreachable!()
+            }
+        }
+    }
+}
diff --git a/derive-macro/src/handle_struct.rs b/derive-macro/src/handle_struct.rs
new file mode 100644
index 0000000..988bd06
--- /dev/null
+++ b/derive-macro/src/handle_struct.rs
@@ -0,0 +1,15 @@
+use proc_macro2::{Ident, TokenStream};
+use quote::quote;
+use syn::DataStruct;
+use super::generate_fields;
+
+pub fn generate(name: &Ident, ty: DataStruct) -> TokenStream {
+    let fields = generate_fields(ty.fields);
+    quote! {
+        impl generate_random::GenerateRandom for #name {
+            fn generate_random<R: rand::Rng + ?Sized>(rng: &mut R) -> Self {
+                Self #fields
+            }
+        }
+    }
+}
diff --git a/derive-macro/src/lib.rs b/derive-macro/src/lib.rs
new file mode 100644
index 0000000..9aa93f5
--- /dev/null
+++ b/derive-macro/src/lib.rs
@@ -0,0 +1,46 @@
+//! This crate provide the [`GenerateRandom`] derive macro
+//! that implements the trait of the same name from the `generate-random` crate.
+//! Refer to the documentation of that crate for more information.
+
+use syn::{DeriveInput, Data, Fields};
+
+mod handle_struct;
+mod handle_enum;
+
+#[proc_macro_derive(GenerateRandom, attributes(weight))]
+pub fn derive_generate_random(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
+    let input: DeriveInput = syn::parse(input).unwrap();
+    match input.data {
+        Data::Struct(ty) => handle_struct::generate(&input.ident, ty),
+        Data::Enum(ty) => handle_enum::generate(&input.ident, ty),
+        Data::Union(_) => panic!("Unions are not supported"),
+    }.into()
+}
+
+fn generate_fields(fields: Fields) -> proc_macro2::TokenStream {
+    use quote::quote;
+    match fields {
+        Fields::Named(fields) => {
+            let fields = fields.named.into_iter()
+                .map(|field| {
+                    let field = field.ident.unwrap();
+                    quote! {
+                        #field: generate_random::GenerateRandom::generate_random(rng),
+                    }
+                })
+                .collect::<proc_macro2::TokenStream>();
+            quote! { { #fields } }
+        }
+        Fields::Unnamed(fields) => {
+            let fields = fields.unnamed.into_iter()
+                .map(|_field| {
+                    quote! {
+                        generate_random::GenerateRandom::generate_random(rng),
+                    }
+                })
+                .collect::<proc_macro2::TokenStream>();
+            quote! { ( #fields ) }
+        }
+        Fields::Unit => quote! {},
+    }
+}