From 571dd1f2d492391816430f91e2f3076b34186a44 Mon Sep 17 00:00:00 2001
From: Robbert van der Helm <mail@robbertvanderhelm.nl>
Date: Tue, 25 Jan 2022 19:55:01 +0100
Subject: [PATCH] Implement the Params derive proc macro

---
 nih_plug_derive/src/lib.rs | 76 +++++++++++++++++++++++++++++++++++---
 1 file changed, 70 insertions(+), 6 deletions(-)

diff --git a/nih_plug_derive/src/lib.rs b/nih_plug_derive/src/lib.rs
index 1b4a90c9..67582d3f 100644
--- a/nih_plug_derive/src/lib.rs
+++ b/nih_plug_derive/src/lib.rs
@@ -1,8 +1,72 @@
-#[cfg(test)]
-mod tests {
-    #[test]
-    fn it_works() {
-        let result = 2 + 2;
-        assert_eq!(result, 4);
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::spanned::Spanned;
+
+#[proc_macro_derive(Params, attributes(id))]
+pub fn derive_params(input: TokenStream) -> TokenStream {
+    let ast = syn::parse_macro_input!(input as syn::DeriveInput);
+
+    let fields = match ast.data {
+        syn::Data::Struct(syn::DataStruct {
+            fields: syn::Fields::Named(named_fields),
+            ..
+        }) => named_fields,
+        _ => {
+            return syn::Error::new(
+                ast.span(),
+                "Deriving Params is only supported on structs with named fields",
+            )
+            .to_compile_error()
+            .into()
+        }
+    };
+
+    // We only care about fields with an `id` attribute. We'll build a that creates a hashmap
+    // containing pointers to those parmaeters.
+    let mut param_insert_tokens = Vec::new();
+    for field in fields.named {
+        let name = match &field.ident {
+            Some(ident) => ident,
+            _ => continue,
+        };
+
+        for attr in &field.attrs {
+            let list = match attr.parse_meta() {
+                Ok(syn::Meta::List(list)) if list.path.is_ident("id") => list,
+                _ => continue,
+            };
+
+            let param_id =
+                match list.nested.first() {
+                    Some(syn::NestedMeta::Lit(syn::Lit::Str(s))) => s.value(),
+                    _ => return syn::Error::new(
+                        attr.span(),
+                        "The id attribute should have a single string argument: #[id(\"foo_bar\")]",
+                    )
+                    .to_compile_error()
+                    .into(),
+                };
+
+            // The specific parameter types know how to convert themselves into the correct ParamPtr
+            // variant
+            param_insert_tokens.push(quote! { param_map.insert(#param_id, self.#name.as_ptr()); });
+        }
     }
+
+    quote! {
+        impl Params for Foo {
+            fn param_map(
+                self: std::pin::Pin<&Self>,
+            ) -> std::collections::HashMap<&'static str, nih_plug::params::ParamPtr> {
+                let mut param_map = std::collections::HashMap::new();
+
+                #(#param_insert_tokens)*
+
+                param_map
+            }
+        }
+    }
+    .into()
 }