Add tree_hash_derive crate

eth2/utils/tree_hash_derive/src/lib.rs

@@ -0,0 +1,125 @@
+#![recursion_limit = "256"]
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, DeriveInput};
+
+/// Returns a Vec of `syn::Ident` for each named field in the struct, whilst filtering out fields
+/// that should not be hashed.
+///
+/// # Panics
+/// Any unnamed struct field (like in a tuple struct) will raise a panic at compile time.
+fn get_hashable_named_field_idents<'a>(struct_data: &'a syn::DataStruct) -> Vec<&'a syn::Ident> {
+    struct_data
+        .fields
+        .iter()
+        .filter_map(|f| {
+            if should_skip_hashing(&f) {
+                None
+            } else {
+                Some(match &f.ident {
+                    Some(ref ident) => ident,
+                    _ => panic!("tree_hash_derive only supports named struct fields."),
+                })
+            }
+        })
+        .collect()
+}
+
+/// Returns true if some field has an attribute declaring it should not be hashedd.
+///
+/// The field attribute is: `#[tree_hash(skip_hashing)]`
+fn should_skip_hashing(field: &syn::Field) -> bool {
+    for attr in &field.attrs {
+        if attr.tts.to_string() == "( skip_hashing )" {
+            return true;
+        }
+    }
+    false
+}
+
+/// Implements `ssz::Encodable` for some `struct`.
+///
+/// Fields are encoded in the order they are defined.
+#[proc_macro_derive(CachedTreeHashSubTree, attributes(tree_hash))]
+pub fn subtree_derive(input: TokenStream) -> TokenStream {
+    let item = parse_macro_input!(input as DeriveInput);
+
+    let name = &item.ident;
+
+    let struct_data = match &item.data {
+        syn::Data::Struct(s) => s,
+        _ => panic!("tree_hash_derive only supports structs."),
+    };
+
+    let idents_a = get_hashable_named_field_idents(&struct_data);
+    let idents_b = idents_a.clone();
+    let idents_c = idents_a.clone();
+    let idents_d = idents_a.clone();
+
+    let output = quote! {
+        impl tree_hash::CachedTreeHashSubTree<#name> for #name {
+            fn item_type() -> tree_hash::ItemType {
+                tree_hash::ItemType::Composite
+            }
+
+            fn new_cache(&self) -> Result<tree_hash::TreeHashCache, tree_hash::Error> {
+                let tree = tree_hash::TreeHashCache::from_leaves_and_subtrees(
+                    self,
+                    vec![
+                        #(
+                            self.#idents_a.new_cache()?,
+                        )*
+                    ],
+                )?;
+
+                Ok(tree)
+            }
+
+            fn btree_overlay(&self, chunk_offset: usize) -> Result<tree_hash::BTreeOverlay, tree_hash::Error> {
+                let mut lengths = vec![];
+
+                #(
+                    lengths.push(tree_hash::BTreeOverlay::new(&self.#idents_b, 0)?.total_nodes());
+                )*
+
+                tree_hash::BTreeOverlay::from_lengths(chunk_offset, lengths)
+            }
+
+            fn packed_encoding(&self) -> Result<Vec<u8>, tree_hash::Error> {
+                Err(tree_hash::Error::ShouldNeverBePacked(Self::item_type()))
+            }
+
+            fn packing_factor() -> usize {
+                1
+            }
+
+            fn update_cache(
+                &self,
+                other: &Self,
+                cache: &mut tree_hash::TreeHashCache,
+                chunk: usize,
+            ) -> Result<usize, tree_hash::Error> {
+                let offset_handler = tree_hash::BTreeOverlay::new(self, chunk)?;
+
+                // Skip past the internal nodes and update any changed leaf nodes.
+                {
+                    let chunk = offset_handler.first_leaf_node()?;
+                    #(
+                        let chunk = self.#idents_c.update_cache(&other.#idents_d, cache, chunk)?;
+                    )*
+                }
+
+                for (&parent, children) in offset_handler.iter_internal_nodes().rev() {
+                    if cache.either_modified(children)? {
+                        cache.modify_chunk(parent, &cache.hash_children(children)?)?;
+                    }
+                }
+
+                Ok(offset_handler.next_node)
+            }
+        }
+    };
+    output.into()
+}