Ark-ff Small Field Support

CHANGELOG.md

@@ -11,6 +11,7 @@
 - (`ark-poly`) Add fast polynomial division
 - (`ark-ec`) Improve GLV scalar multiplication performance by skipping leading zeroes.
 - (`ark-poly`) Make `SparsePolynomial.coeffs` field public
+- [\#1044](https://github.com/arkworks-rs/algebra/pull/1044) Add implementation for small field with native integer types
 
 ### Breaking changes
 

ff-macros/src/lib.rs

@@ -12,6 +12,7 @@ use proc_macro::TokenStream;
 use syn::{Expr, ExprLit, Item, ItemFn, Lit, Meta};
 
 mod montgomery;
+mod small_fp;
 mod unroll;
 
 pub(crate) mod utils;
@@ -74,6 +75,34 @@ pub fn mont_config(input: proc_macro::TokenStream) -> proc_macro::TokenStream {
     .into()
 }
 
+/// Derive the `SmallFpConfig` trait for small prime fields.
+///
+/// The attributes available to this macro are:
+/// * `modulus`: Specify the prime modulus underlying this prime field.
+/// * `generator`: Specify the generator of the multiplicative subgroup.
+/// * `backend`: Specify either "standard" or "montgomery" backend.
+#[proc_macro_derive(SmallFpConfig, attributes(modulus, generator, backend))]
+pub fn small_fp_config(input: TokenStream) -> TokenStream {
+    let ast: syn::DeriveInput = syn::parse(input).unwrap();
+
+    let modulus: u128 = fetch_attr("modulus", &ast.attrs)
+        .expect("Please supply a modulus attribute")
+        .parse()
+        .expect("Modulus should be a number");
+
+    let generator: u128 = fetch_attr("generator", &ast.attrs)
+        .expect("Please supply a generator attribute")
+        .parse()
+        .expect("Generator should be a number");
+
+    let backend: String = fetch_attr("backend", &ast.attrs)
+        .expect("Please supply a backend attribute")
+        .parse()
+        .expect("Backend should be a string");
+
+    small_fp::small_fp_config_helper(modulus, generator, backend, ast.ident).into()
+}
+
 const ARG_MSG: &str = "Failed to parse unroll threshold; must be a positive integer";
 
 /// Attribute used to unroll for loops found inside a function block.
@@ -152,7 +181,7 @@ fn test_str_to_limbs() {
                 if sign == Minus {
                     string.insert(0, '-');
                 }
-                let (is_positive, limbs) = utils::str_to_limbs(&string.to_string());
+                let (is_positive, limbs) = utils::str_to_limbs(&string.clone());
                 assert_eq!(
                     limbs[0],
                     format!("{}u64", signed_number.unsigned_abs() as u64),

ff-macros/src/small_fp/mod.rs

@@ -0,0 +1,54 @@
+mod montgomery_backend;
+mod standard_backend;
+mod utils;
+
+use quote::quote;
+
+/// This function is called by the `#[derive(SmallFp)]` macro and generates
+/// the implementation of the `SmallFpConfig`
+pub(crate) fn small_fp_config_helper(
+    modulus: u128,
+    generator: u128,
+    backend: String,
+    config_name: proc_macro2::Ident,
+) -> proc_macro2::TokenStream {
+    let ty = match modulus {
+        m if m < 1u128 << 8 => quote! { u8 },
+        m if m < 1u128 << 16 => quote! { u16 },
+        m if m < 1u128 << 32 => quote! { u32 },
+        m if m < 1u128 << 64 => quote! { u64 },
+        _ => quote! { u128 },
+    };
+
+    let backend_impl = match backend.as_str() {
+        "standard" => standard_backend::backend_impl(&ty, modulus, generator),
+        "montgomery" => {
+            assert!(modulus < 1u128 << 127,
+                "SmallFpConfig montgomery backend supports only moduli < 2^127. Use MontConfig with BigInt instead of SmallFp."
+            );
+            montgomery_backend::backend_impl(&ty, modulus, generator)
+        },
+
+        _ => panic!("Unknown backend type: {}", backend),
+    };
+
+    let new_impl = match backend.as_str() {
+        "standard" => standard_backend::new(),
+        "montgomery" => montgomery_backend::new(modulus, ty.clone()),
+        _ => panic!("Unknown backend type: {}", backend),
+    };
+
+    quote! {
+        const _: () = {
+            use ark_ff::{SmallFp, SmallFpConfig};
+
+            impl SmallFpConfig for #config_name {
+                #backend_impl
+            }
+
+            impl #config_name {
+                #new_impl
+            }
+        };
+    }
+}