From 8dec84793d200dcb524aa5c397d0a84d38974e7e Mon Sep 17 00:00:00 2001
From: Tom French <15848336+TomAFrench@users.noreply.github.com>
Date: Tue, 22 Oct 2024 17:37:18 +0100
Subject: [PATCH] chore: remove usage of slices in pedersen hash (#6295)

# Description

## Problem\*

Resolves <!-- Link to GitHub Issue -->

## Summary\*

This PR reworks the pedersen hash implementation to work on arrays as
this removes some brillig overhead.

## Additional Context



## Documentation\*

Check one:
- [x] No documentation needed.
- [ ] Documentation included in this PR.
- [ ] **[For Experimental Features]** Documentation to be submitted in a
separate PR.

# PR Checklist\*

- [x] I have tested the changes locally.
- [x] I have formatted the changes with [Prettier](https://prettier.io/)
and/or `cargo fmt` on default settings.
---
 noir_stdlib/src/embedded_curve_ops.nr |  8 +-------
 noir_stdlib/src/hash/mod.nr           | 23 +++++++++++------------
 2 files changed, 12 insertions(+), 19 deletions(-)

diff --git a/noir_stdlib/src/embedded_curve_ops.nr b/noir_stdlib/src/embedded_curve_ops.nr
index a77dd8d47d6..dd5e4285c00 100644
--- a/noir_stdlib/src/embedded_curve_ops.nr
+++ b/noir_stdlib/src/embedded_curve_ops.nr
@@ -112,17 +112,11 @@ pub fn multi_scalar_mul<let N: u32>(
 }
 
 #[foreign(multi_scalar_mul)]
-fn multi_scalar_mul_array_return<let N: u32>(
+pub(crate) fn multi_scalar_mul_array_return<let N: u32>(
     points: [EmbeddedCurvePoint; N],
     scalars: [EmbeddedCurveScalar; N],
 ) -> [Field; 3] {}
 
-#[foreign(multi_scalar_mul)]
-pub(crate) fn multi_scalar_mul_slice(
-    points: [EmbeddedCurvePoint],
-    scalars: [EmbeddedCurveScalar],
-) -> [Field; 3] {}
-
 // docs:start:fixed_base_scalar_mul
 pub fn fixed_base_scalar_mul(scalar: EmbeddedCurveScalar) -> EmbeddedCurvePoint
 // docs:end:fixed_base_scalar_mul
diff --git a/noir_stdlib/src/hash/mod.nr b/noir_stdlib/src/hash/mod.nr
index 97e91f71601..609017d70aa 100644
--- a/noir_stdlib/src/hash/mod.nr
+++ b/noir_stdlib/src/hash/mod.nr
@@ -7,9 +7,8 @@ pub mod sha512;
 
 use crate::default::Default;
 use crate::uint128::U128;
-use crate::collections::vec::Vec;
 use crate::embedded_curve_ops::{
-    EmbeddedCurvePoint, EmbeddedCurveScalar, multi_scalar_mul, multi_scalar_mul_slice,
+    EmbeddedCurvePoint, EmbeddedCurveScalar, multi_scalar_mul, multi_scalar_mul_array_return,
 };
 use crate::meta::derive_via;
 
@@ -57,22 +56,22 @@ pub fn pedersen_hash<let N: u32>(input: [Field; N]) -> Field
 
 #[no_predicates]
 pub fn pedersen_hash_with_separator<let N: u32>(input: [Field; N], separator: u32) -> Field {
-    let mut scalars: Vec<EmbeddedCurveScalar> =
-        Vec::from_slice([EmbeddedCurveScalar { lo: 0, hi: 0 }; N].as_slice()); //Vec::new();
-    for i in 0..N {
-        scalars.set(i, from_field_unsafe(input[i]));
-    }
-    scalars.push(EmbeddedCurveScalar { lo: N as Field, hi: 0 as Field });
+    let mut scalars: [EmbeddedCurveScalar; N + 1] = [EmbeddedCurveScalar { lo: 0, hi: 0 }; N + 1];
+    let mut generators: [EmbeddedCurvePoint; N + 1] =
+        [EmbeddedCurvePoint::point_at_infinity(); N + 1];
     let domain_generators: [EmbeddedCurvePoint; N] =
         derive_generators("DEFAULT_DOMAIN_SEPARATOR".as_bytes(), separator);
-    let mut vec_generators = Vec::new();
+
     for i in 0..N {
-        vec_generators.push(domain_generators[i]);
+        scalars[i] = from_field_unsafe(input[i]);
+        generators[i] = domain_generators[i];
     }
+    scalars[N] = EmbeddedCurveScalar { lo: N as Field, hi: 0 as Field };
+
     let length_generator: [EmbeddedCurvePoint; 1] =
         derive_generators("pedersen_hash_length".as_bytes(), 0);
-    vec_generators.push(length_generator[0]);
-    multi_scalar_mul_slice(vec_generators.slice, scalars.slice)[0]
+    generators[N] = length_generator[0];
+    multi_scalar_mul_array_return(generators, scalars)[0]
 }
 
 #[field(bn254)]