constants::util.rs: Factor out gen_fixed_array() method.

src/constants/util.rs

@@ -40,6 +40,22 @@ pub fn evaluate<C: CurveAffine>(x: u8, coeffs: &[C::Base]) -> C::Base {
         .fold(C::Base::default(), |acc, coeff| acc * x + coeff)
 }
 
+/// Takes in an FnMut closure and returns a constant-length array with elements of
+/// type `Output`.
+pub fn gen_const_array<Input, Output: Copy + Default, const LEN: usize>(
+    mut input: Input,
+    mut closure: impl FnMut(&mut Input, usize) -> Output,
+) -> [Output; LEN] {
+    let mut ret: [Output; LEN] = [Default::default(); LEN];
+    for (bit, val) in ret
+        .iter_mut()
+        .zip((0..LEN).map(|idx| closure(&mut input, idx)))
+    {
+        *bit = val;
+    }
+    ret
+}
+
 #[cfg(test)]
 mod tests {
     use super::decompose_scalar_fixed;

src/primitives/sinsemilla.rs

@@ -4,6 +4,7 @@ use halo2::arithmetic::CurveExt;
 use pasta_curves::pallas;
 use subtle::CtOption;
 
+use crate::constants::util::gen_const_array;
 use crate::spec::extract_p_bottom;
 
 mod addition;
@@ -24,15 +25,9 @@ fn lebs2ip_k(bits: &[bool]) -> u32 {
 /// up to `2^K` - 1.
 pub fn i2lebsp_k(int: usize) -> [bool; K] {
     assert!(int < (1 << K));
-
-    let mut ret = [false; K];
-    for (bit, val) in ret
-        .iter_mut()
-        .zip((0..K).map(|mask| ((int & (1 << mask)) >> mask) == 1))
-    {
-        *bit = val;
-    }
-    ret
+    gen_const_array(int, |int: &mut usize, mask: usize| {
+        ((*int & (1 << mask)) >> mask) == 1
+    })
 }
 
 /// Pads the given iterator (which MUST have length $\leq K * C$) with zero-bits to a

src/tree.rs

@@ -1,13 +1,13 @@
 use crate::{
-    constants::{MERKLE_CRH_PERSONALIZATION, MERKLE_DEPTH_ORCHARD},
+    constants::{util::gen_const_array, MERKLE_CRH_PERSONALIZATION, MERKLE_DEPTH_ORCHARD},
     note::commitment::ExtractedNoteCommitment,
     primitives::sinsemilla::{i2lebsp_k, HashDomain, K},
 };
 use pasta_curves::{arithmetic::FieldExt, pallas};
 
-use ff::{PrimeField, PrimeFieldBits};
+use ff::{Field, PrimeField, PrimeFieldBits};
 use rand::RngCore;
-use std::{convert::TryInto, iter};
+use std::iter;
 
 /// The root of an Orchard commitment tree.
 #[derive(Eq, PartialEq, Clone, Debug)]
@@ -28,13 +28,13 @@ pub struct MerklePath {
 impl MerklePath {
     /// Generates a dummy Merkle path for use in dummy spent notes.
     pub(crate) fn dummy(rng: &mut impl RngCore) -> Self {
+        fn dummy_inner(rng: &mut impl RngCore, _idx: usize) -> pallas::Base {
+            pallas::Base::random(rng)
+        }
+
         MerklePath {
             position: rng.next_u32(),
-            auth_path: (0..MERKLE_DEPTH_ORCHARD)
-                .map(|_| pallas::Base::rand())
-                .collect::<Vec<_>>()
-                .try_into()
-                .unwrap(),
+            auth_path: gen_const_array(rng, dummy_inner),
         }
     }