sinsemilla::merkle.rs: Implement i2lebsp locally.

Also include gen_const_array helper in gadget::utilities.

src/circuit/gadget/sinsemilla/merkle.rs

@@ -4,14 +4,12 @@ use halo2::{
 };
 use pasta_curves::arithmetic::CurveAffine;
 
-use crate::{
-    circuit::gadget::{
-        sinsemilla::{HashDomains, SinsemillaInstructions},
-        utilities::{
-            cond_swap::CondSwapInstructions, transpose_option_array, UtilitiesInstructions,
-        },
+use crate::circuit::gadget::{
+    sinsemilla::{HashDomains, SinsemillaInstructions},
+    utilities::{
+        cond_swap::CondSwapInstructions, gen_const_array, transpose_option_array,
+        UtilitiesInstructions,
     },
-    spec::i2lebsp,
 };
 use std::iter;
 
@@ -27,6 +25,17 @@ pub(crate) const MERKLE_DEPTH_ORCHARD: usize = 32;
 /// Number of bits in a Pallas base field element.
 pub(crate) const L_ORCHARD_BASE: usize = 255;
 
+/// The sequence of bits representing a u64 in little-endian order.
+///
+/// # Panics
+///
+/// Panics if the expected length of the sequence `NUM_BITS` exceeds
+/// 64.
+fn i2lebsp<const NUM_BITS: usize>(int: u64) -> [bool; NUM_BITS] {
+    assert!(NUM_BITS <= 64);
+    gen_const_array(|mask: usize| (int & (1 << mask)) != 0)
+}
+
 /// Instructions to check the validity of a Merkle path of a given `PATH_LENGTH`.
 /// The hash function used is a Sinsemilla instance with `K`-bit words.
 /// The hash function can process `MAX_WORDS` words.

src/circuit/gadget/utilities.rs

@@ -165,6 +165,18 @@ pub fn decompose_word<F: PrimeFieldBits>(
         .collect()
 }
 
+/// Takes in an FnMut closure and returns a constant-length array with elements of
+/// type `Output`.
+pub fn gen_const_array<Output: Copy + Default, const LEN: usize>(
+    mut closure: impl FnMut(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(idx))) {
+        *bit = val;
+    }
+    ret
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;