contracts: improve memory use in digest unpacking

zeth-contracts/contracts/BaseMixer.sol

@@ -139,28 +139,29 @@ contract BaseMixer is MerkleTreeMiMC7, ERC223ReceivingContract {
             "Invalid root: This root doesn't exist"
         );
 
-        // 2. We re-assemble the nullifiers (JSInputs) and check they were not already seen
+        // 2. We re-assemble the nullifiers (JSInputs) and check they were not
+        // already seen
         bytes32[jsIn] memory nfs;
-        uint256[] memory digest_inputs = new uint[](2);
+        uint nf_idx = 0;
         for(uint i = 1; i < 1 + 2*jsIn; i += 2) {
-            digest_inputs[0] = primary_inputs[i];
-            digest_inputs[1] = primary_inputs[i+1];
-            nfs[(i-1)/2] = Bytes.sha256_digest_from_field_elements(digest_inputs);
+            bytes32 nullifier = Bytes.sha256_digest_from_field_elements(
+                primary_inputs[i], primary_inputs[i+1]);
+            nfs[nf_idx] = nullifier;
             require(
-                !nullifiers[nfs[(i-1)/2]],
+                !nullifiers[nullifier],
                 "Invalid nullifier: This nullifier has already been used"
             );
-            nullifiers[nfs[(i-1)/2]] = true;
+            nullifiers[nullifier] = true;
+            ++nf_idx;
         }
 
         // 3. We re-compute h_sig, re-assemble the expected h_sig and check they
         // are equal (i.e. that h_sig re-assembled was correctly generated from
         // vk)
         bytes32 expected_hsig = sha256(abi.encodePacked(nfs, vk));
-
-        digest_inputs[0] = primary_inputs[1 + 2 * (jsIn + jsOut) + 1 + 1];
-        digest_inputs[1] = primary_inputs[1 + 2 * (jsIn + jsOut + 1) + 1];
-        bytes32 hsig = Bytes.sha256_digest_from_field_elements(digest_inputs);
+        bytes32 hsig = Bytes.sha256_digest_from_field_elements(
+            primary_inputs[1 + 2 * (jsIn + jsOut) + 1 + 1],
+            primary_inputs[1 + 2 * (jsIn + jsOut + 1) + 1]);
         require(
             expected_hsig == hsig,
             "Invalid hsig: This hsig does not correspond to the hash of vk and the nfs"
@@ -170,13 +171,11 @@ contract BaseMixer is MerkleTreeMiMC7, ERC223ReceivingContract {
     function assemble_commitments_and_append_to_state(
         uint[] memory primary_inputs) internal {
         // We re-assemble the commitments (JSOutputs)
-        uint256[] memory digest_inputs = new uint[](2);
         for(uint i = 1 + 2 * jsIn ; i < 1 + 2*(jsIn + jsOut); i += 2) {
             // See the way the inputs are ordered in the extended proof
-            digest_inputs[0] = primary_inputs[i];
-            digest_inputs[1] = primary_inputs[i+1];
             bytes32 current_commitment =
-                Bytes.sha256_digest_from_field_elements(digest_inputs);
+                Bytes.sha256_digest_from_field_elements(
+                    primary_inputs[i], primary_inputs[i+1]);
             uint commitmentAddress = insert(current_commitment);
             emit LogAddress(commitmentAddress);
         }

zeth-contracts/contracts/Bytes.sol

@@ -6,13 +6,13 @@ pragma solidity ^0.5.0;
 
 library Bytes {
 
-    function sha256_digest_from_field_elements(uint[] memory input)
+    function sha256_digest_from_field_elements(uint input0, uint input1)
         internal pure returns (bytes32) {
 
         // We know that input[0] is a field element.  Thus, it is encoded on 253
         // bits, and it should be the biggest between inputs[0] and inputs[1].
         // Inputs[0] actually contains 253 bits from the digest
-        bytes32 inverted_input1 = flip_endianness_bytes32(bytes32(input[0]));
+        bytes32 inverted_input1 = flip_endianness_bytes32(bytes32(input0));
 
         // As opposed to inputs[0], inputs[1] is encoded on 253 bits (because it
         // is a field element) but contains ONLY 3 bits from the digest (it is a
@@ -34,7 +34,7 @@ library Bytes {
         // input[1] is a very small number only represented on 3 field bits, so
         // the meaningful data we want to manipulate is only on the last bytes
         // of this bytes32 input
-        bytes1 last_byte_suffix = get_last_byte(bytes32(input[1]));
+        bytes1 last_byte_suffix = get_last_byte(bytes32(input1));
 
         // After selecting the last byte of input[1], we inverse only this byte
         // and we shift 5 times because we have somehting like: 0x4 initally,