make zeros immutable

contracts/MerkleTreeWithHistory.sol

@@ -26,10 +26,9 @@ contract MerkleTreeWithHistory {
   // the following variables are made public for easier testing and debugging and
   // are not supposed to be accessed in regular code
 
-  // filledSubtrees, zeros, and roots could be bytes32[size], but using mappings makes it cheaper because
+  // filledSubtrees and roots could be bytes32[size], but using mappings makes it cheaper because
   // it removes index range check on every interaction
   mapping(uint256 => bytes32) public filledSubtrees;
-  mapping(uint256 => bytes32) public zeros;
   mapping(uint256 => bytes32) public roots;
   uint32 public constant ROOT_HISTORY_SIZE = 30;
   uint32 public currentRootIndex = 0;
@@ -41,14 +40,11 @@ contract MerkleTreeWithHistory {
     levels = _levels;
     hasher = _hasher;
 
-    bytes32 currentZero = bytes32(ZERO_VALUE);
     for (uint32 i = 0; i < _levels; i++) {
-      zeros[i] = currentZero;
-      filledSubtrees[i] = currentZero;
-      currentZero = hashLeftRight(_hasher, currentZero, currentZero);
+      filledSubtrees[i] = zeros(i);
     }
 
-    roots[0] = currentZero;
+    roots[0] = zeros(_levels - 1);
   }
 
   /**
@@ -80,7 +76,7 @@ contract MerkleTreeWithHistory {
     for (uint32 i = 0; i < levels; i++) {
       if (currentIndex % 2 == 0) {
         left = currentLevelHash;
-        right = zeros[i];
+        right = zeros(i);
         filledSubtrees[i] = currentLevelHash;
       } else {
         left = filledSubtrees[i];
@@ -124,4 +120,43 @@ contract MerkleTreeWithHistory {
   function getLastRoot() public view returns (bytes32) {
     return roots[currentRootIndex];
   }
+
+  /// @dev provides Zero (Empty) elements for a MiMC MerkleTree. Up to 32 levels
+  function zeros(uint i) public pure returns(bytes32) {
+    bytes32[32] memory _zeros = [
+      bytes32(0x2fe54c60d3acabf3343a35b6eba15db4821b340f76e741e2249685ed4899af6c),
+      bytes32(0x256a6135777eee2fd26f54b8b7037a25439d5235caee224154186d2b8a52e31d),
+      bytes32(0x1151949895e82ab19924de92c40a3d6f7bcb60d92b00504b8199613683f0c200),
+      bytes32(0x20121ee811489ff8d61f09fb89e313f14959a0f28bb428a20dba6b0b068b3bdb),
+      bytes32(0x0a89ca6ffa14cc462cfedb842c30ed221a50a3d6bf022a6a57dc82ab24c157c9),
+      bytes32(0x24ca05c2b5cd42e890d6be94c68d0689f4f21c9cec9c0f13fe41d566dfb54959),
+      bytes32(0x1ccb97c932565a92c60156bdba2d08f3bf1377464e025cee765679e604a7315c),
+      bytes32(0x19156fbd7d1a8bf5cba8909367de1b624534ebab4f0f79e003bccdd1b182bdb4),
+      bytes32(0x261af8c1f0912e465744641409f622d466c3920ac6e5ff37e36604cb11dfff80),
+      bytes32(0x0058459724ff6ca5a1652fcbc3e82b93895cf08e975b19beab3f54c217d1c007),
+      bytes32(0x1f04ef20dee48d39984d8eabe768a70eafa6310ad20849d4573c3c40c2ad1e30),
+      bytes32(0x1bea3dec5dab51567ce7e200a30f7ba6d4276aeaa53e2686f962a46c66d511e5),
+      bytes32(0x0ee0f941e2da4b9e31c3ca97a40d8fa9ce68d97c084177071b3cb46cd3372f0f),
+      bytes32(0x1ca9503e8935884501bbaf20be14eb4c46b89772c97b96e3b2ebf3a36a948bbd),
+      bytes32(0x133a80e30697cd55d8f7d4b0965b7be24057ba5dc3da898ee2187232446cb108),
+      bytes32(0x13e6d8fc88839ed76e182c2a779af5b2c0da9dd18c90427a644f7e148a6253b6),
+      bytes32(0x1eb16b057a477f4bc8f572ea6bee39561098f78f15bfb3699dcbb7bd8db61854),
+      bytes32(0x0da2cb16a1ceaabf1c16b838f7a9e3f2a3a3088d9e0a6debaa748114620696ea),
+      bytes32(0x24a3b3d822420b14b5d8cb6c28a574f01e98ea9e940551d2ebd75cee12649f9d),
+      bytes32(0x198622acbd783d1b0d9064105b1fc8e4d8889de95c4c519b3f635809fe6afc05),
+      bytes32(0x29d7ed391256ccc3ea596c86e933b89ff339d25ea8ddced975ae2fe30b5296d4),
+      bytes32(0x19be59f2f0413ce78c0c3703a3a5451b1d7f39629fa33abd11548a76065b2967),
+      bytes32(0x1ff3f61797e538b70e619310d33f2a063e7eb59104e112e95738da1254dc3453),
+      bytes32(0x10c16ae9959cf8358980d9dd9616e48228737310a10e2b6b731c1a548f036c48),
+      bytes32(0x0ba433a63174a90ac20992e75e3095496812b652685b5e1a2eae0b1bf4e8fcd1),
+      bytes32(0x019ddb9df2bc98d987d0dfeca9d2b643deafab8f7036562e627c3667266a044c),
+      bytes32(0x2d3c88b23175c5a5565db928414c66d1912b11acf974b2e644caaac04739ce99),
+      bytes32(0x2eab55f6ae4e66e32c5189eed5c470840863445760f5ed7e7b69b2a62600f354),
+      bytes32(0x002df37a2642621802383cf952bf4dd1f32e05433beeb1fd41031fb7eace979d),
+      bytes32(0x104aeb41435db66c3e62feccc1d6f5d98d0a0ed75d1374db457cf462e3a1f427),
+      bytes32(0x1f3c6fd858e9a7d4b0d1f38e256a09d81d5a5e3c963987e2d4b814cfab7c6ebb),
+      bytes32(0x2c7a07d20dff79d01fecedc1134284a8d08436606c93693b67e333f671bf69cc)
+    ];
+    return _zeros[i];
+  }
 }

lib/utils.js

@@ -0,0 +1,28 @@
+const MiMC = require('./MiMC')
+const snarkjs = require('snarkjs')
+
+const hasher = new MiMC()
+const bigInt = snarkjs.bigInt
+
+const toHex = (number, length = 32) =>
+  '0x' +
+  (number instanceof Buffer ? number.toString('hex') : bigInt(number).toString(16)).padStart(length * 2, '0')
+
+function zeros(
+  levels,
+  defaultZero = '21663839004416932945382355908790599225266501822907911457504978515578255421292',
+) {
+  const zeros = []
+
+  let currentZero = defaultZero
+  for (let i = 0; i < levels; i++) {
+    zeros.push(toHex(currentZero))
+    currentZero = hasher.hash(levels, currentZero, currentZero)
+  }
+
+  return zeros
+}
+
+// console.log(zeros(32))
+
+module.exports = { zeros, toHex }