fix: subtree strategy

src/BinaryIMTMemory.sol

@@ -89,25 +89,17 @@ library BinaryIMTMemory {
         revert("IncrementalBinaryTree: defaultZero bad index");
     }
 
-    /// @dev Computes the root of the tree given the leaves.
-    /// @param self: Tree data.
-    /// @param leaves: Leaves in the tree
-    function calcRoot(
-        BinaryIMTMemoryData memory self,
-        uint256 depth,
-        uint256[] memory leaves
-    )
-        public
-        pure
-        returns (uint256)
-    {
+    function calcSubtreeRoot(BinaryIMTMemoryData memory self, uint256[] memory leaves) public pure returns (uint256) {
+        // instead of the depth being 0..19, we go from 10..19.
+        // this way, we preserve the subtree root
+        uint8 depth = 10;
         uint256[2][] memory lastSubtrees = new uint256[2][](depth);
         for (uint256 j = 0; j < leaves.length;) {
             uint256 index = self.numberOfLeaves;
             uint256 hash = leaves[j];
             for (uint8 i = 0; i < depth;) {
                 if (index & 1 == 0) {
-                    lastSubtrees[i] = [hash, defaultZero(i)];
+                    lastSubtrees[i] = [hash, defaultZero(i + depth)];
                 } else {
                     lastSubtrees[i][1] = hash;
                 }

src/RlnBase.sol

@@ -308,18 +308,22 @@ abstract contract RlnBase {
     function root(uint256 shardIndex) public view returns (uint256) {
         BinaryIMTMemoryData memory imtData;
         uint256 idCommitmentIndex = leavesSet[shardIndex];
-        uint256[] memory calcLeaves = new uint256[](idCommitmentIndex);
+        uint256[] memory calcLeaves = new uint256[](1024);
+
+        if (idCommitmentIndex == 0) {
+            return BinaryIMTMemory.calcSubtreeRoot(imtData, calcLeaves);
+        }
         for (uint256 i = 0; i < idCommitmentIndex; i++) {
             uint256 idCommitment = leavesIndex[shardIndex][i];
             uint256 userMessageLimit = userMessageLimits[idCommitment];
             calcLeaves[i] = PoseidonT3.hash([idCommitment, userMessageLimit]);
         }
-        return BinaryIMTMemory.calcRoot(imtData, 10, calcLeaves);
+        return BinaryIMTMemory.calcSubtreeRoot(imtData, calcLeaves);
     }
 
-    function full_root(uint256[] calldata roots) public pure returns (uint256) {
+    function fullRoot(uint256[] calldata roots) public pure returns (uint256) {
         BinaryIMTMemoryData memory imtData;
-        return BinaryIMTMemory.calcRoot(imtData, 10, roots);
+        return BinaryIMTMemory.calcSubtreeRoot(imtData, roots);
     }
 
     function getLeafAtShard(uint256 shardIndex, uint256 index) public view returns (uint256) {

test/Rln.t.sol

@@ -263,6 +263,11 @@ contract RlnTest is Test {
             18_217_334_211_520_937_958_971_536_517_166_530_749_184_547_628_672_204_353_760_850_739_130_586_503_124;
 
         vm.pauseGasMetering();
+        // before we insert into the tree, the kats must pass
+        assertEq(
+            rln.root(0),
+            12_413_880_268_183_407_374_852_357_075_976_609_371_175_688_755_676_981_206_018_884_971_008_854_919_922
+        );
         for (uint256 i = 0; i < idCommitments.length; i++) {
             // default 1 message limit
             rln.register{ value: MEMBERSHIP_DEPOSIT }(idCommitments[i], 1);
@@ -271,7 +276,14 @@ contract RlnTest is Test {
 
         assertEq(
             rln.root(0),
-            1_009_702_141_963_982_084_971_194_921_450_594_432_549_814_353_062_162_436_132_398_913_014_207_593_991
+            2_435_591_050_584_562_785_273_516_085_683_091_349_247_653_357_522_297_485_884_387_709_399_190_797_862
+        );
+
+        uint256[] memory roots = new uint256[](1024);
+        roots[0] = 2_435_591_050_584_562_785_273_516_085_683_091_349_247_653_357_522_297_485_884_387_709_399_190_797_862;
+        assertEq(
+            rln.fullRoot(roots),
+            3_881_711_806_223_377_894_370_509_479_419_308_415_025_371_832_767_296_831_624_691_168_726_496_442_721
         );
     }
 
@@ -291,15 +303,15 @@ contract RlnTest is Test {
         vm.resumeGasMetering();
         assertEq(
             rln.root(0),
-            15_191_547_950_571_875_204_938_158_154_881_229_414_576_184_216_597_114_679_496_514_970_950_611_126_017
+            10_424_638_537_885_815_866_594_133_227_550_095_274_095_476_771_249_698_162_779_016_221_356_823_894_456
         );
     }
 
-    function test__full_root() public {
+    function test__fullRoot() public {
         uint256[] memory roots = new uint256[](1024);
         assertEq(
             // repeat 0 1024 times
-            rln.full_root(roots),
+            rln.fullRoot(roots),
             12_413_880_268_183_407_374_852_357_075_976_609_371_175_688_755_676_981_206_018_884_971_008_854_919_922
         );
     }