feat(rln): trustless root onchain, no storage

src/BinaryIMTMemory.sol

@@ -0,0 +1,126 @@
+// SPDX-License-Identifier: MIT
+pragma solidity ^0.8.4;
+
+import { PoseidonT3 } from "poseidon-solidity/PoseidonT3.sol";
+
+// stripped down version of
+// https://github.com/privacy-scaling-explorations/zk-kit/blob/718a5c2fa0f6cd577cee3fd08373609ac985d3bb/packages/imt.sol/contracts/internal/InternalBinaryIMT.sol
+// that allows getting the root of the rln tree without expensive storage reads/writes
+struct BinaryIMTMemoryData {
+    uint256 root; // Root hash of the tree.
+    uint256 numberOfLeaves; // Number of leaves of the tree.
+    uint256 depth; // Depth of the tree.
+}
+
+/// @title In memory Incremental binary Merkle tree Root calculator
+/// @dev This helper library allows to calculate the root hash of the tree without using storage
+library BinaryIMTMemory {
+    uint8 public constant MAX_DEPTH = 20;
+    uint256 public constant SNARK_SCALAR_FIELD =
+        21_888_242_871_839_275_222_246_405_745_257_275_088_548_364_400_416_034_343_698_204_186_575_808_495_617;
+
+    uint256 public constant Z_0 = 0;
+    uint256 public constant Z_1 =
+        14_744_269_619_966_411_208_579_211_824_598_458_697_587_494_354_926_760_081_771_325_075_741_142_829_156;
+    uint256 public constant Z_2 =
+        7_423_237_065_226_347_324_353_380_772_367_382_631_490_014_989_348_495_481_811_164_164_159_255_474_657;
+    uint256 public constant Z_3 =
+        11_286_972_368_698_509_976_183_087_595_462_810_875_513_684_078_608_517_520_839_298_933_882_497_716_792;
+    uint256 public constant Z_4 =
+        3_607_627_140_608_796_879_659_380_071_776_844_901_612_302_623_152_076_817_094_415_224_584_923_813_162;
+    uint256 public constant Z_5 =
+        19_712_377_064_642_672_829_441_595_136_074_946_683_621_277_828_620_209_496_774_504_837_737_984_048_981;
+    uint256 public constant Z_6 =
+        20_775_607_673_010_627_194_014_556_968_476_266_066_927_294_572_720_319_469_184_847_051_418_138_353_016;
+    uint256 public constant Z_7 =
+        3_396_914_609_616_007_258_851_405_644_437_304_192_397_291_162_432_396_347_162_513_310_381_425_243_293;
+    uint256 public constant Z_8 =
+        21_551_820_661_461_729_022_865_262_380_882_070_649_935_529_853_313_286_572_328_683_688_269_863_701_601;
+    uint256 public constant Z_9 =
+        6_573_136_701_248_752_079_028_194_407_151_022_595_060_682_063_033_565_181_951_145_966_236_778_420_039;
+    uint256 public constant Z_10 =
+        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;
+    uint256 public constant Z_11 =
+        14_271_763_308_400_718_165_336_499_097_156_975_241_954_733_520_325_982_997_864_342_600_795_471_836_726;
+    uint256 public constant Z_12 =
+        20_066_985_985_293_572_387_227_381_049_700_832_219_069_292_839_614_107_140_851_619_262_827_735_677_018;
+    uint256 public constant Z_13 =
+        9_394_776_414_966_240_069_580_838_672_673_694_685_292_165_040_808_226_440_647_796_406_499_139_370_960;
+    uint256 public constant Z_14 =
+        11_331_146_992_410_411_304_059_858_900_317_123_658_895_005_918_277_453_009_197_229_807_340_014_528_524;
+    uint256 public constant Z_15 =
+        15_819_538_789_928_229_930_262_697_811_477_882_737_253_464_456_578_333_862_691_129_291_651_619_515_538;
+    uint256 public constant Z_16 =
+        19_217_088_683_336_594_659_449_020_493_828_377_907_203_207_941_212_636_669_271_704_950_158_751_593_251;
+    uint256 public constant Z_17 =
+        21_035_245_323_335_827_719_745_544_373_081_896_983_162_834_604_456_827_698_288_649_288_827_293_579_666;
+    uint256 public constant Z_18 =
+        6_939_770_416_153_240_137_322_503_476_966_641_397_417_391_950_902_474_480_970_945_462_551_409_848_591;
+    uint256 public constant Z_19 =
+        10_941_962_436_777_715_901_943_463_195_175_331_263_348_098_796_018_438_960_955_633_645_115_732_864_202;
+    uint256 public constant Z_20 =
+        15_019_797_232_609_675_441_998_260_052_101_280_400_536_945_603_062_888_308_240_081_994_073_687_793_470;
+
+    function defaultZero(uint256 index) public pure returns (uint256) {
+        if (index == 0) return Z_0;
+        if (index == 1) return Z_1;
+        if (index == 2) return Z_2;
+        if (index == 3) return Z_3;
+        if (index == 4) return Z_4;
+        if (index == 5) return Z_5;
+        if (index == 6) return Z_6;
+        if (index == 7) return Z_7;
+        if (index == 8) return Z_8;
+        if (index == 9) return Z_9;
+        if (index == 10) return Z_10;
+        if (index == 11) return Z_11;
+        if (index == 12) return Z_12;
+        if (index == 13) return Z_13;
+        if (index == 14) return Z_14;
+        if (index == 15) return Z_15;
+        if (index == 16) return Z_16;
+        if (index == 17) return Z_17;
+        if (index == 18) return Z_18;
+        if (index == 19) return Z_19;
+        if (index == 20) return Z_20;
+        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)
+    {
+        uint256[2][] memory lastSubtrees = new uint256[2][](depth);
+
+        for (uint8 j = 0; j < leaves.length; j++) {
+            uint256 index = self.numberOfLeaves;
+            uint256 hash = leaves[j];
+            for (uint8 i = 0; i < depth;) {
+                if (index & 1 == 0) {
+                    lastSubtrees[i] = [hash, defaultZero(i)];
+                } else {
+                    if (i > 0) {
+                        lastSubtrees[i][0] = lastSubtrees[i][0];
+                    }
+                    lastSubtrees[i][1] = hash;
+                }
+                hash = PoseidonT3.hash(lastSubtrees[i]);
+                index >>= 1;
+                unchecked {
+                    ++i;
+                }
+            }
+            self.root = hash;
+            self.numberOfLeaves += 1;
+        }
+        return self.root;
+    }
+}

src/RlnBase.sol

@@ -3,6 +3,9 @@
 pragma solidity ^0.8.19;
 
 import { IVerifier } from "./IVerifier.sol";
+import { BinaryIMTMemory, BinaryIMTMemoryData } from "./BinaryIMTMemory.sol";
+import { PoseidonT3 } from "poseidon-solidity/PoseidonT3.sol";
+import "forge-std/console2.sol";
 
 /// The tree is full
 error FullTree();
@@ -278,4 +281,15 @@ abstract contract RlnBase {
         }
         return commitments;
     }
+
+    function root() public view returns (uint256) {
+        BinaryIMTMemoryData memory imtData;
+        uint256[] memory leaves = new uint256[](idCommitmentIndex);
+        for (uint256 i = 0; i < idCommitmentIndex; i++) {
+            uint256 idCommitment = indexToCommitment[i];
+            uint256 userMessageLimit = userMessageLimits[idCommitment];
+            leaves[i] = PoseidonT3.hash([idCommitment, userMessageLimit]);
+        }
+        return BinaryIMTMemory.calcRoot(imtData, DEPTH, leaves);
+    }
 }

test/Rln.t.sol

@@ -218,4 +218,60 @@ contract RlnTest is Test {
         rln.withdraw();
         assertEq(rln.withdrawalBalance(to), 0);
     }
+
+    function test__root() public {
+        uint256[] memory idCommitments = new uint256[](20);
+        idCommitments[0] =
+            20_247_267_680_401_005_346_274_578_821_543_189_710_026_653_465_287_274_953_093_311_729_853_323_564_993;
+        idCommitments[1] =
+            17_156_989_550_607_148_435_222_849_763_909_493_047_059_958_780_520_493_064_916_232_951_463_175_150_232;
+        idCommitments[2] =
+            19_407_226_803_745_267_103_608_182_642_384_012_885_199_367_851_532_943_079_325_397_211_138_049_136_059;
+        idCommitments[3] =
+            10_843_159_696_774_792_948_443_591_344_995_886_877_305_260_668_485_645_797_849_607_534_704_262_422_614;
+        idCommitments[4] =
+            15_752_850_412_139_811_972_088_845_441_337_097_950_281_706_419_552_473_737_567_891_612_825_963_973_284;
+        idCommitments[5] =
+            4_519_863_328_057_168_189_298_631_669_175_527_040_337_788_759_018_171_546_425_601_895_038_423_288_676;
+        idCommitments[6] =
+            20_627_603_771_378_896_374_342_882_478_326_393_493_797_675_723_581_644_707_860_442_401_185_847_711_606;
+        idCommitments[7] =
+            15_154_129_217_924_019_401_315_002_167_199_740_344_372_362_483_772_641_456_490_923_254_762_406_760_783;
+        idCommitments[8] =
+            3_015_981_008_465_776_671_380_535_237_073_859_585_910_422_946_858_408_575_400_681_261_858_382_907_193;
+        idCommitments[9] =
+            224_054_746_800_950_089_703_161_552_547_065_908_637_798_421_998_793_333_020_881_640_418_719_302_913;
+        idCommitments[10] =
+            11_312_879_727_214_499_351_626_352_295_289_579_557_712_856_071_583_396_259_776_980_542_429_783_930_998;
+        idCommitments[11] =
+            12_465_380_480_462_031_386_424_255_751_937_172_435_855_917_781_265_550_857_371_960_580_299_590_023_804;
+        idCommitments[12] =
+            10_532_759_670_323_423_548_160_832_454_733_716_332_118_269_972_426_626_732_736_793_882_239_669_630_595;
+        idCommitments[13] =
+            5_363_974_916_211_877_996_441_994_123_101_828_809_420_707_174_378_417_661_497_477_404_431_739_760_929;
+        idCommitments[14] =
+            16_136_233_734_969_897_677_998_619_926_295_968_066_529_681_582_940_418_030_166_845_477_723_796_227_875;
+        idCommitments[15] =
+            19_482_780_886_140_959_996_233_254_660_604_422_414_723_443_061_405_603_031_287_446_813_590_162_051_267;
+        idCommitments[16] =
+            10_229_567_829_413_302_626_314_791_721_752_882_914_790_767_942_876_437_744_444_365_366_344_582_485_888;
+        idCommitments[17] =
+            13_243_196_170_549_739_682_068_942_953_623_914_146_349_583_225_049_284_668_843_899_390_874_999_176_721;
+        idCommitments[18] =
+            10_860_831_981_296_153_559_626_134_370_426_776_811_139_720_916_552_827_925_568_614_682_099_174_768_128;
+        idCommitments[19] =
+            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();
+        for (uint256 i = 0; i < idCommitments.length; i++) {
+            // default 1 message limit
+            rln.register{ value: MEMBERSHIP_DEPOSIT }(idCommitments[i], 1);
+        }
+        vm.resumeGasMetering();
+
+        assertEq(
+            rln.root(),
+            11_878_758_533_199_576_052_254_314_452_742_479_731_463_159_441_555_548_457_402_116_093_772_672_905_513
+        );
+    }
 }