librarires: merkle: MerkleTree: Use zero values in tree init + insert

codegen/merkle-zeros/Cargo.toml

@@ -9,4 +9,3 @@ renegade-constants = { package = "constants", git = "https://github.com/renegade
 clap = { version = "4.5.1", features = ["derive"] }
 anyhow = "1.0"
 tiny-keccak = { version = "2.0", features = ["keccak"] }
-hex = "0.4"

codegen/merkle-zeros/src/main.rs

@@ -2,11 +2,8 @@
 //!
 //! This tool generates a Solidity file with predefined Merkle tree zero values.
 
-// ⚠ ️WARNING: This file is auto-generated by `codegen/merkle-zeros`. Do not edit directly.
-
 use anyhow::{anyhow, Result};
 use clap::Parser;
-use hex;
 use renegade_constants::{Scalar, MERKLE_HEIGHT};
 use renegade_crypto::hash::compute_poseidon_hash;
 use std::fs::File;
@@ -58,6 +55,31 @@ fn generate_solidity_contract() -> Result<String> {
         root
     ));
 
+    // Add an assembly-based getter function for gas-efficient constant-time access
+    contract.push_str("\n\t/// @notice Get zero value for a given height\n");
+    contract.push_str("\t/// @param height The height in the Merkle tree\n");
+    contract.push_str("\t/// @return The zero value for the given height\n");
+    contract
+        .push_str("\tfunction getZeroValue(uint256 height) internal pure returns (uint256) {\n");
+    contract.push_str("\t\t// Require height to be within valid range\n");
+    contract.push_str("\t\trequire(height <= 31, \"MerkleZeros: height must be <= 31\");\n\n");
+
+    contract.push_str("\t\tuint256 result;\n");
+    contract.push_str("\t\tassembly {\n");
+    contract.push_str("\t\t\tswitch height\n");
+
+    // Generate all the assembly cases with direct constant values
+    for i in 0..MERKLE_HEIGHT {
+        contract.push_str(&format!(
+            "\t\t\tcase {} {{ result := ZERO_VALUE_{} }}\n",
+            i, i
+        ));
+    }
+
+    // Remove the default case for assembly since require will handle invalid values
+    contract.push_str("\t\t}\n");
+    contract.push_str("\t}\n");
+
     // Close contract
     contract.push_str("}\n");
     Ok(contract)

src/libraries/merkle/MerkleTree.sol

@@ -4,6 +4,7 @@ pragma solidity ^0.8.0;
 import { BN254 } from "solidity-bn254/BN254.sol";
 import { IHasher } from "../poseidon2/IHasher.sol";
 import { DarkpoolConstants } from "../darkpool/Constants.sol";
+import { MerkleZeros } from "./MerkleZeros.sol";
 
 /// @title MerkleTreeLib
 /// @notice Library for Merkle tree operations
@@ -21,17 +22,24 @@ library MerkleTreeLib {
         mapping(BN254.ScalarField => bool) rootHistory;
     }
 
+    /// @notice Get the zero value for a given height in the Merkle tree
+    /// @param height The height in the Merkle tree
+    /// @return The zero value for the given height
+    function zeroValue(uint256 height) internal pure returns (BN254.ScalarField) {
+        // Use the assembly-based getter for maximum gas efficiency
+        return BN254.ScalarField.wrap(MerkleZeros.getZeroValue(height));
+    }
+
     /// @notice Initialize the Merkle tree
     /// @param tree The tree to initialize
     function initialize(MerkleTree storage tree) internal {
         tree.nextIndex = 0;
         tree.root = BN254.ScalarField.wrap(0);
 
-        // Initialize the sibling path
-        BN254.ScalarField zero = BN254.ScalarField.wrap(0);
+        // Initialize the sibling path array
         tree.siblingPath = new BN254.ScalarField[](DarkpoolConstants.MERKLE_DEPTH);
         for (uint256 i = 0; i < DarkpoolConstants.MERKLE_DEPTH; i++) {
-            tree.siblingPath[i] = zero;
+            tree.siblingPath[i] = zeroValue(i);
         }
     }
 
@@ -78,8 +86,7 @@ library MerkleTreeLib {
             } else {
                 // Right node, the new sibling is in a new sub-tree, and is the zero value
                 // for this depth in the tree
-                // TODO: Use depth-dependent zero values
-                tree.siblingPath[i] = BN254.ScalarField.wrap(0);
+                tree.siblingPath[i] = zeroValue(i);
             }
         }
     }

src/libraries/merkle/MerkleZeros.sol

@@ -38,4 +38,49 @@ library MerkleZeros {
 	uint256 constant public ZERO_VALUE_30 = 7035835480239620343712770214636030506415861196323445446427955599547555378646;
 	uint256 constant public ZERO_VALUE_31 = 3570982782379586050211724779746612745305269241448247085265205218748662232570;
 	uint256 constant public ZERO_VALUE_ROOT = 21822647340628839684360703580761466999432252031123851327655447000077200870350;
+
+	/// @notice Get zero value for a given height
+	/// @param height The height in the Merkle tree
+	/// @return The zero value for the given height
+	function getZeroValue(uint256 height) internal pure returns (uint256) {
+		// Require height to be within valid range
+		require(height <= 31, "MerkleZeros: height must be <= 31");
+
+		uint256 result;
+		assembly {
+			switch height
+			case 0 { result := ZERO_VALUE_0 }
+			case 1 { result := ZERO_VALUE_1 }
+			case 2 { result := ZERO_VALUE_2 }
+			case 3 { result := ZERO_VALUE_3 }
+			case 4 { result := ZERO_VALUE_4 }
+			case 5 { result := ZERO_VALUE_5 }
+			case 6 { result := ZERO_VALUE_6 }
+			case 7 { result := ZERO_VALUE_7 }
+			case 8 { result := ZERO_VALUE_8 }
+			case 9 { result := ZERO_VALUE_9 }
+			case 10 { result := ZERO_VALUE_10 }
+			case 11 { result := ZERO_VALUE_11 }
+			case 12 { result := ZERO_VALUE_12 }
+			case 13 { result := ZERO_VALUE_13 }
+			case 14 { result := ZERO_VALUE_14 }
+			case 15 { result := ZERO_VALUE_15 }
+			case 16 { result := ZERO_VALUE_16 }
+			case 17 { result := ZERO_VALUE_17 }
+			case 18 { result := ZERO_VALUE_18 }
+			case 19 { result := ZERO_VALUE_19 }
+			case 20 { result := ZERO_VALUE_20 }
+			case 21 { result := ZERO_VALUE_21 }
+			case 22 { result := ZERO_VALUE_22 }
+			case 23 { result := ZERO_VALUE_23 }
+			case 24 { result := ZERO_VALUE_24 }
+			case 25 { result := ZERO_VALUE_25 }
+			case 26 { result := ZERO_VALUE_26 }
+			case 27 { result := ZERO_VALUE_27 }
+			case 28 { result := ZERO_VALUE_28 }
+			case 29 { result := ZERO_VALUE_29 }
+			case 30 { result := ZERO_VALUE_30 }
+			case 31 { result := ZERO_VALUE_31 }
+		}
+	}
 }