docs and minor changes

.gitignore

@@ -1,3 +1,5 @@
+circuits/build
+
 # Created by .ignore support plugin (hsz.mobi)
 ### Node template
 # Logs

circuits/merkleTree.circom

@@ -1,6 +1,7 @@
 include "../node_modules/circomlib/circuits/bitify.circom";
 include "../node_modules/circomlib/circuits/mimcsponge.circom";
 
+// Computes MiMC(left + right)
 template HashLeftRight(rounds) {
     signal input left;
     signal input right;
@@ -15,6 +16,8 @@ template HashLeftRight(rounds) {
     hash <== hasher.outs[0];
 }
 
+// if pathIndex == 0 returns (left = inputElement, right = pathElement)
+// if pathIndex == 1 returns (left = pathElement, right = inputElement)
 template Selector() {
     signal input inputElement;
     signal input pathElement;
@@ -39,13 +42,14 @@ template Selector() {
     right <== rightSelector1 + rightSelector2;
 }
 
+// Verifies that merkle proof is correct for given merkle root and a leaf
+// pathIndex input is an array of 0/1 selectors telling whether given pathElement is on the left or right side of merkle path
 template MerkleTree(levels, rounds) {
     signal input leaf;
+    signal input root;
     signal private input pathElements[levels];
     signal private input pathIndex[levels];
 
-    signal output root;
-
     component selectors[levels];
     component hashers[levels];
 
@@ -66,5 +70,5 @@ template MerkleTree(levels, rounds) {
         selectors[i].inputElement <== hashers[i-1].hash;
     }
 
-    root <== hashers[levels - 1].hash;
+    root === hashers[levels - 1].hash;
 }

circuits/withdraw.circom

@@ -2,6 +2,7 @@ include "../node_modules/circomlib/circuits/bitify.circom";
 include "../node_modules/circomlib/circuits/pedersen.circom";
 include "merkleTree.circom";
 
+// computes Pedersen(nullifier + secret)
 template CommitmentHasher() {
     signal input nullifier;
     signal private input secret;
@@ -21,6 +22,7 @@ template CommitmentHasher() {
     hash <== commitment.out[0];
 }
 
+// Verifies that commitment that corresponds to given secret and nullifier is included in the merkle tree of deposits
 template Withdraw(levels, rounds) {
     signal input root;
     signal input nullifier;
@@ -36,10 +38,11 @@ template Withdraw(levels, rounds) {
 
     component tree = MerkleTree(levels, rounds);
     tree.leaf <== hasher.hash;
-    tree.pathElements <== pathElements;
-    tree.pathIndex <== pathIndex;
-
-    root === tree.root;
+    tree.root <== root;
+    for (var i = 0; i < levels; i++) {
+        tree.pathElements[i] <== pathElements[i];
+        tree.pathIndex[i] <== pathIndex[i];
+    }
 
     // TODO: Check if we need some kind of explicit constraints or something
     fee === fee;

contracts/contracts/MerkleTreeWithHistory.sol

@@ -25,15 +25,15 @@ contract MerkleTreeWithHistory {
     filled_subtrees.push(zeros[0]);
 
     for (uint8 i = 1; i < levels; i++) {
-      zeros.push(HashLeftRight(zeros[i-1], zeros[i-1]));
+      zeros.push(hashLeftRight(zeros[i-1], zeros[i-1]));
       filled_subtrees.push(zeros[i]);
     }
 
     roots = new uint256[](ROOT_HISTORY_SIZE);
-    roots[0] = HashLeftRight(zeros[levels - 1], zeros[levels - 1]);
+    roots[0] = hashLeftRight(zeros[levels - 1], zeros[levels - 1]);
   }
 
-  function HashLeftRight(uint256 left, uint256 right) public pure returns (uint256 mimc_hash) {
+  function hashLeftRight(uint256 left, uint256 right) public pure returns (uint256 mimc_hash) {
     uint256 k = 21888242871839275222246405745257275088548364400416034343698204186575808495617;
     uint256 R = 0;
     uint256 C = 0;
@@ -67,7 +67,7 @@ contract MerkleTreeWithHistory {
         right = current_level_hash;
       }
 
-      current_level_hash = HashLeftRight(left, right);
+      current_level_hash = hashLeftRight(left, right);
 
       current_index /= 2;
     }

contracts/contracts/Mixer.sol

@@ -1,15 +1,12 @@
 pragma solidity ^0.5.8;
 
 import "./MerkleTreeWithHistory.sol";
-import "../node_modules/openzeppelin-solidity/contracts/math/SafeMath.sol";
 
 contract IVerifier {
   function verify(uint256[2] memory a, uint256[2][2] memory b, uint256[2] memory c, uint256[4] memory input) public returns(bool);
 }
 
 contract Mixer is MerkleTreeWithHistory {
-  using SafeMath for uint256;
-
   uint256 public transferValue;
   mapping(uint256 => bool) public nullifiers;
   IVerifier verifier;
@@ -17,17 +14,34 @@ contract Mixer is MerkleTreeWithHistory {
   event Deposit(address from, uint256 commitment);
   event Withdraw(address to, uint256 nullifier, uint256 fee);
 
+  /**
+    @dev The constructor
+    @param _verifier the address of SNARK verifier for this contract
+    @param _transferValue the value for all deposits in this contract in wei
+  */
   constructor(address _verifier, uint256 _transferValue) MerkleTreeWithHistory(16, 0) public {
     verifier = IVerifier(_verifier);
     transferValue = _transferValue;
   }
 
+  /**
+    @dev Deposit funds into mixer. The caller must send value equal to `transferValue` of this mixer.
+    @param commitment the note commitment, which is PedersenHash(nullifier + secret)
+  */
   function deposit(uint256 commitment) public payable {
     require(msg.value == transferValue, "Please send `transferValue` ETH along with transaction");
     insert(commitment);
     emit Deposit(msg.sender, commitment);
   }
 
+  /**
+    @dev Withdraw deposit from the mixer. `a`, `b`, and `c` are zkSNARK proof data, and input is an array of circuit public inputs
+    `input` array consists of:
+      - merkle root of all deposits in the mixer
+      - unique deposit nullifier to prevent double spends
+      - the receiver of funds
+      - optional fee that goes to the transaction sender (usually a relay)
+  */
   function withdraw(uint256[2] memory a, uint256[2][2] memory b, uint256[2] memory c, uint256[4] memory input) public {
     uint256 root = input[0];
     uint256 nullifier = input[1];