add IncrementalMerkleTree#pop function

Author: ItsNickBarry

contracts/data/IncrementalMerkleTree.sol

@@ -87,6 +87,37 @@ library IncrementalMerkleTree {
         }
     }
 
+    function pop(Tree storage t) internal {
+        uint256 treeHeight = t.height();
+        uint256 treeSize = t.size() - 1;
+
+        // remove layer if tree has excess capacity
+
+        if (treeSize == (1 << treeHeight) >> 2) {
+            treeHeight--;
+            // TODO: is this pop necessary, or will the row be properly deleted?
+            t.nodes[treeHeight].pop();
+            t.nodes.pop();
+        }
+
+        // remove columns if rows are too long
+
+        uint256 row;
+        uint256 col = treeSize;
+
+        while (row < treeHeight && t.nodes[row].length > col) {
+            t.nodes[row].pop();
+            row++;
+            col = (col + 1) >> 1;
+        }
+
+        // recalculate hashes
+
+        if (treeSize > 0) {
+            t.set(treeSize - 1, t.at(treeSize - 1));
+        }
+    }
+
     /**
      * @notice update existing element in tree
      * @param t Tree struct storage reference

contracts/data/IncrementalMerkleTreeMock.sol

@@ -31,6 +31,10 @@ contract IncrementalMerkleTreeMock {
         tree.push(hash);
     }
 
+    function pop() external {
+        tree.pop();
+    }
+
     function set(uint256 index, bytes32 hash) external {
         tree.set(index, hash);
     }

test/data/IncrementalMerkleTree.ts

@@ -133,6 +133,38 @@ describe('IncrementalMerkleTree', function () {
     });
   });
 
+  describe('#pop', () => {
+    it('updates Merkle root', async () => {
+      const hashes = [];
+
+      for (let i = 0; i < 10; i++) {
+        hashes.push(randomHash());
+        await instance.push(hashes[i]);
+      }
+
+      for (let i = 0; i < hashes.length; i++) {
+        await instance.pop();
+
+        const tree = new MerkleTree(
+          hashes.slice(0, hashes.length - 1 - i),
+          keccak256,
+        );
+
+        // MerkleTree library returns truncated zero hash, so must use hexEqual matcher
+
+        expect(await instance.callStatic.root()).to.hexEqual(tree.getHexRoot());
+      }
+    });
+
+    describe('reverts if', () => {
+      it('tree is size zero', async () => {
+        await expect(instance.pop()).to.be.revertedWithPanic(
+          PANIC_CODES.ARITHMETIC_UNDER_OR_OVERFLOW,
+        );
+      });
+    });
+  });
+
   describe('#set', () => {
     it('updates Merkle root', async () => {
       const hashes = [];