From 62ae879364c3915c2d46e047caec793af90a38b3 Mon Sep 17 00:00:00 2001
From: Lion - dapplion <35266934+dapplion@users.noreply.github.com>
Date: Tue, 31 May 2022 12:37:52 +0700
Subject: [PATCH] Fix ListCompositeType.sliceTo(-1) (#268)

---
 packages/persistent-merkle-tree/src/tree.ts    |  6 ++++++
 packages/ssz/src/viewDU/listComposite.ts       |  2 ++
 .../unit/byType/listComposite/tree.test.ts     | 18 +++++++++++++-----
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/packages/persistent-merkle-tree/src/tree.ts b/packages/persistent-merkle-tree/src/tree.ts
index 456ed085..e2687310 100644
--- a/packages/persistent-merkle-tree/src/tree.ts
+++ b/packages/persistent-merkle-tree/src/tree.ts
@@ -643,6 +643,12 @@ export function treeZeroAfterIndex(rootNode: Node, nodesDepth: number, index: nu
   // goRight = (N & mask) == mask
   // ```
 
+  // Degenerate case where tree is zero after a negative index (-1).
+  // All positive indexes are zero, so the entire tree is zero. Return cached zero node as root.
+  if (index < 0) {
+    return zeroNode(nodesDepth);
+  }
+
   /**
    * Contiguous filled stack of parent nodes. It get filled in the first descent
    * Indexed by depthi
diff --git a/packages/ssz/src/viewDU/listComposite.ts b/packages/ssz/src/viewDU/listComposite.ts
index b4dfbbaf..b9f87caf 100644
--- a/packages/ssz/src/viewDU/listComposite.ts
+++ b/packages/ssz/src/viewDU/listComposite.ts
@@ -39,6 +39,8 @@ export class ListCompositeTreeViewDU<
    * ```
    *
    * To achieve it, rebinds the underlying tree zero-ing all nodes right of `index`.
+   *
+   * Note: Using index = -1, returns an empty list of length 0.
    */
   sliceTo(index: number): this {
     // Commit before getting rootNode to ensure all pending data is in the rootNode
diff --git a/packages/ssz/test/unit/byType/listComposite/tree.test.ts b/packages/ssz/test/unit/byType/listComposite/tree.test.ts
index a60b325e..573db18e 100644
--- a/packages/ssz/test/unit/byType/listComposite/tree.test.ts
+++ b/packages/ssz/test/unit/byType/listComposite/tree.test.ts
@@ -173,16 +173,24 @@ describe("ListCompositeType.sliceTo", () => {
     const listType = new ListCompositeType(ssz.Root, 1024);
     const listView = listType.defaultViewDU();
     const listRoots: string[] = [];
+    const listSerialized: string[] = [];
 
-    for (let i = 0; i < 16; i++) {
-      listView.push(Buffer.alloc(32, i + 1));
-      listRoots.push(toHexString(listView.serialize()));
+    for (let i = -1; i < 16; i++) {
+      // Skip first loop to persist empty case
+      if (i >= 0) {
+        listView.push(Buffer.alloc(32, 0xf + i)); // Avoid 0 case
+      }
+      // Javascript arrays can handle negative indexes (ok for tests)
+      listSerialized[i] = toHexString(listView.serialize());
+      listRoots[i] = toHexString(listView.hashTreeRoot());
     }
 
-    for (let i = 0; i < 16; i++) {
+    // Start at -1 to test the empty case.
+    for (let i = -1; i < 16; i++) {
       const listSlice = listView.sliceTo(i);
       expect(listSlice.length).to.equal(i + 1, `Wrong length at .sliceTo(${i})`);
-      expect(toHexString(listSlice.serialize())).equals(listRoots[i], `Wrong bytes at .sliceTo(${i})`);
+      expect(toHexString(listSlice.serialize())).equals(listSerialized[i], `Wrong serialize at .sliceTo(${i})`);
+      expect(toHexString(listSlice.hashTreeRoot())).equals(listRoots[i], `Wrong root at .sliceTo(${i})`);
     }
   });
 });