feat: add sliceFrom to ListCompositeTreeViewDU (#366)

packages/ssz/src/viewDU/listComposite.ts

@@ -1,4 +1,4 @@
-import {Node, treeZeroAfterIndex} from "@chainsafe/persistent-merkle-tree";
+import {Node, subtreeFillToContents, treeZeroAfterIndex, zeroNode} from "@chainsafe/persistent-merkle-tree";
 import {ByteViews, ValueOf} from "../type/abstract";
 import {CompositeType, CompositeView, CompositeViewDU} from "../type/composite";
 import {ListCompositeType} from "../view/listComposite";
@@ -66,6 +66,47 @@ export class ListCompositeTreeViewDU<
     return this.type.getViewDU(newRootNode) as this;
   }
 
+  /**
+   * Returns a new ListCompositeTreeViewDU instance with the values from `index` to the end of list
+   *
+   * ```ts
+   * const nodes = getChunkNodes()
+   * return listFromChunkNodes(node.slice(index))
+   * ```
+   *
+   * Note: If index === n, returns an empty list of length 0
+   *
+   */
+  sliceFrom(index: number): this {
+    // Commit before getting rootNode to ensure all pending data is in the rootNode
+    this.commit();
+
+    // If negative index, try to make it positive long as |index| < length
+    if (index < 0) {
+      index += this.nodes.length;
+    }
+    // If slicing from 0 or neg index, no slicing is necesary
+    if (index <= 0) {
+      return this;
+    }
+
+    let newChunksNode;
+    let newLength;
+
+    if (index >= this.nodes.length) {
+      newChunksNode = zeroNode(this.type.chunkDepth);
+      newLength = 0;
+    } else {
+      const nodes = this.nodes.slice(index);
+      newChunksNode = subtreeFillToContents(nodes, this.type.chunkDepth);
+      newLength = nodes.length;
+    }
+
+    const newRootNode = this.type.tree_setChunksNode(this._rootNode, newChunksNode, newLength);
+
+    return this.type.getViewDU(newRootNode) as this;
+  }
+
   /**
    * Same method to `type/listComposite.ts` leveraging cached nodes.
    */

packages/ssz/test/unit/byType/listComposite/tree.test.ts

@@ -194,3 +194,22 @@ describe("ListCompositeType.sliceTo", () => {
     }
   });
 });
+
+describe("ListCompositeType.sliceFrom", () => {
+  it("Slice List from multiple length", () => {
+    const listType = new ListCompositeType(ssz.Root, 1024);
+    const listLength = 16;
+    const list = Array.from({length: listLength}, (_, i) => Buffer.alloc(32, i));
+    const listView = listType.toViewDU(list);
+
+    for (let i = -(listLength + 1); i < listLength + 1; i++) {
+      // compare list.slice(i) to listView.sliceFrom(i), they should be equivalent
+      const slicedList = list.slice(i);
+      const slicedListView = listView.sliceFrom(i);
+
+      expect(slicedListView.length).to.equal(slicedList.length);
+      expect(toHexString(slicedListView.serialize())).to.equal(toHexString(listType.serialize(slicedList)));
+      expect(toHexString(slicedListView.hashTreeRoot())).to.equal(toHexString(listType.hashTreeRoot(slicedList)));
+    }
+  });
+});