feat: lag blocks config option

src/indexer.test.ts

@@ -462,6 +462,60 @@ describe("counter contract", () => {
     });
   });
 
+  test("lagBlocks config option", async () => {
+    const mockRpcClient = {
+      ...rpcClient,
+      getLastBlockNumber: vi.fn()
+        .mockResolvedValueOnce(4n)
+        .mockResolvedValueOnce(4n)
+        .mockResolvedValue(4n)
+        .mockResolvedValue(6n)
+    };
+
+    const indexer = createIndexer({
+      chain: {
+        id: 1,
+        rpcClient: mockRpcClient,
+        lagBlocks: 2n,
+      },
+      contracts: Contracts,
+    });
+
+    indexer.on("Counter:Increment", handleIncrement);
+    indexer.on("Counter:Decrement", handleDecrement);
+
+    indexer.subscribeToContract({
+      contract: "Counter",
+      address: "0x0000000000000000000000000000000000000001",
+    });
+
+
+    let tryTwice: boolean;
+    indexer.on("progress", ({ currentBlock, targetBlock }) => {
+      console.log(currentBlock);
+      console.log(targetBlock);
+
+      if (currentBlock == targetBlock && targetBlock == 4n){
+        if (tryTwice){
+          indexer.stop();
+        }
+        tryTwice = true;
+      }
+    });
+
+    await new Promise<void>((resolve) => {
+      indexer.on("stopped", () => {
+        resolve();
+        expect(tryTwice).toBe(true);
+      });
+
+      indexer.watch();
+    });
+
+  });
+
+
+
   test("resumable index with the same indexer instance", async () => {
     const indexer = createIndexer({
       chain: {

src/indexer.ts

@@ -38,6 +38,7 @@ export type Config<TAbis extends Record<string, Abi>, TContext = unknown> = {
     rpcClient: RpcClient;
     pollingIntervalMs?: number;
     maxBlockRange?: bigint;
+    lagBlocks?: bigint; // New field for lag indexing strategy
   };
   context?: TContext;
   logLevel?: keyof typeof LogLevel;
@@ -176,6 +177,11 @@ export function createIndexer<
       //  latest is a moving target
       if (state.finalTargetBlock === "latest") {
         finalTargetBlock = await rpcClient.getLastBlockNumber();
+
+        // Apply lag if specified
+        if (config.chain.lagBlocks) {
+          finalTargetBlock = applyLag(finalTargetBlock);
+        }
       } else {
         finalTargetBlock = state.finalTargetBlock;
       }
@@ -464,6 +470,14 @@ export function createIndexer<
     }
   }
 
+  // Helper function to apply lag
+  function applyLag(blockNumber: bigint): bigint {
+    if (config.chain.lagBlocks && blockNumber > BigInt(config.chain.lagBlocks)) {
+      return blockNumber - BigInt(config.chain.lagBlocks);
+    }
+    return 0n;
+  }
+
   return Object.setPrototypeOf(
     {
       context: config.context,
@@ -524,6 +538,11 @@ export function createIndexer<
 
         if (target === "latest") {
           targetBlock = await rpcClient.getLastBlockNumber();
+
+          // Apply lag if specified
+          if (config.chain.lagBlocks) {
+            targetBlock = applyLag(targetBlock);
+          }
         } else {
           targetBlock = target;
         }