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Usage Patterns

Practical patterns for filtering, parsing, and processing Ethereum event logs.

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Log Filtering

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Filter by Event Signature

import * as EventLog from 'tevm/EventLog';
import * as Event from 'tevm/Event';

// Define Transfer event
const transferEvent = {
  type: "event",
  name: "Transfer",
  inputs: [
    { type: "address", name: "from", indexed: true },
    { type: "address", name: "to", indexed: true },
    { type: "uint256", name: "value", indexed: false }
  ]
};

// Get logs for specific event
async function getTransferLogs(
  provider: Provider,
  tokenAddress: string,
  fromBlock: number,
  toBlock: number
): Promise<EventLog[]> {
  const topic0 = Event.getSelector(transferEvent);

  const logs = await provider.getLogs({
    address: tokenAddress,
    fromBlock,
    toBlock,
    topics: [topic0]
  });

  return logs.map(log => EventLog(log));
}

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Filter by Indexed Parameters

// Filter Transfer events from specific address
async function getTransfersFrom(
  provider: Provider,
  tokenAddress: string,
  fromAddress: string
): Promise<EventLog[]> {
  const topic0 = Event.getSelector(transferEvent);
  const topic1 = Event.encodeIndexed(
    { type: "address" },
    fromAddress
  );

  const logs = await provider.getLogs({
    address: tokenAddress,
    topics: [topic0, topic1]  // topic0 = event sig, topic1 = from
  });

  return logs.map(log => EventLog(log));
}

// Filter by multiple addresses (OR condition)
async function getTransfersFromMultiple(
  provider: Provider,
  tokenAddress: string,
  addresses: string[]
): Promise<EventLog[]> {
  const topic0 = Event.getSelector(transferEvent);
  const topics1 = addresses.map(addr =>
    Event.encodeIndexed({ type: "address" }, addr)
  );

  const logs = await provider.getLogs({
    address: tokenAddress,
    topics: [topic0, topics1]  // Multiple options for topic1
  });

  return logs.map(log => EventLog(log));
}

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Log Parsing

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Decode Event Data

// Parse Transfer event
function parseTransferLog(log: EventLogType): {
  from: string;
  to: string;
  value: bigint;
} {
  // Decode indexed topics
  const from = EventLog.getIndexed(log, 0);  // First indexed param
  const to = EventLog.getIndexed(log, 1);    // Second indexed param

  // Decode non-indexed data
  const decoded = Event.decodeLog(transferEvent, log.data, log.topics);

  return {
    from: decoded.from,
    to: decoded.to,
    value: decoded.value
  };
}

// Batch parsing
function parseTransferLogs(logs: EventLogType[]): Array<{
  from: string;
  to: string;
  value: bigint;
  blockNumber: number;
  transactionHash: string;
}> {
  return logs.map(log => ({
    ...parseTransferLog(log),
    blockNumber: log.blockNumber,
    transactionHash: log.transactionHash
  }));
}

​
Multi-event Parsing

// Parse mixed event types
interface EventRegistry {
  [signature: string]: {
    name: string;
    definition: any;
  };
}

function createEventRegistry(events: any[]): EventRegistry {
  const registry: EventRegistry = {};

  for (const event of events) {
    const sig = Event.getSelector(event);
    registry[sig] = {
      name: event.name,
      definition: event
    };
  }

  return registry;
}

// Parse with registry
function parseLog(
  log: EventLogType,
  registry: EventRegistry
): { name: string; args: any } | null {
  const signature = EventLog.getSignature(log);
  const event = registry[signature];

  if (!event) return null;

  const args = Event.decodeLog(event.definition, log.data, log.topics);

  return {
    name: event.name,
    args
  };
}

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Log Filtering and Sorting

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Filter by Address

// Check if log is from specific address
function isFromAddress(
  log: EventLogType,
  address: string
): boolean {
  return EventLog.matchesAddress(log, address);
}

// Filter logs by address
function filterByAddress(
  logs: EventLogType[],
  address: string
): EventLogType[] {
  return logs.filter(log => EventLog.matchesAddress(log, address));
}

// Filter by multiple addresses
function filterByAddresses(
  logs: EventLogType[],
  addresses: string[]
): EventLogType[] {
  return logs.filter(log =>
    addresses.some(addr => EventLog.matchesAddress(log, addr))
  );
}

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Filter by Topics

// Create topic filter
interface TopicFilter {
  topic0?: string;
  topic1?: string | string[];
  topic2?: string | string[];
  topic3?: string | string[];
}

function matchesTopicFilter(
  log: EventLogType,
  filter: TopicFilter
): boolean {
  return EventLog.matchesTopics(log, [
    filter.topic0,
    filter.topic1,
    filter.topic2,
    filter.topic3
  ]);
}

// Filter logs
function filterLogs(
  logs: EventLogType[],
  filter: TopicFilter
): EventLogType[] {
  return logs.filter(log => matchesTopicFilter(log, filter));
}

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Sort Logs

// Sort by block number and log index
function sortLogs(logs: EventLogType[]): EventLogType[] {
  return EventLog.sortLogs(logs);
}

// Sort in reverse (newest first)
function sortLogsDescending(logs: EventLogType[]): EventLogType[] {
  return EventLog.sortLogs(logs).reverse();
}

// Group by block
function groupByBlock(
  logs: EventLogType[]
): Map<number, EventLogType[]> {
  const groups = new Map<number, EventLogType[]>();

  for (const log of logs) {
    const blockLogs = groups.get(log.blockNumber) ?? [];
    blockLogs.push(log);
    groups.set(log.blockNumber, blockLogs);
  }

  return groups;
}

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Reorg Handling

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Detect Removed Logs

// Check if log was removed due to reorg
function checkForReorgs(logs: EventLogType[]): {
  valid: EventLogType[];
  removed: EventLogType[];
} {
  const valid: EventLogType[] = [];
  const removed: EventLogType[] = [];

  for (const log of logs) {
    if (EventLog.isRemoved(log)) {
      removed.push(log);
    } else {
      valid.push(log);
    }
  }

  return { valid, removed };
}

// Filter out removed logs
function filterRemoved(logs: EventLogType[]): EventLogType[] {
  return logs.filter(log => !EventLog.isRemoved(log));
}

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Reorg-safe Log Processing

// Process logs with reorg handling
async function processLogsWithReorgHandling(
  provider: Provider,
  fromBlock: number,
  toBlock: number,
  confirmations: number = 12
): Promise<void> {
  const confirmedBlock = await provider.getBlockNumber() - confirmations;
  const actualToBlock = Math.min(toBlock, confirmedBlock);

  const logs = await provider.getLogs({
    fromBlock,
    toBlock: actualToBlock
  });

  // Process only confirmed logs
  const validLogs = filterRemoved(logs.map(EventLog.from));

  for (const log of validLogs) {
    await processLog(log);
  }
}

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Real-time Monitoring

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Subscribe to Events

// Monitor Transfer events in real-time
async function monitorTransfers(
  provider: Provider,
  tokenAddress: string,
  callback: (log: EventLogType) => void
): Promise<() => void> {
  const topic0 = Event.getSelector(transferEvent);

  // Subscribe to logs
  const filter = {
    address: tokenAddress,
    topics: [topic0]
  };

  provider.on(filter, (rawLog) => {
    const log = EventLog(rawLog);
    callback(log);
  });

  // Return unsubscribe function
  return () => provider.off(filter);
}

// Usage
const unsubscribe = await monitorTransfers(
  provider,
  tokenAddress,
  (log) => {
    const { from, to, value } = parseTransferLog(log);
    console.log(`Transfer: ${from} → ${to}: ${value}`);
  }
);

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Batch Processing

// Process logs in batches
async function processLogsBatched(
  provider: Provider,
  fromBlock: number,
  toBlock: number,
  batchSize: number = 10000
): Promise<void> {
  for (let start = fromBlock; start <= toBlock; start += batchSize) {
    const end = Math.min(start + batchSize - 1, toBlock);

    const logs = await provider.getLogs({
      fromBlock: start,
      toBlock: end
    });

    const eventLogs = logs.map(EventLog.from);
    await processBatch(eventLogs);

    console.log(`Processed blocks ${start} to ${end}`);
  }
}

​
Aggregation

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Calculate Totals

// Sum transfer amounts
function calculateTotalTransferred(
  logs: EventLogType[]
): bigint {
  let total = 0n;

  for (const log of logs) {
    const { value } = parseTransferLog(log);
    total += value;
  }

  return total;
}

// Group by sender
function groupBySender(
  logs: EventLogType[]
): Map<string, bigint> {
  const totals = new Map<string, bigint>();

  for (const log of logs) {
    const { from, value } = parseTransferLog(log);
    const current = totals.get(from) ?? 0n;
    totals.set(from, current + value);
  }

  return totals;
}

​
Track Balances

// Build balance map from Transfer events
function buildBalanceMap(
  logs: EventLogType[]
): Map<string, bigint> {
  const balances = new Map<string, bigint>();

  const sorted = EventLog.sortLogs(logs);

  for (const log of sorted) {
    const { from, to, value } = parseTransferLog(log);

    // Debit from sender
    if (from !== "0x0000000000000000000000000000000000000000") {
      const fromBalance = balances.get(from) ?? 0n;
      balances.set(from, fromBalance - value);
    }

    // Credit to receiver
    const toBalance = balances.get(to) ?? 0n;
    balances.set(to, toBalance + value);
  }

  return balances;
}

​
Testing

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Mock Logs

// Create test log
function createTestLog(overrides?: Partial<EventLog>): EventLogType {
  return EventLog.create({
    address: "0x1234567890123456789012345678901234567890",
    topics: [
      Event.getSelector(transferEvent),
      Event.encodeIndexed({ type: "address" }, "0xalice"),
      Event.encodeIndexed({ type: "address" }, "0xbob")
    ],
    data: Event.encode([{ type: "uint256" }], [1000n]),
    blockNumber: 100,
    transactionHash: "0xabc123",
    transactionIndex: 0,
    blockHash: "0xblock",
    logIndex: 0,
    removed: false,
    ...overrides
  });
}

// Create batch of test logs
function createTestLogs(count: number): EventLogType[] {
  return Array({ length: count }, (_, i) =>
    createTestLog({
      blockNumber: 100 + i,
      logIndex: i
    })
  );
}