Event signature to topic0, and function selector calculator

Every log a contract emits carries up to four 32-byte topics. For a normal (non-anonymous) event, the first one, topic0, is the keccak-256 hash of the event's canonical signature: its name and parameter types, with no spaces, no parameter names and no indexed keyword.

That is why Transfer(address indexed from, address indexed to, uint256 value) and Transfer(address,address,uint256) have the same topic0, and why ERC-20 and ERC-721 Transfer events share it even though they index different parameters. Explorers, indexers and alert rules all filter logs by topic0.

A function selector is the first four bytes of the keccak-256 hash of a function's canonical signature. It is the first thing in a transaction's input data and tells the contract which function to run; the encoded arguments follow it.

Because only four bytes are kept, different signatures can share a selector. A selector tells you which function was probably called; the full signature, or the contract's ABI, tells you for sure.

The hash is only right when the signature is exactly what the compiler hashes. The tool normalises it for you:

• Parameter names, indexed, memory, calldata and spaces are dropped.
• uint and int become uint256 and int256.
• Structs become tuples of their member types, e.g. (address,uint256)[] for an array of structs.
• The function or event keyword and any returns clause are ignored.