Events

Events are classified as logs on the blockchain. It ranges from log0 to log4.

log[n]

n shows the number of topics in an event. For anonymous events they can have up to 4 indexed event arguments. For the other types of events, the first topic is the event hash, a 32 byte value of the hashed event signature. The subsequent topics are indexed event arguments.

If your anonymous event has x indexed arguments where 0 <= x <= 4, the corresponding log[n] to be used would be log[x].

If your non-anonymous event has x indexed arguments where 0 <= x <= 3, the corresponding log[n] to be used would be log[x + 1].

The log[n] for a non-anonymous event takes in a range of 2 to 6 arguments, as defined by n + 2, the first two being the part of the calldata where non-indexed values are stored according to the ABI Specification. Then the number of indexed events as much as n.

If we have a non-anonymous event with 2 indexed arguments, we will use log3, and log3 would take in 5 arguments.

These five arguments would be log2(a, b, c, d, e), a, the offset, the part of memory where the stored non-indexed values start, b, the size of the memory to be read, c, the event topic, d, the value of the first indexed argument, e, the value of the second indexed argument.

If we have an anonymous event with 2 indexed arguments, we will use log2, and log2 would take in 4 arguments.

These four arguments would be log2(a, b, c, d), a, the offset, the part of memory where the stored non-indexed values start, b, the size of the memory to be read, c, the value of the first indexed argument, d, the value of the second indexed argument.

anonymous events without indexed arguments are emitted using log0.

indexed Event

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    // Event to emit.
    event IndexedEvent(address indexed a, uint256 indexed b);

    bytes32 public constant IndexedEventTopic = 0xfdcfbb6e25802e9ba4d7c62d4a85a10e40219c69383d35be084b401980dd7156;

    function emitIndexedEvent(uint256 x) public {
        assembly {
            log3(0x00, 0x00, IndexedEventTopic, caller(), x)
        }
    }
}

Non-indexed Event

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    // Event to emit.
    event NonIndexedEvent(uint256 a, uint256 b);

    bytes32 public constant NonIndexedEventTopic = 0x8a2dba84c9d33350ff3006c2607aae76d062ae5ac6632d800030613bcf7f74a0;
    
    function emitNonIndexedEvent(uint256 a, uint256 b) public {
        assembly {
            mstore(0x80, a)
            mstore(0xa0, b)
            log1(0x80, 0x40, NonIndexedEventTopic)
        }
    }
}

anonymous indexed Events

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    // Event to emit.
    event AnonymousIndexedEvent(address indexed a, uint256 indexed b) anonymous;

    function emitAnonymousIndexedEvent(uint256 x) public {
        assembly {
            log3(0x00, 0x00, caller(), x)
        }
    }
}

anonymous non-indexed Events

// SPDX-License-Identifier: GPL-3.0
pragma solidity ^0.8.0;

contract Yul {
    // Event to emit.
    event AnonymousNonIndexedEvent(address a, uint256 b) anonymous;
    
    function emitAnonymousNonIndexedEvent(uint256 a, uint256 b) public {
        assembly {
            mstore(0x80, a)
            mstore(0xa0, b)
            log1(0x80, 0x40)
        }
    }
}