LOG Instructions (0xa0-0xa4)

375 gas (base)
+ 375 gas per topic
+ 8 gas per byte of data

// This will revert
function badLog() external view {
    // emit event - reverts in view functions
}

event Transfer(address indexed from, address indexed to, uint256 value);

function transfer(address to, uint256 amount) public {
    balances[msg.sender] -= amount;
    balances[to] += amount;
    emit Transfer(msg.sender, to, amount);  // Compiler generates LOG2
}

// Listen for Transfer events from specific address
const logs = await getLogs({
  address: tokenAddress,
  topics: [
    keccak256("Transfer(address,indexed address,indexed uint256)"),
    null,                                    // Match any 'from'
    "0xaddressToFilterFor"                   // Match specific 'to'
  ]
});

event Approval(address indexed owner, address indexed spender, uint256 value);

function approve(address spender, uint256 amount) public {
    allowances[msg.sender][spender] = amount;
    emit Approval(msg.sender, spender, amount);  // LOG2
    return true;
}

event StringLog(string indexed data);
// Topic is keccak256(data), not the string itself

event DynamicLog(uint256[] indexed arr);
// Topic is keccak256(abi.encode(arr)), not individual values

event Transfer(
  address indexed from,    // Topic 1
  address indexed to,      // Topic 2
  uint256 value           // Data (non-indexed)
);

// Generates: LOG2 with topics=[from, to] and data=abi.encode(value)