Comptime Selectors

eth.zig provides unified keccak.selector() and keccak.hash() functions that work at both compile time and runtime. The caller decides when to evaluate -- the library never restricts you.

Function Selectors

A Solidity function selector is the first 4 bytes of the Keccak-256 hash of the function signature. Use comptime for zero runtime cost:

const eth = @import("eth");

// Evaluated at compile time -- zero runtime cost
const transfer_sel = comptime eth.keccak.selector("transfer(address,uint256)");
// transfer_sel == [4]u8{ 0xa9, 0x05, 0x9c, 0xbb }

const approve_sel = comptime eth.keccak.selector("approve(address,uint256)");
// approve_sel == [4]u8{ 0x09, 0x5e, 0xa7, 0xb3 }

The same function also works at runtime with dynamic strings:

// Runtime -- useful when the signature is only known at runtime
const runtime_sel = eth.keccak.selector(some_signature);

Event Topics

Event topics (used for log filtering) work the same way:

const eth = @import("eth");

const transfer_topic = comptime eth.keccak.hash("Transfer(address,address,uint256)");
// transfer_topic == keccak256("Transfer(address,address,uint256)")

const approval_topic = comptime eth.keccak.hash("Approval(address,address,uint256)");

Why This Matters

Approach	When It Runs	Cost
`comptime eth.keccak.selector(...)`	Compile time	0 ns at runtime
`eth.keccak.selector(runtime_str)`	Runtime	~128 ns per hash

For hot paths (MEV bots, indexers, high-frequency DeFi), eliminating per-call hashing overhead adds up.

ERC-20 Precomputed Selectors

The ERC-20 wrapper provides all standard selectors as comptime constants:

const selectors = eth.erc20.selectors;

selectors.name;          // name()
selectors.symbol;        // symbol()
selectors.decimals;      // decimals()
selectors.totalSupply;   // totalSupply()
selectors.balanceOf;     // balanceOf(address)
selectors.transfer;      // transfer(address,uint256)
selectors.approve;       // approve(address,uint256)
selectors.allowance;     // allowance(address,address)
selectors.transferFrom;  // transferFrom(address,address,uint256)

Custom Selectors

Define selectors for any contract function:

const eth = @import("eth");

// UniswapV2 Router
const swap_sel = comptime eth.keccak.selector(
    "swapExactTokensForTokens(uint256,uint256,address[],address,uint256)"
);

// Aave V3 Pool
const supply_sel = comptime eth.keccak.selector(
    "supply(address,uint256,address,uint16)"
);

// Any custom function
const my_sel = comptime eth.keccak.selector(
    "myFunction(uint256,bytes32,bool)"
);