eth-concepts

Compute the slot as keccak256 of the key concatenated with the mapping's declared slot, both left-padded to 32 bytes. Use `cast index address 0xUser 3 | xargs cast storage 0xContract` to read it in one command.

Transactions specify maxFeePerGas and maxPriorityFeePerGas. The effective price is min(maxFee, baseFee + priorityFee), where the base fee burns and adjusts 12.5% per block toward a 15M gas target. Overpayment of maxFee is refunded.

CALL executes code in the callee's context and can send ETH. DELEGATECALL runs the target's code against the caller's storage and msg.sender, which is how proxies work. STATICCALL forbids any state modification and is used for view functions.

Common causes are unbounded loops, cold storage access costing 2,100 gas per slot, quadratic memory expansion, and the 63/64 rule limiting gas forwarded to nested calls. Trace the failure with `cast run 0xTxHash` to find where gas ran out.

Packing only saves gas when packed variables are read or written in the same transaction. Declaration order determines packing, structs always start a new slot, and you should verify the actual layout with `forge inspect Contract storage-layout`.

Use abi.encode whenever hashing multiple values for signatures or storage keys, because abi.encodePacked is ambiguous for dynamic types and can produce hash collisions. encodePacked is only safe for fixed-length types.