# Relayer Calls

Relayer calls allow an end-user to sign a Ambient command using an off-chain EIP-712 signature. This signed command can then be executed inside an Ethereum transaction sent by a third-party relayer. The user can optionally "tip" the relayer to compensate for the gas cost.

```
function userCmdRelayer (
  uint16 callpathIdx, 
  bytes cmd, 
  bytes conds,
  bytes tip,
  bytes signature)
  returns (bytes memory)
```

`callpathIdx` and `cmd` are the same parameters that would be passed to the standard `userCmd` and will behave identically.

#### Conds

`conds` is a bytestring encoded in a fixed format that governs the conditions of the relayer call. If any of the set conditions are not met at execution call, the transaction will fail. The bytestring is encoded as follows

```
conds = abi.encode(
  deadline,    // uint48 
  alive,       // uint48
  salt,        // bytes32
  nonce,       // uint32
  relayer)     // address 
```

`block.timestamp` must occur before `deadline` and after `alive` 

`nonce` defines a user-specific relayer nonce that increments by one on every relayer call. The user's current on-chain nonce must match the condition or the transaction will falt. This is to prevent replay attacks. Every unique value of `salt` defines a unique nonce track (that starts at 0), this allows for multidimensional nonces.

`relayer` restricts the address of the relayer that can execute the command. Either `msg.sender` or `tx.origin` must match this address or the contract call will revert. If this value is set to `address(0)` the condition is unenforced.

#### tip

The bytestring of `tip` can either be empty (in which case no tip is paid to the relayer) or can be set using the following fixed encoding scheme:

```
tip = abi.encode(
  token,     // address
  amount,    // uint128
  recv)      // address 
```

`token` is the token the tip is being paid in. `amount` is the total amount of the token being paid as a tip to relayer.

`recv` is the recipient of the relayer tip. It can either be set to a specific address (either EOA or smart contract), or can be generically paid based on the following magic values:

* `address(256)` - Tip is paid to the `msg.sender` of the contract caller
* `address(512)` - Tip is paid to the `tx.origin` of the Ethereum transaction.

Tips are always paid from the surplus collateral balance of the end-user. If the end-user has insufficient surplus collateral the contract call will revert.

#### signature

This is the EIP-712 signature of the command and conditions. It is constructed from the standard v, r, and s recovery signature as follows:

```
signature = abi.encode(
  v,       // uint8
  r,       // uint256
  s)       // uint256  
```

The signature is constructed using the EIP-712 standard. The EIP-712 domain hash is constructed as:

```
const domain = {
  name: "CrocSwap",
  chainId: [chain ID],
  verifyingContract: [CrocSwapDex contract address],
  version: "1.0"
}
```

The typed content hash is constructed as

```
CrocRelayerCall: [
  { name: "callpath", type: "uint8"},
  { name: "cmd", type: "bytes" },
  { name: "conds", type: "bytes" },
  { name: "tip", type: "bytes" }
]
```