Outputs

Outputs define which state elements (usually: UTXOs) are produced by a transaction. Each state element can be produced and consumed at most once.

OutputType

enum OutputType : uint8 {
    Transfer = 0,
    Withdraw = 1,
    HTLC = 2,
    Return = 3,
}

Output

name	type	size	description
type	OutputType	1	OutputType.Transfer or OutputType.Withdraw.
token.length	uint8	1	Next field number of bytes.
token	bytes1[]	1*	The token ID (<= 4 bytes). See: token registry.
amount.shift	uint8	1	Amount shifted bytes. See: serialization.
amount.length	uint8	1	Next field number of bytes.
amount	uint8[]	1*	Amount of tokens. See: serialization.
owner.length	uint8	1	Next field number of bytes.
owner	bytes1[]	1*	Either address ID (<= 19 bytes) or address (= 20 bytes) of recipient. See: address registry.

A simple transfer between two users on Fuel or a withdrawal from Fuel back to Ethereum.

OutputHTLC

name	type	size	description
type	OutputType	1	OutputType.HTLC.
token.length	uint8	1	Next field number of bytes.
token	bytes1[]	1*	The token ID (<= 4 bytes). See: token registry.
amount.shift	uint8	1	Amount shifted bytes. See: serialization.
amount.length	uint8	1	Next field number of bytes.
amount	uint8[]	1*	Amount of tokens. See: serialization.
owner.length	uint8	1	Next field number of bytes.
owner	bytes1[]	1*	Either address ID (<= 19 bytes) or address (= 20 bytes) of recipient. See: address registry.
digest	bytes32	32	Hashlock digest. The preimage to this digest must be revealed to spend this output.
expiry	uint32	4	Timelock as an Ethereum block number.
returnOwner.length	uint8	1	Next field number of bytes.
returnOwner	bytes1[]	1*	Either address ID (<= 19 bytes) or address (= 20 bytes) of owner if HTLC expires. See: address registry.

The timelock expiry is an absolute Ethereum block number. Any rollup block committed at or after this Ethereum block number can spend the UTXO with the timelock condition.

See also: HTLC explainer on Bitcoin Wiki.

OutputReturn

name	type	size	description
type	OutputType	1	OutputType.Return.
data.length	uint16	2	Next field number of bytes.
data	bytes1[]	1*	Data blob. Between 1 and 512 bytes, inclusive.

Return outputs are provably prunable, i.e. they are not inserted into the state (the UTXO set). They can be used to record and timestamp arbitrary information to the Fuel chain, useful for building applications with client-side execution.

See also: Bitcoin's OP_RETURN.

UTXO

name	type	size	description
transactionId	bytes32	32	Transaction ID, the hash of the serialized unsigned transaction data.
outputIndex	uint256	32	Index of output in transaction.
outputType	uint256	32	Output's type.
owner	bytes32	32	Owner of UTXO, either a raw address or an address ID.
amount	uint256	32	Amount of tokens.
token	uint256	32	Token ID.
digest	bytes32	32	HTLC only: hashlock.
expiry	uint256	32	HTLC only: timelock.
returnOwner	bytes32	32	HTLC only: return owner.

The UTXO ID is simply the hash of the UTXO.

The HTLC-only fields are set to zero (0) if the output is not an HTLC output.