Block Assembly (Mempool Selection)

Assemble a block from a mempool using a simple policy:

• Greedy by fee rate (fee / size).
• Include a transaction only if all inputs are available.
• Enforce a max block size.

Types

type Outpoint struct {
    TxID  [32]byte
    Index uint32
}

type Tx struct {
    ID       [32]byte
    Size     int
    Fee      uint64
    Inputs   []Outpoint
    Outputs  int
    Coinbase bool
    Reward   uint64 // for coinbase only
}

type UTXOSet map[Outpoint]bool

type Block struct {
    Txs       []Tx // coinbase must be first
    TotalFees uint64
    TotalSize int
}

func AssembleBlock(mempool []Tx, utxo UTXOSet, maxBlockSize int, subsidy uint64) Block

Rules

• Sort mempool by fee rate (higher first). Tie-breakers: higher fee, then lexicographic ID bytes.
• Start with TotalSize = CoinbaseSize (defined below).
• A tx can be included if:
  • TotalSize + tx.Size <= maxBlockSize
  • all Inputs are in the current UTXO set (including outputs created by already-included txs).
• When a tx is included, remove its spent inputs and add its new outputs: (tx.ID, output_index) for output_index in [0, Outputs).
• Coinbase transaction:
  • CoinbaseSize = 100 bytes
  • Reward = subsidy + TotalFees
  • Coinbase=true, Fee=0, Inputs=nil, Outputs=1, ID=zero-hash

Notes

• Inputs contain individually valid non-coinbase candidates with positive sizes, unique nonzero IDs, no duplicate input within one candidate, and nonnegative output counts fitting uint32. Candidates may conflict or depend on each other.
• maxBlockSize >= 100. Size arithmetic fits int; subsidy plus all candidate fees fits uint64. Fee-rate cross-products may still exceed uint64.
• Do not mutate the caller's mempool or UTXO set. Rejected candidates must consume no inputs, even if some of their inputs are available.
• An outpoint is available exactly when utxo[outpoint] == true; a missing key or a false-valued entry is unavailable.
• Transactions that fail the input or size checks are skipped (not retried).
• Sort once and scan once. This policy is neither package-aware nor guaranteed to maximize fees. The generated coinbase output is not available to candidates.
• Return a block with coinbase as the first transaction.