Function change_amount

bitcoin/tx.c:897–920

}

struct amount_sat change_amount(struct amount_sat excess, u32 feerate_perkw,
				size_t total_weight)
{
	size_t outweight;
	struct amount_sat change_fee;

	/* Must be able to pay for its own additional weight */
	outweight = bitcoin_tx_output_weight(BITCOIN_SCRIPTPUBKEY_P2WPKH_LEN);

	/* Rounding can cause off by one errors, so we do this */
	if (!amount_sat_sub(&change_fee,
			    amount_tx_fee(feerate_perkw, outweight + total_weight),
			    amount_tx_fee(feerate_perkw, total_weight)))
		return AMOUNT_SAT(0);

	if (!amount_sat_sub(&excess, excess, change_fee))
		return AMOUNT_SAT(0);

	/* Must be non-dust */
	if (!amount_sat_greater_eq(excess, chainparams->dust_limit))
		return AMOUNT_SAT(0);

	return excess;
}