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51 lines
1.8 KiB
51 lines
1.8 KiB
7 years ago
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#ifndef LIGHTNING_BITCOIN_FEERATE_H
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#define LIGHTNING_BITCOIN_FEERATE_H
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#include "config.h"
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#include <ccan/build_assert/build_assert.h>
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#include <ccan/short_types/short_types.h>
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/* bitcoind considers 250 satoshi per kw to be the minimum acceptable fee:
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* less than this won't even relay.
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*/
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#define BITCOIND_MINRELAYTXFEE_PER_KW 250
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/*
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* But bitcoind uses vbytes (ie. (weight + 3) / 4) for this
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* calculation, rather than weight, meaning we can disagree since we do
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* it sanely (as specified in BOLT #3).
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*/
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#define FEERATE_BITCOIND_SEES(feerate, weight) \
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(((feerate) * (weight)) / 1000 * 1000 / ((weight) + 3))
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/* ie. fee = (feerate * weight) // 1000
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* bitcoind needs (worst-case): fee * 1000 / (weight + 3) >= 250
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*
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* (feerate * weight) // 1000 * 1000 // (weight + 3) >= 250
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*
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* The feerate needs to be higher for lower weight, and our minimum tx weight
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* is 464 (version (4) + count_tx_in (1) + tx_in (32 + 4 + 1 + 4) +
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* count_tx_out (1) + amount (8) + P2WSH (1 + 1 + 32) + witness 1 + 1 + <sig>
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* + 1 + <key>). Assume it's 400 to give a significant safety margin (it
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* only makes 1 difference in the result anyway).
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*/
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#define MINIMUM_TX_WEIGHT 400
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/*
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* This formula is satisfied by a feerate of 253 (hand-search).
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*/
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#define FEERATE_FLOOR 253
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static inline u32 feerate_floor(void)
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{
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/* Assert that bitcoind will see this as above minRelayTxFee */
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BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR, MINIMUM_TX_WEIGHT)
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>= BITCOIND_MINRELAYTXFEE_PER_KW);
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/* And a lesser value won't do */
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BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR-1, MINIMUM_TX_WEIGHT)
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< BITCOIND_MINRELAYTXFEE_PER_KW);
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/* And I'm right about it being OK for larger txs, too */
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BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR, (MINIMUM_TX_WEIGHT*2))
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>= BITCOIND_MINRELAYTXFEE_PER_KW);
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return FEERATE_FLOOR;
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}
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#endif /* LIGHTNING_BITCOIN_FEERATE_H */
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