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bitcoin: expose feerate_floor.

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Onchaind will want it.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
This commit is contained in:
Rusty Russell
2018-06-20 16:17:57 +09:30
committed by Christian Decker
parent 4a5cff8490
commit 0e6c0dbba2
6 changed files with 53 additions and 48 deletions
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50
bitcoin/feerate.h Normal file
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#ifndef LIGHTNING_BITCOIN_FEERATE_H
#define LIGHTNING_BITCOIN_FEERATE_H
#include "config.h"
#include <ccan/build_assert/build_assert.h>
#include <ccan/short_types/short_types.h>
/* bitcoind considers 250 satoshi per kw to be the minimum acceptable fee:
* less than this won't even relay.
*/
#define BITCOIND_MINRELAYTXFEE_PER_KW 250
/*
* But bitcoind uses vbytes (ie. (weight + 3) / 4) for this
* calculation, rather than weight, meaning we can disagree since we do
* it sanely (as specified in BOLT #3).
*/
#define FEERATE_BITCOIND_SEES(feerate, weight) \
(((feerate) * (weight)) / 1000 * 1000 / ((weight) + 3))
/* ie. fee = (feerate * weight) // 1000
* bitcoind needs (worst-case): fee * 1000 / (weight + 3) >= 250
*
* (feerate * weight) // 1000 * 1000 // (weight + 3) >= 250
*
* The feerate needs to be higher for lower weight, and our minimum tx weight
* is 464 (version (4) + count_tx_in (1) + tx_in (32 + 4 + 1 + 4) +
* count_tx_out (1) + amount (8) + P2WSH (1 + 1 + 32) + witness 1 + 1 + <sig>
* + 1 + <key>). Assume it's 400 to give a significant safety margin (it
* only makes 1 difference in the result anyway).
*/
#define MINIMUM_TX_WEIGHT 400
/*
* This formula is satisfied by a feerate of 253 (hand-search).
*/
#define FEERATE_FLOOR 253
static inline u32 feerate_floor(void)
{
/* Assert that bitcoind will see this as above minRelayTxFee */
BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR, MINIMUM_TX_WEIGHT)
>= BITCOIND_MINRELAYTXFEE_PER_KW);
/* And a lesser value won't do */
BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR-1, MINIMUM_TX_WEIGHT)
< BITCOIND_MINRELAYTXFEE_PER_KW);
/* And I'm right about it being OK for larger txs, too */
BUILD_ASSERT(FEERATE_BITCOIND_SEES(FEERATE_FLOOR, (MINIMUM_TX_WEIGHT*2))
>= BITCOIND_MINRELAYTXFEE_PER_KW);
return FEERATE_FLOOR;
}
#endif /* LIGHTNING_BITCOIN_FEERATE_H */