core-lightning/funding.c

#include "funding.h"
#include <assert.h>
#include <ccan/mem/mem.h>
#include <ccan/structeq/structeq.h>
#include <string.h>

uint64_t fee_by_feerate(size_t txsize, uint32_t fee_rate)
{
	/* BOLT #2:
	 * 
	 * The fee for a transaction MUST be calculated by multiplying this
	 * bytecount by the fee rate, dividing by 1000 and truncating
	 * (rounding down) the result to an even number of satoshis.
	 */
	return txsize * fee_rate / 2000 * 2;
}

static uint64_t calculate_fee_msat(size_t num_nondust_htlcs,
				   uint32_t fee_rate)
{
	uint64_t bytes;

	/* BOLT #2:
	 *
	 * A node MUST use the formula 338 + 32 bytes for every
	 * non-dust HTLC as the bytecount for calculating commitment
	 * transaction fees.  Note that the fee requirement is
	 * unchanged, even if the elimination of dust HTLC outputs has
	 * caused a non-zero fee already.
	*/
	bytes = 338 + 32 * num_nondust_htlcs;

	/* milli-satoshis */
	return fee_by_feerate(bytes, fee_rate) * 1000;
}

/* Total, in millisatoshi. */
static uint64_t htlcs_total(const struct channel_htlc *htlcs)
{
	size_t i, n = tal_count(htlcs);
	uint64_t total = 0;

	for (i = 0; i < n; i++)
		total += htlcs[i].msatoshis;
	return total;
}

/* Pay this much fee, if possible.  Return amount unpaid. */
static uint64_t pay_fee(struct channel_oneside *side, uint64_t fee_msat)
{
	if (side->pay_msat >= fee_msat) {
		side->pay_msat -= fee_msat;
		side->fee_msat += fee_msat;
		return 0;
	} else {
		uint64_t remainder = fee_msat - side->pay_msat;
		side->fee_msat += side->pay_msat;
		side->pay_msat = 0;
		return remainder;
	}
}

/* Charge the fee as per BOLT #2 */
static void recalculate_fees(struct channel_oneside *a,
			     struct channel_oneside *b,
			     uint64_t fee_msat)
{
	uint64_t remainder;

	/* Fold in fees, to recalcuate again below. */
	a->pay_msat += a->fee_msat;
	b->pay_msat += b->fee_msat;
	a->fee_msat = b->fee_msat = 0;

	/* BOLT #2:
	 *
	 * 1. If each nodes can afford half the fee from their
	 *    to-`final_key` output, reduce the two to-`final_key`
	 *    outputs accordingly.
	 *
	 * 2. Otherwise, reduce the to-`final_key` output of one node
	 *    which cannot afford the fee to zero (resulting in that
	 *    entire output paying fees).  If the remaining
	 *    to-`final_key` output is greater than the fee remaining,
	 *    reduce it accordingly, otherwise reduce it to zero to
	 *    pay as much fee as possible.
	 */
	remainder = pay_fee(a, fee_msat / 2) + pay_fee(b, fee_msat / 2);

	/* If there's anything left, the other side tries to pay for it. */
	remainder = pay_fee(a, remainder);
	pay_fee(b, remainder);
}

/* a transfers htlc_msat to a HTLC (gains it, if -ve) */
static bool change_funding(uint64_t anchor_satoshis,
			   uint32_t fee_rate,
			   int64_t htlc_msat,
			   struct channel_oneside *a,
			   struct channel_oneside *b,
			   size_t num_nondust_htlcs)
{
	uint64_t fee_msat;

	assert(a->pay_msat + a->fee_msat
	       + b->pay_msat + b->fee_msat
	       + htlcs_total(a->htlcs) + htlcs_total(b->htlcs)
	       == anchor_satoshis * 1000);

	fee_msat = calculate_fee_msat(num_nondust_htlcs, fee_rate);

	/* If A is paying, can it afford it? */
	if (htlc_msat > 0) {
		if (htlc_msat + fee_msat / 2 > a->pay_msat + a->fee_msat)
			return false;
	}

	/* OK, now adjust funds for A, then recalculate fees. */
	a->pay_msat -= htlc_msat;
	recalculate_fees(a, b, fee_msat);

	assert(a->pay_msat + a->fee_msat
	       + b->pay_msat + b->fee_msat
	       + htlcs_total(a->htlcs) + htlcs_total(b->htlcs) + htlc_msat
	       == anchor_satoshis * 1000);
	return true;
}

struct channel_state *initial_funding(const tal_t *ctx,
				      bool am_funder,
				      uint64_t anchor_satoshis,
				      uint32_t fee_rate)
{
	uint64_t fee_msat;
	struct channel_state *cstate = talz(ctx, struct channel_state);

	cstate->a.htlcs = tal_arr(cstate, struct channel_htlc, 0);
	cstate->b.htlcs = tal_arr(cstate, struct channel_htlc, 0);
	cstate->fee_rate = fee_rate;
	cstate->anchor = anchor_satoshis;
	cstate->changes = 0;

	/* Anchor must fit in 32 bit. */
	if (anchor_satoshis >= (1ULL << 32) / 1000)
		return tal_free(cstate);

	fee_msat = calculate_fee_msat(0, fee_rate);
	if (fee_msat > anchor_satoshis * 1000)
		return tal_free(cstate);

	/* Initially, all goes back to funder. */
	cstate->a.pay_msat = anchor_satoshis * 1000 - fee_msat;
	cstate->a.fee_msat = fee_msat;

	/* If B (not A) is funder, invert. */
	if (!am_funder)
		invert_cstate(cstate);

	/* Make sure it checks out. */
	assert(change_funding(anchor_satoshis, fee_rate, 0,
			      &cstate->a, &cstate->b, 0));
	if (am_funder) {
		assert(cstate->a.fee_msat == fee_msat);
		assert(cstate->b.fee_msat == 0);
	} else {
		assert(cstate->b.fee_msat == fee_msat);
		assert(cstate->a.fee_msat == 0);
	}
	return cstate;
}

/* Dust is defined as an output < 546*minRelayTxFee/1000.
 * minRelayTxFee defaults to 1000 satoshi. */
bool is_dust_amount(uint64_t satoshis)
{
	return satoshis < 546;
}

static size_t count_nondust_htlcs(const struct channel_htlc *htlcs)
{
	size_t i, n = tal_count(htlcs), nondust = 0;

	for (i = 0; i < n; i++)
		if (!is_dust_amount(htlcs[i].msatoshis / 1000))
			nondust++;
	return nondust;
}

static size_t total_nondust_htlcs(const struct channel_state *cstate)
{
	return count_nondust_htlcs(cstate->a.htlcs)
		+ count_nondust_htlcs(cstate->b.htlcs);
}

void adjust_fee(struct channel_state *cstate, uint32_t fee_rate)
{
	uint64_t fee_msat;

	fee_msat = calculate_fee_msat(total_nondust_htlcs(cstate), fee_rate);

	recalculate_fees(&cstate->a, &cstate->b, fee_msat);
	cstate->changes++;
}

bool force_fee(struct channel_state *cstate, uint64_t fee)
{
	/* Beware overflow! */
	if (fee > 0xFFFFFFFFFFFFFFFFULL / 1000)
		return false;
	recalculate_fees(&cstate->a, &cstate->b, fee * 1000);
	cstate->changes++;
	return cstate->a.fee_msat + cstate->b.fee_msat == fee * 1000;
}
	
void invert_cstate(struct channel_state *cstate)
{
	struct channel_oneside tmp;

	tmp = cstate->a;
	cstate->a = cstate->b;
	cstate->b = tmp;
}

/* Add a HTLC to @creator if it can afford it. */
static bool add_htlc(struct channel_state *cstate,
		     struct channel_oneside *creator,
		     struct channel_oneside *recipient,
		     u32 msatoshis, const struct abs_locktime *expiry,
		     const struct sha256 *rhash, uint64_t id)
{
	size_t n, nondust;

	assert((creator == &cstate->a && recipient == &cstate->b)
	       || (creator == &cstate->b && recipient == &cstate->a));

	/* Remember to count the new one in total txsize if not dust! */
	nondust = total_nondust_htlcs(cstate);
	if (!is_dust_amount(msatoshis / 1000))
		nondust++;
	
	if (!change_funding(cstate->anchor, cstate->fee_rate,
			    msatoshis, creator, recipient, nondust))
		return false;

	n = tal_count(creator->htlcs);
	tal_resize(&creator->htlcs, n+1);

	creator->htlcs[n].msatoshis = msatoshis;
	creator->htlcs[n].expiry = *expiry;
	creator->htlcs[n].rhash = *rhash;
	creator->htlcs[n].id = id;
	memcheck(&creator->htlcs[n].msatoshis,
		 sizeof(creator->htlcs[n].msatoshis));
	memcheck(&creator->htlcs[n].rhash, sizeof(creator->htlcs[n].rhash));
	cstate->changes++;
	return true;
}

/* Remove htlc from creator, credit it to beneficiary. */
static void remove_htlc(struct channel_state *cstate,
			struct channel_oneside *creator,
			struct channel_oneside *beneficiary,
			struct channel_oneside *non_beneficiary,
			size_t i)
{
	size_t n = tal_count(creator->htlcs);
	size_t nondust;

	assert(i < n);
	assert(creator == &cstate->a || creator == &cstate->b);
	assert((beneficiary == &cstate->a && non_beneficiary == &cstate->b)
	       || (beneficiary == &cstate->b && non_beneficiary == &cstate->a));

	/* Remember to remove this one in total txsize if not dust! */
	nondust = total_nondust_htlcs(cstate);
	if (!is_dust_amount(creator->htlcs[i].msatoshis / 1000)) {
		assert(nondust > 0);
		nondust--;
	}

	/* Can't fail since msatoshis is positive. */
	if (!change_funding(cstate->anchor, cstate->fee_rate,
			    -(int64_t)creator->htlcs[i].msatoshis,
			    beneficiary, non_beneficiary, nondust))
		abort();

	/* Actually remove the HTLC. */
	memmove(creator->htlcs + i, creator->htlcs + i + 1,
		(n - i - 1) * sizeof(*creator->htlcs));
	tal_resize(&creator->htlcs, n-1);
	cstate->changes++;
}

bool funding_a_add_htlc(struct channel_state *cstate,
			u32 msatoshis, const struct abs_locktime *expiry,
			const struct sha256 *rhash, uint64_t id)
{
	return add_htlc(cstate, &cstate->a, &cstate->b,
			msatoshis, expiry, rhash, id);
}

bool funding_b_add_htlc(struct channel_state *cstate,
			u32 msatoshis, const struct abs_locktime *expiry,
			const struct sha256 *rhash, uint64_t id)
{
	return add_htlc(cstate, &cstate->b, &cstate->a,
			msatoshis, expiry, rhash, id);
}

void funding_a_fail_htlc(struct channel_state *cstate, size_t index)
{
	remove_htlc(cstate, &cstate->a, &cstate->a, &cstate->b, index);
}

void funding_b_fail_htlc(struct channel_state *cstate, size_t index)
{
	remove_htlc(cstate, &cstate->b, &cstate->b, &cstate->a, index);
}

void funding_a_fulfill_htlc(struct channel_state *cstate, size_t index)
{
	remove_htlc(cstate, &cstate->a, &cstate->b, &cstate->a, index);
}

void funding_b_fulfill_htlc(struct channel_state *cstate, size_t index)
{
	remove_htlc(cstate, &cstate->b, &cstate->a, &cstate->b, index);
}

size_t funding_find_htlc(struct channel_oneside *creator,
			 const struct sha256 *rhash)
{
	size_t i;

	for (i = 0; i < tal_count(creator->htlcs); i++) {
		if (structeq(&creator->htlcs[i].rhash, rhash))
			break;
	}
	return i;
}

size_t funding_htlc_by_id(struct channel_oneside *creator, uint64_t id)
{
	size_t i;

	for (i = 0; i < tal_count(creator->htlcs); i++) {
		if (creator->htlcs[i].id == id)
			break;
	}
	return i;
}

struct channel_state *copy_funding(const tal_t *ctx,
				   const struct channel_state *cstate)
{
	struct channel_state *cs = tal_dup(ctx, struct channel_state, cstate);

	cs->a.htlcs = tal_dup_arr(cs, struct channel_htlc, cs->a.htlcs,
				  tal_count(cs->a.htlcs), 0);
	cs->b.htlcs = tal_dup_arr(cs, struct channel_htlc, cs->b.htlcs,
				  tal_count(cs->b.htlcs), 0);
	return cs;
}