Function channel_maximum_forward

plugins/renepay/chan_extra.c:193–234

Let `recv` be the maximum amount this channel can receive, this function * computes the maximum amount this channel can forward `send`. * From BOLT7 specification wee need to satisfy the following inequality: * * recv-send >= base_fee + floor(send*proportional_fee/1000000) * * That is equivalent to have * * send <= Bound(recv,send) * * where * * Bound(recv, send) = ((recv - base_fee)*1

 *  send+1 because Bound(recv, send) < Bound_simple(recv) + 2.
 * */
enum renepay_errorcode channel_maximum_forward(struct amount_msat *max_forward,
					       const struct gossmap_chan *chan,
					       const int dir,
					       struct amount_msat recv)
{
	const u64 b = chan->half[dir].base_fee,
		  p = chan->half[dir].proportional_fee;

	const u64 one_million = 1000000;
	u64 x_msat =
	    recv.millisatoshis; /* Raw: need to invert the fee equation */

	// special case, when recv - base_fee <= 0, we cannot forward anything
	if (x_msat <= b) {
		*max_forward = amount_msat(0);
		return RENEPAY_NOERROR;
	}

	x_msat -= b;

	if (mul_overflows_u64(one_million, x_msat))
		return RENEPAY_AMOUNT_OVERFLOW;

	struct amount_msat best_send =
	    AMOUNT_MSAT_INIT((one_million * x_msat) / (one_million + p));

	/* Try to increase the value we send (up tp the last millisat) until we
	 * fail to fulfill the fee inequality. It takes only one iteration
	 * though. */
	for (size_t i = 0; i < 10; ++i) {
		struct amount_msat next_send;
		if (!amount_msat_add(&next_send, best_send, amount_msat(1)))
			return RENEPAY_AMOUNT_OVERFLOW;

		if (check_fee_inequality(recv, next_send, b, p))
			best_send = next_send;
		else
			break;
	}
	*max_forward = best_send;
	return RENEPAY_NOERROR;
}

/* This helper function preserves the uncertainty network invariant after the
 * knowledge is updated. It assumes that the (channel,!dir) knowledge is