Function substract_cycle

plugins/askrene/child/mcf.c:772–813

Substract a flow cycle from the channel allocation. */

/* Substract a flow cycle from the channel allocation. */
static void substract_cycle(const struct gossmap *gossmap,
			    const struct node sink,
			    struct chan_flow *chan_flow, const u32 *prev_idx,
			    const int *prev_dir,
			    const struct gossmap_chan *const *prev_chan)
{
	s64 delta = INFINITE;
	u32 cur_idx;

	/* Compute greatest flow in this cycle. */
	for (cur_idx = sink.idx; cur_idx!=INVALID_INDEX;) {
		const int dir = prev_dir[cur_idx];
		const struct gossmap_chan *const chan = prev_chan[cur_idx];
		const u32 chan_idx = gossmap_chan_idx(gossmap, chan);

		delta = MIN(delta, chan_flow[chan_idx].half[dir]);

		cur_idx = prev_idx[cur_idx];
		if (cur_idx == sink.idx)
			/* we have come back full circle */
			break;
	}
	assert(cur_idx==sink.idx);

	/* Walk again and substract the flow value (delta). */
	assert(delta < INFINITE);
	assert(delta > 0);

	for (cur_idx = sink.idx;cur_idx!=INVALID_INDEX;) {
		const int dir = prev_dir[cur_idx];
		const struct gossmap_chan *const chan = prev_chan[cur_idx];
		const u32 chan_idx = gossmap_chan_idx(gossmap, chan);

		chan_flow[chan_idx].half[dir] -= delta;

		cur_idx = prev_idx[cur_idx];
		if (cur_idx == sink.idx)
			/* we have come back full circle */
			break;
	}
	assert(cur_idx==sink.idx);
}

/* Given a flow in the residual network, build a set of payment flows in the
 * gossmap that corresponds to this flow. */