Special-case the 0th minimum bucket in historical scoring

Points in the 0th minimum bucket either indicate we sent a payment
which is < 1/16,384th of the channel's capacity or, more likely,
we failed to send a payment. In either case, averaging the success
probability across the full range of upper-bounds doesn't make a
whole lot of sense - if we've never managed to send a "real"
payment over a channel, we should be considering it quite poor.

To address this, we special-case the 0th minimum bucket and only
look at the largest-offset max bucket when calculating the success
probability.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -1758,7 +1758,32 @@ mod bucketed_history {
 			}
 
 			let mut cumulative_success_prob_times_billion = 0;
-			for (min_idx, min_bucket) in self.min_liquidity_offset_history.buckets.iter().enumerate() {
+			// Special-case the 0th min bucket - it generally means we failed a payment, so only
+			// consider the highest (i.e. largest-offset-from-max-capacity) max bucket for all
+			// points against the 0th min bucket!
+			if self.min_liquidity_offset_history.buckets[0] != 0 {
+				let mut highest_max_bucket_with_points = 0;
+				let mut total_max_points = 0;
+				for (max_idx, max_bucket) in self.max_liquidity_offset_history.buckets.iter().enumerate() {
+					if *max_bucket >= 32 {
+						highest_max_bucket_with_points = cmp::max(highest_max_bucket_with_points, max_idx);
+					}
+					total_max_points += *max_bucket as u64;
+				}
+				let max_bucket_end_pos = BUCKET_START_POS[32 - highest_max_bucket_with_points] - 1;
+				if payment_pos < max_bucket_end_pos {
+					let bucket_prob_times_billion =
+						(self.min_liquidity_offset_history.buckets[0] as u64) * total_max_points
+							* 1024 * 1024 * 1024 / total_valid_points_tracked;
+					cumulative_success_prob_times_billion += bucket_prob_times_billion *
+						((max_bucket_end_pos - payment_pos) as u64) /
+						// Add an additional one in the divisor as the payment bucket has been
+						// rounded down.
+						(max_bucket_end_pos + 1) as u64;
+				}
+			}
+
+			for (min_idx, min_bucket) in self.min_liquidity_offset_history.buckets.iter().enumerate().skip(1) {
 				let min_bucket_start_pos = BUCKET_START_POS[min_idx];
 				for (max_idx, max_bucket) in self.max_liquidity_offset_history.buckets.iter().enumerate().take(32 - min_idx) {
 					let max_bucket_end_pos = BUCKET_START_POS[32 - max_idx] - 1;