Decay historical liquidity tracking when no new data is added

To avoid scoring based on incredibly old historical liquidity data,
we add a new half-life here which is used to (very slowly) decay
historical liquidity tracking buckets.

lightning/src/routing/scoring.rs

@@ -62,7 +62,7 @@ use util::logger::Logger;
 use util::time::Time;
 
 use prelude::*;
-use core::fmt;
+use core::{cmp, fmt};
 use core::cell::{RefCell, RefMut};
 use core::convert::TryInto;
 use core::ops::{Deref, DerefMut};
@@ -436,6 +436,16 @@ pub struct ProbabilisticScoringParameters {
 	/// [`liquidity_penalty_amount_multiplier_msat`]: Self::liquidity_penalty_amount_multiplier_msat
 	pub historical_liquidity_penalty_amount_multiplier_msat: u64,
 
+	/// If we aren't learning any new datapoints for a channel, the historical liquidity bounds
+	/// tracking can simply live on with increasingly stale data. Instead, when a channel has not
+	/// seen a liquidity estimate update for this amount of time, the historical datapoints are
+	/// decayed by half.
+	///
+	/// Note that after 16 or more half lives all historical data will be completely gone.
+	///
+	/// Default value: 14 days
+	pub historical_no_updates_half_life: Duration,
+
 	/// Manual penalties used for the given nodes. Allows to set a particular penalty for a given
 	/// node. Note that a manual penalty of `u64::max_value()` means the node would not ever be
 	/// considered during path finding.
@@ -509,6 +519,13 @@ impl HistoricalBucketRangeTracker {
 			self.buckets[bucket_idx as usize] = self.buckets[bucket_idx as usize].saturating_add(32);
 		}
 	}
+	/// Decay all buckets by the given number of half-lives. Used to more aggressively remove old
+	/// datapoints as we receive newer information.
+	fn time_decay_data(&mut self, half_lives: u32) {
+		for e in self.buckets.iter_mut() {
+			*e = e.checked_shr(half_lives).unwrap_or(0);
+		}
+	}
 }
 
 impl_writeable_tlv_based!(HistoricalBucketRangeTracker, { (0, buckets, required) });
@@ -645,6 +662,7 @@ impl ProbabilisticScoringParameters {
 			liquidity_penalty_amount_multiplier_msat: 0,
 			historical_liquidity_penalty_multiplier_msat: 0,
 			historical_liquidity_penalty_amount_multiplier_msat: 0,
+			historical_no_updates_half_life: Duration::from_secs(60 * 60 * 24 * 14),
 			manual_node_penalties: HashMap::new(),
 			anti_probing_penalty_msat: 0,
 			considered_impossible_penalty_msat: 0,
@@ -670,6 +688,7 @@ impl Default for ProbabilisticScoringParameters {
 			liquidity_penalty_amount_multiplier_msat: 192,
 			historical_liquidity_penalty_multiplier_msat: 10_000,
 			historical_liquidity_penalty_amount_multiplier_msat: 64,
+			historical_no_updates_half_life: Duration::from_secs(60 * 60 * 24 * 14),
 			manual_node_penalties: HashMap::new(),
 			anti_probing_penalty_msat: 250,
 			considered_impossible_penalty_msat: 1_0000_0000_000,
@@ -810,14 +829,30 @@ impl<L: Deref<Target = u64>, BRT: Deref<Target = HistoricalBucketRangeTracker>,
 			// sending less than 1/16th of a channel's capacity, or 1/8th if we used the top of the
 			// bucket.
 			let mut total_valid_points_tracked = 0;
+			let required_decays = self.now.duration_since(*self.last_updated).as_secs()
+				.checked_div(params.historical_no_updates_half_life.as_secs())
+				.map_or(u32::max_value(), |decays| cmp::min(decays, u32::max_value() as u64) as u32);
+
+			// Rather than actually decaying the individual buckets, which would lose precision, we
+			// simply track whether all buckets would be decayed to zero, in which case we treat it
+			// as if we had no data.
+			let mut is_fully_decayed = true;
+			let mut check_track_bucket_contains_undecayed_points =
+				|bucket_val: u16| if bucket_val.checked_shr(required_decays).unwrap_or(0) > 0 { is_fully_decayed = false; };
+
 			for (min_idx, min_bucket) in self.min_liquidity_offset_history.buckets.iter().enumerate() {
+				check_track_bucket_contains_undecayed_points(*min_bucket);
 				for max_bucket in self.max_liquidity_offset_history.buckets.iter().take(8 - min_idx) {
 					total_valid_points_tracked += (*min_bucket as u64) * (*max_bucket as u64);
+					check_track_bucket_contains_undecayed_points(*max_bucket);
 				}
 			}
-			if total_valid_points_tracked == 0 {
-				// If we don't have any valid points, redo the non-historical calculation with no
-				// liquidity bounds tracked and the historical penalty multipliers.
+			// If the total valid points is smaller than 1.0 (i.e. 32 in our fixed-point scheme),
+			// treat it as if we were fully decayed.
+			if total_valid_points_tracked.checked_shr(required_decays).unwrap_or(0) < 32*32 || is_fully_decayed {
+				// If we don't have any valid points (or, once decayed, we have less than a full
+				// point), redo the non-historical calculation with no liquidity bounds tracked and
+				// the historical penalty multipliers.
 				let max_capacity = self.capacity_msat.saturating_sub(amount_msat).saturating_add(1);
 				let negative_log10_times_2048 =
 					approx::negative_log10_times_2048(max_capacity, self.capacity_msat.saturating_add(1));
@@ -925,6 +960,12 @@ impl<L: DerefMut<Target = u64>, BRT: DerefMut<Target = HistoricalBucketRangeTrac
 	}
 
 	fn update_history_buckets(&mut self) {
+		let half_lives = self.now.duration_since(*self.last_updated).as_secs()
+			.checked_div(self.params.historical_no_updates_half_life.as_secs())
+			.map(|v| v.try_into().unwrap_or(u32::max_value())).unwrap_or(u32::max_value());
+		self.min_liquidity_offset_history.time_decay_data(half_lives);
+		self.max_liquidity_offset_history.time_decay_data(half_lives);
+
 		debug_assert!(*self.min_liquidity_offset_msat <= self.capacity_msat);
 		self.min_liquidity_offset_history.track_datapoint(
 			// Ensure the bucket index we pass is in the range [0, 7], even if the liquidity offset
@@ -947,8 +988,8 @@ impl<L: DerefMut<Target = u64>, BRT: DerefMut<Target = HistoricalBucketRangeTrac
 		} else {
 			self.decayed_offset_msat(*self.max_liquidity_offset_msat)
 		};
-		*self.last_updated = self.now;
 		self.update_history_buckets();
+		*self.last_updated = self.now;
 	}
 
 	/// Adjusts the upper bound of the channel liquidity balance in this direction.
@@ -959,8 +1000,8 @@ impl<L: DerefMut<Target = u64>, BRT: DerefMut<Target = HistoricalBucketRangeTrac
 		} else {
 			self.decayed_offset_msat(*self.min_liquidity_offset_msat)
 		};
-		*self.last_updated = self.now;
 		self.update_history_buckets();
+		*self.last_updated = self.now;
 	}
 }