fix historical liquidity bucket decay

The formula for applying half lives was incorrect. Test coverage added.

Relatively straightforward merge conflicts (code added in
311a083 which was not included
neighbored new code added) fixed in:
 * lightning/src/routing/scoring.rs

Author: joostjager

lightning/src/routing/scoring.rs

@@ -1795,13 +1795,21 @@ mod bucketed_history {
 				self.buckets[bucket] = self.buckets[bucket].saturating_add(BUCKET_FIXED_POINT_ONE);
 			}
 		}
+
+		/// Applies decay at the given half-life to all buckets.
+		fn decay(&mut self, half_lives: f64) {
+			let factor = (1024.0 * powf64(0.5, half_lives)) as u64;
+			for bucket in self.buckets.iter_mut() {
+				*bucket = ((*bucket as u64) * factor / 1024) as u16;
+			}
+		}
 	}
 
 	impl_writeable_tlv_based!(HistoricalBucketRangeTracker, { (0, buckets, required) });
 	impl_writeable_tlv_based!(LegacyHistoricalBucketRangeTracker, { (0, buckets, required) });
 
 	#[derive(Clone, Copy)]
-	#[repr(C)] // Force the fields in memory to be in the order we specify.
+	#[repr(C)]// Force the fields in memory to be in the order we specify.
 	pub(super) struct HistoricalLiquidityTracker {
 		// This struct sits inside a `(u64, ChannelLiquidity)` in memory, and we first read the
 		// liquidity offsets in `ChannelLiquidity` when calculating the non-historical score. This
@@ -1849,13 +1857,8 @@ mod bucketed_history {
 		}
 
 		pub(super) fn decay_buckets(&mut self, half_lives: f64) {
-			let divisor = powf64(2048.0, half_lives) as u64;
-			for bucket in self.min_liquidity_offset_history.buckets.iter_mut() {
-				*bucket = ((*bucket as u64) * 1024 / divisor) as u16;
-			}
-			for bucket in self.max_liquidity_offset_history.buckets.iter_mut() {
-				*bucket = ((*bucket as u64) * 1024 / divisor) as u16;
-			}
+			self.min_liquidity_offset_history.decay(half_lives);
+			self.max_liquidity_offset_history.decay(half_lives);
 			self.recalculate_valid_point_count();
 		}
 
@@ -2049,6 +2052,33 @@ mod bucketed_history {
 			Some((cumulative_success_prob * (1024.0 * 1024.0 * 1024.0)) as u64)
 		}
 	}
+
+	#[cfg(test)]
+	mod tests {
+		use super::HistoricalBucketRangeTracker;
+
+		#[test]
+		fn historical_liquidity_bucket_decay() {
+			let mut bucket = HistoricalBucketRangeTracker::new();
+			bucket.track_datapoint(100, 1000);
+			assert_eq!(
+				bucket.buckets,
+				[
+					0u16, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 32, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+					0, 0, 0, 0, 0, 0, 0
+				]
+			);
+
+			bucket.decay(2.0);
+			assert_eq!(
+				bucket.buckets,
+				[
+					0u16, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 8, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
+					0, 0, 0, 0, 0, 0, 0
+				]
+			);
+		}
+	}
 }
 use bucketed_history::{LegacyHistoricalBucketRangeTracker, HistoricalBucketRangeTracker, DirectedHistoricalLiquidityTracker, HistoricalLiquidityTracker};