f - Use a more accurate approximation

Author: jkczyz

lightning/src/routing/scoring.rs

@@ -1156,16 +1156,17 @@ impl<L: Deref<Target = u64>, BRT: Deref<Target = HistoricalBucketRangeTracker>,
 			0
 		} else {
 			// Decay the offset by the appropriate number of half lives. If half of the next half
-			// life has passed, approximate an additional three-quarter life by summing the results
-			// of taking both the *next two* half lives instead. This helps smooth out the decay.
+			// life has passed, approximate an additional three-quarter life to help smooth out the
+			// decay.
 			let elapsed_time = self.now.duration_since(*self.last_updated).as_secs();
 			let half_decays = elapsed_time / (half_life / 2);
 			let decays = half_decays / 2;
 			let decayed_offset_msat = offset_msat.checked_shr(decays as u32).unwrap_or(0);
 			if half_decays % 2 == 0 {
 				decayed_offset_msat
 			} else {
-				(decayed_offset_msat >> 1) + (decayed_offset_msat >> 2)
+				// 71 / 100 ~= core::f64::consts::FRAC_1_SQRT_2
+				(decayed_offset_msat as u128 * 71 / 100) as u64
 			}
 		}
 	}
@@ -2442,11 +2443,11 @@ mod tests {
 		// Half decay (i.e., three-quarter life)
 		SinceEpoch::advance(Duration::from_secs(1));
 		let usage = ChannelUsage { amount_msat: 128, ..usage };
-		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 18);
+		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 22);
 		let usage = ChannelUsage { amount_msat: 256, ..usage };
-		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 103);
+		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 106);
 		let usage = ChannelUsage { amount_msat: 768, ..usage };
-		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 957);
+		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 916);
 		let usage = ChannelUsage { amount_msat: 896, ..usage };
 		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), u64::max_value());