Calc decayed buckets to decide if we have valid historical points

When we're calculating if, once we apply the unupdated decays, the
historical data tracker has enough data to assign a score, we
previously calculated the decayed points while walking the buckets
as we don't use the decayed buckets anyway (to avoid losing
precision). That is fine, except that as written it decayed
individual buckets additional times.

Instead, here we actually calculate the full set of decayed buckets
and use those to decide if we have valid points. This adds some
additional stack space and may in fact be slower, but will be
useful in the next commit and shouldn't be a huge change.

Author: TheBlueMatt

lightning/src/routing/scoring.rs

@@ -597,7 +597,26 @@ struct HistoricalMinMaxBuckets<'a> {
 
 impl HistoricalMinMaxBuckets<'_> {
 	#[inline]
-	fn calculate_success_probability_times_billion(&self, required_decays: u32, payment_amt_64th_bucket: u8) -> Option<u64> {
+	fn get_decayed_buckets<T: Time>(&self, now: T, last_updated: T, half_life: Duration)
+	-> ([u16; 8], [u16; 8], u32) {
+		let required_decays = now.duration_since(last_updated).as_secs()
+			.checked_div(half_life.as_secs())
+			.map_or(u32::max_value(), |decays| cmp::min(decays, u32::max_value() as u64) as u32);
+		let mut min_buckets = [0; 8];
+		for (idx, bucket) in self.min_liquidity_offset_history.buckets.iter().enumerate() {
+			min_buckets[idx] = (*bucket).checked_shr(required_decays).unwrap_or(0);
+		}
+		let mut max_buckets = [0; 8];
+		for (idx, bucket) in self.max_liquidity_offset_history.buckets.iter().enumerate() {
+			max_buckets[idx] = (*bucket).checked_shr(required_decays).unwrap_or(0);
+		}
+		(min_buckets, max_buckets, required_decays)
+	}
+
+	#[inline]
+	fn calculate_success_probability_times_billion<T: Time>(
+		&self, now: T, last_updated: T, half_life: Duration, payment_amt_64th_bucket: u8)
+	-> Option<u64> {
 		// If historical penalties are enabled, calculate the penalty by walking the set of
 		// historical liquidity bucket (min, max) combinations (where min_idx < max_idx) and, for
 		// each, calculate the probability of success given our payment amount, then total the
@@ -619,23 +638,22 @@ impl HistoricalMinMaxBuckets<'_> {
 		// 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;
 
-		// 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; };
+		// Check if all our buckets are zero, once decayed and treat it as if we had no data. We
+		// don't actually use the decayed buckets, though, as that would lose precision.
+		let (decayed_min_buckets, decayed_max_buckets, required_decays) =
+			self.get_decayed_buckets(now, last_updated, half_life);
+		if decayed_min_buckets.iter().all(|v| *v == 0) || decayed_max_buckets.iter().all(|v| *v == 0) {
+			return None;
+		}
 
 		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 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 total_valid_points_tracked.checked_shr(required_decays).unwrap_or(0) < 32*32 {
 			return None;
 		}
 
@@ -942,9 +960,6 @@ impl<L: Deref<Target = u64>, BRT: Deref<Target = HistoricalBucketRangeTracker>,
 
 		if params.historical_liquidity_penalty_multiplier_msat != 0 ||
 		   params.historical_liquidity_penalty_amount_multiplier_msat != 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);
 			let payment_amt_64th_bucket = amount_msat * 64 / self.capacity_msat;
 			debug_assert!(payment_amt_64th_bucket <= 64);
 			if payment_amt_64th_bucket > 64 { return res; }
@@ -954,7 +969,9 @@ impl<L: Deref<Target = u64>, BRT: Deref<Target = HistoricalBucketRangeTracker>,
 				max_liquidity_offset_history: &self.max_liquidity_offset_history,
 			};
 			if let Some(cumulative_success_prob_times_billion) = buckets
-					.calculate_success_probability_times_billion(required_decays, payment_amt_64th_bucket as u8) {
+				.calculate_success_probability_times_billion(self.now, *self.last_updated,
+					params.historical_no_updates_half_life, payment_amt_64th_bucket as u8)
+			{
 				let historical_negative_log10_times_2048 = approx::negative_log10_times_2048(cumulative_success_prob_times_billion + 1, 1024 * 1024 * 1024);
 				res = res.saturating_add(Self::combined_penalty_msat(amount_msat,
 					historical_negative_log10_times_2048, params.historical_liquidity_penalty_multiplier_msat,