Expose the historical success probability calculation itself

In 3f32f60ae7 we exposed the
historical success probability buckets directly, with a long method
doc explaining how to use it. While this is great for logging
exactly what the internal model thinks, its also helpful to let
users know what the internal model thinks the success probability
is directly, allowing them to compare route success probabilities.

Here we do so but only for the historical tracking buckets.

lightning/src/routing/scoring.rs

@@ -939,6 +939,9 @@ impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, T: Time> ProbabilisticScorerU
 	///
 	/// Because the datapoints are decayed slowly over time, values will eventually return to
 	/// `Some(([0; 8], [0; 8]))`.
+	///
+	/// In order to fetch a single success probability from the buckets provided here, as used in
+	/// the scoring model, see [`Self::historical_estimated_payment_success_probability`].
 	pub fn historical_estimated_channel_liquidity_probabilities(&self, scid: u64, target: &NodeId)
 	-> Option<([u16; 8], [u16; 8])> {
 		let graph = self.network_graph.read_only();
@@ -953,7 +956,7 @@ impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, T: Time> ProbabilisticScorerU
 						min_liquidity_offset_history: &dir_liq.min_liquidity_offset_history,
 						max_liquidity_offset_history: &dir_liq.max_liquidity_offset_history,
 					};
-					let (min_buckets, mut max_buckets, _) = buckets.get_decayed_buckets(T::now(),
+					let (min_buckets, mut max_buckets, _) = buckets.get_decayed_buckets(dir_liq.now,
 						*dir_liq.last_updated, self.decay_params.historical_no_updates_half_life);
 					// Note that the liquidity buckets are an offset from the edge, so we inverse
 					// the max order to get the probabilities from zero.
@@ -964,6 +967,39 @@ impl<G: Deref<Target = NetworkGraph<L>>, L: Deref, T: Time> ProbabilisticScorerU
 		}
 		None
 	}
+
+	/// Query the probability of payment success sending the given `amount_msat` over the channel
+	/// with `scid` towards the given `target` node, based on the historical estimated liquidity
+	/// bounds.
+	///
+	/// These are the same bounds as returned by
+	/// [`Self::historical_estimated_channel_liquidity_probabilities`] (but not those returned by
+	/// [`Self::estimated_channel_liquidity_range`]).
+	pub fn historical_estimated_payment_success_probability(
+		&self, scid: u64, target: &NodeId, amount_msat: u64)
+	-> Option<f64> {
+		let graph = self.network_graph.read_only();
+
+		if let Some(chan) = graph.channels().get(&scid) {
+			if let Some(liq) = self.channel_liquidities.get(&scid) {
+				if let Some((directed_info, source)) = chan.as_directed_to(target) {
+					let capacity_msat = directed_info.effective_capacity().as_msat();
+					let dir_liq = liq.as_directed(source, target, 0, capacity_msat, self.decay_params);
+
+					let buckets = HistoricalMinMaxBuckets {
+						min_liquidity_offset_history: &dir_liq.min_liquidity_offset_history,
+						max_liquidity_offset_history: &dir_liq.max_liquidity_offset_history,
+					};
+
+					return buckets.calculate_success_probability_times_billion(dir_liq.now,
+						*dir_liq.last_updated, self.decay_params.historical_no_updates_half_life,
+						amount_msat, capacity_msat
+					).map(|p| p as f64 / (1024 * 1024 * 1024) as f64);
+				}
+			}
+		}
+		None
+	}
 }
 
 impl<T: Time> ChannelLiquidity<T> {
@@ -2847,6 +2883,8 @@ mod tests {
 		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 47);
 		assert_eq!(scorer.historical_estimated_channel_liquidity_probabilities(42, &target),
 			None);
+		assert_eq!(scorer.historical_estimated_payment_success_probability(42, &target, 42),
+			None);
 
 		scorer.payment_path_failed(&payment_path_for_amount(1), 42);
 		assert_eq!(scorer.channel_penalty_msat(42, &source, &target, usage, &params), 2048);
@@ -2854,6 +2892,10 @@ mod tests {
 		// octile.
 		assert_eq!(scorer.historical_estimated_channel_liquidity_probabilities(42, &target),
 			Some(([32, 0, 0, 0, 0, 0, 0, 0], [32, 0, 0, 0, 0, 0, 0, 0])));
+		assert_eq!(scorer.historical_estimated_payment_success_probability(42, &target, 1),
+			Some(1.0));
+		assert_eq!(scorer.historical_estimated_payment_success_probability(42, &target, 500),
+			Some(0.0));
 
 		// Even after we tell the scorer we definitely have enough available liquidity, it will
 		// still remember that there was some failure in the past, and assign a non-0 penalty.
@@ -2863,6 +2905,17 @@ mod tests {
 		assert_eq!(scorer.historical_estimated_channel_liquidity_probabilities(42, &target),
 			Some(([31, 0, 0, 0, 0, 0, 0, 32], [31, 0, 0, 0, 0, 0, 0, 32])));
 
+		// The exact success probability is a bit complicated and involves integer rounding, so we
+		// simply check bounds here.
+		let five_hundred_prob =
+			scorer.historical_estimated_payment_success_probability(42, &target, 500).unwrap();
+		assert!(five_hundred_prob > 0.5);
+		assert!(five_hundred_prob < 0.52);
+		let one_prob =
+			scorer.historical_estimated_payment_success_probability(42, &target, 1).unwrap();
+		assert!(one_prob < 1.0);
+		assert!(one_prob > 0.99);
+
 		// Advance the time forward 16 half-lives (which the docs claim will ensure all data is
 		// gone), and check that we're back to where we started.
 		SinceEpoch::advance(Duration::from_secs(10 * 16));
@@ -2871,6 +2924,7 @@ mod tests {
 		// data entirely instead.
 		assert_eq!(scorer.historical_estimated_channel_liquidity_probabilities(42, &target),
 			Some(([0; 8], [0; 8])));
+		assert_eq!(scorer.historical_estimated_payment_success_probability(42, &target, 1), None);
 
 		let mut usage = ChannelUsage {
 			amount_msat: 100,