Revert "(XXX: Migration) Reduce bucket size somewhat"

This reverts commit 31c12a6.

lightning/src/routing/scoring.rs

@@ -1695,40 +1695,38 @@ mod bucketed_history {
 		buckets: [u16; 32],
 	}
 
-	/// Buckets are stored in fixed point numbers with a 4 bit fractional part. Thus, the value
-	/// "one" is 16, or this constant.
-	pub const BUCKET_FIXED_POINT_ONE: u16 = 16;
+	/// Buckets are stored in fixed point numbers with a 5 bit fractional part. Thus, the value
+	/// "one" is 32, or this constant.
+	pub const BUCKET_FIXED_POINT_ONE: u16 = 32;
 
 	impl HistoricalBucketRangeTracker {
 		pub(super) fn new() -> Self { Self { buckets: [0; 32] } }
 		fn track_datapoint(&mut self, liquidity_offset_msat: u64, capacity_msat: u64) {
 			// We have 32 leaky buckets for min and max liquidity. Each bucket tracks the amount of time
-			// we spend in each bucket as a 13-bit fixed-point number with a 4 bit fractional part.
+			// we spend in each bucket as a 16-bit fixed-point number with a 5 bit fractional part.
 			//
-			// Each time we update our liquidity estimate, we add 16 (1.0 in our fixed-point system) to
+			// Each time we update our liquidity estimate, we add 32 (1.0 in our fixed-point system) to
 			// the buckets for the current min and max liquidity offset positions.
 			//
-			// We then decay each bucket by multiplying by 511/512 (avoiding dividing by a
-			// non-power-of-two). This ensures we can't actually overflow the u13 - when we get to
-			// 8,176 adding 16 and decaying by 511/512 leaves us back at 8,176.
+			// We then decay each bucket by multiplying by 2047/2048 (avoiding dividing by a
+			// non-power-of-two). This ensures we can't actually overflow the u16 - when we get to
+			// 63,457 adding 32 and decaying by 2047/2048 leaves us back at 63,457.
 			//
-			// In total, this allows us to track data for the last 1,000 or so payments across a given
-			// channel per bucket.
+			// In total, this allows us to track data for the last 8,000 or so payments across a given
+			// channel.
 			//
-			// These constants are a balance - we try to fit in 2 bytes per bucket to reduce
-			// overhead and must fit in 13 bits to allow us to square bucket weights without
-			// overflowing into a 128-bit integer to track total points. We also need to balance
-			// having more bits in the decimal part (to ensure decay isn't too non-linear) with
-			// having too few bits in the mantissa, causing us to not store very many datapoints.
+			// These constants are a balance - we try to fit in 2 bytes per bucket to reduce overhead,
+			// and need to balance having more bits in the decimal part (to ensure decay isn't too
+			// non-linear) with having too few bits in the mantissa, causing us to not store very many
+			// datapoints.
 			//
 			// The constants were picked experimentally, selecting a decay amount that restricts us
 			// from overflowing buckets without having to cap them manually.
 
 			let pos: u16 = amount_to_pos(liquidity_offset_msat, capacity_msat);
 			if pos < POSITION_TICKS {
 				for e in self.buckets.iter_mut() {
-					*e = ((*e as u32) * 511 / 512) as u16;
-					debug_assert!(*e < (1 << 11));
+					*e = ((*e as u32) * 2047 / 2048) as u16;
 				}
 				let bucket = pos_to_bucket(pos);
 				self.buckets[bucket] = self.buckets[bucket].saturating_add(BUCKET_FIXED_POINT_ONE);