proto/bloom: add BloomFilter::fpr() and use it for the false-positive-rate

The false-positive-rate primitive (fill ratio raised to the hash count) was
inlined at three sites. Hoist it into a BloomFilter::fpr() method and call it
from all three; the threshold/policy logic stays at the call sites. Behavior
and result bytes are unchanged.

src/node/bloom.rs

@@ -87,7 +87,7 @@ impl Node {
         // operator to see one clear message, not spam.
         let max_fpr = self.config().node.bloom.max_inbound_fpr;
         let out_fill = sent_filter.fill_ratio();
-        let out_fpr = out_fill.powi(sent_filter.hash_count() as i32);
+        let out_fpr = sent_filter.fpr();
         if out_fpr > max_fpr {
             let now = std::time::Instant::now();
             let should_warn = self
@@ -213,7 +213,7 @@ impl Node {
         // to wipe a victim's contribution to aggregation.
         let max_fpr = self.config().node.bloom.max_inbound_fpr;
         let fill = announce.filter.fill_ratio();
-        let fpr = fill.powi(announce.filter.hash_count() as i32);
+        let fpr = announce.filter.fpr();
         if fpr > max_fpr {
             self.metrics()
                 .bloom

src/proto/bloom/core.rs

@@ -139,6 +139,11 @@ impl BloomFilter {
         self.count_ones() as f64 / self.num_bits as f64
     }
 
+    /// Current false-positive rate: fill ratio raised to the hash count.
+    pub fn fpr(&self) -> f64 {
+        self.fill_ratio().powi(self.hash_count as i32)
+    }
+
     /// Estimate the number of elements in the filter.
     ///
     /// Uses the formula: n = -(m/k) * ln(1 - X/m)
@@ -160,7 +165,7 @@ impl BloomFilter {
         }
 
         let fill = x / m;
-        let fpr = fill.powi(self.hash_count as i32);
+        let fpr = self.fpr();
         if fpr > max_fpr {
             return None;
         }