improve partitioned filter size estimation

Author: marvin-j97

src/table/filter/mod.rs

@@ -40,17 +40,19 @@ impl BloomConstructionPolicy {
         }
     }
 
+    /// Returns the estimated filter size in bytes.
     #[must_use]
-    pub fn estimated_key_bits(&self, n: usize) -> f32 {
+    pub fn estimated_filter_size(&self, n: usize) -> usize {
         #[expect(
             clippy::cast_precision_loss,
             reason = "truncation is fine because this is an estimation"
         )]
         match self {
-            Self::BitsPerKey(bpk) => *bpk,
+            Self::BitsPerKey(bpk) => (*bpk * (n as f32)) as usize / 8,
             Self::FalsePositiveRate(fpr) => {
                 let m = StandardBloomFilterBuilder::calculate_m(n, *fpr);
-                (m / n) as f32
+                let bpk = (m / n) as f32;
+                (bpk * (n as f32)) as usize / 8
             }
         }
     }

src/table/writer/filter/partitioned.rs

@@ -163,16 +163,9 @@ impl<W: std::io::Write + std::io::Seek> FilterWriter<W> for PartitionedFilterWri
     fn register_key(&mut self, key: &UserKey) -> crate::Result<()> {
         self.bloom_hash_buffer.push(Builder::get_hash(key));
 
-        #[expect(
-            clippy::cast_possible_truncation,
-            clippy::cast_sign_loss,
-            reason = "truncation is fine because this is an estimation"
-        )]
-        let estimated_key_bits =
-            self.bloom_policy
-                .estimated_key_bits(self.bloom_hash_buffer.len()) as usize;
-
-        self.approx_filter_size += estimated_key_bits;
+        self.approx_filter_size = self
+            .bloom_policy
+            .estimated_filter_size(self.bloom_hash_buffer.len());
 
         self.last_key = Some(key.clone());