Optimized null_count (#442)

benches/bitmap_ops.rs

@@ -18,6 +18,33 @@ fn add_benchmark(c: &mut Criterion) {
                 assert!(r.null_count() > 0);
             })
         });
+
+        let offset = ((size as f64) * 0.1) as usize;
+        let len = ((size as f64) * 0.85) as usize;
+
+        c.bench_function(
+            &format!("bitmap count zeros 85% slice 2^{}", log2_size),
+            |b| {
+                b.iter(|| {
+                    let r = bitmap.clone().slice(offset, len);
+                    assert!(r.null_count() > 0);
+                })
+            },
+        );
+
+        let offset = ((size as f64) * 0.2) as usize;
+        let len = ((size as f64) * 0.51) as usize;
+
+        c.bench_function(
+            &format!("bitmap count zeros 51% slice 2^{}", log2_size),
+            |b| {
+                b.iter(|| {
+                    let r = bitmap.clone().slice(offset, len);
+                    assert!(r.null_count() > 0);
+                })
+            },
+        );
+
         let bitmap1 = bitmap.clone().slice(1, size - 1);
         c.bench_function(&format!("bitmap not 2^{}", log2_size), |b| {
             b.iter(|| {

src/bitmap/immutable.rs

@@ -121,9 +121,19 @@ impl Bitmap {
     /// The caller must ensure that `self.offset + offset + length <= self.len()`
     #[inline]
     pub unsafe fn slice_unchecked(mut self, offset: usize, length: usize) -> Self {
+        // count the smallest chunk
+        if length < self.length / 2 {
+            // count the null values in the slice
+            self.null_count = count_zeros(&self.bytes, offset, length);
+        } else {
+            // subtract the null count of the chunks we slice off
+            let start_end = self.offset + offset + length;
+            let head_count = count_zeros(&self.bytes, self.offset, offset);
+            let tail_count = count_zeros(&self.bytes, start_end, self.length - length - offset);
+            self.null_count -= head_count + tail_count;
+        }
         self.offset += offset;
         self.length = length;
-        self.null_count = count_zeros(&self.bytes, self.offset, self.length);
         self
     }