Improve bitmap slice unchecked (#1574)

src/bitmap/immutable.rs

@@ -172,11 +172,15 @@ 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) {
-        // first guard a no-op slice so that we don't do a bitcount
-        // if there isn't any data sliced
-        if !(offset == 0 && length == self.length) {
-            // count the smallest chunk
-            if length < self.length / 2 {
+        // we don't do a bitcount in the following cases:
+        // 1. if there isn't any data sliced.
+        // 2. if this [`Bitmap`] is all true or all false.
+        if !(offset == 0 && length == self.length || self.unset_bits == 0) {
+            // if `self.unset_bits == self.length` is false, we count the smallest chunk
+            // and do a bitcount.
+            if self.unset_bits == self.length {
+                self.unset_bits = length;
+            } else if length < self.length / 2 {
                 // count the null values in the slice
                 self.unset_bits = count_zeros(&self.bytes, self.offset + offset, length);
             } else {
@@ -186,9 +190,9 @@ impl Bitmap {
                 let tail_count = count_zeros(&self.bytes, start_end, self.length - length - offset);
                 self.unset_bits -= head_count + tail_count;
             }
-            self.offset += offset;
-            self.length = length;
         }
+        self.offset += offset;
+        self.length = length;
     }
 
     /// Slices `self`, offsetting by `offset` and truncating up to `length` bits.