Merge pull request #123 from hacatu/master

Added `nth` to `BitVec::Iter` so that `.skip(n)` gets optimized

Author: pczarn

benches/bench.rs

@@ -260,3 +260,23 @@ fn bench_erathostenes_set_all(b: &mut test::Bencher) {
         black_box(&mut sieve);
     });
 }
+
+#[bench]
+fn bench_iter_skip(b: &mut test::Bencher) {
+    let start = 3 << 20;
+    let p = 16411;
+    let g = 9749; // 9749 is a primitive root modulo 16411, so we can generate numbers mod p in a seemingly random order
+    let end = start + p;
+    let mut tbl = BitVec::from_elem(end, false);
+    let mut r = g;
+    for i in start..end {
+        tbl.set(i, r & 1 != 0);
+        r = r * g % p;
+    }
+    b.iter(|| {
+        black_box(&mut tbl);
+        // start is large relative to end-start, so before Iterator::nth was implemented for bitvec this would
+        // have been much slower
+        black_box(tbl.iter().skip(start).filter(|&v| v).count());
+    });
+}

src/lib.rs

@@ -1937,6 +1937,13 @@ impl<'a, B: BitBlock> Iterator for Iter<'a, B> {
         self.range.next().map(|i| self.bit_vec.get(i).unwrap())
     }
 
+    fn nth(&mut self, n: usize) -> Option<Self::Item> {
+        // This override is used by the compiler to optimize Iterator::skip.
+        // Without this, the default implementation of Iterator::nth is used, which walks over
+        // the whole iterator up to n.
+        self.range.nth(n).and_then(|i| self.bit_vec.get(i))
+    }
+
     fn size_hint(&self) -> (usize, Option<usize>) {
         self.range.size_hint()
     }