Fix #124 Record iteration when recording only zeros (#125)

As pointed in the comment in the corresponding issues, we want to ensure
we trigger the picker logic at least once to keep the invariant of the
loop, that is to say that the picking logic has been executed at least
once for last_picked_index, which is not the case for zero when there
are only zeros.

To do so we use and Option, and initialize to None, to make sure the
pick logic is ran at least once.

I added a test that collect the record and check for their count.

Fixes #124

src/iterators/mod.rs

@@ -74,7 +74,7 @@ pub struct HistogramIterator<'a, T: 'a + Counter, P: PickyIterator<T>> {
     count_since_last_iteration: u64,
     count_at_index: T,
     current_index: usize,
-    last_picked_index: usize,
+    last_picked_index: Option<usize>,
     max_value_index: usize,
     fresh: bool,
     ended: bool,
@@ -152,7 +152,7 @@ impl<'a, T: Counter, P: PickyIterator<T>> HistogramIterator<'a, T, P> {
             count_since_last_iteration: 0,
             count_at_index: T::zero(),
             current_index: 0,
-            last_picked_index: 0,
+            last_picked_index: None,
             max_value_index: h.index_for(h.max()).expect("Either 0 or an existing index"),
             picker,
             fresh: true,
@@ -186,7 +186,7 @@ where
             }
 
             // Have we already picked the index with the last non-zero count in the histogram?
-            if self.last_picked_index >= self.max_value_index {
+            if self.last_picked_index >= Some(self.max_value_index) {
                 // is the picker done?
                 if !self.picker.more(self.current_index) {
                     self.ended = true;
@@ -243,7 +243,7 @@ where
                 // step multiple times without advancing the index.
 
                 self.count_since_last_iteration = 0;
-                self.last_picked_index = self.current_index;
+                self.last_picked_index = Some(self.current_index);
                 return Some(val);
             }
 

tests/histogram.rs

@@ -560,3 +560,10 @@ fn subtract_underflow_guarded_by_per_value_count_check() {
         h.subtract(h2).unwrap_err()
     );
 }
+
+#[test]
+fn recorded_only_zeros() {
+    let mut h = Histogram::<u64>::new(1).unwrap();
+    h += 0;
+    assert_eq!(h.iter_recorded().count(), 1);
+}