fix for RoaringBitmap#130 append perf 10x slower than collect

Author: saik0

src/bitmap/iter.rs

@@ -184,13 +184,35 @@ impl RoaringBitmap {
         &mut self,
         iterator: I,
     ) -> Result<u64, NonSortedIntegers> {
-        let mut count = 0;
+        // Name shadowed to prevent accidentally referencing the param
+        let mut iterator = iterator.into_iter();
+
+        let mut prev: u32 = match iterator.next() {
+            None => return Ok(0),
+            Some(first) => {
+                if let Some(max) = self.max() {
+                    if first <= max {
+                        return Err(NonSortedIntegers { valid_until: 0 });
+                    }
+                }
+
+                first
+            }
+        };
+
+        self.insert(prev);
+        let mut count = 1;
+
+        // It is now guaranteed that so long as the values are iterator are monotonically
+        // increasing they must also be the greatest in the set.
 
         for value in iterator {
-            if self.push(value) {
-                count += 1;
-            } else {
+            if value <= prev {
                 return Err(NonSortedIntegers { valid_until: count });
+            } else {
+                self.insert(value);
+                prev = value;
+                count += 1;
             }
         }