Merge RoaringBitmap#131

131: Append perf bugfix r=Kerollmops a=saik0

Fixes RoaringBitmap#130 

## Cause

Where _N_ is the length of the iterator: _N_ calls to `RoaringBitmap::push` required N calls to`RoaringBitmap::max`, which is not cheap.

## Solution

Compare only the first item in the iterator to the bitmap max, then compare each element to its predecessor.

## Future work

This could be strictly faster than inserting, given that it's always inserting into the last container (or pushing a new container)

## Before
![Screen Shot 2022-01-09 at 9 35 20 PM](https://user-images.githubusercontent.com/997050/148730686-152f7232-4ef0-43da-b37f-7bef6d1502da.png)
![Screen Shot 2022-01-09 at 9 35 36 PM](https://user-images.githubusercontent.com/997050/148730691-a15045a7-0abc-47c3-8208-b95aa406a681.png)


## After

![Screen Shot 2022-01-09 at 11 13 35 PM](https://user-images.githubusercontent.com/997050/148729666-55c48265-d38a-4865-a92d-ceb83efdb500.png)
![Screen Shot 2022-01-09 at 11 13 43 PM](https://user-images.githubusercontent.com/997050/148729673-41dfc7e1-62ca-4cc4-8c7c-8cd3b59adc4c.png)



Co-authored-by: saik0 <[email protected]>

Author: bors[bot]

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;
             }
         }
 

tests/push.rs

@@ -25,6 +25,32 @@ fn append() {
     test_from_sorted_iter!(vec![1, 2, 4, 5, 7, 8, 9], RoaringBitmap);
 }
 
+#[test]
+fn append_empty() {
+    assert_eq!(RoaringBitmap::new().append(vec![]), Ok(0u64))
+}
+
+#[test]
+fn append_error() {
+    match [100u32].iter().cloned().collect::<RoaringBitmap>().append(vec![10, 20, 0]) {
+        Ok(_) => {
+            panic!("The 0th element in the iterator was < the max of the bitmap")
+        }
+        Err(non_sorted_error) => {
+            assert_eq!(non_sorted_error.valid_until(), 0)
+        }
+    }
+
+    match [100u32].iter().cloned().collect::<RoaringBitmap>().append(vec![200, 201, 201]) {
+        Ok(_) => {
+            panic!("The 3rd element in the iterator was < 2nd")
+        }
+        Err(non_sorted_error) => {
+            assert_eq!(non_sorted_error.valid_until(), 2)
+        }
+    }
+}
+
 #[test]
 fn append_tree() {
     test_from_sorted_iter!((0..1_000_000).map(|x| 13 * x).collect::<Vec<u64>>(), RoaringTreemap);