feat: RangeSet::split_off (#11)

utils/src/range/mod.rs

@@ -127,6 +127,40 @@ impl<T: Copy + Ord> RangeSet<T> {
     pub fn max(&self) -> Option<T> {
         self.ranges.last().map(|range| range.end)
     }
+
+    /// Splits the set into two at the provided value.
+    ///
+    /// Returns a new set containing all the existing elements `>= at`. After the call,
+    /// the original set will be left containing the elements `< at`.
+    ///
+    /// # Panics
+    ///
+    /// Panics if `at` is not in the set.
+    pub fn split_off(&mut self, at: &T) -> Self {
+        // Find the index of the range containing `at`
+        let idx = self
+            .ranges
+            .iter()
+            .position(|range| range.contains(at))
+            .expect("`at` is in the set");
+
+        // Split off the range containing `at` and all the ranges to the right.
+        let mut split_ranges = self.ranges.split_off(idx);
+
+        // If the first range starts before `at` we have to push those values back
+        // into the existing set and truncate.
+        if *at > split_ranges[0].start {
+            self.ranges.push(Range {
+                start: split_ranges[0].start,
+                end: *at,
+            });
+            split_ranges[0].start = *at;
+        }
+
+        Self {
+            ranges: split_ranges,
+        }
+    }
 }
 
 impl<T: Copy + Ord> RangeSet<T>
@@ -349,6 +383,7 @@ impl<T: Copy + Ord> RangeSubset<RangeSet<T>> for Range<T> {
 #[allow(clippy::all)]
 mod tests {
     use super::*;
+    use rstest::*;
 
     #[test]
     fn test_range_disjoint() {
@@ -444,4 +479,34 @@ mod tests {
         _ = iter.next();
         assert_eq!(iter.len(), 2);
     }
+
+    #[rstest]
+    #[case(RangeSet::from([(0..1)]), 0)]
+    #[case(RangeSet::from([(0..5)]), 1)]
+    #[case(RangeSet::from([(0..5), (6..10)]), 4)]
+    #[case(RangeSet::from([(0..5), (6..10)]), 6)]
+    #[case(RangeSet::from([(0..5), (6..10)]), 9)]
+    fn test_range_set_split_off(#[case] set: RangeSet<usize>, #[case] at: usize) {
+        let mut a = set.clone();
+        let b = a.split_off(&at);
+
+        assert!(a
+            .ranges
+            .last()
+            .map(|range| !range.is_empty())
+            .unwrap_or(true));
+        assert!(b
+            .ranges
+            .first()
+            .map(|range| !range.is_empty())
+            .unwrap_or(true));
+        assert_eq!(a.len() + b.len(), set.len());
+        assert!(a.iter().chain(b.iter()).eq(set.iter()));
+    }
+
+    #[test]
+    #[should_panic = "`at` is in the set"]
+    fn test_range_set_split_off_panic_not_in_set() {
+        RangeSet::from([0..1]).split_off(&1);
+    }
 }