common/rust: add UpToTwo type

common/rust/src/lib.rs

@@ -2,4 +2,62 @@
 
 mod section_range;
 
+use std::mem;
+
 pub use section_range::{EmptyRange, InvalidSectionString, SectionRange};
+
+pub enum UpToTwo<T> {
+    Zero,
+    One(T),
+    Two(T, T),
+}
+
+impl<T> UpToTwo<T> {
+    pub fn is_empty(&self) -> bool {
+        matches!(self, UpToTwo::Zero)
+    }
+
+    pub fn first(&self) -> Option<&T> {
+        match self {
+            UpToTwo::Zero => None,
+            UpToTwo::One(x) | UpToTwo::Two(x, _) => Some(x),
+        }
+    }
+
+    pub fn second(&self) -> Option<&T> {
+        match self {
+            UpToTwo::Zero | UpToTwo::One(_) => None,
+            UpToTwo::Two(_, x) => Some(x),
+        }
+    }
+}
+
+impl<T> IntoIterator for UpToTwo<T> {
+    type Item = T;
+
+    type IntoIter = UpToTwoIter<T>;
+
+    fn into_iter(self) -> Self::IntoIter {
+        UpToTwoIter(self)
+    }
+}
+
+pub struct UpToTwoIter<T>(UpToTwo<T>);
+
+impl<T> Iterator for UpToTwoIter<T> {
+    type Item = T;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        let mut content = UpToTwo::Zero;
+        mem::swap(&mut self.0, &mut content);
+
+        let (ret, mut content) = match content {
+            UpToTwo::Zero => (None, UpToTwo::Zero),
+            UpToTwo::One(x) => (Some(x), UpToTwo::Zero),
+            UpToTwo::Two(x, y) => (Some(x), UpToTwo::One(y)),
+        };
+        mem::swap(&mut self.0, &mut content);
+
+        ret
+    }
+}