move common_elements util into unit

Signed-off-by: Jason Volk <jason@zemos.net>

src/core/utils/algorithm.rs

@@ -0,0 +1,25 @@
+use std::cmp::Ordering;
+
+#[allow(clippy::impl_trait_in_params)]
+pub fn common_elements(
+	mut iterators: impl Iterator<Item = impl Iterator<Item = Vec<u8>>>, check_order: impl Fn(&[u8], &[u8]) -> Ordering,
+) -> Option<impl Iterator<Item = Vec<u8>>> {
+	let first_iterator = iterators.next()?;
+	let mut other_iterators = iterators.map(Iterator::peekable).collect::<Vec<_>>();
+
+	Some(first_iterator.filter(move |target| {
+		other_iterators.iter_mut().all(|it| {
+			while let Some(element) = it.peek() {
+				match check_order(element, target) {
+					Ordering::Greater => return false, // We went too far
+					Ordering::Equal => return true,    // Element is in both iters
+					Ordering::Less => {
+						// Keep searching
+						it.next();
+					},
+				}
+			}
+			false
+		})
+	}))
+}