Merge inserts with inserts

backend/reconcile/src/utils/find_common_overlap.rs

@@ -0,0 +1,66 @@
+use crate::Token;
+
+/// Given two lists of tokens, returns the offset in the first (old) list from which the two lists have the same tokens until the end of the first list.
+/// Thus, the suffix of the old list from the offset to the end is equal to a prefix of the new list.
+///
+/// If there is no overlap, the function returns the maxmium offset, the length of the old list.
+///
+/// ## Example
+/// ```
+/// old: [0, 1, 9, 0, 2, 5]
+/// new:       [9, 0, 2, 5, 1]
+/// ```
+/// > results in an offset of 2
+pub fn find_common_overlap<T>(old: &[Token<T>], new: &[Token<T>]) -> usize
+where
+    T: PartialEq + Clone,
+{
+    let minimum_offset = old.len().saturating_sub(new.len());
+    for offset in minimum_offset..old.len() {
+        if old.iter().skip(offset).zip(new.iter()).all(|(a, b)| a == b) {
+            return offset;
+        }
+    }
+
+    old.len()
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use pretty_assertions::assert_eq;
+
+    #[test]
+    fn test_common_overlap() {
+        assert_eq!(find_common_overlap(&["".into()], &["".into()]), 0);
+
+        assert_eq!(
+            find_common_overlap(
+                &["a".into(), "b".into(), "c".into()],
+                &["b".into(), "c".into(), "a".into()]
+            ),
+            1
+        );
+
+        assert_eq!(
+            find_common_overlap(
+                &["a".into(), "a".into(), "a".into()],
+                &["a".into(), "b".into(), "c".into()]
+            ),
+            2
+        );
+
+        assert_eq!(
+            find_common_overlap(
+                &["a".into(), "b".into(), "c".into()],
+                &["d".into(), "e".into(), "a".into()]
+            ),
+            3
+        );
+
+        assert_eq!(
+            find_common_overlap(&["a".into(), "a".into()], &["a".into()]),
+            1
+        );
+    }
+}

backend/reconcile/src/utils/mod.rs

@@ -1,5 +1,6 @@
 pub mod common_prefix_len;
 pub mod common_suffix_len;
+pub mod find_common_overlap;
 pub mod merge_iters;
 pub mod ordered_operation;
 pub mod side;