1 files changed, 77 insertions, 0 deletions
diff --git a/src/document/store.rs b/src/document/store.rs
index 89058fb..13a3fc3 100644
--- a/src/document/store.rs
+++ b/src/document/store.rs
@@ -15,6 +15,12 @@ pub(crate) enum StoreError {
     Conflict,
     #[error("value does not exist")]
     NotFound,
+    /// An insert where there is no list.
+    #[error("not a list")]
+    NotAList,
+    /// An insert past a list's end.
+    #[error("index {index} is past the list's end ({length} items)")]
+    PastTheEnd { index: usize, length: usize },
     #[error(transparent)]
     Doc(#[from] automerge::AutomergeError),
     #[error(transparent)]
@@ -357,6 +363,29 @@ pub(crate) fn write_path(
     Ok(())
 }
 
+/// Inserts `value` into the list at `path`: at `index` (the items from there on move
+/// up one), or at the end without one. Returns where it went. An index past the end is
+/// an error (`PastTheEnd`), as is a path without a list (`NotAList`, `NotFound`).
+pub(crate) fn insert_path(
+    doc: &mut AutoCommit,
+    path: &[String],
+    index: Option<usize>,
+    value: &JsonValue,
+) -> Result<usize, StoreError> {
+    let list = match resolve_at(doc, path)? {
+        None => return Err(StoreError::NotFound),
+        Some(Resolved::Other(Value::Object(ObjType::List), id)) => id,
+        Some(_) => return Err(StoreError::NotAList),
+    };
+    let length = doc.length(&list);
+    let index = index.unwrap_or(length);
+    if index > length {
+        return Err(StoreError::PastTheEnd { index, length });
+    }
+    write_value(doc, &list, Slot::Index(index), value)?;
+    Ok(index)
+}
+
 /// The merged maps at `path`, creating missing maps on the way.
 fn ensure_maps(doc: &mut AutoCommit, path: &[String]) -> Result<Vec<ObjId>, StoreError> {
     let mut maps = vec![automerge::ROOT];
@@ -806,3 +835,51 @@ fn nested<T>(
     }
     out
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::json;
+
+    fn path(at: &str) -> Vec<String> {
+        at.split('/').map(str::to_owned).collect()
+    }
+
+    #[test]
+    fn inserts_into_lists() {
+        let mut doc = AutoCommit::new();
+        write_path(&mut doc, &path("list"), &json!(["b", "d"])).unwrap();
+        assert_eq!(
+            insert_path(&mut doc, &path("list"), Some(0), &json!("a")).unwrap(),
+            0
+        );
+        assert_eq!(
+            insert_path(&mut doc, &path("list"), Some(2), &json!("c")).unwrap(),
+            2
+        );
+        assert_eq!(
+            insert_path(&mut doc, &path("list"), None, &json!({"e": 5})).unwrap(),
+            4
+        );
+        assert_eq!(
+            read_path(&doc, &path("list")).unwrap(),
+            Some(json!(["a", "b", "c", "d", {"e": 5}]))
+        );
+        assert!(matches!(
+            insert_path(&mut doc, &path("list"), Some(6), &json!(1)),
+            Err(StoreError::PastTheEnd {
+                index: 6,
+                length: 5
+            })
+        ));
+        write_path(&mut doc, &path("map"), &json!({"k": 1})).unwrap();
+        assert!(matches!(
+            insert_path(&mut doc, &path("map"), None, &json!(1)),
+            Err(StoreError::NotAList)
+        ));
+        assert!(matches!(
+            insert_path(&mut doc, &path("nothing"), None, &json!(1)),
+            Err(StoreError::NotFound)
+        ));
+    }
+}