op_walk: add support for op_id+ (children) operator

A possible use case is when doing some archaeology around a certain operation.

The current implementation is quadratic if + is repeated. Suppose op_id is
usually close to the current op heads, I think it'll practically work better
than building a reverse lookup table.

docs/operation-log.md

@@ -17,8 +17,13 @@ The operation log allows you to undo an operation (`jj [op] undo`), which doesn'
 need to be the most recent one. It also lets you restore the entire repo to the
 way it looked at an earlier point (`jj op restore`).
 
-When referring to operations, you can use `@` to represent the current operation
-as well as the `-` operator (e.g. `@-`) to get the parent of an operation.
+When referring to operations, you can use `@` to represent the current
+operation.
+
+The following operators are supported:
+
+* `x-`: Parents of `x` (e.g. `@-`)
+* `x+`: Children of `x`
 
 
 ## Concurrent operations

lib/src/op_walk.rs

@@ -101,22 +101,27 @@ fn resolve_single_op(
     get_current_op: impl FnOnce() -> Result<Operation, OpsetEvaluationError>,
     op_str: &str,
 ) -> Result<Operation, OpsetEvaluationError> {
-    let op_symbol = op_str.trim_end_matches('-');
+    let op_symbol = op_str.trim_end_matches(['-', '+']);
     let op_postfix = &op_str[op_symbol.len()..];
+    let head_ops = op_postfix
+        .contains('+')
+        .then(|| get_current_head_ops(op_store, op_heads_store))
+        .transpose()?;
     let mut operation = match op_symbol {
         "@" => get_current_op(),
         s => resolve_single_op_from_store(op_store, op_heads_store, s),
     }?;
-    for _ in op_postfix.chars() {
-        let mut parent_ops = operation.parents();
-        let Some(op) = parent_ops.next().transpose()? else {
-            return Err(OpsetResolutionError::EmptyOperations(op_str.to_owned()).into());
+    for c in op_postfix.chars() {
+        let mut neighbor_ops = match c {
+            '-' => operation.parents().try_collect()?,
+            '+' => find_child_ops(head_ops.as_ref().unwrap(), operation.id())?,
+            _ => unreachable!(),
+        };
+        operation = match neighbor_ops.len() {
+            0 => Err(OpsetResolutionError::EmptyOperations(op_str.to_owned()))?,
+            1 => neighbor_ops.pop().unwrap(),
+            _ => Err(OpsetResolutionError::MultipleOperations(op_str.to_owned()))?,
         };
-        if parent_ops.next().is_some() {
-            return Err(OpsetResolutionError::MultipleOperations(op_str.to_owned()).into());
-        }
-        drop(parent_ops);
-        operation = op;
     }
     Ok(operation)
 }
@@ -178,6 +183,20 @@ fn get_current_head_ops(
         .try_collect()
 }
 
+/// Looks up children of the `root_op_id` by traversing from the `head_ops`.
+///
+/// This will be slow if the `root_op_id` is far away (or unreachable) from the
+/// `head_ops`.
+fn find_child_ops(
+    head_ops: &[Operation],
+    root_op_id: &OperationId,
+) -> OpStoreResult<Vec<Operation>> {
+    walk_ancestors(head_ops)
+        .take_while(|res| res.as_ref().map_or(true, |op| op.id() != root_op_id))
+        .filter_ok(|op| op.parent_ids().iter().any(|id| id == root_op_id))
+        .try_collect()
+}
+
 #[derive(Clone, Debug, Eq, Hash, PartialEq)]
 struct OperationByEndTime(Operation);
 

lib/tests/test_operations.rs

@@ -262,7 +262,7 @@ fn test_resolve_op_id() {
 }
 
 #[test]
-fn test_resolve_op_parents() {
+fn test_resolve_op_parents_children() {
     // Use monotonic timestamp to stabilize merge order of transactions
     let settings = testutils::user_settings();
     let test_repo = TestRepo::init_with_settings(&settings);
@@ -275,6 +275,7 @@ fn test_resolve_op_parents() {
         operations.push(repo.operation().clone());
     }
 
+    // Parent
     let op2_id_hex = operations[2].id().hex();
     assert_eq!(
         op_walk::resolve_op_with_repo(&repo, &format!("{op2_id_hex}-")).unwrap(),
@@ -292,6 +293,30 @@ fn test_resolve_op_parents() {
         ))
     );
 
+    // Child
+    let op0_id_hex = operations[0].id().hex();
+    assert_eq!(
+        op_walk::resolve_op_with_repo(&repo, &format!("{op0_id_hex}+")).unwrap(),
+        operations[1]
+    );
+    assert_eq!(
+        op_walk::resolve_op_with_repo(&repo, &format!("{op0_id_hex}++")).unwrap(),
+        operations[2]
+    );
+    assert_matches!(
+        op_walk::resolve_op_with_repo(&repo, &format!("{op0_id_hex}+++")),
+        Err(OpsetEvaluationError::OpsetResolution(
+            OpsetResolutionError::EmptyOperations(_)
+        ))
+    );
+
+    // Child of parent
+    assert_eq!(
+        op_walk::resolve_op_with_repo(&repo, &format!("{op2_id_hex}--+")).unwrap(),
+        operations[1]
+    );
+
+    // Merge and fork
     let tx1 = repo.start_transaction(&settings);
     let tx2 = repo.start_transaction(&settings);
     repo = testutils::commit_transactions(&settings, vec![tx1, tx2]);
@@ -302,4 +327,11 @@ fn test_resolve_op_parents() {
             OpsetResolutionError::MultipleOperations(_)
         ))
     );
+    let op2_id_hex = operations[2].id().hex();
+    assert_matches!(
+        op_walk::resolve_op_with_repo(&repo, &format!("{op2_id_hex}+")),
+        Err(OpsetEvaluationError::OpsetResolution(
+            OpsetResolutionError::MultipleOperations(_)
+        ))
+    );
 }