From 9b976a6c4add3719b8f9afef53578ff778b50aa1 Mon Sep 17 00:00:00 2001
From: tv <tv@krebsco.de>
Date: Sun, 1 Nov 2026 22:29:54 +0100
Subject: api: add support for counters

---
 src/api/mod.rs        |  7 ++++---
 src/api/store.rs      | 27 ++++++++++++++++++++++--
 src/document/store.rs | 57 +++++++++++++++++++++++++++++++++++++++++++++++++++
 3 files changed, 86 insertions(+), 5 deletions(-)

(limited to 'src')

diff --git a/src/api/mod.rs b/src/api/mod.rs
index 8ce855e..dfefcc8 100644
--- a/src/api/mod.rs
+++ b/src/api/mod.rs
@@ -201,9 +201,10 @@ impl IntoResponse for ApiError {
 /// client's; others are logged with `context`, and are a bare `500`.
 pub(crate) fn store_status(error: StoreError, context: &str) -> ApiError {
     let status = match error {
-        StoreError::Conflict | StoreError::NotAList | StoreError::PastTheEnd { .. } => {
-            StatusCode::CONFLICT
-        }
+        StoreError::Conflict
+        | StoreError::NotAList
+        | StoreError::PastTheEnd { .. }
+        | StoreError::NotACounter => StatusCode::CONFLICT,
         StoreError::NotFound | StoreError::UnknownHeads => StatusCode::NOT_FOUND,
         StoreError::Rejoining => StatusCode::SERVICE_UNAVAILABLE,
         other => {
diff --git a/src/api/store.rs b/src/api/store.rs
index 8b09487..7039008 100644
--- a/src/api/store.rs
+++ b/src/api/store.rs
@@ -77,6 +77,13 @@ macro_rules! read_at {
     };
 }
 
+#[derive(Debug, Deserialize)]
+pub(crate) struct PostQuery {
+    /// Adds this to the counter at the path instead of inserting the body
+    /// (`store::increment_path`).
+    increment: Option<i64>,
+}
+
 #[derive(Debug, Deserialize)]
 pub(crate) struct DeleteQuery {
     #[serde(default)]
@@ -355,11 +362,14 @@ fn body_value(headers: &HeaderMap, body: &Bytes) -> Result<JsonValue, StatusCode
 /// `POST /store/{key}`: inserts the body into a list, `key` ending in the index it
 /// goes to (`store::insert_path`: the items from there on move up; the length, or `-1`,
 /// appends); `201` with where it went (`{"index": n}`). Not a list there, or an index
-/// past its end: `409`, saying which.
+/// past its end: `409`, saying which. With `?increment=n`, adds `n` to the counter at
+/// `key` instead (`store::increment_path`), answering with its value then; not a
+/// counter there: `409`.
 pub(crate) async fn post_store_entry(
     State(state): State<AppState>,
     Extension(user): Extension<AuthenticatedUser>,
     Path(key): Path<String>,
+    Query(query): Query<PostQuery>,
     RawQuery(raw_query): RawQuery,
     headers: HeaderMap,
     body: Bytes,
@@ -368,9 +378,22 @@ pub(crate) async fn post_store_entry(
         return Err(StatusCode::SERVICE_UNAVAILABLE.into());
     }
     let path = parse_write_path(&state, &user, &key, "POST")?;
-    if let Some(redirect) = to_positive(&state, &path, true, None, raw_query.as_deref()).await? {
+    // An increment names an item (`-1`: the last); an insert where it goes (`-1`: the end).
+    let insertion = query.increment.is_none();
+    if let Some(redirect) =
+        to_positive(&state, &path, insertion, None, raw_query.as_deref()).await?
+    {
         return Ok(redirect);
     }
+    if let Some(by) = query.increment {
+        let value = state
+            .mutate_doc_for(Some(user.0.clone()), move |doc| {
+                store::increment_path(doc, &path, by)
+            })
+            .await
+            .map_err(|e| store_status(e, "Failed to increment a counter"))?;
+        return response_for_value(&JsonValue::from(value), headers.get(ACCEPT));
+    }
     let index = path.last().and_then(|leaf| store::list_index(leaf));
     let value = body_value(&headers, &body)?;
     let inserted = state
diff --git a/src/document/store.rs b/src/document/store.rs
index c16767b..13824b9 100644
--- a/src/document/store.rs
+++ b/src/document/store.rs
@@ -94,6 +94,9 @@ pub(crate) enum StoreError {
     /// An index past a list's end (`length` items).
     #[error("index {index} is past the list's end ({length} items)")]
     PastTheEnd { index: usize, length: usize },
+    /// An increment of something that isn't a counter (`increment_path`).
+    #[error("not a counter")]
+    NotACounter,
     /// A read at heads the document doesn't have (`at::At`).
     #[error("this node doesn't have these heads (yet)")]
     UnknownHeads,
@@ -744,6 +747,60 @@ fn parent_for_write(doc: &mut impl Transactable, parents: &[String]) -> Result<P
     Ok(current)
 }
 
+/// Adds `by` to the counter at `path`, creating one with `by` where there is nothing
+/// (in a map: a list's items are only incremented); returns its value then. Increments
+/// of one counter on several nodes add up, where writes of numbers would leave one.
+/// Anything else there (the newest of concurrent values, as a read resolves them) is
+/// `NotACounter`.
+pub(crate) fn increment_path(
+    doc: &mut impl Transactable,
+    path: &[String],
+    by: i64,
+) -> Result<i64, StoreError> {
+    let (leaf, parents) = path.split_last().ok_or(StoreError::NotFound)?;
+    let (parent, prop): (ObjId, Prop) = match parent_for_write(doc, parents)? {
+        Parent::Maps(maps) => {
+            let holds = candidates(doc, &maps, leaf)?;
+            if holds.is_empty() {
+                doc.put(&maps[0], leaf.as_str(), ScalarValue::counter(by))?;
+                return Ok(by);
+            }
+            // The map that holds the newest value.
+            let newest = &holds[0].1;
+            let map = maps
+                .iter()
+                .find(|map| {
+                    doc.get_all(*map, leaf.as_str())
+                        .is_ok_and(|all| all.iter().any(|(_, id)| id == newest))
+                })
+                .cloned()
+                .ok_or(StoreError::NotFound)?;
+            (map, Prop::Map(leaf.clone()))
+        }
+        Parent::List(list) => {
+            let index = list_index(leaf).ok_or(StoreError::NotFound)?;
+            if index >= doc.length(&list) {
+                return Err(StoreError::NotFound);
+            }
+            (list, Prop::Seq(index))
+        }
+    };
+    counter_at(doc, &parent, prop.clone())?;
+    doc.increment(&parent, prop.clone(), by)?;
+    counter_at(doc, &parent, prop)
+}
+
+/// The value of the counter `prop` of `parent` holds (the newest of its values).
+fn counter_at(doc: &impl ReadDoc, parent: &ObjId, prop: Prop) -> Result<i64, StoreError> {
+    match doc.get(parent, prop)? {
+        Some((Value::Scalar(scalar), _)) => match scalar.as_ref() {
+            ScalarValue::Counter(counter) => Ok(i64::from(counter)),
+            _ => Err(StoreError::NotACounter),
+        },
+        _ => Err(StoreError::NotACounter),
+    }
+}
+
 /// Inserts `value` into a list: `path` ends in the index it goes to (the items from
 /// there on move up one; the list's length appends). An index past the end is
 /// `PastTheEnd`; a parent that isn't a list `NotAList`; anything but an index there, or
-- 
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