1 files changed, 506 insertions, 0 deletions
diff --git a/src/store.rs b/src/store.rs
new file mode 100644
index 0000000..08fe5c0
--- /dev/null
+++ b/src/store.rs
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+//! JSON view over the Automerge document with conflict-aware merging.
+
+use anyhow::anyhow;
+use automerge::{
+    transaction::Transactable, ActorId, AutoCommit, ObjId, ObjType, ReadDoc, ScalarValue, Value,
+};
+use base64::{engine::general_purpose::STANDARD as BASE64, Engine};
+use serde::Serialize;
+use serde_json::Value as JsonValue;
+use std::collections::BTreeSet;
+
+#[derive(Debug, thiserror::Error)]
+pub enum StoreError {
+    #[error("path crosses a non-map value")]
+    Conflict,
+    #[error("value does not exist")]
+    NotFound,
+    #[error(transparent)]
+    Doc(#[from] automerge::AutomergeError),
+    #[error(transparent)]
+    Other(#[from] anyhow::Error),
+}
+
+/// Automerge's total order on operations: Lamport counter, then actor.
+fn op_order(id: &ObjId) -> (u64, Option<&ActorId>) {
+    match id {
+        ObjId::Root => (0, None),
+        ObjId::Id(counter, actor, _) => (*counter, Some(actor)),
+    }
+}
+
+#[derive(Debug, Clone)]
+enum Resolved {
+    /// Concurrent maps presented as one. Newest first; `[0]` is canonical for writes.
+    Maps(Vec<ObjId>),
+    /// Any other value: the newest concurrent candidate.
+    Other(Value<'static>, ObjId),
+}
+
+/// All concurrent values of `key` across a merged set of parent maps, newest first.
+fn candidates(
+    doc: &AutoCommit,
+    parents: &[ObjId],
+    key: &str,
+) -> Result<Vec<(Value<'static>, ObjId)>, StoreError> {
+    let mut all = Vec::new();
+    for parent in parents {
+        for (value, id) in doc.get_all(parent, key)? {
+            all.push((value.into_owned(), id));
+        }
+    }
+    all.sort_by(|a, b| op_order(&b.1).cmp(&op_order(&a.1)));
+    Ok(all)
+}
+
+fn resolve(
+    doc: &AutoCommit,
+    parents: &[ObjId],
+    key: &str,
+    log: Option<&mut ConflictLog>,
+) -> Result<Option<Resolved>, StoreError> {
+    let mut all = candidates(doc, parents, key)?;
+    if all.is_empty() {
+        return Ok(None);
+    }
+
+    let merged = matches!(all[0].0, Value::Object(ObjType::Map));
+
+    if all.len() > 1 {
+        if let Some(log) = log {
+            log.record(doc, key, &all, merged)?;
+        }
+    }
+
+    if merged {
+        let maps = all
+            .into_iter()
+            .filter(|(value, _)| matches!(value, Value::Object(ObjType::Map)))
+            .map(|(_, id)| id)
+            .collect();
+        Ok(Some(Resolved::Maps(maps)))
+    } else {
+        let (value, id) = all.swap_remove(0);
+        Ok(Some(Resolved::Other(value, id)))
+    }
+}
+
+fn merged_keys(doc: &AutoCommit, maps: &[ObjId]) -> BTreeSet<String> {
+    maps.iter().flat_map(|map| doc.keys(map)).collect()
+}
+
+fn resolved_to_json(
+    doc: &AutoCommit,
+    resolved: &Resolved,
+    mut log: Option<&mut ConflictLog>,
+) -> Result<JsonValue, StoreError> {
+    let maps = match resolved {
+        Resolved::Maps(maps) => maps,
+        Resolved::Other(Value::Scalar(scalar), _) => {
+            return Ok(scalar_to_json(scalar.clone().into_owned())?)
+        }
+        Resolved::Other(Value::Object(_), id) => return object_to_json(doc, id, log),
+    };
+
+    let mut out = serde_json::Map::new();
+
+    if let [map] = maps.as_slice() {
+        // Fast path: one pass; get_all only for keys Automerge flags as conflicted.
+        for item in doc.map_range(map, ..) {
+            let key = item.key.to_string();
+
+            let child = if item.conflict {
+                match resolve(doc, maps, &key, log.as_deref_mut())? {
+                    Some(child) => child,
+                    None => continue,
+                }
+            } else {
+                let id = item.id();
+                match Value::from(item.value) {
+                    Value::Object(ObjType::Map) => Resolved::Maps(vec![id]),
+                    value => Resolved::Other(value.into_owned(), id),
+                }
+            };
+
+            let json = nested(&mut log, &key, |log| resolved_to_json(doc, &child, log))?;
+            out.insert(key, json);
+        }
+    } else {
+        // Several concurrent maps: union of their keys.
+        for key in merged_keys(doc, maps) {
+            let Some(child) = resolve(doc, maps, &key, log.as_deref_mut())? else {
+                continue;
+            };
+            let json = nested(&mut log, &key, |log| resolved_to_json(doc, &child, log))?;
+            out.insert(key, json);
+        }
+    }
+
+    Ok(JsonValue::Object(out))
+}
+
+fn object_to_json(
+    doc: &AutoCommit,
+    id: &ObjId,
+    mut log: Option<&mut ConflictLog>,
+) -> Result<JsonValue, StoreError> {
+    match doc.object_type(id)? {
+        ObjType::Map => resolved_to_json(doc, &Resolved::Maps(vec![id.clone()]), log),
+        ObjType::List => {
+            let mut out = Vec::new();
+            for (index, item) in doc.list_range(id, ..).enumerate() {
+                let child_id = item.id();
+                out.push(match Value::from(item.value) {
+                    Value::Object(_) => nested(&mut log, &index.to_string(), |log| {
+                        object_to_json(doc, &child_id, log)
+                    })?,
+                    Value::Scalar(scalar) => scalar_to_json(scalar.into_owned())?,
+                });
+            }
+            Ok(JsonValue::Array(out))
+        }
+        ObjType::Text => Ok(JsonValue::String(doc.text(id)?)),
+        ObjType::Table => Err(anyhow!("Automerge tables are not supported").into()),
+    }
+}
+
+fn delete_in_all(doc: &mut AutoCommit, maps: &[ObjId], key: &str) -> Result<(), StoreError> {
+    for map in maps {
+        if doc.get(map, key)?.is_some() {
+            doc.delete(map, key)?;
+        }
+    }
+    Ok(())
+}
+
+fn read_path_inner(
+    doc: &AutoCommit,
+    path: &[String],
+    mut log: Option<&mut ConflictLog>,
+) -> Result<Option<JsonValue>, StoreError> {
+    let mut current = Resolved::Maps(vec![automerge::ROOT]);
+
+    for (index, segment) in path.iter().enumerate() {
+        let Resolved::Maps(maps) = &current else {
+            return Ok(None);
+        };
+        if let Some(log) = log.as_deref_mut() {
+            // The last segment is the requested value itself, not an ancestor.
+            log.in_ancestors = index + 1 < path.len();
+        }
+        match resolve(doc, maps, segment, log.as_deref_mut())? {
+            Some(next) => current = next,
+            None => return Ok(None),
+        }
+        if let Some(log) = log.as_deref_mut() {
+            log.path.push(segment.clone());
+        }
+    }
+
+    if let Some(log) = log.as_deref_mut() {
+        log.in_ancestors = false;
+    }
+    Ok(Some(resolved_to_json(doc, &current, log)?))
+}
+
+/// Reads a path; the empty path is the whole store.
+pub fn read_path(doc: &AutoCommit, path: &[String]) -> Result<Option<JsonValue>, StoreError> {
+    read_path_inner(doc, path, None)
+}
+
+pub fn read_path_with_conflicts(
+    doc: &AutoCommit,
+    path: &[String],
+) -> Result<Option<(JsonValue, Vec<ConflictReport>)>, StoreError> {
+    let mut log = ConflictLog { want_values: true, ..Default::default() };
+    let value = read_path_inner(doc, path, Some(&mut log))?;
+    Ok(value.map(|value| (value, log.reports)))
+}
+
+/// For streams: the value, plus whether the path or subtree involves any conflict.
+/// Reports are collected without values, so this costs little more than read_path.
+pub fn read_path_traced(
+    doc: &AutoCommit,
+    path: &[String],
+) -> Result<(Option<JsonValue>, bool), StoreError> {
+    let mut log = ConflictLog::default();
+    let value = read_path_inner(doc, path, Some(&mut log))?;
+    Ok((value, !log.reports.is_empty()))
+}
+
+/// Keys of the merged map at `path`; empty if it doesn't exist or isn't a map.
+pub fn keys_at(doc: &AutoCommit, path: &[String]) -> Result<Vec<String>, StoreError> {
+    let mut maps = vec![automerge::ROOT];
+    for segment in path {
+        maps = match resolve(doc, &maps, segment, None)? {
+            Some(Resolved::Maps(next)) => next,
+            _ => return Ok(Vec::new()),
+        };
+    }
+    Ok(merged_keys(doc, &maps).into_iter().collect())
+}
+
+/// Writes (replaces) the value at `path`, creating intermediate maps.
+pub fn write_path(
+    doc: &mut AutoCommit,
+    path: &[String],
+    value: &JsonValue,
+) -> Result<(), StoreError> {
+    let (leaf, parents) = path.split_last().ok_or(StoreError::NotFound)?;
+
+    let mut maps = vec![automerge::ROOT];
+    for segment in parents {
+        maps = match resolve(doc, &maps, segment, None)? {
+            Some(Resolved::Maps(next)) => next,
+            Some(Resolved::Other(..)) => return Err(StoreError::Conflict),
+            // A concurrent creation elsewhere is merged on read.
+            None => vec![doc.put_object(&maps[0], segment.as_str(), ObjType::Map)?],
+        };
+    }
+
+    // Replace semantics: clear the key in every merged parent, or an old map
+    // would merge into the new value.
+    delete_in_all(doc, &maps, leaf)?;
+    json_to_automerge(doc, &maps[0], leaf, value)?;
+    Ok(())
+}
+
+/// Deletes the value at `path`, optionally pruning ancestors that become empty.
+pub fn delete_path(doc: &mut AutoCommit, path: &[String], prune: bool) -> Result<(), StoreError> {
+    let (leaf, parents) = path.split_last().ok_or(StoreError::NotFound)?;
+
+    // levels[d] = merged maps at path[..d]
+    let mut levels = vec![vec![automerge::ROOT]];
+    for segment in parents {
+        let next = match resolve(doc, levels.last().expect("non-empty"), segment, None)? {
+            Some(Resolved::Maps(maps)) => maps,
+            _ => return Err(StoreError::NotFound),
+        };
+        levels.push(next);
+    }
+
+    let maps = levels.last().expect("non-empty");
+    if resolve(doc, maps, leaf, None)?.is_none() {
+        return Err(StoreError::NotFound);
+    }
+    delete_in_all(doc, maps, leaf)?;
+
+    if prune {
+        // Never prune the root (depth 0).
+        for depth in (1..levels.len()).rev() {
+            if !merged_keys(doc, &levels[depth]).is_empty() {
+                break;
+            }
+            delete_in_all(doc, &levels[depth - 1], &path[depth - 1])?;
+        }
+    }
+
+    Ok(())
+}
+
+/// Where a value is placed inside its parent object.
+#[derive(Clone, Copy)]
+enum Slot<'a> {
+    /// A key in a map.
+    Key(&'a str),
+    /// An index in a list. The value is inserted here, not overwritten.
+    Index(usize),
+}
+
+fn put_object(
+    doc: &mut AutoCommit,
+    parent: &ObjId,
+    slot: Slot,
+    obj_type: ObjType,
+) -> Result<ObjId, StoreError> {
+    Ok(match slot {
+        Slot::Key(key) => doc.put_object(parent, key, obj_type)?,
+        Slot::Index(index) => doc.insert_object(parent, index, obj_type)?,
+    })
+}
+
+fn put_scalar(
+    doc: &mut AutoCommit,
+    parent: &ObjId,
+    slot: Slot,
+    value: impl Into<ScalarValue>,
+) -> Result<(), StoreError> {
+    match slot {
+        Slot::Key(key) => doc.put(parent, key, value)?,
+        Slot::Index(index) => doc.insert(parent, index, value)?,
+    }
+    Ok(())
+}
+
+fn write_value(
+    doc: &mut AutoCommit,
+    parent: &ObjId,
+    slot: Slot,
+    value: &JsonValue,
+) -> Result<(), StoreError> {
+    match value {
+        JsonValue::Object(object) => {
+            let id = put_object(doc, parent, slot, ObjType::Map)?;
+            for (child_key, child_value) in object {
+                write_value(doc, &id, Slot::Key(child_key), child_value)?;
+            }
+        }
+        JsonValue::Array(items) => {
+            let id = put_object(doc, parent, slot, ObjType::List)?;
+            for (index, item) in items.iter().enumerate() {
+                write_value(doc, &id, Slot::Index(index), item)?;
+            }
+        }
+        JsonValue::String(value) => put_scalar(doc, parent, slot, value.as_str())?,
+        JsonValue::Bool(value) => put_scalar(doc, parent, slot, *value)?,
+        JsonValue::Number(value) => {
+            if let Some(value) = value.as_i64() {
+                put_scalar(doc, parent, slot, value)?;
+            } else if let Some(value) = value.as_u64() {
+                put_scalar(doc, parent, slot, value)?;
+            } else if let Some(value) = value.as_f64() {
+                put_scalar(doc, parent, slot, value)?;
+            } else {
+                return Err(anyhow!("Unsupported JSON number").into());
+            }
+        }
+        JsonValue::Null => put_scalar(doc, parent, slot, ScalarValue::Null)?,
+    }
+
+    Ok(())
+}
+
+fn json_to_automerge(
+    doc: &mut AutoCommit,
+    parent: &ObjId,
+    key: &str,
+    value: &JsonValue,
+) -> Result<(), StoreError> {
+    write_value(doc, parent, Slot::Key(key), value)
+}
+
+fn scalar_to_json(scalar: ScalarValue) -> Result<JsonValue, StoreError> {
+    match scalar {
+        ScalarValue::Null => Ok(JsonValue::Null),
+        ScalarValue::Boolean(b) => Ok(JsonValue::Bool(b)),
+        ScalarValue::Int(i) => Ok(JsonValue::Number(i.into())),
+        ScalarValue::Uint(u) => Ok(JsonValue::Number(u.into())),
+        ScalarValue::F64(f) => serde_json::Number::from_f64(f)
+            .map(JsonValue::Number)
+            .ok_or(anyhow!("Non-finite float cannot be represented as JSON").into()),
+        ScalarValue::Str(s) => Ok(JsonValue::String(s.to_string())),
+        ScalarValue::Bytes(b) => Ok(JsonValue::String(BASE64.encode(b))),
+        ScalarValue::Counter(c) => Ok(JsonValue::Number(i64::from(c).into())),
+        ScalarValue::Timestamp(t) => Ok(JsonValue::Number(t.into())),
+        ScalarValue::Unknown { .. } => Err(anyhow!("Unsupported Automerge scalar type").into()),
+    }
+}
+
+// --- conflict log ------------------------------------------------------------
+
+#[derive(Debug, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub enum Resolution {
+    MergedMaps,
+    NewestWins,
+}
+
+#[derive(Debug, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct CandidateReport {
+    pub op_id: String,
+    pub selected: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub value: Option<JsonValue>,
+}
+
+#[derive(Debug, Serialize)]
+#[serde(rename_all = "camelCase")]
+pub struct ConflictReport {
+    pub path: Vec<String>,
+    /// On the way to the requested path; reported without candidate values.
+    #[serde(skip_serializing_if = "std::ops::Not::not")]
+    pub ancestor: bool,
+    pub resolution: Resolution,
+    pub candidates: Vec<CandidateReport>,
+}
+
+#[derive(Default)]
+struct ConflictLog {
+    path: Vec<String>,
+    reports: Vec<ConflictReport>,
+    /// Render candidate values (only within the requested subtree).
+    want_values: bool,
+    /// Currently walking the ancestors of the requested path.
+    in_ancestors: bool,
+}
+
+fn op_id_string(id: &ObjId) -> String {
+    match id {
+        ObjId::Root => "_root".to_owned(),
+        ObjId::Id(counter, actor, _) => format!("{counter}@{actor}"),
+    }
+}
+
+fn value_to_json(doc: &AutoCommit, value: &Value<'_>, id: &ObjId) -> Result<JsonValue, StoreError> {
+    match value {
+        Value::Scalar(scalar) => Ok(scalar_to_json(scalar.clone().into_owned())?),
+        Value::Object(_) => object_to_json(doc, id, None),
+    }
+}
+
+impl ConflictLog {
+    fn record(
+        &mut self,
+        doc: &AutoCommit,
+        key: &str,
+        all: &[(Value<'static>, ObjId)],
+        merged: bool,
+    ) -> Result<(), StoreError> {
+        let with_values = self.want_values && !self.in_ancestors;
+
+        let candidates = all
+            .iter()
+            .enumerate()
+            .map(|(index, (value, id))| {
+                let selected = if merged {
+                    matches!(value, Value::Object(ObjType::Map))
+                } else {
+                    index == 0
+                };
+                Ok(CandidateReport {
+                    op_id: op_id_string(id),
+                    selected,
+                    value: if with_values { Some(value_to_json(doc, value, id)?) } else { None },
+                })
+            })
+            .collect::<Result<Vec<_>, StoreError>>()?;
+
+        let mut path = self.path.clone();
+        path.push(key.to_owned());
+
+        self.reports.push(ConflictReport {
+            path,
+            ancestor: self.in_ancestors,
+            resolution: if merged { Resolution::MergedMaps } else { Resolution::NewestWins },
+            candidates,
+        });
+        Ok(())
+    }
+}
+
+/// Runs `f` with `segment` appended to the conflict path.
+fn nested<T>(
+    log: &mut Option<&mut ConflictLog>,
+    segment: &str,
+    f: impl FnOnce(Option<&mut ConflictLog>) -> T,
+) -> T {
+    if let Some(log) = log.as_deref_mut() {
+        log.path.push(segment.to_owned());
+    }
+    let out = f(log.as_deref_mut());
+    if let Some(log) = log.as_deref_mut() {
+        log.path.pop();
+    }
+    out
+}