feat: branching service (branch_from_event + switch + metadata) (T94)

chat/services/branching.py

@@ -0,0 +1,107 @@
+"""Branching service (T94, Phase 4).
+
+Wraps branches state with validation + event emission. Phase 4 ships
+the data model and creation/switching APIs; the read-side filter
+(event readers consulting is_active) is a Phase 4.5+ follow-up — for
+now branches are metadata-only and the existing event readers remain
+branch-agnostic. The drawer UI (T98) drives create/switch via these
+helpers.
+"""
+
+from __future__ import annotations
+from sqlite3 import Connection
+
+from chat.eventlog.log import append_and_apply
+from chat.state.branches import get_branch, list_branches, active_branch  # noqa: F401
+
+
+def branch_from_event(
+    conn: Connection,
+    *,
+    name: str,
+    origin_event_id: int,
+    chat_id: str | None = None,
+) -> int:
+    """Create a new named branch forking from origin_event_id.
+
+    Emits a branch_created event. Returns the new branch's row id.
+    Raises ValueError if name already exists or origin_event_id doesn't
+    correspond to a real event."""
+    if not name or not name.strip():
+        raise ValueError("branch name must be non-empty")
+
+    if get_branch(conn, name) is not None:
+        raise ValueError(f"branch {name!r} already exists")
+
+    # Validate origin_event_id is a real event id (or 0 for the bootstrap case
+    # which only main uses).
+    if origin_event_id < 0:
+        raise ValueError(f"origin_event_id must be >= 0, got {origin_event_id}")
+    if origin_event_id > 0:
+        row = conn.execute(
+            "SELECT 1 FROM event_log WHERE id = ?", (origin_event_id,)
+        ).fetchone()
+        if row is None:
+            raise ValueError(
+                f"origin_event_id {origin_event_id} does not exist in event_log"
+            )
+
+    append_and_apply(
+        conn,
+        kind="branch_created",
+        payload={
+            "name": name,
+            "origin_event_id": origin_event_id,
+            "head_event_id": origin_event_id,  # head starts at origin
+            "chat_id": chat_id,
+        },
+    )
+
+    branch = get_branch(conn, name)
+    if branch is None:
+        # Should be unreachable if append_and_apply worked.
+        raise RuntimeError(f"branch {name!r} not found after creation")
+    return branch["id"]
+
+
+def switch_active_branch(conn: Connection, *, name: str) -> None:
+    """Make the named branch active. Emits branch_switched."""
+    if get_branch(conn, name) is None:
+        raise ValueError(f"branch {name!r} does not exist")
+
+    append_and_apply(
+        conn,
+        kind="branch_switched",
+        payload={"name": name},
+    )
+
+
+def list_branches_with_metadata(
+    conn: Connection, chat_id: str | None = None
+) -> list[dict]:
+    """List branches with computed event_count metadata.
+
+    event_count = head_event_id - origin_event_id + 1 (when both are set)
+                  OR head_event_id (when origin is 0, e.g., main branch)
+                  OR 0 (when head <= origin, which is the bootstrap state)
+    """
+    branches = list_branches(conn, chat_id)
+    enriched = []
+    for b in branches:
+        origin = b["origin_event_id"]
+        head = b["head_event_id"]
+        if head < origin:
+            event_count = 0
+        elif origin == 0:
+            event_count = head
+        else:
+            event_count = head - origin + 1
+        enriched.append({**b, "event_count": event_count})
+    return enriched
+
+
+__all__ = [
+    "branch_from_event",
+    "switch_active_branch",
+    "list_branches_with_metadata",
+]