From 029b046f6666b39fe019cbc4b1f1a49a87a76e85 Mon Sep 17 00:00:00 2001
From: Jack Harper <jack.harper@stfc.ac.uk>
Date: Thu, 1 Oct 2026 15:50:20 +0100
Subject: [PATCH 1/2] fill out default nexus structure using streamed events as
 a sensible default

---
 src/howl.rs | 75 ++++++++++++++++++++++++++++++++++++++++++++++++++++-
 1 file changed, 74 insertions(+), 1 deletion(-)

diff --git a/src/howl.rs b/src/howl.rs
index f01575d..60895fb 100644
--- a/src/howl.rs
+++ b/src/howl.rs
@@ -24,6 +24,7 @@ use rand::prelude::ThreadRng;
 use rand_distr::{Distribution, Normal};
 use rdkafka::ClientConfig;
 use rdkafka::producer::{BaseRecord, DefaultProducerContext, ThreadedProducer};
+use serde_json::json;
 use uuid::Uuid;
 
 fn generate_run_start<'a>(
@@ -55,7 +56,7 @@ fn generate_run_start<'a>(
                 });
             fbb.create_string(&json_structure.to_string())
         })
-        .unwrap_or_else(|| fbb.create_string("{}"));
+        .unwrap_or_else(|| fbb.create_string(generate_default_nexus_structure(config).as_str()));
 
     let det_spec_map_buf = SpectraDetectorMapping::create(fbb, &args);
     let file_name = format!("{}.nxs", Uuid::new_v4());
@@ -87,6 +88,38 @@ fn generate_run_start<'a>(
     fbb.finished_data()
 }
 
+fn generate_default_nexus_structure(config: &HowlConfig) -> String {
+    json!(
+        {
+        "children": [
+            {
+                "type": "group",
+                "name": "raw_data_1",
+                "children": [
+                    {
+                        "type": "group",
+                        "name": "events",
+                        "children": [
+                            {
+                                "type": "stream",
+                                "stream": {
+                                    "topic": config.event_topic,
+                                    "source": "saluki_howl",
+                                    "writer_module": "ev44",
+                                },
+                            },
+                        ],
+                        "attributes": [{"name": "NX_class", "values": "NXentry"}],
+                    },
+                ],
+                "attributes": [{"name": "NX_class", "values": "NXentry"}],
+            }
+        ]
+    }
+    )
+    .to_string()
+}
+
 fn generate_run_stop<'a>(fbb: &'a mut FlatBufferBuilder<'_>, job_id: &str) -> &'a [u8] {
     fbb.reset();
     let stop_time = SystemTime::now()
@@ -379,3 +412,43 @@ pub fn howl(conf: &HowlConfig) {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use serde_json::Value;
+
+    #[test]
+    fn test_generate_default_nexus_structure_substitutes_topic_correctly() {
+        let event_topic = "myevents";
+        let conf = HowlConfig {
+            broker: "blah",
+            event_topic,
+            run_info_topic: "blah",
+            messages_per_frame: 1,
+            frames_per_second: 1,
+            frames_per_run: 1,
+            veto_probability: 0.5,
+            event_message_config: &EventMessageConfig {
+                events_per_message: 0,
+                tof_peak: 0.0,
+                tof_sigma: 0.0,
+                det_min: 0,
+                det_max: 0,
+            },
+
+            fast: false,
+            kafka_config: None,
+            nexus_structure_path: None,
+        };
+
+        let out = generate_default_nexus_structure(&conf);
+
+        let j: Value = serde_json::from_str(&out).unwrap();
+
+        assert_eq!(
+            j["children"][0]["children"][0]["children"][0]["stream"]["topic"],
+            event_topic
+        )
+    }
+}

From 8fcb7512bfa95c9959c96e33be3dff015e84790b Mon Sep 17 00:00:00 2001
From: Jack Harper <jack.harper@stfc.ac.uk>
Date: Thu, 1 Oct 2026 17:18:46 +0100
Subject: [PATCH 2/2] make default nexus structure match new fw

---
 src/howl.rs | 48 ++++++++++++++++++++++++------------------------
 1 file changed, 24 insertions(+), 24 deletions(-)

diff --git a/src/howl.rs b/src/howl.rs
index 60895fb..679d6a8 100644
--- a/src/howl.rs
+++ b/src/howl.rs
@@ -90,33 +90,33 @@ fn generate_run_start<'a>(
 
 fn generate_default_nexus_structure(config: &HowlConfig) -> String {
     json!(
+            {
+      "children": [
         {
-        "children": [
+          "type": "group",
+          "name": "raw_data_1",
+          "attributes": {
+            "NX_class": "NXentry"
+          },
+          "children": [
             {
-                "type": "group",
-                "name": "raw_data_1",
-                "children": [
-                    {
-                        "type": "group",
-                        "name": "events",
-                        "children": [
-                            {
-                                "type": "stream",
-                                "stream": {
-                                    "topic": config.event_topic,
-                                    "source": "saluki_howl",
-                                    "writer_module": "ev44",
-                                },
-                            },
-                        ],
-                        "attributes": [{"name": "NX_class", "values": "NXentry"}],
-                    },
-                ],
-                "attributes": [{"name": "NX_class", "values": "NXentry"}],
+              "type": "group",
+              "name": "detector_1_events",
+              "children": [
+                {
+                  "module": "NXevent_data",
+                  "config": {
+                    "topic": config.event_topic
+                  }
+                }
+              ]
             }
-        ]
+          ]
+        }
+      ]
     }
-    )
+
+        )
     .to_string()
 }
 
@@ -447,7 +447,7 @@ mod tests {
         let j: Value = serde_json::from_str(&out).unwrap();
 
         assert_eq!(
-            j["children"][0]["children"][0]["children"][0]["stream"]["topic"],
+            j["children"][0]["children"][0]["children"][0]["config"]["topic"],
             event_topic
         )
     }