Narrow engine type auxiliary stem frontier

crates/rrt-runtime/src/inspect/engine_types.rs

@@ -172,6 +172,8 @@ pub struct EngineTypesInspectionReport {
     pub car_auxiliary_stem_counts: BTreeMap<String, usize>,
     pub car_auxiliary_stem_relation_counts: BTreeMap<String, usize>,
     pub car_auxiliary_stem_distinct_pair_counts: BTreeMap<String, usize>,
+    pub car_auxiliary_stem_distinct_pattern_counts: BTreeMap<String, usize>,
+    pub car_auxiliary_stem_distinct_pair_family_stems: BTreeMap<String, Vec<String>>,
     pub internal_ne_profile_pk4_match_count: usize,
     pub internal_ne_profile_pk4_missing_count: usize,
     pub locomotive_pair_internal_ne_profile_pk4_match_count: usize,
@@ -439,6 +441,13 @@ pub fn inspect_engine_types_dir(
             .iter()
             .filter_map(distinct_car_auxiliary_stem_pair_label),
     );
+    let car_auxiliary_stem_distinct_pattern_counts = count_owned_values(
+        family_entries
+            .iter()
+            .filter_map(classify_distinct_car_auxiliary_stem_pattern),
+    );
+    let car_auxiliary_stem_distinct_pair_family_stems =
+        build_distinct_car_auxiliary_stem_pair_family_stems(&family_entries);
     let lco_companion_stem_counts = count_named_values(
         family_entries
             .iter()
@@ -611,6 +620,8 @@ pub fn inspect_engine_types_dir(
         car_auxiliary_stem_counts,
         car_auxiliary_stem_relation_counts,
         car_auxiliary_stem_distinct_pair_counts,
+        car_auxiliary_stem_distinct_pattern_counts,
+        car_auxiliary_stem_distinct_pair_family_stems,
         internal_ne_profile_pk4_match_count,
         internal_ne_profile_pk4_missing_count,
         locomotive_pair_internal_ne_profile_pk4_match_count,
@@ -1010,6 +1021,35 @@ fn distinct_car_auxiliary_stem_pair_label(family: &EngineTypeFamilyEntry) -> Opt
     ))
 }
 
+fn classify_distinct_car_auxiliary_stem_pattern(family: &EngineTypeFamilyEntry) -> Option<String> {
+    let pair_label = distinct_car_auxiliary_stem_pair_label(family)?;
+    let internal_root = strip_terminal_role_letter(family.internal_stem.as_deref()?)?;
+    let auxiliary_root = strip_terminal_role_letter(family.auxiliary_stem.as_deref()?)?;
+    if internal_root.eq_ignore_ascii_case(auxiliary_root) {
+        return Some("paired_opposite_role_suffix".to_string());
+    }
+    Some(format!("other_distinct_auxiliary_stem:{pair_label}"))
+}
+
+fn build_distinct_car_auxiliary_stem_pair_family_stems(
+    families: &[EngineTypeFamilyEntry],
+) -> BTreeMap<String, Vec<String>> {
+    let mut grouped = BTreeMap::<String, Vec<String>>::new();
+    for family in families {
+        let Some(pair_label) = distinct_car_auxiliary_stem_pair_label(family) else {
+            continue;
+        };
+        grouped
+            .entry(pair_label)
+            .or_default()
+            .push(family.canonical_stem.clone());
+    }
+    for stems in grouped.values_mut() {
+        stems.sort();
+    }
+    grouped
+}
+
 fn strip_terminal_role_letter(value: &str) -> Option<&str> {
     let last = value.chars().last()?;
     matches!(last, 'L' | 'T' | 'l' | 't').then(|| {
@@ -1421,6 +1461,58 @@ mod tests {
         assert_eq!(ladders.get("55.000000 -> 85.000000"), Some(&1));
     }
 
+    #[test]
+    fn classifies_distinct_auxiliary_stem_patterns() {
+        let paired = EngineTypeFamilyEntry {
+            canonical_stem: "class_a1t".to_string(),
+            internal_stem: Some("ClassA1T".to_string()),
+            auxiliary_stem: Some("ClassA1L".to_string()),
+            ..minimal_family_entry()
+        };
+        let reordered = EngineTypeFamilyEntry {
+            canonical_stem: "class_qjl".to_string(),
+            internal_stem: Some("classqjl".to_string()),
+            auxiliary_stem: Some("qjclassl".to_string()),
+            ..minimal_family_entry()
+        };
+
+        assert_eq!(
+            classify_distinct_car_auxiliary_stem_pattern(&paired),
+            Some("paired_opposite_role_suffix".to_string())
+        );
+        assert_eq!(
+            classify_distinct_car_auxiliary_stem_pattern(&reordered),
+            Some("other_distinct_auxiliary_stem:classqjl -> qjclassl".to_string())
+        );
+    }
+
+    #[test]
+    fn groups_distinct_auxiliary_stem_pairs_by_family() {
+        let grouped = build_distinct_car_auxiliary_stem_pair_family_stems(&[
+            EngineTypeFamilyEntry {
+                canonical_stem: "class_a1t".to_string(),
+                internal_stem: Some("ClassA1T".to_string()),
+                auxiliary_stem: Some("ClassA1L".to_string()),
+                ..minimal_family_entry()
+            },
+            EngineTypeFamilyEntry {
+                canonical_stem: "class_qjt".to_string(),
+                internal_stem: Some("classqjt".to_string()),
+                auxiliary_stem: Some("qjclasst".to_string()),
+                ..minimal_family_entry()
+            },
+        ]);
+
+        assert_eq!(
+            grouped.get("ClassA1T -> ClassA1L"),
+            Some(&vec!["class_a1t".to_string()])
+        );
+        assert_eq!(
+            grouped.get("classqjt -> qjclasst"),
+            Some(&vec!["class_qjt".to_string()])
+        );
+    }
+
     #[test]
     fn builds_lco_low_cardinality_lane_counts() {
         let lane_counts = build_lco_low_cardinality_lane_counts(