From d36ab23c468859ce9b322e349b79cb41f51ba54a Mon Sep 17 00:00:00 2001
From: Orual <orual@nonbinary.computer>
Date: Sun, 2 Aug 2026 16:29:55 -0400
Subject: [PATCH] PM-78: derive model identity from actual model graph

Use canonical LDraw paths for scene and summary identities, and preserve parent-child include positions in occurrence identifiers so parsed graphs remain round-trippable.

Epic: PM-86
Task: PM-78
---
 crates/polymodel-ldraw-core/src/lib.rs       |  8 +++---
 crates/polymodel-ldraw-core/src/traversal.rs | 30 +++++++++++---------
 crates/polymodel-ldraw-core/src/types.rs     |  1 -
 3 files changed, 21 insertions(+), 18 deletions(-)

diff --git a/crates/polymodel-ldraw-core/src/lib.rs b/crates/polymodel-ldraw-core/src/lib.rs
index 71e46cd..ac9dbbf 100644
--- a/crates/polymodel-ldraw-core/src/lib.rs
+++ b/crates/polymodel-ldraw-core/src/lib.rs
@@ -102,11 +102,11 @@ mod tests {
         let first = LdrawParser
             .parse_bytes(bytes, options(&first_ledger))
             .unwrap();
-        assert_eq!(first.scene.model_id, "model-00000001");
-        assert_eq!(first.scene.instance_ids.len(), 1);
+        assert_eq!(first.scene.model_id, "root.ldr");
+        assert_eq!(first.scene.instance_ids, vec!["root.ldr→child.dat#1"]);
         assert_eq!(
-            first.scene.instance_ids,
-            vec!["instance-00000001".to_string()]
+            first.models.iter().map(|model| model.id.as_str()).collect::<Vec<_>>(),
+            vec!["root.ldr", "child.dat"]
         );
 
         let second_ledger = ReservationLedger::new();
diff --git a/crates/polymodel-ldraw-core/src/traversal.rs b/crates/polymodel-ldraw-core/src/traversal.rs
index e361cd0..6dd5c41 100644
--- a/crates/polymodel-ldraw-core/src/traversal.rs
+++ b/crates/polymodel-ldraw-core/src/traversal.rs
@@ -16,7 +16,7 @@ pub(crate) fn traverse(
     if models.is_empty() {
         return Ok((
             SceneRecord {
-                model_id: "model-00000001".into(),
+                model_id: String::new(),
                 instance_ids: Vec::new(),
                 triangles: 0,
                 quads: 0,
@@ -56,7 +56,7 @@ pub(crate) fn traverse(
     let mut reflection = false;
 
     let root_key = models[0].key.clone();
-    ids.insert(root_key.clone(), "model-00000001".into());
+    ids.insert(root_key.clone(), models[0].path.to_string());
     stack.push_back(Visit::Enter {
         index: 0,
         transform: Transform::default(),
@@ -86,9 +86,9 @@ pub(crate) fn traverse(
                 }
                 let first_parse = !completed.contains(&key);
                 let model_id = ids
-                    .get(&key)
-                    .cloned()
-                    .unwrap_or_else(|| format!("model-{:08}", ids.len() + 1));
+                    .entry(key.clone())
+                    .or_insert_with(|| model.path.to_string())
+                    .clone();
                 if !summaries
                     .iter()
                     .any(|summary: &ModelSummary| summary.id == model_id)
@@ -139,20 +139,28 @@ pub(crate) fn traverse(
 
                 stack.push_back(Visit::Exit { key: key.clone() });
                 if first_parse {
-                    for include in model.includes.iter().rev() {
+                    for (include_position, include) in model.includes.iter().enumerate().rev() {
                         if options.cancellation.cancelled {
                             return Err(ParseError::Cancelled);
                         }
                         counters
                             .add(LimitKind::Instances, 1, &options.limits)
                             .map_err(|name| limit_error(name, Some(include.span)))?;
-                        instance_ids.push(format!("instance-{:08}", counters.instances));
                         let target = models.iter().position(|candidate| {
                             candidate
                                 .path
                                 .as_str()
                                 .eq_ignore_ascii_case(&include.name.replace('\\', "/"))
                         });
+                        let child_name = target
+                            .map(|index| models[index].path.to_string())
+                            .unwrap_or_else(|| include.name.replace('\\', "/"));
+                        instance_ids.push(format!(
+                            "{}→{}#{}",
+                            model.path,
+                            child_name,
+                            include_position + 1
+                        ));
                         let child_depth = depth
                             .checked_add(1)
                             .ok_or(ParseError::Overflow("include depth"))?;
@@ -197,11 +205,7 @@ pub(crate) fn traverse(
                             continue;
                         }
                         if !ids.contains_key(&child.key) {
-                            let next_id = ids
-                                .len()
-                                .checked_add(1)
-                                .ok_or(ParseError::Overflow("model id"))?;
-                            ids.insert(child.key.clone(), format!("model-{next_id:08}"));
+                            ids.insert(child.key.clone(), child.path.to_string());
                         }
                         stack.push_back(Visit::Enter {
                             index: target,
@@ -217,7 +221,7 @@ pub(crate) fn traverse(
         }
     }
     let scene = SceneRecord {
-        model_id: "model-00000001".into(),
+        model_id: models[0].path.to_string(),
         instance_ids,
         triangles: scene_triangles,
         quads: scene_quads,
diff --git a/crates/polymodel-ldraw-core/src/types.rs b/crates/polymodel-ldraw-core/src/types.rs
index 728aa7c..b91e9e3 100644
--- a/crates/polymodel-ldraw-core/src/types.rs
+++ b/crates/polymodel-ldraw-core/src/types.rs
@@ -11,7 +11,6 @@ pub const SCHEMA_VERSION: &str = "ldraw-canonical-v1";
 pub const DEFAULT_ROOT_NAME: &str = "model.ldr";
 pub const DEFAULT_COLOUR: u16 = 16;
 pub const DEFAULT_PROVENANCE: &str = "local-parse";
-pub const ROOT_MODEL_ID: &str = "model-00000001";
 pub const DEFAULT_RESOURCE_BYTES: u64 = 100 * 1024 * 1024;
 pub const DEFAULT_LINE_BYTES: u64 = 1024 * 1024;
 pub const DEFAULT_FILES: u64 = 16_384;
-- 
2.51.2