Densify tree ids when merging skeletons (#8643)

* Densify tree ids when merging skeletons

* changelog

Author: fm3

CHANGELOG.unreleased.md

@@ -41,6 +41,7 @@ For upgrade instructions, please check the [migration guide](MIGRATIONS.released
 - Fixed a rare bug where segment bounding box would not be displayed correctly, with the request potentially even crashing the server. [#8590](https://github.com/scalableminds/webknossos/pull/8590)
 - Fixed a rare bug where download requests would terminate without sending the whole annotation. [#8624](https://github.com/scalableminds/webknossos/pull/8624)
 - Fixed that deletion of dataset would lead to an error. [#8639](https://github.com/scalableminds/webknossos/pull/8639)
+- Fixed a bug where merging annotations with large tree IDs could lead to an error. [#8643](https://github.com/scalableminds/webknossos/pull/8643)
 
 ### Removed
 - The old "Selective Segment Visibility" feature that allowed to only see the active and the hovered segment was removed. From now on the visibility of segments can be controlled with checkboxes in the segment list and with the "Hide unlisted segments" toggle in the layer settings. [#8546](https://github.com/scalableminds/webknossos/pull/8546)

webknossos-tracingstore/app/com/scalableminds/webknossos/tracingstore/tracings/skeleton/TreeUtils.scala

@@ -33,20 +33,27 @@ object TreeUtils {
     else
       trees.map(_.treeId).max
 
+  private def densifyTreeIds(trees: Seq[Tree]): Seq[Tree] =
+    trees.sortBy(_.treeId).zipWithIndex.map {
+      case (tree, index) => tree.withTreeId(index + 1)
+    }
+
   def mergeTrees(sourceTrees: Seq[Tree],
                  targetTrees: Seq[Tree],
                  nodeMapping: FunctionalNodeMapping,
                  groupMapping: FunctionalGroupMapping): Seq[Tree] = {
-    val treeMaxId = maxTreeId(targetTrees)
+    val targetTreesDensified = densifyTreeIds(targetTrees)
+    val sourceTreesDensified = densifyTreeIds(sourceTrees)
+    val treeMaxId = maxTreeId(targetTreesDensified)
 
-    val sourceNodeIds: Set[Int] = sourceTrees.flatMap(_.nodes.map(_.id)).toSet
+    val sourceNodeIds: Set[Int] = sourceTreesDensified.flatMap(_.nodes.map(_.id)).toSet
 
-    val mappedSourceTrees = sourceTrees.map(
+    val mappedSourceTrees = sourceTreesDensified.map(
       tree =>
         applyNodeMapping(tree.withTreeId(tree.treeId + treeMaxId), nodeMapping, sourceNodeIds)
           .copy(groupId = tree.groupId.map(groupMapping(_))))
 
-    targetTrees ++ mappedSourceTrees
+    targetTreesDensified ++ mappedSourceTrees
   }
 
   private def applyNodeMapping(tree: Tree, f: Int => Int, sourceNodeIds: Set[Int]) =