[Tech Debt]: Assembler.Synchronize is O(n²) — 89 s to assemble a 50 000-element model #347

Description

Assembler.Synchronize resolves each new DTO with dtos.Single(x => x.Id == identifier) inside a
per-element loop (SysML2.NET.Dal/Assembler.cs:110). Single cannot short-circuit — it scans the
whole sequence to prove uniqueness — so on a cold cache this is n scans of n elements: ~2.5 billion
comparisons at 50 000 elements.

The identifier is already on the DTO being sought, so the lookup is entirely avoidable.

Measured impact

CoreCLR, .NET 10.0.11 Release, synthetic SysML v2 containment tree via Serializer.MessagePack:

Assembly is ~450× the cost of deserialization at 50 000 elements, growing at roughly n^2.4–n^3.2.
This is runtime-independent — the same code under Blazor WebAssembly takes 3.11 s for just 4 999
elements.

Proposed fix

Replace the Except + Single combination with a single pass over the DTOs:

foreach (var dto in dtoList)
{
    if (this.Cache.ContainsKey(dto.Id)) continue;

    var poco = elementFactory.Create(dto);
    this.Cache.AddOrUpdate(poco.Id, new Lazy<Core.POCO.Root.Elements.IElement>(() => poco), (key, oldValue) => oldValue);
    addedCount++;
}

existingIdentifiers, dtoIdentifiers and newIdentifiers all become dead. Should be O(n) and
sub-second at 50 000.

Behaviour change to confirm: duplicate Ids currently throw InvalidOperationException from
Single; under this loop the first occurrence wins. Preferable to a hard failure mid-assembly, but
worth an explicit decision.

Related smaller problems in the same method

• Line 115 — newIdentifiers.Count() re-runs the lazy Except. Arguments are evaluated even
  when Debug logging is off, so this full extra pass is paid in production. Use a counter.
• Lines 92–99 — when TryRemove returns false it assigns null to lazyPoco, and the else
  branch then dereferences lazyPoco.Value.GetType().Name → NullReferenceException. Reachable via
  a duplicate in deletedIdentifiers.
• Line 86 — "A total of {0} identifiers..." is passed the List<Guid> instead of its .Count.
• dtos is enumerated 4+ times (lines 77, 106, 110×n, 119). Materialise once; a lazy caller
  sequence is currently re-evaluated on every pass.

Tasks

• Materialise dtos once; replace the Except/Single lookup with the single-pass loop.
• Use a counter instead of newIdentifiers.Count().
• Decide and document duplicate-Id behaviour.
• Fix the NullReferenceException and the {0} argument in the deletion loop.
• Add a regression test asserting linear scaling, so a reintroduced quadratic scan fails CI.
• SysML2.NET.Dal.Tests green, in particular AssemblerTestFixture.

Out of scope

Parallelising assembly; changes to ElementFactory, UpdateReferenceProperties or the Cache type;
the ~522 MB managed-memory footprint at 50 000 elements (separate investigation).