@buildtovalue/simulation

Headless BPMN token-simulation engine — "does the model behave the way I expect?". Walks a diagram with animated-token semantics, entirely without a DOM, so the React overlay (Handoff 7A-2) is a thin rendering layer over a fully-tested core. Consumes only @buildtovalue/core. Zero runtime dependencies.

import { SimulationEngine, CoverageTracker } from '@buildtovalue/simulation';

const engine = new SimulationEngine(diagram);
const coverage = new CoverageTracker(engine.graph); // survives engine.reset()

while (engine.canAdvance) {
  const choice = engine.pendingChoice;
  if (choice) {
    engine.choose({ kind: 'exclusive', gateway: choice.nodeId, edge: choice.options[0].edgeId });
  } else {
    engine.advance(); // one token, one hop — deterministic
  }
}
coverage.record(engine.state.traversedEdges);

Semantics

Exact for XOR (exclusive), AND (parallel) and event-based gateways, and for boundary events:

• XOR / event-based split — pauses for a decision (pendingChoice); one branch is taken.
• AND split — emits one token per outgoing flow; AND join waits for a token on every incoming flow (join-wait → join-fire), so the classic trap (an XOR-split feeding an AND-join) deadlocks exactly where the soundness analysis predicts it — the two agree by construction (tests/soundnessAgreement.test.ts).
• Boundary events — fired while a token rests on the host: interrupting moves the token onto the boundary; non-interrupting spawns a second token.

OR (inclusive) is approximate — declaredly (cerca §0.1): the split is a manual multi-select and the join fires once no other live token can still reach it. engine.hasApproximateSemantics is true whenever an OR gateway participates, and every OR-join transition is flagged approximate. See docs/limitations.md.

Coverage & scenarios

• CoverageTracker enumerates the structural paths of the graph — the same graph the soundness analysis reasons over (Handoff 7 §7.2) — and reports N of M exercised. It is held across engine.reset(), so restarting a session keeps coverage.
• A session is a replayable scenario: engine.scenario captures only the ordered decisions; SimulationEngine.replay(diagram, scenario) reproduces the run bit-for-bit. canonicalizeScenario / hashScenario give the stable artifact + #hash used for the ledger evidence (Handoff 7A-3).

Decoupling

simulation never imports soundness, replay, registry, library or react — pinned by tests/independence.test.ts. Integrations (versioned scenarios in the library, session → ledger) happen through host adapters, never a direct import.