feat: add @uncaged/uwf-moderator with JSONata evaluation engine

5 tests passing:  transition, condition match, fallback,
missing role error, output expansion.

Refs #309, #311

packages/uwf-moderator/src/evaluate.ts

@@ -0,0 +1,82 @@
+import type { ModeratorContext, WorkflowPayload } from "@uncaged/uwf-protocol";
+import jsonata from "jsonata";
+
+import type { Result } from "./types.js";
+
+const START_ROLE = "$START";
+
+function isTruthy(value: unknown): boolean {
+  if (value === null || value === undefined) {
+    return false;
+  }
+  if (typeof value === "boolean") {
+    return value;
+  }
+  if (typeof value === "number") {
+    return value !== 0 && !Number.isNaN(value);
+  }
+  if (typeof value === "string") {
+    return value.length > 0;
+  }
+  return true;
+}
+
+function evaluateJsonata(expression: string, context: ModeratorContext): Result<unknown, Error> {
+  try {
+    const result = jsonata(expression).evaluate(context);
+    return { ok: true, value: result };
+  } catch (error) {
+    return {
+      ok: false,
+      error: error instanceof Error ? error : new Error(String(error)),
+    };
+  }
+}
+
+function currentRole(context: ModeratorContext): string {
+  if (context.steps.length === 0) {
+    return START_ROLE;
+  }
+  return context.steps[context.steps.length - 1].role;
+}
+
+export function evaluate(
+  workflow: WorkflowPayload,
+  context: ModeratorContext,
+): Result<string, Error> {
+  const role = currentRole(context);
+  const transitions = workflow.graph[role];
+  if (transitions === undefined) {
+    return {
+      ok: false,
+      error: new Error(`no transitions defined for role "${role}"`),
+    };
+  }
+
+  for (const transition of transitions) {
+    if (transition.condition === null) {
+      return { ok: true, value: transition.role };
+    }
+
+    const conditionDef = workflow.conditions[transition.condition];
+    if (conditionDef === undefined) {
+      return {
+        ok: false,
+        error: new Error(`unknown condition "${transition.condition}"`),
+      };
+    }
+
+    const evalResult = evaluateJsonata(conditionDef.expression, context);
+    if (!evalResult.ok) {
+      return evalResult;
+    }
+    if (isTruthy(evalResult.value)) {
+      return { ok: true, value: transition.role };
+    }
+  }
+
+  return {
+    ok: false,
+    error: new Error(`no transition matched for role "${role}"`),
+  };
+}