Merge cost-benefit/ev-calculation: calculateTaskEv pure function with 30 tests

src/analysis/cost-benefit.ts

@@ -1 +1,60 @@
-// calculateTaskEv, workflowCost, computeEffectiveP
+import type { EvConfig, EvResult } from "../schema/results.js";
+
+/**
+ * Calculate the expected value (EV) of a task.
+ *
+ * Pure math function — takes numeric inputs, returns EV result.
+ * No graph dependency.
+ *
+ * Formula:
+ *   expectedRetries = (1 - p) / p  when p > 0, else 0  (geometric series)
+ *   C_success = scopeCost * impactWeight
+ *   C_fail    = scopeCost * impactWeight + fallbackCost + timeLost * expectedRetries
+ *   EV        = p * C_success + (1 - p) * C_fail
+ *
+ * When `config.retries` is provided and > 0, `expectedRetries` is capped at `retries`.
+ * When `config.valueRate` is non-zero, the final EV is multiplied by `valueRate`.
+ *
+ * @param p - Probability of success (0 to 1)
+ * @param scopeCost - Cost estimate from scope (1.0–5.0)
+ * @param impactWeight - Impact weight (1.0–3.0)
+ * @param config - Optional configuration: retries, fallbackCost, timeLost, valueRate
+ * @returns EvResult with ev, pSuccess, and expectedRetries
+ */
+export function calculateTaskEv(
+  p: number,
+  scopeCost: number,
+  impactWeight: number,
+  config?: EvConfig,
+): EvResult {
+  const retries = config?.retries ?? 0;
+  const fallbackCost = config?.fallbackCost ?? 0;
+  const timeLost = config?.timeLost ?? 0;
+  const valueRate = config?.valueRate ?? 0;
+
+  // Expected retries: geometric series (1-p)/p when p > 0
+  let expectedRetries = p > 0 ? (1 - p) / p : 0;
+
+  // Cap at configured max retries when retries > 0
+  if (retries > 0 && expectedRetries > retries) {
+    expectedRetries = retries;
+  }
+
+  // C_success and C_fail: impactWeight scales scopeCost
+  const cSuccess = scopeCost * impactWeight;
+  const cFail = scopeCost * impactWeight + fallbackCost + timeLost * expectedRetries;
+
+  // EV = P_success * C_success + (1 - P_success) * C_fail
+  let ev = p * cSuccess + (1 - p) * cFail;
+
+  // Apply value rate conversion when configured
+  if (valueRate !== 0) {
+    ev = ev * valueRate;
+  }
+
+  return { ev, pSuccess: p, expectedRetries };
+}
+
+// Placeholder for future implementation
+// export function workflowCost(...)  { ... }
+// export function computeEffectiveP(...) { ... }