# Fix: "Achievable" status ignores credit sharing with achieved specs

**Date:** 2026-03-27
**Type:** Bugfix

## Problem

`determineStatuses()` marks specializations as "achievable" based solely on per-specialization upper bounds (`computeUpperBounds`), which ignore credit sharing between specializations. A spec can show "achievable" (upper bound >= 9) even when it's infeasible alongside the already-achieved higher-priority specs because shared courses have committed their credits elsewhere.

**Reproduction scenario:**
- 11 courses selected, Fall 1 open
- Priority: CRF > STR > LCM > MGT
- LCM upper bound = 10 (from spr1-collaboration, spr3-M&A, fall1-managing-change, fall3-corporate-governance)
- CRF + STR achieved, consuming credits from spr3 and fall3
- LCM shows "Achievable" but `checkFeasibility([all courses], ['CRF', 'STR', 'LCM'])` is infeasible for all S2 choices
- Selecting Managing Change for Fall 1 achieves MGT (priority 4) instead of LCM (priority 3)

## Root Cause

`determineStatuses()` in `optimizer.ts:146-185` checks only:
1. Whether the spec is in the achieved set
2. Whether the required course gate passes
3. Whether `computeUpperBounds` >= 9 (per-spec, ignoring sharing)

It never checks whether the spec is actually feasible alongside the achieved set.

## Fix

In `determineStatuses()`, after a non-achieved spec passes the upper bound check, add a feasibility check: call `checkWithS2(selectedCourseIds, [...achieved, specId])`. If infeasible, mark as `unreachable` instead of `achievable`.

### Changes

**`optimizer.ts` — `determineStatuses` function:**
- After the upper bound check passes (currently falls through to `statuses[spec.id] = 'achievable'`), add:
  ```ts
  const testSet = [...achieved, spec.id];
  const feasResult = checkWithS2(selectedCourseIds, testSet);
  if (!feasResult.feasible) {
    statuses[spec.id] = 'unreachable';
    continue;
  }
  ```
- No new parameters needed — `selectedCourseIds` is already passed to the function.

### What doesn't change

- No new status types — reuses existing `unreachable`
- No UI changes — `unreachable` already renders correctly with grey styling
- `computeUpperBounds` unchanged — still used for credit bar display
- `AllocationResult` type unchanged
- `checkWithS2` helper already exists in the same file

### Test updates

- Add a test for the bug scenario: given the specific course selection and CRF > STR > LCM > MGT ranking, verify LCM status is `unreachable` (not `achievable`) when CRF and STR are achieved
- Existing test `marks achievable when required course is in open set` should be unaffected (uses all-open sets with no achieved specs, so feasibility check passes trivially)

### Performance

14 specializations total, at most ~10 non-achieved specs to check. Each check is a small LP solve. Negligible overhead.