# Step 4: element-definition-storage

## Read First

Read these files before editing:

- `/AGENTS.md`
- `/docs/AGENT_RULES.md`
- `/docs/ADR.md`
- `/docs/ARCHITECTURE.md`
- `/docs/implementation-plans/domain-model-foundation-implementation-plan.md`
- `/include/fesa/core/Domain.hpp`
- `/include/fesa/core/Node.hpp`
- `/include/fesa/core/ModelTypes.hpp`
- `/tests/core/domain_storage_test.cpp`

## Task

Add element definition storage without implementing element stiffness or MITC4 formulation.

Allowed files:

- Create `/include/fesa/core/ElementDefinition.hpp`
- Modify `/include/fesa/core/Domain.hpp`
- Modify `/src/core/Domain.cpp`
- Modify `/tests/core/domain_storage_test.cpp`

Required behavior:

- `ElementDefinition` stores `ElementId`, type enum/string for `MITC4`, four `NodeId` connectivity entries, and a `PropertyId`.
- `Domain::addElement(ElementDefinition)` inserts an element.
- Duplicate element ids throw `std::invalid_argument`.
- Element connectivity must contain exactly four node ids for the MITC4 definition.
- Adding an element with a missing node id throws `std::invalid_argument`.
- `Domain::findElement(ElementId)` returns pointer or `nullptr`.
- `Domain::element(ElementId)` returns const reference or throws `std::out_of_range`.
- `Domain::elementCount()` returns the stored element count.

## Tests To Write First

Write failing tests before production changes:

- Add an element with four existing nodes and retrieve connectivity in order.
- Duplicate element id throws.
- Element referencing a missing node throws.
- Missing element lookup follows the `findElement` and `element` contracts.

Run targeted CTest and verify RED before implementation.

## Acceptance Criteria

Run:

```powershell
cmake --build build/msvc-debug --config Debug
ctest --test-dir build/msvc-debug --output-on-failure -C Debug -R domain
python scripts/validate_workspace.py
```

Update `/phases/domain-model-foundation/index.json` step 4 with `completed`, `error`, or `blocked`.

## Do Not

- Do not implement element stiffness, Jacobians, shape functions, sparse assembly, or solver behavior.
- Do not add parser behavior.