# Step 3: node-and-domain-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/ModelTypes.hpp`
- `/CMakeLists.txt`

## Task

Implement the first useful `Node` and `Domain` slice.

Allowed files:

- Create `/include/fesa/core/Node.hpp`
- Create `/include/fesa/core/Domain.hpp`
- Create `/src/core/Domain.cpp`
- Create or modify `/tests/core/domain_storage_test.cpp`
- Modify `/CMakeLists.txt` to compile the source and register the test

Required behavior:

- `Node` stores `NodeId` and three coordinates as `double`.
- `Domain::addNode(Node)` inserts a node.
- Duplicate node ids throw `std::invalid_argument`.
- `Domain::findNode(NodeId)` returns a pointer or `nullptr`.
- `Domain::node(NodeId)` returns a const reference or throws `std::out_of_range`.
- `Domain::nodeCount()` returns the stored node count.
- `Domain` must not expose mutable node references.

## Tests To Write First

Write failing tests before creating production headers/sources:

- Add and retrieve one node by id.
- Missing node returns `nullptr` from `findNode`.
- Missing node throws from `node`.
- Duplicate node id throws.

Run the targeted CTest and verify failure due to missing API. Then implement the minimal code.

## 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 3 with `completed`, `error`, or `blocked`.

## Do Not

- Do not add elements, materials, properties, sets, loads, or steps yet.
- Do not store equation ids, displacement vectors, residuals, reactions, or solver state.