# Step 0 Audit: Phase 1 Rebaseline

Date: 2026-05-04
Author: Codex

## Objective
Audit the existing Phase 1 C++ implementation against the paper-based `docs/MITC4_FORMULATION.md` reset before changing solver behavior.

This step did not modify production C++ code. The existing implementation remains useful historical scaffolding, but it is not authoritative for the rebuilt MITC4 element until each layer is revalidated through the rebaseline steps.

## Required Sources Read
- `AGENTS.md`
- `PLAN.md`
- `PROGRESS.md`
- `docs/README.md`
- `docs/HARNESS_ENGINEERING.md`
- `docs/PRD.md`
- `docs/ARCHITECTURE.md`
- `docs/ADR.md`
- `docs/NUMERICAL_CONVENTIONS.md`
- `docs/MITC4_FORMULATION.md`
- `phases/1-linear-static-mitc4-rebaseline/step0.md`

## Code Areas Inspected
- `include/fesa/fesa.hpp`
- `tests/test_main.cpp`
- `CMakeLists.txt`
- `scripts/validate_workspace.py`
- `references/quad_01.inp`
- `references/quad_02.inp`

## High-Level Finding
The existing implementation has a working single-header Phase 1 skeleton: parser, domain validation, DOF mapping, dense test solver, in-memory results, displacement CSV comparison, full-vector reaction recovery, and a basic linear static workflow.

The existing MITC4 kernel is not compatible with the revised formulation contract. It uses a simplified midsurface local basis, 2D projected derivatives, separated membrane/bending/shear factors, direct shear row construction at tying points, 2 x 2 midsurface-only integration, and `drilling_stiffness_scale = 1.0e-6`. The revised contract requires degenerated-continuum geometry, director/local rotation handling, convected covariant strain rows, MITC transverse shear tying on tensor components, `2 x 2 x 2` Gauss integration, and drilling stabilization based on `1.0e-3 * min_positive_diagonal(K_local_without_drilling)`.

## Retain With Revalidation
These areas should be kept as starting points, but each must be revalidated by the assigned rebaseline step before being treated as Phase 1 evidence.

| Area | Current Evidence | Rebaseline Owner |
|---|---|---|
| CMake/CTest validation harness | CMake files and `scripts/validate_workspace.py` run a real C++ test executable. | P1R-02 |
| Core numeric and DOF aliases | `Real`, int64 ids/equations, and Abaqus DOF 1..6 mapping exist. | P1R-02 |
| Diagnostics model | Error/warning diagnostics and code-based checks exist. | P1R-02, P1R-04 |
| Parser subset skeleton | `*Node`, `*Element TYPE=S4`, `*Nset`, `*Elset`, `*Material`, `*Elastic`, `*Shell Section`, `*Boundary`, `*Cload`, `*Step`, `*Static`, and `*End Step` are represented. | P1R-03 |
| Unsupported feature rejection | Generic unsupported keyword rejection plus `S4R` and `NLGEOM=YES` checks exist. | P1R-01, P1R-03 |
| Domain validation | Missing element nodes, shell section/material links, load/boundary targets, no active elements, and no load warnings are present. | P1R-04 |
| DofManager foundation | Six-DOF full order, constrained/free partition, equation numbering, and full-vector reconstruction exist outside Node/Element. | P1R-05 |
| Result field structure | In-memory step/frame fields for `U` and `RF` exist. | P1R-06 |
| CSV displacement comparator | Required Abaqus displacement columns are parsed and compared by node id with abs/rel tolerance support. | P1R-06, P1R-14 |
| Linear static flow | Reduced free-DOF solve, full vector reconstruction, and `R_full = K_full * U_full - F_full` are implemented. | P1R-12, P1R-13 |

## Rewrite Required
These areas conflict with `docs/MITC4_FORMULATION.md` and should be rebuilt under the matching contracts, not patched casually.

| Area | Conflict | Rebaseline Owner |
|---|---|---|
| MITC4 local basis | Current `computeLocalBasis` uses averaged edge directions from the four coordinates. The revised contract requires element-center midsurface normal, nodal director axes `V1/V2/Vn`, deterministic fallback axes, and integration local bases. | P1R-07 |
| Geometry and Jacobian | Current code projects the element to local 2D `xy` and uses bilinear planar derivatives only. The revised contract requires degenerated-continuum geometry with through-thickness coordinate `zeta`, covariant bases `g_i`, and invalid basis/Jacobian diagnostics. | P1R-07, P1R-09 |
| Local rotation transform | Current transform rotates three translations and three rotations with one basis block. The revised contract requires mapping global rotations to local `[alpha, beta, gamma]`, where drilling `gamma` does not enter physical strain. | P1R-08, P1R-10 |
| Strain rows | Current B matrix is an 8-row engineering shell split, not the documented convected covariant strain vector `[eps_11, eps_22, eps_33, gamma_23, gamma_13, gamma_12]`. | P1R-08 |
| MITC shear tying | Current `addStandardShearRow` builds Cartesian-like `gamma_xz/gamma_yz` rows at midside derivatives, then interpolates them. The revised contract ties convected tensor components `eps_13` and `eps_23` from direct covariant rows at A/B/C/D. | P1R-08 |
| Material and integration | Current code uses pre-integrated membrane, bending, and shear factors at `2 x 2` midsurface points. The revised contract requires local plane-stress matrix with shear correction, material transform as needed, and `2 x 2 x 2` Gauss integration. | P1R-09 |
| Drilling stabilization | Current default is `1.0e-6` and the stiffness is assembled through a drilling row using `E * thickness * scale`. The revised contract requires `1.0e-3 * min_positive_diagonal(K_local_without_drilling)` added to local `gamma` diagonals before transformation. | P1R-10 |
| MITC4 tests | Existing element test only checks shape partition, stiffness symmetry, and one uniform translation. The revised contract requires director basis, rotation transform, tying-row finite difference, MITC interpolation, rigid-body, drilling, patch, and locking-sensitivity tests. | P1R-07 through P1R-11 |

## Revalidate Before Reference Regression
These areas are promising but not sufficient for stored-reference credibility yet.

| Area | Required Revalidation |
|---|---|
| `quad_02` reference path | `quad_02.inp` uses `TYPE=S4`, but also `Part/Assembly/Instance` and `*Density`. Step 1 must either add a normalized Phase 1-compatible derivative input or create an explicit parser compatibility plan. |
| Parser rejection policy | The parser must continue rejecting `S4R`, `Part/Assembly/Instance`, `*Density`, `*Include`, pressure loads, nonzero prescribed displacement, and `NLGEOM=YES` unless a later contract changes the subset. |
| Singular diagnostics | Existing diagnostics cover some conditions, but untouched free DOFs, rotational/drilling risks, invalid element geometry, non-positive thickness paths, and solver zero-pivot messages need stronger coverage. |
| Reaction output | Full-vector reaction recovery exists, but `RF` should remain verified by equilibrium tests until user-provided Abaqus reaction CSV artifacts are available. |
| Assembly and solver boundary | Current dense matrix and Gaussian solver are acceptable for deterministic tests, but the architecture requires a future sparse/MKL adapter boundary and int64 sparse path. |

## Deferred Or Out Of Scope
- Abaqus `S4R`, reduced integration, and hourglass control.
- `Part/Assembly/Instance` support unless Step 1 or a later parser contract explicitly adds it.
- `*Density` as material support for Phase 1 static analysis.
- `NLGEOM=YES`, geometric nonlinearity, pressure loads, RBE2/RBE3, nonzero prescribed displacements, dynamics, heat transfer, composite sections, stress/strain/resultant outputs, and mesh quality diagnostics.
- Tuning drilling stiffness to match one reference case.

## Step Handoff
Proceed to P1R-01 reference onboarding.

P1R-01 must decide the `quad_02` compatibility path without silently expanding parser support. P1R-02 can proceed in parallel afterward for core harness guardrails, but production MITC4 work should wait until the reference and guardrail foundations are explicit.