Skip fiber stretch/rate computation when active stress model doesn't need it

[dseyler]

Current situation

As discussed in issue #604, this PR adds flags stating whether each active stress model needs to use fiber stretch or stretch rate. Computing these quantities at each time step was quite expensive (~5% of total runtime for 500k element simulation), so it makes sense to skip when the active stress model does not need them.

In active_stress.h:

  /// Whether this model uses fiber stretch.
  const bool needs_fiber_stretch;

  /// Whether this model uses fiber stretch rate.
  const bool needs_fiber_stretch_rate;

In each active stress model:

NashPanfilov() : ActiveStressODE(/* n_state_variables = */ 1,
                                  /* needs_fiber_stretch = */ false,
                                  /* needs_fiber_stretch_rate = */ false) {}

Then Integrator::predictor() determines when stretch and stretch rate should be computed:

    for (const auto &dmn : eq.dmn) {
        if (dmn.active_stress != nullptr) {
          need_fiber_stretch = true;
          need_fiber_stretch_rate = true;
          have_active_stress = true;
          need_fiber_stretch |= dmn.active_stress->needs_fiber_stretch;
          need_fiber_stretch_rate |=
              dmn.active_stress->needs_fiber_stretch_rate;
.
.
.

    if (need_fiber_stretch && fiber_stretch_eq_index >= 0) {
      for (const auto &mesh : com_mod.msh) {
        Vector<double> tmp(mesh.nNo);

        post::fib_stretch(com_mod, fiber_stretch_eq_index, mesh, Dn, tmp);
        for (int a = 0; a < mesh.nNo; ++a)
          fiber_stretch[mesh.gN[a]] = tmp[a];

Release Notes

• Added needs_fiber_stretch and needs_fiber_stretch_rate for each active stress model
• Integrator::predictor() only computes stretch/rate when needed

Testing

Please ensure that the PR meets the testing requirements set by GitHub Actions.

In addition, please ensure all modified and new code is covered by tests.

Code of Conduct & Contributing Guidelines

• I agree to follow the Code of Conduct and Contributing Guidelines.

[claude]

Claude Code Review

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[Copilot]

Pull request overview

This PR adds per–active stress model flags indicating whether fiber stretch and/or fiber stretch rate are required, and updates Integrator::predictor() to compute those quantities only when needed (reducing per-step overhead for models that don’t use them).

Changes:

• Add needs_fiber_stretch / needs_fiber_stretch_rate flags to the ActiveStress base class and plumb them through ActiveStressODE.
• Update active stress model constructors to declare their stretch / stretch-rate requirements.
• Update Integrator::predictor() to allocate stretch vectors when active stress is present, but only compute stretch/rate when required by at least one model.

Reviewed changes

Copilot reviewed 7 out of 7 changed files in this pull request and generated 4 comments.

Show a summary per file

File	Description
Code/Source/solver/Integrator.cpp	Computes fiber stretch/rate conditionally based on active stress model needs while preserving required allocations.
Code/Source/solver/active_stress.h	Adds required flags to the ActiveStress API via constructor + public const members.
Code/Source/solver/active_stress_ode.h	Extends ActiveStressODE constructor to forward stretch/rate requirement flags.
Code/Source/solver/active_stress_uniform_unsteady.h	Declares uniform-unsteady model does not require stretch or stretch rate.
Code/Source/solver/active_stress_uniform_steady.h	Declares uniform-steady model does not require stretch or stretch rate.
Code/Source/solver/active_stress_regazzoni.h	Declares Regazzoni model requires stretch and stretch rate.
Code/Source/solver/active_stress_nash_panfilov.h	Declares Nash–Panfilov model does not require stretch or stretch rate.

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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 72.73%. Comparing base (a3413e0) to head (0e51c7f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #610   +/-   ##
=======================================
  Coverage   72.73%   72.73%           
=======================================
  Files         255      255           
  Lines       39290    39300   +10     
  Branches     6726     6726           
=======================================
+ Hits        28576    28586   +10     
  Misses      10472    10472           
  Partials      242      242           

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