1: #include <petsc/private/linesearchimpl.h>
  2: #include <petsc/private/snesimpl.h>

  4: static PetscErrorCode SNESLineSearchApply_Basic(SNESLineSearch linesearch)
  5: {
  6:   PetscBool changed_y, changed_w;
  7:   Vec       X, F, Y, W;
  8:   SNES      snes;
  9:   PetscReal gnorm, xnorm, ynorm, lambda, fnorm;

 11:   PetscFunctionBegin;
 12:   PetscCall(SNESLineSearchGetVecs(linesearch, &X, &F, &Y, &W, NULL));
 13:   PetscCall(SNESLineSearchGetNorms(linesearch, &xnorm, &gnorm, &ynorm));
 14:   PetscCall(SNESLineSearchGetLambda(linesearch, &lambda));
 15:   PetscCall(SNESLineSearchGetSNES(linesearch, &snes));
 16:   PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_SUCCEEDED));

 18:   /* precheck */
 19:   PetscCall(SNESLineSearchPreCheck(linesearch, X, Y, &changed_y));

 21:   /* update */
 22:   PetscCall(VecWAXPY(W, -lambda, Y, X));
 23:   if (linesearch->ops->viproject) PetscCall((*linesearch->ops->viproject)(snes, W));

 25:   /* postcheck */
 26:   PetscCall(SNESLineSearchPostCheck(linesearch, X, Y, W, &changed_y, &changed_w));
 27:   if (changed_y) {
 28:     if (!changed_w) PetscCall(VecWAXPY(W, -lambda, Y, X));
 29:     if (linesearch->ops->viproject) PetscCall((*linesearch->ops->viproject)(snes, W));
 30:   }
 31:   if (linesearch->norms || snes->iter < snes->max_its - 1) {
 32:     PetscCall((*linesearch->ops->snesfunc)(snes, W, F));
 33:     PetscCall(VecNorm(F, NORM_2, &fnorm));
 34:     if (PetscIsInfOrNanReal(fnorm)) {
 35:       PetscCall(SNESLineSearchSetReason(linesearch, SNES_LINESEARCH_FAILED_DOMAIN));
 36:       PetscFunctionReturn(PETSC_SUCCESS);
 37:     }
 38:   }
 39:   if (linesearch->norms) {
 40:     PetscCall(VecNormBegin(Y, NORM_2, &linesearch->ynorm));
 41:     PetscCall(VecNormBegin(W, NORM_2, &linesearch->xnorm));
 42:     PetscCall(VecNormEnd(Y, NORM_2, &linesearch->ynorm));
 43:     PetscCall(VecNormEnd(W, NORM_2, &linesearch->xnorm));

 45:     if (linesearch->ops->vinorm) {
 46:       linesearch->fnorm = gnorm;

 48:       PetscCall((*linesearch->ops->vinorm)(snes, F, W, &linesearch->fnorm));
 49:     } else linesearch->fnorm = fnorm;
 50:   }

 52:   /* copy the solution over */
 53:   PetscCall(VecCopy(W, X));
 54:   PetscFunctionReturn(PETSC_SUCCESS);
 55: }

 57: /*MC
 58:    SNESLINESEARCHBASIC - This line search implementation is not a line
 59:    search at all; it simply uses the full step $x_{k+1} = x_k - \lambda Y_k$ with $\lambda=1$.
 60:    Alternatively, $\lambda$ can be configured to be a constant damping factor by setting `snes_linesearch_damping`.
 61:    Thus, this routine is intended for methods with well-scaled updates; i.e. Newton's method (`SNESNEWTONLS`), on
 62:    well-behaved problems. Also named `SNESLINESEARCHNONE`.

 64:    Options Database Keys:
 65: +  -snes_linesearch_damping <1.0>     - step length is scaled by this factor
 66: -  -snes_linesearch_norms <true>      - whether to compute norms or not (`SNESLineSearchSetComputeNorms()`)

 68:    Note:
 69:    For methods with ill-scaled updates (`SNESNRICHARDSON`, `SNESNCG`), a small
 70:    damping parameter may yield satisfactory, but slow convergence, despite
 71:    the lack of the line search.

 73:    Level: advanced

 75: .seealso: [](ch_snes), `SNES`, `SNESLineSearch`, `SNESLineSearchType`, `SNESGetLineSearch()`, `SNESLineSearchCreate()`, `SNESLineSearchSetType()`, `SNESLineSearchSetDamping()`, `SNESLineSearchSetComputeNorms()`
 76: M*/
 77: PETSC_EXTERN PetscErrorCode SNESLineSearchCreate_Basic(SNESLineSearch linesearch)
 78: {
 79:   PetscFunctionBegin;
 80:   linesearch->ops->apply          = SNESLineSearchApply_Basic;
 81:   linesearch->ops->destroy        = NULL;
 82:   linesearch->ops->setfromoptions = NULL;
 83:   linesearch->ops->reset          = NULL;
 84:   linesearch->ops->view           = NULL;
 85:   linesearch->ops->setup          = NULL;
 86:   PetscFunctionReturn(PETSC_SUCCESS);
 87: }