#include <../src/snes/impls/tr/trimpl.h> /*I   "petscsnes.h"   I*/

typedef struct {
  SNES snes;
  /*  Information on the regular SNES convergence test; which may have been user provided */
  PetscErrorCode (*convtest)(KSP, PetscInt, PetscReal, KSPConvergedReason *, void *);
  PetscErrorCode (*convdestroy)(void *);
  void *convctx;
} SNES_TR_KSPConverged_Ctx;

static PetscErrorCode SNESTR_KSPConverged_Private(KSP ksp, PetscInt n, PetscReal rnorm, KSPConvergedReason *reason, void *cctx) {
  SNES_TR_KSPConverged_Ctx *ctx  = (SNES_TR_KSPConverged_Ctx *)cctx;
  SNES                      snes = ctx->snes;
  SNES_NEWTONTR            *neP  = (SNES_NEWTONTR *)snes->data;
  Vec                       x;
  PetscReal                 nrm;

  PetscFunctionBegin;
  PetscCall((*ctx->convtest)(ksp, n, rnorm, reason, ctx->convctx));
  if (*reason) PetscCall(PetscInfo(snes, "Default or user provided convergence test KSP iterations=%" PetscInt_FMT ", rnorm=%g\n", n, (double)rnorm));
  /* Determine norm of solution */
  PetscCall(KSPBuildSolution(ksp, NULL, &x));
  PetscCall(VecNorm(x, NORM_2, &nrm));
  if (nrm >= neP->delta) {
    PetscCall(PetscInfo(snes, "Ending linear iteration early, delta=%g, length=%g\n", (double)neP->delta, (double)nrm));
    *reason = KSP_CONVERGED_STEP_LENGTH;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESTR_KSPConverged_Destroy(void *cctx) {
  SNES_TR_KSPConverged_Ctx *ctx = (SNES_TR_KSPConverged_Ctx *)cctx;

  PetscFunctionBegin;
  PetscCall((*ctx->convdestroy)(ctx->convctx));
  PetscCall(PetscFree(ctx));
  PetscFunctionReturn(0);
}

/*
   SNESTR_Converged_Private -test convergence JUST for
   the trust region tolerance.
*/
static PetscErrorCode SNESTR_Converged_Private(SNES snes, PetscInt it, PetscReal xnorm, PetscReal pnorm, PetscReal fnorm, SNESConvergedReason *reason, void *dummy) {
  SNES_NEWTONTR *neP = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  *reason = SNES_CONVERGED_ITERATING;
  if (neP->delta < xnorm * snes->deltatol) {
    PetscCall(PetscInfo(snes, "Converged due to trust region param %g<%g*%g\n", (double)neP->delta, (double)xnorm, (double)snes->deltatol));
    *reason = SNES_DIVERGED_TR_DELTA;
  } else if (snes->nfuncs >= snes->max_funcs && snes->max_funcs >= 0) {
    PetscCall(PetscInfo(snes, "Exceeded maximum number of function evaluations: %" PetscInt_FMT "\n", snes->max_funcs));
    *reason = SNES_DIVERGED_FUNCTION_COUNT;
  }
  PetscFunctionReturn(0);
}

/*@C
   SNESNewtonTRSetPreCheck - Sets a user function that is called before the search step has been determined.
       Allows the user a chance to change or override the decision of the line search routine.

   Deprecated use `SNESNEWTONDCTRDC`

   Logically Collective on snes

   Input Parameters:
+  snes - the nonlinear solver object
.  func - [optional] function evaluation routine, see `SNESNewtonTRPreCheck()`  for the calling sequence
-  ctx  - [optional] user-defined context for private data for the function evaluation routine (may be NULL)

   Level: intermediate

   Note:
   This function is called BEFORE the function evaluation within the `SNESNEWTONTR` solver.

.seealso: `SNESNEWTONDCTRDC`, `SNESNEWTONDCTR`, `SNESNewtonTRPreCheck()`, `SNESNewtonTRGetPreCheck()`, `SNESNewtonTRSetPostCheck()`, `SNESNewtonTRGetPostCheck()`
@*/
PetscErrorCode SNESNewtonTRSetPreCheck(SNES snes, PetscErrorCode (*func)(SNES, Vec, Vec, PetscBool *, void *), void *ctx) {
  SNES_NEWTONTR *tr = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (func) tr->precheck = func;
  if (ctx) tr->precheckctx = ctx;
  PetscFunctionReturn(0);
}

/*@C
   SNESNewtonTRGetPreCheck - Gets the pre-check function

   Deprecated use `SNESNEWTONDCTRDC`

   Not collective

   Input Parameter:
.  snes - the nonlinear solver context

   Output Parameters:
+  func - [optional] function evaluation routine, see for the calling sequence `SNESNewtonTRPreCheck()`
-  ctx  - [optional] user-defined context for private data for the function evaluation routine (may be NULL)

   Level: intermediate

.seealso: `SNESNEWTONDCTRDC`, `SNESNEWTONDCTR`, `SNESNewtonTRSetPreCheck()`, `SNESNewtonTRPreCheck()`
@*/
PetscErrorCode SNESNewtonTRGetPreCheck(SNES snes, PetscErrorCode (**func)(SNES, Vec, Vec, PetscBool *, void *), void **ctx) {
  SNES_NEWTONTR *tr = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (func) *func = tr->precheck;
  if (ctx) *ctx = tr->precheckctx;
  PetscFunctionReturn(0);
}

/*@C
   SNESNewtonTRSetPostCheck - Sets a user function that is called after the search step has been determined but before the next
       function evaluation. Allows the user a chance to change or override the decision of the line search routine

   Deprecated use `SNESNEWTONDCTRDC`

   Logically Collective on snes

   Input Parameters:
+  snes - the nonlinear solver object
.  func - [optional] function evaluation routine, see `SNESNewtonTRPostCheck()`  for the calling sequence
-  ctx  - [optional] user-defined context for private data for the function evaluation routine (may be NULL)

   Level: intermediate

   Note:
   This function is called BEFORE the function evaluation within the `SNESNEWTONTR` solver while the function set in
   `SNESLineSearchSetPostCheck()` is called AFTER the function evaluation.

.seealso: `SNESNEWTONDCTRDC`, `SNESNEWTONDCTR`, `SNESNewtonTRPostCheck()`, `SNESNewtonTRGetPostCheck()`
@*/
PetscErrorCode SNESNewtonTRSetPostCheck(SNES snes, PetscErrorCode (*func)(SNES, Vec, Vec, Vec, PetscBool *, PetscBool *, void *), void *ctx) {
  SNES_NEWTONTR *tr = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (func) tr->postcheck = func;
  if (ctx) tr->postcheckctx = ctx;
  PetscFunctionReturn(0);
}

/*@C
   SNESNewtonTRGetPostCheck - Gets the post-check function

   Deprecated use `SNESNEWTONDCTRDC`

   Not collective

   Input Parameter:
.  snes - the nonlinear solver context

   Output Parameters:
+  func - [optional] function evaluation routine, see for the calling sequence `SNESNewtonTRPostCheck()`
-  ctx  - [optional] user-defined context for private data for the function evaluation routine (may be NULL)

   Level: intermediate

.seealso: `SNESNEWTONDCTRDC`, `SNESNEWTONDCTR`, `SNESNewtonTRSetPostCheck()`, `SNESNewtonTRPostCheck()`
@*/
PetscErrorCode SNESNewtonTRGetPostCheck(SNES snes, PetscErrorCode (**func)(SNES, Vec, Vec, Vec, PetscBool *, PetscBool *, void *), void **ctx) {
  SNES_NEWTONTR *tr = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  if (func) *func = tr->postcheck;
  if (ctx) *ctx = tr->postcheckctx;
  PetscFunctionReturn(0);
}

/*@C
   SNESNewtonTRPreCheck - Called before the step has been determined in `SNESNEWTONTR`

   Deprecated use `SNESNEWTONDCTRDC`

   Logically Collective on snes

   Input Parameters:
+  snes - the solver
.  X - The last solution
-  Y - The step direction

   Output Parameters:
.  changed_Y - Indicator that the step direction Y has been changed.

   Level: developer

.seealso: `SNESNEWTONDCTRDC`, `SNESNEWTONDCTR`, `SNESNewtonTRSetPreCheck()`, `SNESNewtonTRGetPreCheck()`
@*/
static PetscErrorCode SNESNewtonTRPreCheck(SNES snes, Vec X, Vec Y, PetscBool *changed_Y) {
  SNES_NEWTONTR *tr = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  *changed_Y = PETSC_FALSE;
  if (tr->precheck) {
    PetscCall((*tr->precheck)(snes, X, Y, changed_Y, tr->precheckctx));
    PetscValidLogicalCollectiveBool(snes, *changed_Y, 4);
  }
  PetscFunctionReturn(0);
}

/*@C
   SNESNewtonTRPostCheck - Called after the step has been determined in `SNESNEWTONTR` but before the function evaluation

   Deprecated use `SNESNEWTONDCTRDC`

   Logically Collective on snes

   Input Parameters:
+  snes - the solver
.  X - The last solution
.  Y - The full step direction
-  W - The updated solution, W = X - Y

   Output Parameters:
+  changed_Y - indicator if step has been changed
-  changed_W - Indicator if the new candidate solution W has been changed.

   Note:
     If Y is changed then W is recomputed as X - Y

   Level: developer

.seealso: `SNESNEWTONDCTRDC`, `SNESNEWTONDCTR`, `SNESNewtonTRSetPostCheck()`, `SNESNewtonTRGetPostCheck()`
@*/
static PetscErrorCode SNESNewtonTRPostCheck(SNES snes, Vec X, Vec Y, Vec W, PetscBool *changed_Y, PetscBool *changed_W) {
  SNES_NEWTONTR *tr = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  *changed_Y = PETSC_FALSE;
  *changed_W = PETSC_FALSE;
  if (tr->postcheck) {
    PetscCall((*tr->postcheck)(snes, X, Y, W, changed_Y, changed_W, tr->postcheckctx));
    PetscValidLogicalCollectiveBool(snes, *changed_Y, 5);
    PetscValidLogicalCollectiveBool(snes, *changed_W, 6);
  }
  PetscFunctionReturn(0);
}

/*
   SNESSolve_NEWTONTR - Implements Newton's Method with a very simple trust
   region approach for solving systems of nonlinear equations.

*/
static PetscErrorCode SNESSolve_NEWTONTR(SNES snes) {
  SNES_NEWTONTR            *neP = (SNES_NEWTONTR *)snes->data;
  Vec                       X, F, Y, G, Ytmp, W;
  PetscInt                  maxits, i, lits;
  PetscReal                 rho, fnorm, gnorm, gpnorm, xnorm = 0, delta, nrm, ynorm, norm1;
  PetscScalar               cnorm;
  KSP                       ksp;
  SNESConvergedReason       reason   = SNES_CONVERGED_ITERATING;
  PetscBool                 breakout = PETSC_FALSE;
  SNES_TR_KSPConverged_Ctx *ctx;
  PetscErrorCode (*convtest)(KSP, PetscInt, PetscReal, KSPConvergedReason *, void *), (*convdestroy)(void *);
  void *convctx;

  PetscFunctionBegin;
  PetscCheck(!snes->xl && !snes->xu && !snes->ops->computevariablebounds, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_WRONGSTATE, "SNES solver %s does not support bounds", ((PetscObject)snes)->type_name);

  maxits = snes->max_its;  /* maximum number of iterations */
  X      = snes->vec_sol;  /* solution vector */
  F      = snes->vec_func; /* residual vector */
  Y      = snes->work[0];  /* work vectors */
  G      = snes->work[1];
  Ytmp   = snes->work[2];
  W      = snes->work[3];

  PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
  snes->iter = 0;
  PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));

  /* Set the linear stopping criteria to use the More' trick. */
  PetscCall(SNESGetKSP(snes, &ksp));
  PetscCall(KSPGetConvergenceTest(ksp, &convtest, &convctx, &convdestroy));
  if (convtest != SNESTR_KSPConverged_Private) {
    PetscCall(PetscNew(&ctx));
    ctx->snes = snes;
    PetscCall(KSPGetAndClearConvergenceTest(ksp, &ctx->convtest, &ctx->convctx, &ctx->convdestroy));
    PetscCall(KSPSetConvergenceTest(ksp, SNESTR_KSPConverged_Private, ctx, SNESTR_KSPConverged_Destroy));
    PetscCall(PetscInfo(snes, "Using Krylov convergence test SNESTR_KSPConverged_Private\n"));
  }

  if (!snes->vec_func_init_set) {
    PetscCall(SNESComputeFunction(snes, X, F)); /* F(X) */
  } else snes->vec_func_init_set = PETSC_FALSE;

  PetscCall(VecNorm(F, NORM_2, &fnorm)); /* fnorm <- || F || */
  SNESCheckFunctionNorm(snes, fnorm);
  PetscCall(VecNorm(X, NORM_2, &xnorm)); /* xnorm <- || X || */
  PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
  snes->norm = fnorm;
  PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
  delta      = xnorm ? neP->delta0 * xnorm : neP->delta0;
  neP->delta = delta;
  PetscCall(SNESLogConvergenceHistory(snes, fnorm, 0));
  PetscCall(SNESMonitor(snes, 0, fnorm));

  /* test convergence */
  PetscUseTypeMethod(snes, converged, snes->iter, 0.0, 0.0, fnorm, &snes->reason, snes->cnvP);
  if (snes->reason) PetscFunctionReturn(0);

  for (i = 0; i < maxits; i++) {
    /* Call general purpose update function */
    PetscTryTypeMethod(snes, update, snes->iter);

    /* Solve J Y = F, where J is Jacobian matrix */
    PetscCall(SNESComputeJacobian(snes, X, snes->jacobian, snes->jacobian_pre));
    SNESCheckJacobianDomainerror(snes);
    PetscCall(KSPSetOperators(snes->ksp, snes->jacobian, snes->jacobian_pre));
    PetscCall(KSPSolve(snes->ksp, F, Ytmp));
    PetscCall(KSPGetIterationNumber(snes->ksp, &lits));
    snes->linear_its += lits;

    PetscCall(PetscInfo(snes, "iter=%" PetscInt_FMT ", linear solve iterations=%" PetscInt_FMT "\n", snes->iter, lits));
    PetscCall(VecNorm(Ytmp, NORM_2, &nrm));
    norm1 = nrm;

    while (1) {
      PetscBool changed_y;
      PetscBool changed_w;
      PetscCall(VecCopy(Ytmp, Y));
      nrm = norm1;

      /* Scale Y if need be and predict new value of F norm */
      if (nrm >= delta) {
        nrm    = delta / nrm;
        gpnorm = (1.0 - nrm) * fnorm;
        cnorm  = nrm;
        PetscCall(PetscInfo(snes, "Scaling direction by %g\n", (double)nrm));
        PetscCall(VecScale(Y, cnorm));
        nrm   = gpnorm;
        ynorm = delta;
      } else {
        gpnorm = 0.0;
        PetscCall(PetscInfo(snes, "Direction is in Trust Region\n"));
        ynorm = nrm;
      }
      /* PreCheck() allows for updates to Y prior to W <- X - Y */
      PetscCall(SNESNewtonTRPreCheck(snes, X, Y, &changed_y));
      PetscCall(VecWAXPY(W, -1.0, Y, X)); /* W <- X - Y */
      PetscCall(SNESNewtonTRPostCheck(snes, X, Y, W, &changed_y, &changed_w));
      if (changed_y) PetscCall(VecWAXPY(W, -1.0, Y, X));
      PetscCall(VecCopy(Y, snes->vec_sol_update));
      PetscCall(SNESComputeFunction(snes, W, G)); /*  F(X-Y) = G */
      PetscCall(VecNorm(G, NORM_2, &gnorm));      /* gnorm <- || g || */
      SNESCheckFunctionNorm(snes, gnorm);
      if (fnorm == gpnorm) rho = 0.0;
      else rho = (fnorm * fnorm - gnorm * gnorm) / (fnorm * fnorm - gpnorm * gpnorm);

      /* Update size of trust region */
      if (rho < neP->mu) delta *= neP->delta1;
      else if (rho < neP->eta) delta *= neP->delta2;
      else delta *= neP->delta3;
      PetscCall(PetscInfo(snes, "fnorm=%g, gnorm=%g, ynorm=%g\n", (double)fnorm, (double)gnorm, (double)ynorm));
      PetscCall(PetscInfo(snes, "gpred=%g, rho=%g, delta=%g\n", (double)gpnorm, (double)rho, (double)delta));

      neP->delta = delta;
      if (rho > neP->sigma) break;
      PetscCall(PetscInfo(snes, "Trying again in smaller region\n"));

      /* check to see if progress is hopeless */
      neP->itflag = PETSC_FALSE;
      PetscCall(SNESTR_Converged_Private(snes, snes->iter, xnorm, ynorm, fnorm, &reason, snes->cnvP));
      if (!reason) PetscUseTypeMethod(snes, converged, snes->iter, xnorm, ynorm, fnorm, &reason, snes->cnvP);
      if (reason == SNES_CONVERGED_SNORM_RELATIVE) reason = SNES_DIVERGED_INNER;
      if (reason) {
        /* We're not progressing, so return with the current iterate */
        PetscCall(SNESMonitor(snes, i + 1, fnorm));
        breakout = PETSC_TRUE;
        break;
      }
      snes->numFailures++;
    }
    if (!breakout) {
      /* Update function and solution vectors */
      fnorm = gnorm;
      PetscCall(VecCopy(G, F));
      PetscCall(VecCopy(W, X));
      /* Monitor convergence */
      PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
      snes->iter  = i + 1;
      snes->norm  = fnorm;
      snes->xnorm = xnorm;
      snes->ynorm = ynorm;
      PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
      PetscCall(SNESLogConvergenceHistory(snes, snes->norm, lits));
      PetscCall(SNESMonitor(snes, snes->iter, snes->norm));
      /* Test for convergence, xnorm = || X || */
      neP->itflag = PETSC_TRUE;
      if (snes->ops->converged != SNESConvergedSkip) PetscCall(VecNorm(X, NORM_2, &xnorm));
      PetscUseTypeMethod(snes, converged, snes->iter, xnorm, ynorm, fnorm, &reason, snes->cnvP);
      if (reason) break;
    } else break;
  }

  if (i == maxits) {
    PetscCall(PetscInfo(snes, "Maximum number of iterations has been reached: %" PetscInt_FMT "\n", maxits));
    if (!reason) reason = SNES_DIVERGED_MAX_IT;
  }
  PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
  snes->reason = reason;
  PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
  if (convtest != SNESTR_KSPConverged_Private) {
    PetscCall(KSPGetAndClearConvergenceTest(ksp, &ctx->convtest, &ctx->convctx, &ctx->convdestroy));
    PetscCall(PetscFree(ctx));
    PetscCall(KSPSetConvergenceTest(ksp, convtest, convctx, convdestroy));
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESSetUp_NEWTONTR(SNES snes) {
  PetscFunctionBegin;
  PetscCall(SNESSetWorkVecs(snes, 4));
  PetscCall(SNESSetUpMatrices(snes));
  PetscFunctionReturn(0);
}

PetscErrorCode SNESReset_NEWTONTR(SNES snes) {
  PetscFunctionBegin;
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESDestroy_NEWTONTR(SNES snes) {
  PetscFunctionBegin;
  PetscCall(SNESReset_NEWTONTR(snes));
  PetscCall(PetscFree(snes->data));
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESSetFromOptions_NEWTONTR(SNES snes, PetscOptionItems *PetscOptionsObject) {
  SNES_NEWTONTR *ctx = (SNES_NEWTONTR *)snes->data;

  PetscFunctionBegin;
  PetscOptionsHeadBegin(PetscOptionsObject, "SNES trust region options for nonlinear equations");
  PetscCall(PetscOptionsReal("-snes_trtol", "Trust region tolerance", "SNESSetTrustRegionTolerance", snes->deltatol, &snes->deltatol, NULL));
  PetscCall(PetscOptionsReal("-snes_tr_mu", "mu", "None", ctx->mu, &ctx->mu, NULL));
  PetscCall(PetscOptionsReal("-snes_tr_eta", "eta", "None", ctx->eta, &ctx->eta, NULL));
  PetscCall(PetscOptionsReal("-snes_tr_sigma", "sigma", "None", ctx->sigma, &ctx->sigma, NULL));
  PetscCall(PetscOptionsReal("-snes_tr_delta0", "delta0", "None", ctx->delta0, &ctx->delta0, NULL));
  PetscCall(PetscOptionsReal("-snes_tr_delta1", "delta1", "None", ctx->delta1, &ctx->delta1, NULL));
  PetscCall(PetscOptionsReal("-snes_tr_delta2", "delta2", "None", ctx->delta2, &ctx->delta2, NULL));
  PetscCall(PetscOptionsReal("-snes_tr_delta3", "delta3", "None", ctx->delta3, &ctx->delta3, NULL));
  PetscOptionsHeadEnd();
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESView_NEWTONTR(SNES snes, PetscViewer viewer) {
  SNES_NEWTONTR *tr = (SNES_NEWTONTR *)snes->data;
  PetscBool      iascii;

  PetscFunctionBegin;
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
  if (iascii) {
    PetscCall(PetscViewerASCIIPrintf(viewer, "  Trust region tolerance %g (-snes_trtol)\n", (double)snes->deltatol));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  mu=%g, eta=%g, sigma=%g\n", (double)tr->mu, (double)tr->eta, (double)tr->sigma));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  delta0=%g, delta1=%g, delta2=%g, delta3=%g\n", (double)tr->delta0, (double)tr->delta1, (double)tr->delta2, (double)tr->delta3));
  }
  PetscFunctionReturn(0);
}

/*MC
      SNESNEWTONTR - Newton based nonlinear solver that uses a trust region

      Deprecated use `SNESNEWTONTRDC`

   Options Database Keys:
+    -snes_trtol <tol> - trust region tolerance
.    -snes_tr_mu <mu> - trust region parameter
.    -snes_tr_eta <eta> - trust region parameter
.    -snes_tr_sigma <sigma> - trust region parameter
.    -snes_tr_delta0 <delta0> -  initial size of the trust region is delta0*norm2(x)
.    -snes_tr_delta1 <delta1> - trust region parameter
.    -snes_tr_delta2 <delta2> - trust region parameter
-    -snes_tr_delta3 <delta3> - trust region parameter

   Reference:
.  - *  "The Minpack Project", by More', Sorensen, Garbow, Hillstrom, pages 88-111 of "Sources and Development
   of Mathematical Software", Wayne Cowell, editor.

   Level: intermediate

.seealso: `SNESNEWTONTRDC`, `SNESCreate()`, `SNES`, `SNESSetType()`, `SNESNEWTONLS`, `SNESSetTrustRegionTolerance()`
M*/
PETSC_EXTERN PetscErrorCode SNESCreate_NEWTONTR(SNES snes) {
  SNES_NEWTONTR *neP;

  PetscFunctionBegin;
  snes->ops->setup          = SNESSetUp_NEWTONTR;
  snes->ops->solve          = SNESSolve_NEWTONTR;
  snes->ops->destroy        = SNESDestroy_NEWTONTR;
  snes->ops->setfromoptions = SNESSetFromOptions_NEWTONTR;
  snes->ops->view           = SNESView_NEWTONTR;
  snes->ops->reset          = SNESReset_NEWTONTR;

  snes->usesksp = PETSC_TRUE;
  snes->usesnpc = PETSC_FALSE;

  snes->alwayscomputesfinalresidual = PETSC_TRUE;

  PetscCall(PetscNewLog(snes, &neP));
  snes->data  = (void *)neP;
  neP->mu     = 0.25;
  neP->eta    = 0.75;
  neP->delta  = 0.0;
  neP->delta0 = 0.2;
  neP->delta1 = 0.3;
  neP->delta2 = 0.75;
  neP->delta3 = 2.0;
  neP->sigma  = 0.0001;
  neP->itflag = PETSC_FALSE;
  neP->rnorm0 = 0.0;
  neP->ttol   = 0.0;
  PetscFunctionReturn(0);
}