impls/composite/snescomposite.c

/*
      Defines a SNES that can consist of a collection of SNESes
*/
#include <petsc/private/snesimpl.h> /*I "petscsnes.h" I*/
#include <petscblaslapack.h>

const char *const SNESCompositeTypes[] = {"ADDITIVE", "MULTIPLICATIVE", "ADDITIVEOPTIMAL", "SNESCompositeType", "SNES_COMPOSITE", NULL};

typedef struct _SNES_CompositeLink *SNES_CompositeLink;
struct _SNES_CompositeLink {
  SNES               snes;
  PetscReal          dmp;
  Vec                X;
  SNES_CompositeLink next;
  SNES_CompositeLink previous;
};

typedef struct {
  SNES_CompositeLink head;
  PetscInt           nsnes;
  SNESCompositeType  type;
  Vec                Xorig;
  PetscInt           innerFailures; /* the number of inner failures we've seen */

  /* context for ADDITIVEOPTIMAL */
  Vec         *Xes, *Fes; /* solution and residual vectors for the subsolvers */
  PetscReal   *fnorms;    /* norms of the solutions */
  PetscScalar *h;         /* the matrix formed as q_ij = (rdot_i, rdot_j) */
  PetscScalar *g;         /* the dotproducts of the previous function with the candidate functions */
  PetscBLASInt n;         /* matrix dimension -- nsnes */
  PetscBLASInt nrhs;      /* the number of right hand sides */
  PetscBLASInt lda;       /* the padded matrix dimension */
  PetscBLASInt ldb;       /* the padded vector dimension */
  PetscReal   *s;         /* the singular values */
  PetscScalar *beta;      /* the RHS and combination */
  PetscReal    rcond;     /* the exit condition */
  PetscBLASInt rank;      /* the effective rank */
  PetscScalar *work;      /* the work vector */
  PetscReal   *rwork;     /* the real work vector used for complex */
  PetscBLASInt lwork;     /* the size of the work vector */
  PetscBLASInt info;      /* the output condition */

  PetscReal rtol; /* restart tolerance for accepting the combination */
  PetscReal stol; /* restart tolerance for the combination */
} SNES_Composite;

static PetscErrorCode SNESCompositeApply_Multiplicative(SNES snes, Vec X, Vec B, Vec F, PetscReal *fnorm) {
  SNES_Composite     *jac  = (SNES_Composite *)snes->data;
  SNES_CompositeLink  next = jac->head;
  Vec                 FSub;
  SNESConvergedReason reason;

  PetscFunctionBegin;
  PetscCheck(next, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_WRONGSTATE, "No composite SNESes supplied via SNESCompositeAddSNES() or -snes_composite_sneses");
  if (snes->normschedule == SNES_NORM_ALWAYS) PetscCall(SNESSetInitialFunction(next->snes, F));
  PetscCall(SNESSolve(next->snes, B, X));
  PetscCall(SNESGetConvergedReason(next->snes, &reason));
  if (reason < 0 && reason != SNES_DIVERGED_MAX_IT) {
    jac->innerFailures++;
    if (jac->innerFailures >= snes->maxFailures) {
      snes->reason = SNES_DIVERGED_INNER;
      PetscFunctionReturn(0);
    }
  }

  while (next->next) {
    /* only copy the function over in the case where the functions correspond */
    if (next->snes->npcside == PC_RIGHT && next->snes->normschedule != SNES_NORM_NONE) {
      PetscCall(SNESGetFunction(next->snes, &FSub, NULL, NULL));
      next = next->next;
      PetscCall(SNESSetInitialFunction(next->snes, FSub));
    } else {
      next = next->next;
    }
    PetscCall(SNESSolve(next->snes, B, X));
    PetscCall(SNESGetConvergedReason(next->snes, &reason));
    if (reason < 0 && reason != SNES_DIVERGED_MAX_IT) {
      jac->innerFailures++;
      if (jac->innerFailures >= snes->maxFailures) {
        snes->reason = SNES_DIVERGED_INNER;
        PetscFunctionReturn(0);
      }
    }
  }
  if (next->snes->npcside == PC_RIGHT) {
    PetscCall(SNESGetFunction(next->snes, &FSub, NULL, NULL));
    PetscCall(VecCopy(FSub, F));
    if (fnorm) {
      if (snes->xl && snes->xu) {
        PetscCall(SNESVIComputeInactiveSetFnorm(snes, F, X, fnorm));
      } else {
        PetscCall(VecNorm(F, NORM_2, fnorm));
      }
      SNESCheckFunctionNorm(snes, *fnorm);
    }
  } else if (snes->normschedule == SNES_NORM_ALWAYS) {
    PetscCall(SNESComputeFunction(snes, X, F));
    if (fnorm) {
      if (snes->xl && snes->xu) {
        PetscCall(SNESVIComputeInactiveSetFnorm(snes, F, X, fnorm));
      } else {
        PetscCall(VecNorm(F, NORM_2, fnorm));
      }
      SNESCheckFunctionNorm(snes, *fnorm);
    }
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESCompositeApply_Additive(SNES snes, Vec X, Vec B, Vec F, PetscReal *fnorm) {
  SNES_Composite     *jac  = (SNES_Composite *)snes->data;
  SNES_CompositeLink  next = jac->head;
  Vec                 Y, Xorig;
  SNESConvergedReason reason;

  PetscFunctionBegin;
  Y = snes->vec_sol_update;
  if (!jac->Xorig) PetscCall(VecDuplicate(X, &jac->Xorig));
  Xorig = jac->Xorig;
  PetscCall(VecCopy(X, Xorig));
  PetscCheck(next, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_WRONGSTATE, "No composite SNESes supplied via SNESCompositeAddSNES() or -snes_composite_sneses");
  if (snes->normschedule == SNES_NORM_ALWAYS) {
    PetscCall(SNESSetInitialFunction(next->snes, F));
    while (next->next) {
      next = next->next;
      PetscCall(SNESSetInitialFunction(next->snes, F));
    }
  }
  next = jac->head;
  PetscCall(VecCopy(Xorig, Y));
  PetscCall(SNESSolve(next->snes, B, Y));
  PetscCall(SNESGetConvergedReason(next->snes, &reason));
  if (reason < 0 && reason != SNES_DIVERGED_MAX_IT) {
    jac->innerFailures++;
    if (jac->innerFailures >= snes->maxFailures) {
      snes->reason = SNES_DIVERGED_INNER;
      PetscFunctionReturn(0);
    }
  }
  PetscCall(VecAXPY(Y, -1.0, Xorig));
  PetscCall(VecAXPY(X, next->dmp, Y));
  while (next->next) {
    next = next->next;
    PetscCall(VecCopy(Xorig, Y));
    PetscCall(SNESSolve(next->snes, B, Y));
    PetscCall(SNESGetConvergedReason(next->snes, &reason));
    if (reason < 0 && reason != SNES_DIVERGED_MAX_IT) {
      jac->innerFailures++;
      if (jac->innerFailures >= snes->maxFailures) {
        snes->reason = SNES_DIVERGED_INNER;
        PetscFunctionReturn(0);
      }
    }
    PetscCall(VecAXPY(Y, -1.0, Xorig));
    PetscCall(VecAXPY(X, next->dmp, Y));
  }
  if (snes->normschedule == SNES_NORM_ALWAYS) {
    PetscCall(SNESComputeFunction(snes, X, F));
    if (fnorm) {
      if (snes->xl && snes->xu) {
        PetscCall(SNESVIComputeInactiveSetFnorm(snes, F, X, fnorm));
      } else {
        PetscCall(VecNorm(F, NORM_2, fnorm));
      }
      SNESCheckFunctionNorm(snes, *fnorm);
    }
  }
  PetscFunctionReturn(0);
}

/* approximately solve the overdetermined system:

 2*F(x_i)\cdot F(\x_j)\alpha_i = 0
 \alpha_i                      = 1

 Which minimizes the L2 norm of the linearization of:
 ||F(\sum_i \alpha_i*x_i)||^2

 With the constraint that \sum_i\alpha_i = 1
 Where x_i is the solution from the ith subsolver.
 */
static PetscErrorCode SNESCompositeApply_AdditiveOptimal(SNES snes, Vec X, Vec B, Vec F, PetscReal *fnorm) {
  SNES_Composite     *jac  = (SNES_Composite *)snes->data;
  SNES_CompositeLink  next = jac->head;
  Vec                *Xes = jac->Xes, *Fes = jac->Fes;
  PetscInt            i, j;
  PetscScalar         tot, total, ftf;
  PetscReal           min_fnorm;
  PetscInt            min_i;
  SNESConvergedReason reason;

  PetscFunctionBegin;
  PetscCheck(next, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_WRONGSTATE, "No composite SNESes supplied via SNESCompositeAddSNES() or -snes_composite_sneses");

  if (snes->normschedule == SNES_NORM_ALWAYS) {
    next = jac->head;
    PetscCall(SNESSetInitialFunction(next->snes, F));
    while (next->next) {
      next = next->next;
      PetscCall(SNESSetInitialFunction(next->snes, F));
    }
  }

  next = jac->head;
  i    = 0;
  PetscCall(VecCopy(X, Xes[i]));
  PetscCall(SNESSolve(next->snes, B, Xes[i]));
  PetscCall(SNESGetConvergedReason(next->snes, &reason));
  if (reason < 0 && reason != SNES_DIVERGED_MAX_IT) {
    jac->innerFailures++;
    if (jac->innerFailures >= snes->maxFailures) {
      snes->reason = SNES_DIVERGED_INNER;
      PetscFunctionReturn(0);
    }
  }
  while (next->next) {
    i++;
    next = next->next;
    PetscCall(VecCopy(X, Xes[i]));
    PetscCall(SNESSolve(next->snes, B, Xes[i]));
    PetscCall(SNESGetConvergedReason(next->snes, &reason));
    if (reason < 0 && reason != SNES_DIVERGED_MAX_IT) {
      jac->innerFailures++;
      if (jac->innerFailures >= snes->maxFailures) {
        snes->reason = SNES_DIVERGED_INNER;
        PetscFunctionReturn(0);
      }
    }
  }

  /* all the solutions are collected; combine optimally */
  for (i = 0; i < jac->n; i++) {
    for (j = 0; j < i + 1; j++) PetscCall(VecDotBegin(Fes[i], Fes[j], &jac->h[i + j * jac->n]));
    PetscCall(VecDotBegin(Fes[i], F, &jac->g[i]));
  }

  for (i = 0; i < jac->n; i++) {
    for (j = 0; j < i + 1; j++) {
      PetscCall(VecDotEnd(Fes[i], Fes[j], &jac->h[i + j * jac->n]));
      if (i == j) jac->fnorms[i] = PetscSqrtReal(PetscRealPart(jac->h[i + j * jac->n]));
    }
    PetscCall(VecDotEnd(Fes[i], F, &jac->g[i]));
  }

  ftf = (*fnorm) * (*fnorm);

  for (i = 0; i < jac->n; i++) {
    for (j = i + 1; j < jac->n; j++) jac->h[i + j * jac->n] = jac->h[j + i * jac->n];
  }

  for (i = 0; i < jac->n; i++) {
    for (j = 0; j < jac->n; j++) jac->h[i + j * jac->n] = jac->h[i + j * jac->n] - jac->g[j] - jac->g[i] + ftf;
    jac->beta[i] = ftf - jac->g[i];
  }

  jac->info  = 0;
  jac->rcond = -1.;
  PetscCall(PetscFPTrapPush(PETSC_FP_TRAP_OFF));
#if defined(PETSC_USE_COMPLEX)
  PetscCallBLAS("LAPACKgelss", LAPACKgelss_(&jac->n, &jac->n, &jac->nrhs, jac->h, &jac->lda, jac->beta, &jac->lda, jac->s, &jac->rcond, &jac->rank, jac->work, &jac->lwork, jac->rwork, &jac->info));
#else
  PetscCallBLAS("LAPACKgelss", LAPACKgelss_(&jac->n, &jac->n, &jac->nrhs, jac->h, &jac->lda, jac->beta, &jac->lda, jac->s, &jac->rcond, &jac->rank, jac->work, &jac->lwork, &jac->info));
#endif
  PetscCall(PetscFPTrapPop());
  PetscCheck(jac->info >= 0, PetscObjectComm((PetscObject)snes), PETSC_ERR_LIB, "Bad argument to GELSS");
  PetscCheck(jac->info <= 0, PetscObjectComm((PetscObject)snes), PETSC_ERR_LIB, "SVD failed to converge");
  tot   = 0.;
  total = 0.;
  for (i = 0; i < jac->n; i++) {
    PetscCheck(!snes->errorifnotconverged || !PetscIsInfOrNanScalar(jac->beta[i]), PetscObjectComm((PetscObject)snes), PETSC_ERR_LIB, "SVD generated inconsistent output");
    PetscCall(PetscInfo(snes, "%" PetscInt_FMT ": %g\n", i, (double)PetscRealPart(jac->beta[i])));
    tot += jac->beta[i];
    total += PetscAbsScalar(jac->beta[i]);
  }
  PetscCall(VecScale(X, (1. - tot)));
  PetscCall(VecMAXPY(X, jac->n, jac->beta, Xes));
  PetscCall(SNESComputeFunction(snes, X, F));

  if (snes->xl && snes->xu) {
    PetscCall(SNESVIComputeInactiveSetFnorm(snes, F, X, fnorm));
  } else {
    PetscCall(VecNorm(F, NORM_2, fnorm));
  }

  /* take the minimum-normed candidate if it beats the combination by a factor of rtol or the combination has stagnated */
  min_fnorm = jac->fnorms[0];
  min_i     = 0;
  for (i = 0; i < jac->n; i++) {
    if (jac->fnorms[i] < min_fnorm) {
      min_fnorm = jac->fnorms[i];
      min_i     = i;
    }
  }

  /* stagnation or divergence restart to the solution of the solver that failed the least */
  if (PetscRealPart(total) < jac->stol || min_fnorm * jac->rtol < *fnorm) {
    PetscCall(VecCopy(jac->Xes[min_i], X));
    PetscCall(VecCopy(jac->Fes[min_i], F));
    *fnorm = min_fnorm;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESSetUp_Composite(SNES snes) {
  DM                 dm;
  SNES_Composite    *jac  = (SNES_Composite *)snes->data;
  SNES_CompositeLink next = jac->head;
  PetscInt           n    = 0, i;
  Vec                F;

  PetscFunctionBegin;
  PetscCall(SNESGetDM(snes, &dm));

  if (snes->ops->computevariablebounds) {
    /* SNESVI only ever calls computevariablebounds once, so calling it once here is justified */
    if (!snes->xl) PetscCall(VecDuplicate(snes->vec_sol, &snes->xl));
    if (!snes->xu) PetscCall(VecDuplicate(snes->vec_sol, &snes->xu));
    PetscUseTypeMethod(snes, computevariablebounds, snes->xl, snes->xu);
  }

  while (next) {
    n++;
    PetscCall(SNESSetDM(next->snes, dm));
    PetscCall(SNESSetApplicationContext(next->snes, snes->user));
    if (snes->xl && snes->xu) {
      if (snes->ops->computevariablebounds) {
        PetscCall(SNESVISetComputeVariableBounds(next->snes, snes->ops->computevariablebounds));
      } else {
        PetscCall(SNESVISetVariableBounds(next->snes, snes->xl, snes->xu));
      }
    }

    next = next->next;
  }
  jac->nsnes = n;
  PetscCall(SNESGetFunction(snes, &F, NULL, NULL));
  if (jac->type == SNES_COMPOSITE_ADDITIVEOPTIMAL) {
    PetscCall(VecDuplicateVecs(F, jac->nsnes, &jac->Xes));
    PetscCall(PetscMalloc1(n, &jac->Fes));
    PetscCall(PetscMalloc1(n, &jac->fnorms));
    next = jac->head;
    i    = 0;
    while (next) {
      PetscCall(SNESGetFunction(next->snes, &F, NULL, NULL));
      jac->Fes[i] = F;
      PetscCall(PetscObjectReference((PetscObject)F));
      next = next->next;
      i++;
    }
    /* allocate the subspace direct solve area */
    jac->nrhs = 1;
    jac->lda  = jac->nsnes;
    jac->ldb  = jac->nsnes;
    jac->n    = jac->nsnes;

    PetscCall(PetscMalloc1(jac->n * jac->n, &jac->h));
    PetscCall(PetscMalloc1(jac->n, &jac->beta));
    PetscCall(PetscMalloc1(jac->n, &jac->s));
    PetscCall(PetscMalloc1(jac->n, &jac->g));
    jac->lwork = 12 * jac->n;
#if defined(PETSC_USE_COMPLEX)
    PetscCall(PetscMalloc1(jac->lwork, &jac->rwork));
#endif
    PetscCall(PetscMalloc1(jac->lwork, &jac->work));
  }

  PetscFunctionReturn(0);
}

static PetscErrorCode SNESReset_Composite(SNES snes) {
  SNES_Composite    *jac  = (SNES_Composite *)snes->data;
  SNES_CompositeLink next = jac->head;

  PetscFunctionBegin;
  while (next) {
    PetscCall(SNESReset(next->snes));
    next = next->next;
  }
  PetscCall(VecDestroy(&jac->Xorig));
  if (jac->Xes) PetscCall(VecDestroyVecs(jac->nsnes, &jac->Xes));
  if (jac->Fes) PetscCall(VecDestroyVecs(jac->nsnes, &jac->Fes));
  PetscCall(PetscFree(jac->fnorms));
  PetscCall(PetscFree(jac->h));
  PetscCall(PetscFree(jac->s));
  PetscCall(PetscFree(jac->g));
  PetscCall(PetscFree(jac->beta));
  PetscCall(PetscFree(jac->work));
  PetscCall(PetscFree(jac->rwork));
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESDestroy_Composite(SNES snes) {
  SNES_Composite    *jac  = (SNES_Composite *)snes->data;
  SNES_CompositeLink next = jac->head, next_tmp;

  PetscFunctionBegin;
  PetscCall(SNESReset_Composite(snes));
  while (next) {
    PetscCall(SNESDestroy(&next->snes));
    next_tmp = next;
    next     = next->next;
    PetscCall(PetscFree(next_tmp));
  }
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeSetType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeAddSNES_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeGetSNES_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeSetDamping_C", NULL));
  PetscCall(PetscFree(snes->data));
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESSetFromOptions_Composite(SNES snes, PetscOptionItems *PetscOptionsObject) {
  SNES_Composite    *jac  = (SNES_Composite *)snes->data;
  PetscInt           nmax = 8, i;
  SNES_CompositeLink next;
  char              *sneses[8];
  PetscReal          dmps[8];
  PetscBool          flg;

  PetscFunctionBegin;
  PetscOptionsHeadBegin(PetscOptionsObject, "Composite preconditioner options");
  PetscCall(PetscOptionsEnum("-snes_composite_type", "Type of composition", "SNESCompositeSetType", SNESCompositeTypes, (PetscEnum)jac->type, (PetscEnum *)&jac->type, &flg));
  if (flg) PetscCall(SNESCompositeSetType(snes, jac->type));
  PetscCall(PetscOptionsStringArray("-snes_composite_sneses", "List of composite solvers", "SNESCompositeAddSNES", sneses, &nmax, &flg));
  if (flg) {
    for (i = 0; i < nmax; i++) {
      PetscCall(SNESCompositeAddSNES(snes, sneses[i]));
      PetscCall(PetscFree(sneses[i])); /* deallocate string sneses[i], which is allocated in PetscOptionsStringArray() */
    }
  }
  PetscCall(PetscOptionsRealArray("-snes_composite_damping", "Damping of the additive composite solvers", "SNESCompositeSetDamping", dmps, &nmax, &flg));
  if (flg) {
    for (i = 0; i < nmax; i++) PetscCall(SNESCompositeSetDamping(snes, i, dmps[i]));
  }
  PetscCall(PetscOptionsReal("-snes_composite_stol", "Step tolerance for restart on the additive composite solvers", "", jac->stol, &jac->stol, NULL));
  PetscCall(PetscOptionsReal("-snes_composite_rtol", "Residual tolerance for the additive composite solvers", "", jac->rtol, &jac->rtol, NULL));
  PetscOptionsHeadEnd();

  next = jac->head;
  while (next) {
    PetscCall(SNESSetFromOptions(next->snes));
    next = next->next;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESView_Composite(SNES snes, PetscViewer viewer) {
  SNES_Composite    *jac  = (SNES_Composite *)snes->data;
  SNES_CompositeLink next = jac->head;
  PetscBool          iascii;

  PetscFunctionBegin;
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
  if (iascii) {
    PetscCall(PetscViewerASCIIPrintf(viewer, "  type - %s\n", SNESCompositeTypes[jac->type]));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  SNESes on composite preconditioner follow\n"));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  ---------------------------------\n"));
  }
  if (iascii) PetscCall(PetscViewerASCIIPushTab(viewer));
  while (next) {
    PetscCall(SNESView(next->snes, viewer));
    next = next->next;
  }
  if (iascii) {
    PetscCall(PetscViewerASCIIPopTab(viewer));
    PetscCall(PetscViewerASCIIPrintf(viewer, "  ---------------------------------\n"));
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESCompositeSetType_Composite(SNES snes, SNESCompositeType type) {
  SNES_Composite *jac = (SNES_Composite *)snes->data;

  PetscFunctionBegin;
  jac->type = type;
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESCompositeAddSNES_Composite(SNES snes, SNESType type) {
  SNES_Composite    *jac;
  SNES_CompositeLink next, ilink;
  PetscInt           cnt = 0;
  const char        *prefix;
  char               newprefix[20];
  DM                 dm;

  PetscFunctionBegin;
  PetscCall(PetscNewLog(snes, &ilink));
  ilink->next = NULL;
  PetscCall(SNESCreate(PetscObjectComm((PetscObject)snes), &ilink->snes));
  PetscCall(PetscLogObjectParent((PetscObject)snes, (PetscObject)ilink->snes));
  PetscCall(SNESGetDM(snes, &dm));
  PetscCall(SNESSetDM(ilink->snes, dm));
  PetscCall(SNESSetTolerances(ilink->snes, snes->abstol, snes->rtol, snes->stol, 1, snes->max_funcs));
  PetscCall(PetscObjectCopyFortranFunctionPointers((PetscObject)snes, (PetscObject)ilink->snes));
  jac  = (SNES_Composite *)snes->data;
  next = jac->head;
  if (!next) {
    jac->head       = ilink;
    ilink->previous = NULL;
  } else {
    cnt++;
    while (next->next) {
      next = next->next;
      cnt++;
    }
    next->next      = ilink;
    ilink->previous = next;
  }
  PetscCall(SNESGetOptionsPrefix(snes, &prefix));
  PetscCall(SNESSetOptionsPrefix(ilink->snes, prefix));
  PetscCall(PetscSNPrintf(newprefix, sizeof(newprefix), "sub_%d_", (int)cnt));
  PetscCall(SNESAppendOptionsPrefix(ilink->snes, newprefix));
  PetscCall(PetscObjectIncrementTabLevel((PetscObject)ilink->snes, (PetscObject)snes, 1));
  PetscCall(SNESSetType(ilink->snes, type));
  PetscCall(SNESSetNormSchedule(ilink->snes, SNES_NORM_FINAL_ONLY));

  ilink->dmp = 1.0;
  jac->nsnes++;
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESCompositeGetSNES_Composite(SNES snes, PetscInt n, SNES *subsnes) {
  SNES_Composite    *jac;
  SNES_CompositeLink next;
  PetscInt           i;

  PetscFunctionBegin;
  jac  = (SNES_Composite *)snes->data;
  next = jac->head;
  for (i = 0; i < n; i++) {
    PetscCheck(next->next, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_INCOMP, "Not enough SNESes in composite preconditioner");
    next = next->next;
  }
  *subsnes = next->snes;
  PetscFunctionReturn(0);
}

/*@C
   SNESCompositeSetType - Sets the type of composite preconditioner.

   Logically Collective on snes

   Input Parameters:
+  snes - the preconditioner context
-  type - `SNES_COMPOSITE_ADDITIVE` (default), `SNES_COMPOSITE_MULTIPLICATIVE`

   Options Database Key:
.  -snes_composite_type <type: one of multiplicative, additive, special> - Sets composite preconditioner type

   Level: Developer

.seealso: `SNES_COMPOSITE_ADDITIVE`, `SNES_COMPOSITE_MULTIPLICATIVE`, `SNESCompositeType`, `SNESCOMPOSITE`
@*/
PetscErrorCode SNESCompositeSetType(SNES snes, SNESCompositeType type) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveEnum(snes, type, 2);
  PetscTryMethod(snes, "SNESCompositeSetType_C", (SNES, SNESCompositeType), (snes, type));
  PetscFunctionReturn(0);
}

/*@C
   SNESCompositeAddSNES - Adds another `SNES` to the `SNESCOMPOSITE`

   Collective on snes

   Input Parameters:
+  snes - the snes context of type `SNESCOMPOSITE`
-  type - the type of the new solver

   Level: Developer

.seealso: `SNES`, `SNESCOMPOSITE`, `SNESCompositeGetSNES()`
@*/
PetscErrorCode SNESCompositeAddSNES(SNES snes, SNESType type) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscTryMethod(snes, "SNESCompositeAddSNES_C", (SNES, SNESType), (snes, type));
  PetscFunctionReturn(0);
}

/*@
   SNESCompositeGetSNES - Gets one of the `SNES` objects in the composite  `SNESCOMPOSITE`

   Not Collective

   Input Parameters:
+  snes - the snes context
-  n - the number of the composed snes requested

   Output Parameters:
.  subsnes - the `SNES` requested

   Level: Developer

.seealso: `SNES`, `SNESCOMPOSITE`, `SNESCompositeAddSNES()`
@*/
PetscErrorCode SNESCompositeGetSNES(SNES snes, PetscInt n, SNES *subsnes) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidPointer(subsnes, 3);
  PetscUseMethod(snes, "SNESCompositeGetSNES_C", (SNES, PetscInt, SNES *), (snes, n, subsnes));
  PetscFunctionReturn(0);
}

/*@
   SNESCompositeGetNumber - Get the number of subsolvers in the `SNESCOMPOSITE`

   Logically Collective on snes

   Input Parameter:
   snes - the snes context

   Output Parameter:
   n - the number of subsolvers

   Level: Developer

.seealso: `SNES`, `SNESCOMPOSITE`, `SNESCompositeAddSNES()`, `SNESCompositeGetSNES()`
@*/
PetscErrorCode SNESCompositeGetNumber(SNES snes, PetscInt *n) {
  SNES_Composite    *jac;
  SNES_CompositeLink next;

  PetscFunctionBegin;
  jac  = (SNES_Composite *)snes->data;
  next = jac->head;

  *n = 0;
  while (next) {
    *n   = *n + 1;
    next = next->next;
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESCompositeSetDamping_Composite(SNES snes, PetscInt n, PetscReal dmp) {
  SNES_Composite    *jac;
  SNES_CompositeLink next;
  PetscInt           i;

  PetscFunctionBegin;
  jac  = (SNES_Composite *)snes->data;
  next = jac->head;
  for (i = 0; i < n; i++) {
    PetscCheck(next->next, PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_INCOMP, "Not enough SNESes in composite preconditioner");
    next = next->next;
  }
  next->dmp = dmp;
  PetscFunctionReturn(0);
}

/*@
   SNESCompositeSetDamping - Sets the damping of a subsolver when using `SNES_COMPOSITE_ADDITIVE` `SNESCOMPOSITE`

   Not Collective

   Input Parameters:
+  snes - the snes context
.  n - the number of the snes requested
-  dmp - the damping

   Level: intermediate

.seealso: `SNES`, `SNESCOMPOSITE`, `SNESCompositeAddSNES()`, `SNESCompositeGetSNES()`,
          `SNES_COMPOSITE_ADDITIVE`, `SNES_COMPOSITE_MULTIPLICATIVE`, `SNESCompositeType`, `SNESCompositeSetType()`
@*/
PetscErrorCode SNESCompositeSetDamping(SNES snes, PetscInt n, PetscReal dmp) {
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscUseMethod(snes, "SNESCompositeSetDamping_C", (SNES, PetscInt, PetscReal), (snes, n, dmp));
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESSolve_Composite(SNES snes) {
  Vec              F, X, B, Y;
  PetscInt         i;
  PetscReal        fnorm = 0.0, xnorm = 0.0, snorm = 0.0;
  SNESNormSchedule normtype;
  SNES_Composite  *comp = (SNES_Composite *)snes->data;

  PetscFunctionBegin;
  X = snes->vec_sol;
  F = snes->vec_func;
  B = snes->vec_rhs;
  Y = snes->vec_sol_update;

  PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
  snes->iter          = 0;
  snes->norm          = 0.;
  comp->innerFailures = 0;
  PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
  snes->reason = SNES_CONVERGED_ITERATING;
  PetscCall(SNESGetNormSchedule(snes, &normtype));
  if (normtype == SNES_NORM_ALWAYS || normtype == SNES_NORM_INITIAL_ONLY || normtype == SNES_NORM_INITIAL_FINAL_ONLY) {
    if (!snes->vec_func_init_set) {
      PetscCall(SNESComputeFunction(snes, X, F));
    } else snes->vec_func_init_set = PETSC_FALSE;

    if (snes->xl && snes->xu) {
      PetscCall(SNESVIComputeInactiveSetFnorm(snes, F, X, &fnorm));
    } else {
      PetscCall(VecNorm(F, NORM_2, &fnorm)); /* fnorm <- ||F||  */
    }
    SNESCheckFunctionNorm(snes, fnorm);
    PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
    snes->iter = 0;
    snes->norm = fnorm;
    PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
    PetscCall(SNESLogConvergenceHistory(snes, snes->norm, 0));
    PetscCall(SNESMonitor(snes, 0, snes->norm));

    /* test convergence */
    PetscUseTypeMethod(snes, converged, 0, 0.0, 0.0, fnorm, &snes->reason, snes->cnvP);
    if (snes->reason) PetscFunctionReturn(0);
  } else {
    PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
    PetscCall(SNESLogConvergenceHistory(snes, snes->norm, 0));
    PetscCall(SNESMonitor(snes, 0, snes->norm));
  }

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

    /* Copy the state before modification by application of the composite solver;
       we will subtract the new state after application */
    PetscCall(VecCopy(X, Y));

    if (comp->type == SNES_COMPOSITE_ADDITIVE) {
      PetscCall(SNESCompositeApply_Additive(snes, X, B, F, &fnorm));
    } else if (comp->type == SNES_COMPOSITE_MULTIPLICATIVE) {
      PetscCall(SNESCompositeApply_Multiplicative(snes, X, B, F, &fnorm));
    } else if (comp->type == SNES_COMPOSITE_ADDITIVEOPTIMAL) {
      PetscCall(SNESCompositeApply_AdditiveOptimal(snes, X, B, F, &fnorm));
    } else SETERRQ(PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_WRONGSTATE, "Unsupported SNESComposite type");
    if (snes->reason < 0) break;

    /* Compute the solution update for convergence testing */
    PetscCall(VecAYPX(Y, -1.0, X));

    if ((i == snes->max_its - 1) && (normtype == SNES_NORM_INITIAL_FINAL_ONLY || normtype == SNES_NORM_FINAL_ONLY)) {
      PetscCall(SNESComputeFunction(snes, X, F));

      if (snes->xl && snes->xu) {
        PetscCall(VecNormBegin(X, NORM_2, &xnorm));
        PetscCall(VecNormBegin(Y, NORM_2, &snorm));
        PetscCall(SNESVIComputeInactiveSetFnorm(snes, F, X, &fnorm));
        PetscCall(VecNormEnd(X, NORM_2, &xnorm));
        PetscCall(VecNormEnd(Y, NORM_2, &snorm));
      } else {
        PetscCall(VecNormBegin(F, NORM_2, &fnorm));
        PetscCall(VecNormBegin(X, NORM_2, &xnorm));
        PetscCall(VecNormBegin(Y, NORM_2, &snorm));

        PetscCall(VecNormEnd(F, NORM_2, &fnorm));
        PetscCall(VecNormEnd(X, NORM_2, &xnorm));
        PetscCall(VecNormEnd(Y, NORM_2, &snorm));
      }
      SNESCheckFunctionNorm(snes, fnorm);
    } else if (normtype == SNES_NORM_ALWAYS) {
      PetscCall(VecNormBegin(X, NORM_2, &xnorm));
      PetscCall(VecNormBegin(Y, NORM_2, &snorm));
      PetscCall(VecNormEnd(X, NORM_2, &xnorm));
      PetscCall(VecNormEnd(Y, NORM_2, &snorm));
    }
    /* Monitor convergence */
    PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
    snes->iter  = i + 1;
    snes->norm  = fnorm;
    snes->xnorm = xnorm;
    snes->ynorm = snorm;
    PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
    PetscCall(SNESLogConvergenceHistory(snes, snes->norm, 0));
    PetscCall(SNESMonitor(snes, snes->iter, snes->norm));
    /* Test for convergence */
    if (normtype == SNES_NORM_ALWAYS) PetscUseTypeMethod(snes, converged, snes->iter, xnorm, snorm, fnorm, &snes->reason, snes->cnvP);
    if (snes->reason) break;
  }
  if (normtype == SNES_NORM_ALWAYS) {
    if (i == snes->max_its) {
      PetscCall(PetscInfo(snes, "Maximum number of iterations has been reached: %" PetscInt_FMT "\n", snes->max_its));
      if (!snes->reason) snes->reason = SNES_DIVERGED_MAX_IT;
    }
  } else if (!snes->reason) snes->reason = SNES_CONVERGED_ITS;
  PetscFunctionReturn(0);
}

/*MC
     SNESCOMPOSITE - Build a preconditioner by composing together several nonlinear solvers

   Options Database Keys:
+  -snes_composite_type <type: one of multiplicative, additive, symmetric_multiplicative, special> - Sets composite preconditioner type
-  -snes_composite_sneses - <snes0,snes1,...> list of SNESes to compose

   Level: intermediate

   References:
.  * - Peter R. Brune, Matthew G. Knepley, Barry F. Smith, and Xuemin Tu, "Composing Scalable Nonlinear Algebraic Solvers",
   SIAM Review, 57(4), 2015

.seealso: `SNES`, `SNESCOMPOSITE`, `SNESCompositeAddSNES()`, `SNESCompositeGetSNES()`,
          `SNES_COMPOSITE_ADDITIVE`, `SNES_COMPOSITE_MULTIPLICATIVE`, `SNESCompositeType`, `SNESCompositeSetType()`,
          `SNESCompositeSetDamping()`
M*/

PETSC_EXTERN PetscErrorCode SNESCreate_Composite(SNES snes) {
  SNES_Composite *jac;

  PetscFunctionBegin;
  PetscCall(PetscNewLog(snes, &jac));

  snes->ops->solve          = SNESSolve_Composite;
  snes->ops->setup          = SNESSetUp_Composite;
  snes->ops->reset          = SNESReset_Composite;
  snes->ops->destroy        = SNESDestroy_Composite;
  snes->ops->setfromoptions = SNESSetFromOptions_Composite;
  snes->ops->view           = SNESView_Composite;

  snes->usesksp = PETSC_FALSE;

  snes->alwayscomputesfinalresidual = PETSC_FALSE;

  snes->data  = (void *)jac;
  jac->type   = SNES_COMPOSITE_ADDITIVEOPTIMAL;
  jac->Fes    = NULL;
  jac->Xes    = NULL;
  jac->fnorms = NULL;
  jac->nsnes  = 0;
  jac->head   = NULL;
  jac->stol   = 0.1;
  jac->rtol   = 1.1;

  jac->h     = NULL;
  jac->s     = NULL;
  jac->beta  = NULL;
  jac->work  = NULL;
  jac->rwork = NULL;

  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeSetType_C", SNESCompositeSetType_Composite));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeAddSNES_C", SNESCompositeAddSNES_Composite));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeGetSNES_C", SNESCompositeGetSNES_Composite));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESCompositeSetDamping_C", SNESCompositeSetDamping_Composite));
  PetscFunctionReturn(0);
}