#include <petsc/private/snesimpl.h> /*I "petscsnes.h" I*/

static SNESMSType SNESMSDefault = SNESMSM62;
static PetscBool  SNESMSRegisterAllCalled;
static PetscBool  SNESMSPackageInitialized;

typedef struct _SNESMSTableau *SNESMSTableau;
struct _SNESMSTableau {
  char      *name;
  PetscInt   nstages;    /* Number of stages */
  PetscInt   nregisters; /* Number of registers */
  PetscReal  stability;  /* Scaled stability region */
  PetscReal *gamma;      /* Coefficients of 3S* method */
  PetscReal *delta;      /* Coefficients of 3S* method */
  PetscReal *betasub;    /* Subdiagonal of beta in Shu-Osher form */
};

typedef struct _SNESMSTableauLink *SNESMSTableauLink;
struct _SNESMSTableauLink {
  struct _SNESMSTableau tab;
  SNESMSTableauLink     next;
};
static SNESMSTableauLink SNESMSTableauList;

typedef struct {
  SNESMSTableau tableau; /* Tableau in low-storage form */
  PetscReal     damping; /* Damping parameter, like length of (pseudo) time step */
} SNES_MS;

/*@C
  SNESMSRegisterAll - Registers all of the multi-stage methods in `SNESMS`

  Logically Collective

  Level: developer

.seealso: [](ch_snes), `SNES`, `SNESMS`, `SNESMSRegisterDestroy()`
@*/
PetscErrorCode SNESMSRegisterAll(void)
{
  PetscFunctionBegin;
  if (SNESMSRegisterAllCalled) PetscFunctionReturn(PETSC_SUCCESS);
  SNESMSRegisterAllCalled = PETSC_TRUE;

  {
    PetscReal alpha[1] = {1.0};
    PetscCall(SNESMSRegister(SNESMSEULER, 1, 1, 1.0, NULL, NULL, alpha));
  }

  {
    PetscReal gamma[3][6] = {
      {0.0000000000000000E+00, -7.0304722367110606E-01, -1.9836719667506464E-01, -1.6023843981863788E+00, 9.4483822882855284E-02,  -1.4204296130641869E-01},
      {1.0000000000000000E+00, 1.1111025767083920E+00,  5.6150921583923230E-01,  7.4151723494934041E-01,  3.1714538168600587E-01,  4.6479276238548706E-01 },
      {0.0000000000000000E+00, 0.0000000000000000E+00,  0.0000000000000000E+00,  6.7968174970583317E-01,  -4.1755042846051737E-03, -1.9115668129923846E-01}
    };
    PetscReal delta[6]   = {1.0000000000000000E+00, 5.3275427433201750E-01, 6.0143544663985238E-01, 4.5874077053842177E-01, 2.7544386906104651E-01, 0.0000000000000000E+00};
    PetscReal betasub[6] = {8.4753115429481929E-01, 7.4018896368655618E-01, 6.5963574086583309E-03, 4.6747795645517759E-01, 1.3314545813643919E-01, 5.3260800028018784E-01};
    PetscCall(SNESMSRegister(SNESMSM62, 6, 3, 1.0, &gamma[0][0], delta, betasub));
  }

  { /* Jameson (1983) */
    PetscReal alpha[4] = {0.25, 0.5, 0.55, 1.0};
    PetscCall(SNESMSRegister(SNESMSJAMESON83, 4, 1, 1.0, NULL, NULL, alpha));
  }

  { /* Van Leer, Tai, and Powell (1989) 1 stage, order 1 */
    PetscReal alpha[1] = {1.0};
    PetscCall(SNESMSRegister(SNESMSVLTP11, 1, 1, 0.5, NULL, NULL, alpha));
  }
  { /* Van Leer, Tai, and Powell (1989) 2 stage, order 1 */
    PetscReal alpha[2] = {0.3333, 1.0};
    PetscCall(SNESMSRegister(SNESMSVLTP21, 2, 1, 1.0, NULL, NULL, alpha));
  }
  { /* Van Leer, Tai, and Powell (1989) 3 stage, order 1 */
    PetscReal alpha[3] = {0.1481, 0.4000, 1.0};
    PetscCall(SNESMSRegister(SNESMSVLTP31, 3, 1, 1.5, NULL, NULL, alpha));
  }
  { /* Van Leer, Tai, and Powell (1989) 4 stage, order 1 */
    PetscReal alpha[4] = {0.0833, 0.2069, 0.4265, 1.0};
    PetscCall(SNESMSRegister(SNESMSVLTP41, 4, 1, 2.0, NULL, NULL, alpha));
  }
  { /* Van Leer, Tai, and Powell (1989) 5 stage, order 1 */
    PetscReal alpha[5] = {0.0533, 0.1263, 0.2375, 0.4414, 1.0};
    PetscCall(SNESMSRegister(SNESMSVLTP51, 5, 1, 2.5, NULL, NULL, alpha));
  }
  { /* Van Leer, Tai, and Powell (1989) 6 stage, order 1 */
    PetscReal alpha[6] = {0.0370, 0.0851, 0.1521, 0.2562, 0.4512, 1.0};
    PetscCall(SNESMSRegister(SNESMSVLTP61, 6, 1, 3.0, NULL, NULL, alpha));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESMSRegisterDestroy - Frees the list of schemes that were registered by `SNESMSRegister()`.

  Logically Collective

  Level: developer

.seealso: [](ch_snes), `SNES`, `SNESMS`, `SNESMSRegister()`, `SNESMSRegisterAll()`
@*/
PetscErrorCode SNESMSRegisterDestroy(void)
{
  SNESMSTableauLink link;

  PetscFunctionBegin;
  while ((link = SNESMSTableauList)) {
    SNESMSTableau t   = &link->tab;
    SNESMSTableauList = link->next;

    PetscCall(PetscFree(t->name));
    PetscCall(PetscFree(t->gamma));
    PetscCall(PetscFree(t->delta));
    PetscCall(PetscFree(t->betasub));
    PetscCall(PetscFree(link));
  }
  SNESMSRegisterAllCalled = PETSC_FALSE;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESMSInitializePackage - This function initializes everything in the `SNESMS` package. It is called
  from `SNESInitializePackage()`.

  Level: developer

.seealso: [](ch_snes), `SNES`, `SNESMS`, `SNESMSRegister()`, `SNESMSRegisterAll()`, `PetscInitialize()`
@*/
PetscErrorCode SNESMSInitializePackage(void)
{
  PetscFunctionBegin;
  if (SNESMSPackageInitialized) PetscFunctionReturn(PETSC_SUCCESS);
  SNESMSPackageInitialized = PETSC_TRUE;

  PetscCall(SNESMSRegisterAll());
  PetscCall(PetscRegisterFinalize(SNESMSFinalizePackage));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESMSFinalizePackage - This function destroys everything in the `SNESMS` package. It is
  called from `PetscFinalize()`.

  Level: developer

.seealso: [](ch_snes), `SNES`, `SNESMS`, `SNESMSRegister()`, `SNESMSRegisterAll()`, `SNESMSInitializePackage()`, `PetscFinalize()`
@*/
PetscErrorCode SNESMSFinalizePackage(void)
{
  PetscFunctionBegin;
  SNESMSPackageInitialized = PETSC_FALSE;

  PetscCall(SNESMSRegisterDestroy());
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESMSRegister - register a multistage scheme for `SNESMS`

  Logically Collective

  Input Parameters:
+ name       - identifier for method
. nstages    - number of stages
. nregisters - number of registers used by low-storage implementation
. stability  - scaled stability region
. gamma      - coefficients, see Ketcheson's paper {cite}`ketcheson2010runge`
. delta      - coefficients, see Ketcheson's paper {cite}`ketcheson2010runge`
- betasub    - subdiagonal of Shu-Osher form

  Level: advanced

  Notes:
  The notation is described in {cite}`ketcheson2010runge` Ketcheson (2010) Runge-Kutta methods with minimum storage implementations.

  Many multistage schemes are of the form

  $$
  \begin{align*}
  X_0 = X^{(n)} \\
  X_k = X_0 + \alpha_k * F(X_{k-1}), k = 1,\ldots,s \\
  X^{(n+1)} = X_s
  \end{align*}
  $$

  These methods can be registered with
.vb
   SNESMSRegister("name",s,1,stability,NULL,NULL,alpha);
.ve

.seealso: [](ch_snes), `SNES`, `SNESMS`
@*/
PetscErrorCode SNESMSRegister(SNESMSType name, PetscInt nstages, PetscInt nregisters, PetscReal stability, const PetscReal gamma[], const PetscReal delta[], const PetscReal betasub[])
{
  SNESMSTableauLink link;
  SNESMSTableau     t;

  PetscFunctionBegin;
  PetscAssertPointer(name, 1);
  PetscCheck(nstages >= 1, PETSC_COMM_SELF, PETSC_ERR_ARG_OUTOFRANGE, "Must have at least one stage");
  if (gamma || delta) {
    PetscCheck(nregisters == 3, PETSC_COMM_SELF, PETSC_ERR_SUP, "Only support for methods written in 3-register form");
    PetscAssertPointer(gamma, 5);
    PetscAssertPointer(delta, 6);
  } else {
    PetscCheck(nregisters == 1, PETSC_COMM_SELF, PETSC_ERR_SUP, "Only support for methods written in 1-register form");
  }
  PetscAssertPointer(betasub, 7);

  PetscCall(SNESMSInitializePackage());
  PetscCall(PetscNew(&link));
  t = &link->tab;
  PetscCall(PetscStrallocpy(name, &t->name));
  t->nstages    = nstages;
  t->nregisters = nregisters;
  t->stability  = stability;

  if (gamma && delta) {
    PetscCall(PetscMalloc1(nstages * nregisters, &t->gamma));
    PetscCall(PetscMalloc1(nstages, &t->delta));
    PetscCall(PetscArraycpy(t->gamma, gamma, nstages * nregisters));
    PetscCall(PetscArraycpy(t->delta, delta, nstages));
  }
  PetscCall(PetscMalloc1(nstages, &t->betasub));
  PetscCall(PetscArraycpy(t->betasub, betasub, nstages));

  link->next        = SNESMSTableauList;
  SNESMSTableauList = link;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
  X - initial state, updated in-place.
  F - residual, computed at the initial X on input
*/
static PetscErrorCode SNESMSStep_3Sstar(SNES snes, Vec X, Vec F)
{
  SNES_MS         *ms      = (SNES_MS *)snes->data;
  SNESMSTableau    t       = ms->tableau;
  const PetscInt   nstages = t->nstages;
  const PetscReal *gamma = t->gamma, *delta = t->delta, *betasub = t->betasub;
  Vec              S1 = X, S2 = snes->work[1], S3 = snes->work[2], Y = snes->work[0], S1copy = snes->work[3];

  PetscFunctionBegin;
  PetscCall(VecZeroEntries(S2));
  PetscCall(VecCopy(X, S3));
  for (PetscInt i = 0; i < nstages; i++) {
    Vec               Ss[]     = {S1copy, S2, S3, Y};
    const PetscScalar scoeff[] = {gamma[0 * nstages + i] - 1, gamma[1 * nstages + i], gamma[2 * nstages + i], -betasub[i] * ms->damping};

    PetscCall(VecAXPY(S2, delta[i], S1));
    if (i > 0) PetscCall(SNESComputeFunction(snes, S1, F));
    PetscCall(KSPSolve(snes->ksp, F, Y));
    PetscCall(VecCopy(S1, S1copy));
    PetscCall(VecMAXPY(S1, 4, scoeff, Ss));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*
  X - initial state, updated in-place.
  F - residual, computed at the initial X on input
*/
static PetscErrorCode SNESMSStep_Basic(SNES snes, Vec X, Vec F)
{
  SNES_MS         *ms    = (SNES_MS *)snes->data;
  SNESMSTableau    tab   = ms->tableau;
  const PetscReal *alpha = tab->betasub, h = ms->damping;
  PetscInt         i, nstages              = tab->nstages;
  Vec              X0 = snes->work[0];

  PetscFunctionBegin;
  PetscCall(VecCopy(X, X0));
  for (i = 0; i < nstages; i++) {
    if (i > 0) PetscCall(SNESComputeFunction(snes, X, F));
    PetscCall(KSPSolve(snes->ksp, F, X));
    PetscCall(VecAYPX(X, -alpha[i] * h, X0));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESMSStep_Step(SNES snes, Vec X, Vec F)
{
  SNES_MS      *ms  = (SNES_MS *)snes->data;
  SNESMSTableau tab = ms->tableau;

  PetscFunctionBegin;
  if (tab->gamma && tab->delta) {
    PetscCall(SNESMSStep_3Sstar(snes, X, F));
  } else {
    PetscCall(SNESMSStep_Basic(snes, X, F));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESMSStep_Norms(SNES snes, PetscInt iter, Vec F)
{
  PetscReal fnorm;

  PetscFunctionBegin;
  if (SNESNeedNorm_Private(snes, iter)) {
    PetscCall(VecNorm(F, NORM_2, &fnorm)); /* fnorm <- ||F||  */
    SNESCheckFunctionNorm(snes, fnorm);
    /* Monitor convergence */
    PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
    snes->iter = iter;
    snes->norm = fnorm;
    PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
    PetscCall(SNESLogConvergenceHistory(snes, snes->norm, 0));
    /* Test for convergence */
    PetscCall(SNESConverged(snes, snes->iter, 0.0, 0.0, fnorm));
    PetscCall(SNESMonitor(snes, snes->iter, snes->norm));
  } else if (iter > 0) {
    PetscCall(PetscObjectSAWsTakeAccess((PetscObject)snes));
    snes->iter = iter;
    PetscCall(PetscObjectSAWsGrantAccess((PetscObject)snes));
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESSolve_MS(SNES snes)
{
  Vec      X = snes->vec_sol, F = snes->vec_func;
  PetscInt i;

  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);
  PetscCall(PetscCitationsRegister(SNESCitation, &SNEScite));

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

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

  PetscCall(SNESMSStep_Norms(snes, 0, F));
  if (snes->reason) PetscFunctionReturn(PETSC_SUCCESS);

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

    if (i == 0 && snes->jacobian) {
      /* This method does not require a Jacobian, but it is usually preconditioned by PBJacobi */
      PetscCall(SNESComputeJacobian(snes, snes->vec_sol, snes->jacobian, snes->jacobian_pre));
      SNESCheckJacobianDomainerror(snes);
      PetscCall(KSPSetOperators(snes->ksp, snes->jacobian, snes->jacobian_pre));
    }

    PetscCall(SNESMSStep_Step(snes, X, F));

    if (i < snes->max_its - 1 || SNESNeedNorm_Private(snes, i + 1)) PetscCall(SNESComputeFunction(snes, X, F));

    PetscCall(SNESMSStep_Norms(snes, i + 1, F));
    if (snes->reason) PetscFunctionReturn(PETSC_SUCCESS);
  }
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESSetUp_MS(SNES snes)
{
  SNES_MS      *ms    = (SNES_MS *)snes->data;
  SNESMSTableau tab   = ms->tableau;
  PetscInt      nwork = tab->nregisters + 1; // +1 because VecMAXPY() in SNESMSStep_3Sstar()
                                             // needs an extra work vec

  PetscFunctionBegin;
  PetscCall(SNESSetWorkVecs(snes, nwork));
  PetscCall(SNESSetUpMatrices(snes));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESReset_MS(SNES snes)
{
  PetscFunctionBegin;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESDestroy_MS(SNES snes)
{
  PetscFunctionBegin;
  PetscCall(SNESReset_MS(snes));
  PetscCall(PetscFree(snes->data));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSGetType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSSetType_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSGetDamping_C", NULL));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSSetDamping_C", NULL));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESView_MS(SNES snes, PetscViewer viewer)
{
  PetscBool     iascii;
  SNES_MS      *ms  = (SNES_MS *)snes->data;
  SNESMSTableau tab = ms->tableau;

  PetscFunctionBegin;
  PetscCall(PetscObjectTypeCompare((PetscObject)viewer, PETSCVIEWERASCII, &iascii));
  if (iascii) PetscCall(PetscViewerASCIIPrintf(viewer, "  multi-stage method type: %s\n", tab->name));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESSetFromOptions_MS(SNES snes, PetscOptionItems *PetscOptionsObject)
{
  PetscFunctionBegin;
  PetscOptionsHeadBegin(PetscOptionsObject, "SNES MS options");
  {
    SNESMSTableauLink link;
    PetscInt          count, choice;
    PetscBool         flg;
    const char      **namelist;
    SNESMSType        mstype;
    PetscReal         damping;
    PetscBool         norms = PETSC_FALSE;

    PetscCall(SNESMSGetType(snes, &mstype));
    for (link = SNESMSTableauList, count = 0; link; link = link->next, count++);
    PetscCall(PetscMalloc1(count, (char ***)&namelist));
    for (link = SNESMSTableauList, count = 0; link; link = link->next, count++) namelist[count] = link->tab.name;
    PetscCall(PetscOptionsEList("-snes_ms_type", "Multistage smoother type", "SNESMSSetType", (const char *const *)namelist, count, mstype, &choice, &flg));
    if (flg) PetscCall(SNESMSSetType(snes, namelist[choice]));
    PetscCall(PetscFree(namelist));
    PetscCall(SNESMSGetDamping(snes, &damping));
    PetscCall(PetscOptionsReal("-snes_ms_damping", "Damping for multistage method", "SNESMSSetDamping", damping, &damping, &flg));
    if (flg) PetscCall(SNESMSSetDamping(snes, damping));

    /* deprecated option */
    PetscCall(PetscOptionsDeprecated("-snes_ms_norms", NULL, "3.20", "Use -snes_norm_schedule always"));
    PetscCall(PetscOptionsBool("-snes_ms_norms", NULL, NULL, norms, &norms, NULL));
    if (norms) PetscCall(SNESSetNormSchedule(snes, SNES_NORM_ALWAYS));
  }
  PetscOptionsHeadEnd();
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESMSGetType_MS(SNES snes, SNESMSType *mstype)
{
  SNES_MS      *ms  = (SNES_MS *)snes->data;
  SNESMSTableau tab = ms->tableau;

  PetscFunctionBegin;
  *mstype = tab->name;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESMSSetType_MS(SNES snes, SNESMSType mstype)
{
  SNES_MS          *ms = (SNES_MS *)snes->data;
  SNESMSTableauLink link;
  PetscBool         match;

  PetscFunctionBegin;
  if (ms->tableau) {
    PetscCall(PetscStrcmp(ms->tableau->name, mstype, &match));
    if (match) PetscFunctionReturn(PETSC_SUCCESS);
  }
  for (link = SNESMSTableauList; link; link = link->next) {
    PetscCall(PetscStrcmp(link->tab.name, mstype, &match));
    if (match) {
      if (snes->setupcalled) PetscCall(SNESReset_MS(snes));
      ms->tableau = &link->tab;
      if (snes->setupcalled) PetscCall(SNESSetUp_MS(snes));
      PetscFunctionReturn(PETSC_SUCCESS);
    }
  }
  SETERRQ(PetscObjectComm((PetscObject)snes), PETSC_ERR_ARG_UNKNOWN_TYPE, "Could not find '%s'", mstype);
}

/*@C
  SNESMSGetType - Get the type of multistage smoother `SNESMS`

  Not Collective

  Input Parameter:
. snes - nonlinear solver context

  Output Parameter:
. mstype - type of multistage method

  Level: advanced

.seealso: [](ch_snes), `SNESMS`, `SNESMSSetType()`, `SNESMSType`
@*/
PetscErrorCode SNESMSGetType(SNES snes, SNESMSType *mstype)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscAssertPointer(mstype, 2);
  PetscUseMethod(snes, "SNESMSGetType_C", (SNES, SNESMSType *), (snes, mstype));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESMSSetType - Set the type of multistage smoother `SNESMS`

  Logically Collective

  Input Parameters:
+ snes   - nonlinear solver context
- mstype - type of multistage method

  Level: advanced

.seealso: [](ch_snes), `SNESMS`, `SNESMSGetType()`, `SNESMSType`
@*/
PetscErrorCode SNESMSSetType(SNES snes, SNESMSType mstype)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscAssertPointer(mstype, 2);
  PetscTryMethod(snes, "SNESMSSetType_C", (SNES, SNESMSType), (snes, mstype));
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESMSGetDamping_MS(SNES snes, PetscReal *damping)
{
  SNES_MS *ms = (SNES_MS *)snes->data;

  PetscFunctionBegin;
  *damping = ms->damping;
  PetscFunctionReturn(PETSC_SUCCESS);
}

static PetscErrorCode SNESMSSetDamping_MS(SNES snes, PetscReal damping)
{
  SNES_MS *ms = (SNES_MS *)snes->data;

  PetscFunctionBegin;
  ms->damping = damping;
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESMSGetDamping - Get the damping parameter of `SNESMS` multistage scheme

  Not Collective

  Input Parameter:
. snes - nonlinear solver context

  Output Parameter:
. damping - damping parameter

  Level: advanced

.seealso: [](ch_snes), `SNESMSSetDamping()`, `SNESMS`
@*/
PetscErrorCode SNESMSGetDamping(SNES snes, PetscReal *damping)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscAssertPointer(damping, 2);
  PetscUseMethod(snes, "SNESMSGetDamping_C", (SNES, PetscReal *), (snes, damping));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*@C
  SNESMSSetDamping - Set the damping parameter for a `SNESMS` multistage scheme

  Logically Collective

  Input Parameters:
+ snes    - nonlinear solver context
- damping - damping parameter

  Level: advanced

.seealso: [](ch_snes), `SNESMSGetDamping()`, `SNESMS`
@*/
PetscErrorCode SNESMSSetDamping(SNES snes, PetscReal damping)
{
  PetscFunctionBegin;
  PetscValidHeaderSpecific(snes, SNES_CLASSID, 1);
  PetscValidLogicalCollectiveReal(snes, damping, 2);
  PetscTryMethod(snes, "SNESMSSetDamping_C", (SNES, PetscReal), (snes, damping));
  PetscFunctionReturn(PETSC_SUCCESS);
}

/*MC
      SNESMS - multi-stage smoothers

      Options Database Keys:
+     -snes_ms_type    - type of multi-stage smoother
-     -snes_ms_damping - damping for multi-stage method

      Level: advanced

      Notes:
      These multistage methods are explicit Runge-Kutta methods that are often used as smoothers for
      FAS multigrid for transport problems. In the linear case, these are equivalent to polynomial smoothers (such as Chebyshev).

      Multi-stage smoothers should usually be preconditioned by point-block Jacobi to ensure proper scaling and to normalize the wave speeds.

      The methods are specified in low storage form (Ketcheson 2010). New methods can be registered with `SNESMSRegister()`.

      See {cite}`ketcheson2010runge`, {cite}`jameson1983`, {cite}`pierce1997preconditioned`, and {cite}`va1981design`

.seealso: [](ch_snes), `SNESCreate()`, `SNES`, `SNESSetType()`, `SNESMS`, `SNESFAS`, `KSPCHEBYSHEV`, `SNESMSSetDamping()`, `SNESMSGetDamping()`,
          `SNESMSSetType()`, `SNESMSGetType()`
M*/

PETSC_EXTERN PetscErrorCode SNESCreate_MS(SNES snes)
{
  SNES_MS *ms;

  PetscFunctionBegin;
  PetscCall(SNESMSInitializePackage());

  snes->ops->setup          = SNESSetUp_MS;
  snes->ops->solve          = SNESSolve_MS;
  snes->ops->destroy        = SNESDestroy_MS;
  snes->ops->setfromoptions = SNESSetFromOptions_MS;
  snes->ops->view           = SNESView_MS;
  snes->ops->reset          = SNESReset_MS;

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

  snes->alwayscomputesfinalresidual = PETSC_FALSE;

  PetscCall(PetscNew(&ms));
  snes->data  = (void *)ms;
  ms->damping = 0.9;

  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSGetType_C", SNESMSGetType_MS));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSSetType_C", SNESMSSetType_MS));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSGetDamping_C", SNESMSGetDamping_MS));
  PetscCall(PetscObjectComposeFunction((PetscObject)snes, "SNESMSSetDamping_C", SNESMSSetDamping_MS));

  PetscCall(SNESMSSetType(snes, SNESMSDefault));
  PetscFunctionReturn(PETSC_SUCCESS);
}
