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

typedef struct {
  PetscBool transpose_solve;
} SNES_KSPONLY;

static PetscErrorCode SNESSolve_KSPONLY(SNES snes) {
  SNES_KSPONLY *ksponly = (SNES_KSPONLY *)snes->data;
  PetscInt      lits;
  Vec           Y, X, F;

  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);

  snes->numFailures            = 0;
  snes->numLinearSolveFailures = 0;
  snes->reason                 = SNES_CONVERGED_ITERATING;
  snes->iter                   = 0;
  snes->norm                   = 0.0;

  X = snes->vec_sol;
  F = snes->vec_func;
  Y = snes->vec_sol_update;

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

  if (snes->numbermonitors) {
    PetscReal fnorm;
    PetscCall(VecNorm(F, NORM_2, &fnorm));
    PetscCall(SNESMonitor(snes, 0, fnorm));
  }

  /* Call general purpose update function */
  PetscTryTypeMethod(snes, update, 0);

  /* 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));
  if (ksponly->transpose_solve) {
    PetscCall(KSPSolveTranspose(snes->ksp, F, Y));
  } else {
    PetscCall(KSPSolve(snes->ksp, F, Y));
  }
  snes->reason = SNES_CONVERGED_ITS;
  SNESCheckKSPSolve(snes);

  PetscCall(KSPGetIterationNumber(snes->ksp, &lits));
  PetscCall(PetscInfo(snes, "iter=%" PetscInt_FMT ", linear solve iterations=%" PetscInt_FMT "\n", snes->iter, lits));
  snes->iter++;

  /* Take the computed step. */
  PetscCall(VecAXPY(X, -1.0, Y));
  if (snes->numbermonitors) {
    PetscReal fnorm;
    PetscCall(SNESComputeFunction(snes, X, F));
    PetscCall(VecNorm(F, NORM_2, &fnorm));
    PetscCall(SNESMonitor(snes, 1, fnorm));
  }
  PetscFunctionReturn(0);
}

static PetscErrorCode SNESSetUp_KSPONLY(SNES snes) {
  PetscFunctionBegin;
  PetscCall(SNESSetUpMatrices(snes));
  PetscFunctionReturn(0);
}

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

/*MC
      SNESKSPONLY - Nonlinear solver that performs one Newton step and does not compute any norms.
      The main purpose of this solver is to solve linear problems using the `SNES` interface, without
      any additional overhead in the form of vector operations.

   Level: beginner

.seealso: `SNES`, `SNESType`, `SNESCreate()`, `SNES`, `SNESSetType()`, `SNESNEWTONLS`, `SNESNEWTONTR`
M*/
PETSC_EXTERN PetscErrorCode SNESCreate_KSPONLY(SNES snes) {
  SNES_KSPONLY *ksponly;

  PetscFunctionBegin;
  snes->ops->setup          = SNESSetUp_KSPONLY;
  snes->ops->solve          = SNESSolve_KSPONLY;
  snes->ops->destroy        = SNESDestroy_KSPONLY;
  snes->ops->setfromoptions = NULL;
  snes->ops->view           = NULL;
  snes->ops->reset          = NULL;

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

  snes->alwayscomputesfinalresidual = PETSC_FALSE;

  PetscCall(PetscNewLog(snes, &ksponly));
  snes->data = (void *)ksponly;
  PetscFunctionReturn(0);
}

/*MC
      SNESKSPTRANSPOSEONLY - Nonlinear solver that performs one Newton step and does not compute any norms.
      The main purpose of this solver is to solve transposed linear problems using the `SNES` interface, without
      any additional overhead in the form of vector operations within adjoint solvers.

   Level: beginner

.seealso: `SNES`, `SNESType`, `SNESCreate()`, `SNES`, `SNESSetType()`, `SNESKSPTRANSPOSEONLY`, `SNESNEWTONLS`, `SNESNEWTONTR`
M*/
PETSC_EXTERN PetscErrorCode SNESCreate_KSPTRANSPOSEONLY(SNES snes) {
  SNES_KSPONLY *kspo;

  PetscFunctionBegin;
  PetscCall(SNESCreate_KSPONLY(snes));
  PetscCall(PetscObjectChangeTypeName((PetscObject)snes, SNESKSPTRANSPOSEONLY));
  kspo                  = (SNES_KSPONLY *)snes->data;
  kspo->transpose_solve = PETSC_TRUE;
  PetscFunctionReturn(0);
}