Date: 2026-06-01T23:13:32+00:00

Actual source code: dgefa5.c

      1: /*
      2:       Inverts 5 by 5 matrix using gaussian elimination with partial pivoting.

      4:        Used by the sparse factorization routines in
      5:      src/mat/impls/baij/seq

      7:        This is a combination of the Linpack routines
      8:     dgefa() and dgedi() specialized for a size of 5.

     10: */
     11: #include <petscsys.h>
     12: #include <petsc/private/kernels/blockinvert.h>

     14: PetscErrorCode PetscKernel_A_gets_inverse_A_5(MatScalar *a, PetscInt *ipvt, MatScalar *work, PetscReal shift, PetscBool allowzeropivot, PetscBool *zeropivotdetected)
     15: {
     16:   PetscInt   i__2, i__3, kp1, j, k, l, ll, i, kb, k3;
     17:   PetscInt   k4, j3;
     18:   MatScalar *aa, *ax, *ay, stmp;
     19:   MatReal    tmp, max;

     21:   PetscFunctionBegin;
     22:   if (zeropivotdetected) *zeropivotdetected = PETSC_FALSE;
     23:   shift = .25 * shift * (1.e-12 + PetscAbsScalar(a[0]) + PetscAbsScalar(a[6]) + PetscAbsScalar(a[12]) + PetscAbsScalar(a[18]) + PetscAbsScalar(a[24]));

     25:   /* Parameter adjustments */
     26:   a -= 6;

     28:   for (k = 1; k <= 4; ++k) {
     29:     kp1 = k + 1;
     30:     k3  = 5 * k;
     31:     k4  = k3 + k;

     33:     /* find l = pivot index */
     34:     i__2 = 6 - k;
     35:     aa   = &a[k4];
     36:     max  = PetscAbsScalar(aa[0]);
     37:     l    = 1;
     38:     for (ll = 1; ll < i__2; ll++) {
     39:       tmp = PetscAbsScalar(aa[ll]);
     40:       if (tmp > max) {
     41:         max = tmp;
     42:         l   = ll + 1;
     43:       }
     44:     }
     45:     l += k - 1;
     46:     ipvt[k - 1] = l;

     48:     if (a[l + k3] == 0.0) {
     49:       if (shift == 0.0) {
     50:         PetscCheck(allowzeropivot, PETSC_COMM_SELF, PETSC_ERR_MAT_LU_ZRPVT, "Zero pivot, row %" PetscInt_FMT, k - 1);
     51:         PetscCall(PetscInfo(NULL, "Zero pivot, row %" PetscInt_FMT "\n", k - 1));
     52:         *zeropivotdetected = PETSC_TRUE;
     53:       } else {
     54:         /* SHIFT is applied to SINGLE diagonal entry; does this make any sense? */
     55:         a[l + k3] = shift;
     56:       }
     57:     }

     59:     /* interchange if necessary */
     60:     if (l != k) {
     61:       stmp      = a[l + k3];
     62:       a[l + k3] = a[k4];
     63:       a[k4]     = stmp;
     64:     }

     66:     /* compute multipliers */
     67:     stmp = -1. / a[k4];
     68:     i__2 = 5 - k;
     69:     aa   = &a[1 + k4];
     70:     for (ll = 0; ll < i__2; ll++) aa[ll] *= stmp;

     72:     /* row elimination with column indexing */
     73:     ax = &a[k4 + 1];
     74:     for (j = kp1; j <= 5; ++j) {
     75:       j3   = 5 * j;
     76:       stmp = a[l + j3];
     77:       if (l != k) {
     78:         a[l + j3] = a[k + j3];
     79:         a[k + j3] = stmp;
     80:       }

     82:       i__3 = 5 - k;
     83:       ay   = &a[1 + k + j3];
     84:       for (ll = 0; ll < i__3; ll++) ay[ll] += stmp * ax[ll];
     85:     }
     86:   }
     87:   ipvt[4] = 5;
     88:   if (a[30] == 0.0) {
     89:     PetscCheck(PetscLikely(allowzeropivot), PETSC_COMM_SELF, PETSC_ERR_MAT_LU_ZRPVT, "Zero pivot, row 4");
     90:     PetscCall(PetscInfo(NULL, "Zero pivot, row 4\n"));
     91:     *zeropivotdetected = PETSC_TRUE;
     92:   }

     94:   /* Now form the inverse */
     95:   /* compute inverse(u) */
     96:   for (k = 1; k <= 5; ++k) {
     97:     k3    = 5 * k;
     98:     k4    = k3 + k;
     99:     a[k4] = 1.0 / a[k4];
    100:     stmp  = -a[k4];
    101:     i__2  = k - 1;
    102:     aa    = &a[k3 + 1];
    103:     for (ll = 0; ll < i__2; ll++) aa[ll] *= stmp;
    104:     kp1 = k + 1;
    105:     if (5 < kp1) continue;
    106:     ax = aa;
    107:     for (j = kp1; j <= 5; ++j) {
    108:       j3        = 5 * j;
    109:       stmp      = a[k + j3];
    110:       a[k + j3] = 0.0;
    111:       ay        = &a[j3 + 1];
    112:       for (ll = 0; ll < k; ll++) ay[ll] += stmp * ax[ll];
    113:     }
    114:   }

    116:   /* form inverse(u)*inverse(l) */
    117:   for (kb = 1; kb <= 4; ++kb) {
    118:     k   = 5 - kb;
    119:     k3  = 5 * k;
    120:     kp1 = k + 1;
    121:     aa  = a + k3;
    122:     for (i = kp1; i <= 5; ++i) {
    123:       work[i - 1] = aa[i];
    124:       aa[i]       = 0.0;
    125:     }
    126:     for (j = kp1; j <= 5; ++j) {
    127:       stmp = work[j - 1];
    128:       ax   = &a[5 * j + 1];
    129:       ay   = &a[k3 + 1];
    130:       ay[0] += stmp * ax[0];
    131:       ay[1] += stmp * ax[1];
    132:       ay[2] += stmp * ax[2];
    133:       ay[3] += stmp * ax[3];
    134:       ay[4] += stmp * ax[4];
    135:     }
    136:     l = ipvt[k - 1];
    137:     if (l != k) {
    138:       ax    = &a[k3 + 1];
    139:       ay    = &a[5 * l + 1];
    140:       stmp  = ax[0];
    141:       ax[0] = ay[0];
    142:       ay[0] = stmp;
    143:       stmp  = ax[1];
    144:       ax[1] = ay[1];
    145:       ay[1] = stmp;
    146:       stmp  = ax[2];
    147:       ax[2] = ay[2];
    148:       ay[2] = stmp;
    149:       stmp  = ax[3];
    150:       ax[3] = ay[3];
    151:       ay[3] = stmp;
    152:       stmp  = ax[4];
    153:       ax[4] = ay[4];
    154:       ay[4] = stmp;
    155:     }
    156:   }
    157:   PetscFunctionReturn(PETSC_SUCCESS);
    158: }