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            /* Sparse matrices in compressed column-oriented form */
#include "Csparse.h"
#include "Tsparse.h"
#include "chm_common.h"

SEXP Csparse_validate(SEXP x)
{
    /* NB: we do *NOT* check a potential 'x' slot here, at all */
    SEXP pslot = GET_SLOT(x, Matrix_pSym),
    islot = GET_SLOT(x, Matrix_iSym);
    Rboolean sorted, strictly;
    int j, k,
    *dims = INTEGER(GET_SLOT(x, Matrix_DimSym)),
    nrow = dims[0],
    ncol = dims[1],
    *xp = INTEGER(pslot),
    *xi = INTEGER(islot);

    if (length(pslot) != dims[1] + 1)
    return mkString(_("slot p must have length = ncol(.) + 1"));
    if (xp[0] != 0)
    return mkString(_("first element of slot p must be zero"));
    if (length(islot) < xp[ncol]) /* allow larger slots from over-allocation!*/
    return
        mkString(_("last element of slot p must match length of slots i and x"));
    for (j = 0; j < length(islot); j++) {
    if (xi[j] < 0 || xi[j] >= nrow)
        return mkString(_("all row indices must be between 0 and nrow-1"));
    }
    sorted = TRUE; strictly = TRUE;
    for (j = 0; j < ncol; j++) {
    if (xp[j] > xp[j+1])
        return mkString(_("slot p must be non-decreasing"));
    if(sorted)
        for (k = xp[j] + 1; k < xp[j + 1]; k++) {
        if (xi[k] < xi[k - 1])
            sorted = FALSE;
        else if (xi[k] == xi[k - 1])
            strictly = FALSE;
        }
    }
    if (!sorted) {
    CHM_SP chx = AS_CHM_SP(x);
    R_CheckStack();

    cholmod_sort(chx, &c);
    /* Now re-check that row indices are *strictly* increasing
     * (and not just increasing) within each column : */
    for (j = 0; j < ncol; j++) {
        for (k = xp[j] + 1; k < xp[j + 1]; k++)
        if (xi[k] == xi[k - 1])
            return mkString(_("slot i is not *strictly* increasing inside a column (even after cholmod_sort)"));
    }

    } else if(!strictly) {  /* sorted, but not strictly */
    return mkString(_("slot i is not *strictly* increasing inside a column"));
    }
    return ScalarLogical(1);
}

SEXP Rsparse_validate(SEXP x)
{
    /* NB: we do *NOT* check a potential 'x' slot here, at all */
    SEXP pslot = GET_SLOT(x, Matrix_pSym),
    jslot = GET_SLOT(x, Matrix_jSym);
    Rboolean sorted, strictly;
    int i, k,
    *dims = INTEGER(GET_SLOT(x, Matrix_DimSym)),
    nrow = dims[0],
    ncol = dims[1],
    *xp = INTEGER(pslot),
    *xj = INTEGER(jslot);

    if (length(pslot) != dims[0] + 1)
    return mkString(_("slot p must have length = nrow(.) + 1"));
    if (xp[0] != 0)
    return mkString(_("first element of slot p must be zero"));
    if (length(jslot) < xp[nrow]) /* allow larger slots from over-allocation!*/
    return
        mkString(_("last element of slot p must match length of slots j and x"));
    for (i = 0; i < length(jslot); i++) {
    if (xj[i] < 0 || xj[i] >= ncol)
        return mkString(_("all column indices must be between 0 and ncol-1"));
    }
    sorted = TRUE; strictly = TRUE;
    for (i = 0; i < nrow; i++) {
    if (xp[i] > xp[i+1])
        return mkString(_("slot p must be non-decreasing"));
    if(sorted)
        for (k = xp[i] + 1; k < xp[i + 1]; k++) {
        if (xj[k] < xj[k - 1])
            sorted = FALSE;
        else if (xj[k] == xj[k - 1])
            strictly = FALSE;
        }
    }
    if (!sorted)
    /* cannot easily use cholmod_sort(.) ... -> "error out" :*/
    return mkString(_("slot j is not increasing inside a column"));
    else if(!strictly) /* sorted, but not strictly */
    return mkString(_("slot j is not *strictly* increasing inside a column"));

    return ScalarLogical(1);
}


/* Called from ../R/Csparse.R : */
/* Can only return [dln]geMatrix (no symm/triang);
 * FIXME: replace by non-CHOLMOD code ! */
SEXP Csparse_to_dense(SEXP x)
{
    CHM_SP chxs = AS_CHM_SP(x);
    /* This loses the symmetry property, since cholmod_dense has none,
     * BUT, much worse (FIXME!), it also transforms CHOLMOD_PATTERN ("n") matrices
     * to numeric (CHOLMOD_REAL) ones : */
    CHM_DN chxd = cholmod_sparse_to_dense(chxs, &c);
    int Rkind = (chxs->xtype == CHOLMOD_PATTERN)? -1 : Real_kind(x);
    R_CheckStack();

    return chm_dense_to_SEXP(chxd, 1, Rkind, GET_SLOT(x, Matrix_DimNamesSym));
}

SEXP Csparse_to_nz_pattern(SEXP x, SEXP tri)
{
    CHM_SP chxs = AS_CHM_SP(x);
    CHM_SP chxcp = cholmod_copy(chxs, chxs->stype, CHOLMOD_PATTERN, &c);
    int tr = asLogical(tri);
    R_CheckStack();

    return chm_sparse_to_SEXP(chxcp, 1/*do_free*/,
                  tr ? ((*uplo_P(x) == 'U') ? 1 : -1) : 0,
                  0, tr ? diag_P(x) : "",
                  GET_SLOT(x, Matrix_DimNamesSym));
}

SEXP Csparse_to_matrix(SEXP x)
{
    return chm_dense_to_matrix(cholmod_sparse_to_dense(AS_CHM_SP(x), &c),
                   1 /*do_free*/, GET_SLOT(x, Matrix_DimNamesSym));
}

SEXP Csparse_to_Tsparse(SEXP x, SEXP tri)
{
    CHM_SP chxs = AS_CHM_SP(x);
    CHM_TR chxt = cholmod_sparse_to_triplet(chxs, &c);
    int tr = asLogical(tri);
    int Rkind = (chxs->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    R_CheckStack();

    return chm_triplet_to_SEXP(chxt, 1,
                   tr ? ((*uplo_P(x) == 'U') ? 1 : -1) : 0,
                   Rkind, tr ? diag_P(x) : "",
                   GET_SLOT(x, Matrix_DimNamesSym));
}

/* this used to be called  sCMatrix_to_gCMatrix(..)   [in ./dsCMatrix.c ]: */
SEXP Csparse_symmetric_to_general(SEXP x)
{
    CHM_SP chx = AS_CHM_SP(x), chgx;
    int Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    R_CheckStack();

    if (!(chx->stype))
    error(_("Nonsymmetric matrix in Csparse_symmetric_to_general"));
    chgx = cholmod_copy(chx, /* stype: */ 0, chx->xtype, &c);
    /* xtype: pattern, "real", complex or .. */
    return chm_sparse_to_SEXP(chgx, 1, 0, Rkind, "",
                  GET_SLOT(x, Matrix_DimNamesSym));
}

SEXP Csparse_general_to_symmetric(SEXP x, SEXP uplo)
{
    CHM_SP chx = AS_CHM_SP(x), chgx;
    int uploT = (*CHAR(STRING_ELT(uplo,0)) == 'U') ? 1 : -1;
    int Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    R_CheckStack();

    chgx = cholmod_copy(chx, /* stype: */ uploT, chx->xtype, &c);
    /* xtype: pattern, "real", complex or .. */
    return chm_sparse_to_SEXP(chgx, 1, 0, Rkind, "",
                  GET_SLOT(x, Matrix_DimNamesSym));
}

SEXP Csparse_transpose(SEXP x, SEXP tri)
{
    /* TODO: lgCMatrix & igC* currently go via double prec. cholmod -
     *       since cholmod (& cs) lacks sparse 'int' matrices */
    CHM_SP chx = AS_CHM_SP(x);
    int Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    CHM_SP chxt = cholmod_transpose(chx, chx->xtype, &c);
    SEXP dn = PROTECT(duplicate(GET_SLOT(x, Matrix_DimNamesSym))), tmp;
    int tr = asLogical(tri);
    R_CheckStack();

    tmp = VECTOR_ELT(dn, 0);    /* swap the dimnames */
    SET_VECTOR_ELT(dn, 0, VECTOR_ELT(dn, 1));
    SET_VECTOR_ELT(dn, 1, tmp);
    UNPROTECT(1);
    return chm_sparse_to_SEXP(chxt, 1, /* SWAP 'uplo' for triangular */
                  tr ? ((*uplo_P(x) == 'U') ? -1 : 1) : 0,
                  Rkind, tr ? diag_P(x) : "", dn);
}

SEXP Csparse_Csparse_prod(SEXP a, SEXP b)
{
    CHM_SP
    cha = AS_CHM_SP(Csparse_diagU2N(a)),
    chb = AS_CHM_SP(Csparse_diagU2N(b)),
    chc = cholmod_ssmult(cha, chb, /*out_stype:*/ 0,
                 cha->xtype, /*out sorted:*/ 1, &c);
    const char *cl_a = class_P(a), *cl_b = class_P(b);
    char diag[] = {'\0', '\0'};
    int uploT = 0;
    SEXP dn = allocVector(VECSXP, 2);
    R_CheckStack();

    /* Preserve triangularity and even unit-triangularity if appropriate.
     * Note that in that case, the multiplication itself should happen
     * faster.  But there's no support for that in CHOLMOD */

    /* UGLY hack -- rather should have (fast!) C-level version of
     *       is(a, "triangularMatrix") etc */
    if (cl_a[1] == 't' && cl_b[1] == 't')
    /* FIXME: fails for "Cholesky","BunchKaufmann"..*/
    if(*uplo_P(a) == *uplo_P(b)) { /* both upper, or both lower tri. */
        uploT = (*uplo_P(a) == 'U') ? 1 : -1;
        if(*diag_P(a) == 'U' && *diag_P(b) == 'U') { /* return UNIT-triag. */
        /* "remove the diagonal entries": */
        chm_diagN2U(chc, uploT, /* do_realloc */ FALSE);
        diag[0]= 'U';
        }
        else diag[0]= 'N';
    }
    SET_VECTOR_ELT(dn, 0,   /* establish dimnames */
           duplicate(VECTOR_ELT(GET_SLOT(a, Matrix_DimNamesSym), 0)));
    SET_VECTOR_ELT(dn, 1,
           duplicate(VECTOR_ELT(GET_SLOT(b, Matrix_DimNamesSym), 1)));
    return chm_sparse_to_SEXP(chc, 1, uploT, /*Rkind*/0, diag, dn);
}

SEXP Csparse_Csparse_crossprod(SEXP a, SEXP b, SEXP trans)
{
    int tr = asLogical(trans);
    CHM_SP
    cha = AS_CHM_SP(Csparse_diagU2N(a)),
    chb = AS_CHM_SP(Csparse_diagU2N(b)),
    chTr, chc;
    const char *cl_a = class_P(a), *cl_b = class_P(b);
    char diag[] = {'\0', '\0'};
    int uploT = 0;
    SEXP dn = allocVector(VECSXP, 2);
    R_CheckStack();

    chTr = cholmod_transpose((tr) ? chb : cha, chb->xtype, &c);
    chc = cholmod_ssmult((tr) ? cha : chTr, (tr) ? chTr : chb,
             /*out_stype:*/ 0, cha->xtype, /*out sorted:*/ 1, &c);
    cholmod_free_sparse(&chTr, &c);

    /* Preserve triangularity and unit-triangularity if appropriate;
     * see Csparse_Csparse_prod() for comments */
    if (cl_a[1] == 't' && cl_b[1] == 't')
    if(*uplo_P(a) != *uplo_P(b)) { /* one 'U', the other 'L' */
        uploT = (*uplo_P(b) == 'U') ? 1 : -1;
        if(*diag_P(a) == 'U' && *diag_P(b) == 'U') { /* return UNIT-triag. */
        chm_diagN2U(chc, uploT, /* do_realloc */ FALSE);
        diag[0]= 'U';
        }
        else diag[0]= 'N';
    }

    SET_VECTOR_ELT(dn, 0,   /* establish dimnames */
           duplicate(VECTOR_ELT(GET_SLOT(a, Matrix_DimNamesSym), (tr) ? 0 : 1)));
    SET_VECTOR_ELT(dn, 1,
           duplicate(VECTOR_ELT(GET_SLOT(b, Matrix_DimNamesSym), (tr) ? 0 : 1)));
    return chm_sparse_to_SEXP(chc, 1, uploT, /*Rkind*/0, diag, dn);
}

SEXP Csparse_dense_prod(SEXP a, SEXP b)
{
    CHM_SP cha = AS_CHM_SP(Csparse_diagU2N(a));
    SEXP b_M = PROTECT(mMatrix_as_dgeMatrix(b));
    CHM_DN chb = AS_CHM_DN(b_M);
    CHM_DN chc = cholmod_allocate_dense(cha->nrow, chb->ncol, cha->nrow,
                    chb->xtype, &c);
    SEXP dn = PROTECT(allocVector(VECSXP, 2));
    double one[] = {1,0}, zero[] = {0,0};
    R_CheckStack();

    cholmod_sdmult(cha, 0, one, zero, chb, chc, &c);
    SET_VECTOR_ELT(dn, 0,   /* establish dimnames */
           duplicate(VECTOR_ELT(GET_SLOT(a, Matrix_DimNamesSym), 0)));
    SET_VECTOR_ELT(dn, 1,
           duplicate(VECTOR_ELT(GET_SLOT(b_M, Matrix_DimNamesSym), 1)));
    UNPROTECT(2);
    return chm_dense_to_SEXP(chc, 1, 0, dn);
}

SEXP Csparse_dense_crossprod(SEXP a, SEXP b)
{
    CHM_SP cha = AS_CHM_SP(Csparse_diagU2N(a));
    SEXP b_M = PROTECT(mMatrix_as_dgeMatrix(b));
    CHM_DN chb = AS_CHM_DN(b_M);
    CHM_DN chc = cholmod_allocate_dense(cha->ncol, chb->ncol, cha->ncol,
                    chb->xtype, &c);
    SEXP dn = PROTECT(allocVector(VECSXP, 2));
    double one[] = {1,0}, zero[] = {0,0};
    R_CheckStack();

    cholmod_sdmult(cha, 1, one, zero, chb, chc, &c);
    SET_VECTOR_ELT(dn, 0,   /* establish dimnames */
           duplicate(VECTOR_ELT(GET_SLOT(a, Matrix_DimNamesSym), 1)));
    SET_VECTOR_ELT(dn, 1,
           duplicate(VECTOR_ELT(GET_SLOT(b_M, Matrix_DimNamesSym), 1)));
    UNPROTECT(2);
    return chm_dense_to_SEXP(chc, 1, 0, dn);
}

/* Computes   x'x  or  x x' -- *also* for Tsparse (triplet = TRUE)
   see Csparse_Csparse_crossprod above for  x'y and x y' */
SEXP Csparse_crossprod(SEXP x, SEXP trans, SEXP triplet)
{
    int trip = asLogical(triplet),
    tr   = asLogical(trans); /* gets reversed because _aat is tcrossprod */
    CHM_TR cht = trip ? AS_CHM_TR(Tsparse_diagU2N(x)) : (CHM_TR) NULL;
    CHM_SP chcp, chxt,
    chx = (trip ?
           cholmod_triplet_to_sparse(cht, cht->nnz, &c) :
           AS_CHM_SP(Csparse_diagU2N(x)));
    SEXP dn = PROTECT(allocVector(VECSXP, 2));
    R_CheckStack();

    if (!tr) chxt = cholmod_transpose(chx, chx->xtype, &c);
    chcp = cholmod_aat((!tr) ? chxt : chx, (int *) NULL, 0, chx->xtype, &c);
    if(!chcp) {
    UNPROTECT(1);
    error(_("Csparse_crossprod(): error return from cholmod_aat()"));
    }
    cholmod_band_inplace(0, chcp->ncol, chcp->xtype, chcp, &c);
    chcp->stype = 1;
    if (trip) cholmod_free_sparse(&chx, &c);
    if (!tr) cholmod_free_sparse(&chxt, &c);
    SET_VECTOR_ELT(dn, 0,   /* establish dimnames */
           duplicate(VECTOR_ELT(GET_SLOT(x, Matrix_DimNamesSym),
                    (tr) ? 0 : 1)));
    SET_VECTOR_ELT(dn, 1, duplicate(VECTOR_ELT(dn, 0)));
    UNPROTECT(1);
    return chm_sparse_to_SEXP(chcp, 1, 0, 0, "", dn);
}

SEXP Csparse_drop(SEXP x, SEXP tol)
{
    CHM_SP chx = AS_CHM_SP(x);
    CHM_SP ans = cholmod_copy(chx, chx->stype, chx->xtype, &c);
    double dtol = asReal(tol);
    int Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    R_CheckStack();

    if(!cholmod_drop(dtol, ans, &c))
    error(_("cholmod_drop() failed"));
    return chm_sparse_to_SEXP(ans, 1, 0, Rkind, "",
                  GET_SLOT(x, Matrix_DimNamesSym));
}

SEXP Csparse_horzcat(SEXP x, SEXP y)
{
    CHM_SP chx = AS_CHM_SP(x), chy = AS_CHM_SP(y);
    int Rkind = 0; /* only for "d" - FIXME */
    R_CheckStack();

    /* FIXME: currently drops dimnames */
    return chm_sparse_to_SEXP(cholmod_horzcat(chx, chy, 1, &c),
                  1, 0, Rkind, "", R_NilValue);
}

SEXP Csparse_vertcat(SEXP x, SEXP y)
{
    CHM_SP chx = AS_CHM_SP(x), chy = AS_CHM_SP(y);
    int Rkind = 0; /* only for "d" - FIXME */
    R_CheckStack();

    /* FIXME: currently drops dimnames */
    return chm_sparse_to_SEXP(cholmod_vertcat(chx, chy, 1, &c),
                  1, 0, Rkind, "", R_NilValue);
}

SEXP Csparse_band(SEXP x, SEXP k1, SEXP k2)
{
    CHM_SP chx = AS_CHM_SP(x);
    int Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    CHM_SP ans = cholmod_band(chx, asInteger(k1), asInteger(k2), chx->xtype, &c);
    R_CheckStack();

    return chm_sparse_to_SEXP(ans, 1, 0, Rkind, "",
                  GET_SLOT(x, Matrix_DimNamesSym));
}

SEXP Csparse_diagU2N(SEXP x)
{
    const char *cl = class_P(x);
    /* dtCMatrix, etc; [1] = the second character =?= 't' for triangular */
    if (cl[1] != 't' || *diag_P(x) != 'U') {
    /* "trivially fast" when not triangular (<==> no 'diag' slot),
       or not *unit* triangular */
    return (x);
    }
    else { /* unit triangular (diag='U'): "fill the diagonal" & diag:= "N" */
    CHM_SP chx = AS_CHM_SP(x);
    CHM_SP eye = cholmod_speye(chx->nrow, chx->ncol, chx->xtype, &c);
    double one[] = {1, 0};
    CHM_SP ans = cholmod_add(chx, eye, one, one, TRUE, TRUE, &c);
    int uploT = (*uplo_P(x) == 'U') ? 1 : -1;
    int Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;

    R_CheckStack();
    cholmod_free_sparse(&eye, &c);
    return chm_sparse_to_SEXP(ans, 1, uploT, Rkind, "N",
                  GET_SLOT(x, Matrix_DimNamesSym));
    }
}

SEXP Csparse_diagN2U(SEXP x)
{
    const char *cl = class_P(x);
    /* dtCMatrix, etc; [1] = the second character =?= 't' for triangular */
    if (cl[1] != 't' || *diag_P(x) != 'N') {
    /* "trivially fast" when not triangular (<==> no 'diag' slot),
       or already *unit* triangular */
    return (x);
    }
    else { /* triangular with diag='N'): now drop the diagonal */
    /* duplicate, since chx will be modified: */
    CHM_SP chx = AS_CHM_SP(duplicate(x));
    int uploT = (*uplo_P(x) == 'U') ? 1 : -1,
        Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    R_CheckStack();

    chm_diagN2U(chx, uploT, /* do_realloc */ FALSE);

    return chm_sparse_to_SEXP(chx, /*dofree*/ 0/* or 1 ?? */,
                  uploT, Rkind, "U",
                  GET_SLOT(x, Matrix_DimNamesSym));
    }
}

SEXP Csparse_submatrix(SEXP x, SEXP i, SEXP j)
{
    CHM_SP chx = AS_CHM_SP(x);
    int rsize = (isNull(i)) ? -1 : LENGTH(i),
    csize = (isNull(j)) ? -1 : LENGTH(j);
    int Rkind = (chx->xtype != CHOLMOD_PATTERN) ? Real_kind(x) : 0;
    R_CheckStack();

    if (rsize >= 0 && !isInteger(i))
    error(_("Index i must be NULL or integer"));
    if (csize >= 0 && !isInteger(j))
    error(_("Index j must be NULL or integer"));

    return chm_sparse_to_SEXP(cholmod_submatrix(chx, INTEGER(i), rsize,
                        INTEGER(j), csize,
                        TRUE, TRUE, &c),
                  1, 0, Rkind, "",
                  /* FIXME: drops dimnames */ R_NilValue);
}

SEXP Csparse_MatrixMarket(SEXP x, SEXP fname)
{
    FILE *f = fopen(CHAR(asChar(fname)), "w");

    if (!f)
    error(_("failure to open file \"%s\" for writing"),
          CHAR(asChar(fname)));
    if (!cholmod_write_sparse(f, AS_CHM_SP(Csparse_diagU2N(x)),
                  (CHM_SP)NULL, (char*) NULL, &c))
    error(_("cholmod_write_sparse returned error code"));
    fclose(f);
    return R_NilValue;
}


/**
 * Extract the diagonal entries from *triangular* Csparse matrix  __or__ a
 * cholmod_sparse factor (LDL = TRUE).
 *
 * @param n  dimension of the matrix.
 * @param x_p  'p' (column pointer) slot contents
 * @param x_x  'x' (non-zero entries) slot contents
 * @param perm 'perm' (= permutation vector) slot contents
 * @param resultKind a (SEXP) string indicating which kind of result is desired.
 *
 * @return  a SEXP, either a (double) number or a length n-vector of diagonal entries
 */
SEXP diag_tC_ptr(int n, int *x_p, double *x_x, int *perm, SEXP resultKind)
/*                                ^^^^^^ FIXME[Generalize] to int / ... */
{
    const char* res_ch = CHAR(STRING_ELT(resultKind,0));
    enum diag_kind { diag, diag_backpermuted, trace, prod, sum_log
    } res_kind = ((!strcmp(res_ch, "trace")) ? trace :
          ((!strcmp(res_ch, "sumLog")) ? sum_log :
           ((!strcmp(res_ch, "prod")) ? prod :
            ((!strcmp(res_ch, "diag")) ? diag :
             ((!strcmp(res_ch, "diagBack")) ? diag_backpermuted :
              -1)))));
    int i, n_x, i_from = 0;
    SEXP ans = PROTECT(allocVector(REALSXP,
/*                                 ^^^^  FIXME[Generalize] */
                   (res_kind == diag ||
                    res_kind == diag_backpermuted) ? n : 1));
    double *v = REAL(ans);
/*  ^^^^^^      ^^^^  FIXME[Generalize] */

#define for_DIAG(v_ASSIGN)                      \
    for(i = 0; i < n; i++, i_from += n_x) {             \
    /* looking at i-th column */                    \
    n_x = x_p[i+1] - x_p[i];/* #{entries} in this column */ \
    v_ASSIGN;                           \
    }

    /* NOTA BENE: we assume  -- uplo = "L" i.e. lower triangular matrix
     *            for uplo = "U" (makes sense with a "dtCMatrix" !),
     *            should use  x_x[i_from + (nx - 1)] instead of x_x[i_from],
     *            where nx = (x_p[i+1] - x_p[i])
     */

    switch(res_kind) {
    case trace:
    v[0] = 0.;
    for_DIAG(v[0] += x_x[i_from]);
    break;

    case sum_log:
    v[0] = 0.;
    for_DIAG(v[0] += log(x_x[i_from]));
    break;

    case prod:
    v[0] = 1.;
    for_DIAG(v[0] *= x_x[i_from]);
    break;

    case diag:
    for_DIAG(v[i] = x_x[i_from]);
    break;

    case diag_backpermuted:
    for_DIAG(v[i] = x_x[i_from]);
    /* now back_permute : */

    error(_("resultKind = 'diagBack' (back-permuted) is not yet implemented"));
    break;

    default: /* -1 from above */
    error("diag_tC(): invalid 'resultKind'");
    /* Wall: */ ans = R_NilValue; v = REAL(ans);
    }

    UNPROTECT(1);
    return ans;
}

/**
 * Extract the diagonal entries from *triangular* Csparse matrix  __or__ a
 * cholmod_sparse factor (LDL = TRUE).
 *
 * @param pslot  'p' (column pointer)   slot of Csparse matrix/factor
 * @param xslot  'x' (non-zero entries) slot of Csparse matrix/factor
 * @param perm_slot  'perm' (= permutation vector) slot of corresponding CHMfactor
 * @param resultKind a (SEXP) string indicating which kind of result is desired.
 *
 * @return  a SEXP, either a (double) number or a length n-vector of diagonal entries
 */
SEXP diag_tC(SEXP pslot, SEXP xslot, SEXP perm_slot, SEXP resultKind)
{
    int n = length(pslot) - 1, /* n = ncol(.) = nrow(.) */
    *x_p  = INTEGER(pslot),
    *perm = INTEGER(perm_slot);
    double *x_x = REAL(xslot);
/*  ^^^^^^        ^^^^ FIXME[Generalize] to INTEGER(.) / LOGICAL(.) / ... xslot !*/

    return diag_tC_ptr(n, x_p, x_x, perm, resultKind);
}