#include "blaswrap.h"
#include "f2c.h"

/* Subroutine */ int dgttrf_(integer *n, doublereal *dl, doublereal *d__, 
	doublereal *du, doublereal *du2, integer *ipiv, integer *info)
{
/*  -- LAPACK routine (version 3.1) --   
       Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..   
       November 2006   


    Purpose   
    =======   

    DGTTRF computes an LU factorization of a real tridiagonal matrix A   
    using elimination with partial pivoting and row interchanges.   

    The factorization has the form   
       A = L * U   
    where L is a product of permutation and unit lower bidiagonal   
    matrices and U is upper triangular with nonzeros in only the main   
    diagonal and first two superdiagonals.   

    Arguments   
    =========   

    N       (input) INTEGER   
            The order of the matrix A.   

    DL      (input/output) DOUBLE PRECISION array, dimension (N-1)   
            On entry, DL must contain the (n-1) sub-diagonal elements of   
            A.   

            On exit, DL is overwritten by the (n-1) multipliers that   
            define the matrix L from the LU factorization of A.   

    D       (input/output) DOUBLE PRECISION array, dimension (N)   
            On entry, D must contain the diagonal elements of A.   

            On exit, D is overwritten by the n diagonal elements of the   
            upper triangular matrix U from the LU factorization of A.   

    DU      (input/output) DOUBLE PRECISION array, dimension (N-1)   
            On entry, DU must contain the (n-1) super-diagonal elements   
            of A.   

            On exit, DU is overwritten by the (n-1) elements of the first   
            super-diagonal of U.   

    DU2     (output) DOUBLE PRECISION array, dimension (N-2)   
            On exit, DU2 is overwritten by the (n-2) elements of the   
            second super-diagonal of U.   

    IPIV    (output) INTEGER array, dimension (N)   
            The pivot indices; for 1 <= i <= n, row i of the matrix was   
            interchanged with row IPIV(i).  IPIV(i) will always be either   
            i or i+1; IPIV(i) = i indicates a row interchange was not   
            required.   

    INFO    (output) INTEGER   
            = 0:  successful exit   
            < 0:  if INFO = -k, the k-th argument had an illegal value   
            > 0:  if INFO = k, U(k,k) is exactly zero. The factorization   
                  has been completed, but the factor U is exactly   
                  singular, and division by zero will occur if it is used   
                  to solve a system of equations.   

    =====================================================================   


       Parameter adjustments */
    /* System generated locals */
    integer i__1;
    doublereal d__1, d__2;
    /* Local variables */
    static integer i__;
    static doublereal fact, temp;
    extern /* Subroutine */ int xerbla_(char *, integer *);

    --ipiv;
    --du2;
    --du;
    --d__;
    --dl;

    /* Function Body */
    *info = 0;
    if (*n < 0) {
	*info = -1;
	i__1 = -(*info);
	xerbla_("DGTTRF", &i__1);
	return 0;
    }

/*     Quick return if possible */

    if (*n == 0) {
	return 0;
    }

/*     Initialize IPIV(i) = i and DU2(I) = 0 */

    i__1 = *n;
    for (i__ = 1; i__ <= i__1; ++i__) {
	ipiv[i__] = i__;
/* L10: */
    }
    i__1 = *n - 2;
    for (i__ = 1; i__ <= i__1; ++i__) {
	du2[i__] = 0.;
/* L20: */
    }

    i__1 = *n - 2;
    for (i__ = 1; i__ <= i__1; ++i__) {
	if ((d__1 = d__[i__], abs(d__1)) >= (d__2 = dl[i__], abs(d__2))) {

/*           No row interchange required, eliminate DL(I) */

	    if (d__[i__] != 0.) {
		fact = dl[i__] / d__[i__];
		dl[i__] = fact;
		d__[i__ + 1] -= fact * du[i__];
	    }
	} else {

/*           Interchange rows I and I+1, eliminate DL(I) */

	    fact = d__[i__] / dl[i__];
	    d__[i__] = dl[i__];
	    dl[i__] = fact;
	    temp = du[i__];
	    du[i__] = d__[i__ + 1];
	    d__[i__ + 1] = temp - fact * d__[i__ + 1];
	    du2[i__] = du[i__ + 1];
	    du[i__ + 1] = -fact * du[i__ + 1];
	    ipiv[i__] = i__ + 1;
	}
/* L30: */
    }
    if (*n > 1) {
	i__ = *n - 1;
	if ((d__1 = d__[i__], abs(d__1)) >= (d__2 = dl[i__], abs(d__2))) {
	    if (d__[i__] != 0.) {
		fact = dl[i__] / d__[i__];
		dl[i__] = fact;
		d__[i__ + 1] -= fact * du[i__];
	    }
	} else {
	    fact = d__[i__] / dl[i__];
	    d__[i__] = dl[i__];
	    dl[i__] = fact;
	    temp = du[i__];
	    du[i__] = d__[i__ + 1];
	    d__[i__ + 1] = temp - fact * d__[i__ + 1];
	    ipiv[i__] = i__ + 1;
	}
    }

/*     Check for a zero on the diagonal of U. */

    i__1 = *n;
    for (i__ = 1; i__ <= i__1; ++i__) {
	if (d__[i__] == 0.) {
	    *info = i__;
	    goto L50;
	}
/* L40: */
    }
L50:

    return 0;

/*     End of DGTTRF */

} /* dgttrf_ */