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

/* Subroutine */ int spttrf_(integer *n, real *d__, real *e, integer *info)
{
/*  -- LAPACK routine (version 3.1) --   
       Univ. of Tennessee, Univ. of California Berkeley and NAG Ltd..   
       November 2006   


    Purpose   
    =======   

    SPTTRF computes the L*D*L' factorization of a real symmetric   
    positive definite tridiagonal matrix A.  The factorization may also   
    be regarded as having the form A = U'*D*U.   

    Arguments   
    =========   

    N       (input) INTEGER   
            The order of the matrix A.  N >= 0.   

    D       (input/output) REAL array, dimension (N)   
            On entry, the n diagonal elements of the tridiagonal matrix   
            A.  On exit, the n diagonal elements of the diagonal matrix   
            D from the L*D*L' factorization of A.   

    E       (input/output) REAL array, dimension (N-1)   
            On entry, the (n-1) subdiagonal elements of the tridiagonal   
            matrix A.  On exit, the (n-1) subdiagonal elements of the   
            unit bidiagonal factor L from the L*D*L' factorization of A.   
            E can also be regarded as the superdiagonal of the unit   
            bidiagonal factor U from the U'*D*U factorization of A.   

    INFO    (output) INTEGER   
            = 0: successful exit   
            < 0: if INFO = -k, the k-th argument had an illegal value   
            > 0: if INFO = k, the leading minor of order k is not   
                 positive definite; if k < N, the factorization could not   
                 be completed, while if k = N, the factorization was   
                 completed, but D(N) <= 0.   

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


       Test the input parameters.   

       Parameter adjustments */
    /* System generated locals */
    integer i__1;
    /* Local variables */
    static integer i__, i4;
    static real ei;
    extern /* Subroutine */ int xerbla_(char *, integer *);

    --e;
    --d__;

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

/*     Quick return if possible */

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

/*     Compute the L*D*L' (or U'*D*U) factorization of A. */

    i4 = (*n - 1) % 4;
    i__1 = i4;
    for (i__ = 1; i__ <= i__1; ++i__) {
	if (d__[i__] <= 0.f) {
	    *info = i__;
	    goto L30;
	}
	ei = e[i__];
	e[i__] = ei / d__[i__];
	d__[i__ + 1] -= e[i__] * ei;
/* L10: */
    }

    i__1 = *n - 4;
    for (i__ = i4 + 1; i__ <= i__1; i__ += 4) {

/*        Drop out of the loop if d(i) <= 0: the matrix is not positive   
          definite. */

	if (d__[i__] <= 0.f) {
	    *info = i__;
	    goto L30;
	}

/*        Solve for e(i) and d(i+1). */

	ei = e[i__];
	e[i__] = ei / d__[i__];
	d__[i__ + 1] -= e[i__] * ei;

	if (d__[i__ + 1] <= 0.f) {
	    *info = i__ + 1;
	    goto L30;
	}

/*        Solve for e(i+1) and d(i+2). */

	ei = e[i__ + 1];
	e[i__ + 1] = ei / d__[i__ + 1];
	d__[i__ + 2] -= e[i__ + 1] * ei;

	if (d__[i__ + 2] <= 0.f) {
	    *info = i__ + 2;
	    goto L30;
	}

/*        Solve for e(i+2) and d(i+3). */

	ei = e[i__ + 2];
	e[i__ + 2] = ei / d__[i__ + 2];
	d__[i__ + 3] -= e[i__ + 2] * ei;

	if (d__[i__ + 3] <= 0.f) {
	    *info = i__ + 3;
	    goto L30;
	}

/*        Solve for e(i+3) and d(i+4). */

	ei = e[i__ + 3];
	e[i__ + 3] = ei / d__[i__ + 3];
	d__[i__ + 4] -= e[i__ + 3] * ei;
/* L20: */
    }

/*     Check d(n) for positive definiteness. */

    if (d__[*n] <= 0.f) {
	*info = *n;
    }

L30:
    return 0;

/*     End of SPTTRF */

} /* spttrf_ */