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#include "cs.h"
/* compute the etree of A (using triu(A), or A'A without forming A'A */
int *cs_etree (const cs *A, int ata)
{
int i, k, p, m, n, inext, *Ap, *Ai, *w, *parent, *ancestor, *prev ;
if (!CS_CSC (A)) return (NULL) ; /* check inputs */
m = A->m ; n = A->n ; Ap = A->p ; Ai = A->i ;
parent = cs_malloc (n, sizeof (int)) ; /* allocate result */
w = cs_malloc (n + (ata ? m : 0), sizeof (int)) ; /* get workspace */
if (!w || !parent) return (cs_idone (parent, NULL, w, 0)) ;
ancestor = w ; prev = w + n ;
if (ata) for (i = 0 ; i < m ; i++) prev [i] = -1 ;
for (k = 0 ; k < n ; k++)
{
parent [k] = -1 ; /* node k has no parent yet */
ancestor [k] = -1 ; /* nor does k have an ancestor */
for (p = Ap [k] ; p < Ap [k+1] ; p++)
{
i = ata ? (prev [Ai [p]]) : (Ai [p]) ;
for ( ; i != -1 && i < k ; i = inext) /* traverse from i to k */
{
inext = ancestor [i] ; /* inext = ancestor of i */
ancestor [i] = k ; /* path compression */
if (inext == -1) parent [i] = k ; /* no anc., parent is k */
}
if (ata) prev [Ai [p]] = k ;
}
}
return (cs_idone (parent, NULL, w, 1)) ;
}
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