cs_spsolve.c
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```00001 #include "cs.h"
00002 /* solve Gx=b(:,k), where G is either upper (lo=0) or lower (lo=1) triangular */
00003 int cs_spsolve (cs *G, const cs *B, int k, int *xi, double *x, const int *pinv,
00004     int lo)
00005 {
00006     int j, J, p, q, px, top, n, *Gp, *Gi, *Bp, *Bi ;
00007     double *Gx, *Bx ;
00008     if (!CS_CSC (G) || !CS_CSC (B) || !xi || !x) return (-1) ;
00009     Gp = G->p ; Gi = G->i ; Gx = G->x ; n = G->n ;
00010     Bp = B->p ; Bi = B->i ; Bx = B->x ;
00011     top = cs_reach (G, B, k, xi, pinv) ;        /* xi[top..n-1]=Reach(B(:,k)) */
00012     for (p = top ; p < n ; p++) x [xi [p]] = 0 ;    /* clear x */
00013     for (p = Bp [k] ; p < Bp [k+1] ; p++) x [Bi [p]] = Bx [p] ; /* scatter B */
00014     for (px = top ; px < n ; px++)
00015     {
00016         j = xi [px] ;                               /* x(j) is nonzero */
00017         J = pinv ? (pinv [j]) : j ;                 /* j maps to col J of G */
00018         if (J < 0) continue ;                       /* column J is empty */
00019         x [j] /= Gx [lo ? (Gp [J]) : (Gp [J+1]-1)] ;/* x(j) /= G(j,j) */
00020         p = lo ? (Gp [J]+1) : (Gp [J]) ;            /* lo: L(j,j) 1st entry */
00021         q = lo ? (Gp [J+1]) : (Gp [J+1]-1) ;        /* up: U(j,j) last entry */
00022         for ( ; p < q ; p++)
00023         {
00024             x [Gi [p]] -= Gx [p] * x [j] ;          /* x(i) -= G(i,j) * x(j) */
00025         }
00026     }