// CSparse/Source/cs_spsolve: solve x=L\b or U\b where x and b are both sparse
// CSparse, Copyright (c) 2006-2022, Timothy A. Davis. All Rights Reserved.
// SPDX-License-Identifier: LGPL-2.1+
#include "cs.h"
/* solve Gx=b(:,k), where G is either upper (lo=0) or lower (lo=1) triangular */
csi cs_spsolve (cs *G, const cs *B, csi k, csi *xi, double *x, const csi *pinv,
    csi lo)
{
    csi j, J, p, q, px, top, n, *Gp, *Gi, *Bp, *Bi ;
    double *Gx, *Bx ;
    if (!CS_CSC (G) || !CS_CSC (B) || !xi || !x) return (-1) ;
    Gp = G->p ; Gi = G->i ; Gx = G->x ; n = G->n ;
    Bp = B->p ; Bi = B->i ; Bx = B->x ;
    top = cs_reach (G, B, k, xi, pinv) ;        /* xi[top..n-1]=Reach(B(:,k)) */
    for (p = top ; p < n ; p++) x [xi [p]] = 0 ;    /* clear x */
    for (p = Bp [k] ; p < Bp [k+1] ; p++) x [Bi [p]] = Bx [p] ; /* scatter B */
    for (px = top ; px < n ; px++)
    {
        j = xi [px] ;                               /* x(j) is nonzero */
        J = pinv ? (pinv [j]) : j ;                 /* j maps to col J of G */
        if (J < 0) continue ;                       /* column J is empty */
        x [j] /= Gx [lo ? (Gp [J]) : (Gp [J+1]-1)] ;/* x(j) /= G(j,j) */
        p = lo ? (Gp [J]+1) : (Gp [J]) ;            /* lo: L(j,j) 1st entry */
        q = lo ? (Gp [J+1]) : (Gp [J+1]-1) ;        /* up: U(j,j) last entry */
        for ( ; p < q ; p++)
        {
            x [Gi [p]] -= Gx [p] * x [j] ;          /* x(i) -= G(i,j) * x(j) */
        }
    }
    return (top) ;                                  /* return top of stack */
}