// CXSparse/Source/cs_qr: sparse QR factorization
// CXSparse, Copyright (c) 2006-2022, Timothy A. Davis. All Rights Reserved.
// SPDX-License-Identifier: LGPL-2.1+
#include "cs.h"
/* sparse QR factorization [V,beta,pinv,R] = qr (A) */
csn *cs_qr (const cs *A, const css *S)
{
    CS_ENTRY *Rx, *Vx, *Ax, *x ;
    double *Beta ;
    CS_INT i, k, p, n, vnz, p1, top, m2, len, col, rnz, *s, *leftmost, *Ap, *Ai,
        *parent, *Rp, *Ri, *Vp, *Vi, *w, *pinv, *q ;
    cs *R, *V ;
    csn *N ;
    if (!CS_CSC (A) || !S) return (NULL) ;
    n = A->n ; Ap = A->p ; Ai = A->i ; Ax = A->x ;
    q = S->q ; parent = S->parent ; pinv = S->pinv ; m2 = S->m2 ;
    vnz = S->lnz ; rnz = S->unz ; leftmost = S->leftmost ;
    w = cs_malloc (m2+n, sizeof (CS_INT)) ;            /* get CS_INT workspace */
    x = cs_malloc (m2, sizeof (CS_ENTRY)) ;           /* get CS_ENTRY workspace */
    N = cs_calloc (1, sizeof (csn)) ;               /* allocate result */
    if (!w || !x || !N) return (cs_ndone (N, NULL, w, x, 0)) ;
    s = w + m2 ;                                    /* s is size n */
    for (k = 0 ; k < m2 ; k++) x [k] = 0 ;          /* clear workspace x */
    N->L = V = cs_spalloc (m2, n, vnz, 1, 0) ;      /* allocate result V */
    N->U = R = cs_spalloc (m2, n, rnz, 1, 0) ;      /* allocate result R */
    N->B = Beta = cs_malloc (n, sizeof (double)) ;  /* allocate result Beta */
    if (!R || !V || !Beta) return (cs_ndone (N, NULL, w, x, 0)) ;
    Rp = R->p ; Ri = R->i ; Rx = R->x ;
    Vp = V->p ; Vi = V->i ; Vx = V->x ;
    for (i = 0 ; i < m2 ; i++) w [i] = -1 ; /* clear w, to mark nodes */
    rnz = 0 ; vnz = 0 ;
    for (k = 0 ; k < n ; k++)               /* compute V and R */
    {
        Rp [k] = rnz ;                      /* R(:,k) starts here */
        Vp [k] = p1 = vnz ;                 /* V(:,k) starts here */
        w [k] = k ;                         /* add V(k,k) to pattern of V */
        Vi [vnz++] = k ;
        top = n ;
        col = q ? q [k] : k ;
        for (p = Ap [col] ; p < Ap [col+1] ; p++)   /* find R(:,k) pattern */
        {
            i = leftmost [Ai [p]] ;         /* i = min(find(A(i,q))) */
            for (len = 0 ; w [i] != k ; i = parent [i]) /* traverse up to k */
            {
                s [len++] = i ;
                w [i] = k ;
            }
            while (len > 0) s [--top] = s [--len] ; /* push path on stack */
            i = pinv [Ai [p]] ;             /* i = permuted row of A(:,col) */
            x [i] = Ax [p] ;                /* x (i) = A(:,col) */
            if (i > k && w [i] < k)         /* pattern of V(:,k) = x (k+1:m) */
            {
                Vi [vnz++] = i ;            /* add i to pattern of V(:,k) */
                w [i] = k ;
            }
        }
        for (p = top ; p < n ; p++) /* for each i in pattern of R(:,k) */
        {
            i = s [p] ;                     /* R(i,k) is nonzero */
            cs_happly (V, i, Beta [i], x) ; /* apply (V(i),Beta(i)) to x */
            Ri [rnz] = i ;                  /* R(i,k) = x(i) */
            Rx [rnz++] = x [i] ;
            x [i] = 0 ;
            if (parent [i] == k) vnz = cs_scatter (V, i, 0, w, NULL, k, V, vnz);
        }
        for (p = p1 ; p < vnz ; p++)        /* gather V(:,k) = x */
        {
            Vx [p] = x [Vi [p]] ;
            x [Vi [p]] = 0 ;
        }
        Ri [rnz] = k ;                     /* R(k,k) = norm (x) */
        Rx [rnz++] = cs_house (Vx+p1, Beta+k, vnz-p1) ; /* [v,beta]=house(x) */
    }
    Rp [n] = rnz ;                          /* finalize R */
    Vp [n] = vnz ;                          /* finalize V */
    return (cs_ndone (N, NULL, w, x, 1)) ;  /* success */
}
