// CSparse/Source/cs_amd: approximate minimum degree // CSparse, Copyright (c) 2006-2022, Timothy A. Davis. All Rights Reserved. // SPDX-License-Identifier: LGPL-2.1+ #include "cs.h" /* clear w */ static csi cs_wclear (csi mark, csi lemax, csi *w, csi n) { csi k ; if (mark < 2 || (mark + lemax < 0)) { for (k = 0 ; k < n ; k++) if (w [k] != 0) w [k] = 1 ; mark = 2 ; } return (mark) ; /* at this point, w [0..n-1] < mark holds */ } /* keep off-diagonal entries; drop diagonal entries */ static csi cs_diag (csi i, csi j, double aij, void *other) { return (i != j) ; } /* p = amd(A+A') if symmetric is true, or amd(A'A) otherwise */ csi *cs_amd (csi order, const cs *A) /* order 0:natural, 1:Chol, 2:LU, 3:QR */ { cs *C, *A2, *AT ; csi *Cp, *Ci, *last, *W, *len, *nv, *next, *P, *head, *elen, *degree, *w, *hhead, *ATp, *ATi, d, dk, dext, lemax = 0, e, elenk, eln, i, j, k, k1, k2, k3, jlast, ln, dense, nzmax, mindeg = 0, nvi, nvj, nvk, mark, wnvi, ok, cnz, nel = 0, p, p1, p2, p3, p4, pj, pk, pk1, pk2, pn, q, n, m, t ; csi h ; /* --- Construct matrix C ----------------------------------------------- */ if (!CS_CSC (A) || order <= 0 || order > 3) return (NULL) ; /* check */ AT = cs_transpose (A, 0) ; /* compute A' */ if (!AT) return (NULL) ; m = A->m ; n = A->n ; dense = CS_MAX (16, 10 * sqrt ((double) n)) ; /* find dense threshold */ dense = CS_MIN (n-2, dense) ; if (order == 1 && n == m) { C = cs_add (A, AT, 0, 0) ; /* C = A+A' */ } else if (order == 2) { ATp = AT->p ; /* drop dense columns from AT */ ATi = AT->i ; for (p2 = 0, j = 0 ; j < m ; j++) { p = ATp [j] ; /* column j of AT starts here */ ATp [j] = p2 ; /* new column j starts here */ if (ATp [j+1] - p > dense) continue ; /* skip dense col j */ for ( ; p < ATp [j+1] ; p++) ATi [p2++] = ATi [p] ; } ATp [m] = p2 ; /* finalize AT */ A2 = cs_transpose (AT, 0) ; /* A2 = AT' */ C = A2 ? cs_multiply (AT, A2) : NULL ; /* C=A'*A with no dense rows */ cs_spfree (A2) ; } else { C = cs_multiply (AT, A) ; /* C=A'*A */ } cs_spfree (AT) ; if (!C) return (NULL) ; cs_fkeep (C, &cs_diag, NULL) ; /* drop diagonal entries */ Cp = C->p ; cnz = Cp [n] ; P = cs_malloc (n+1, sizeof (csi)) ; /* allocate result */ W = cs_malloc (8*(n+1), sizeof (csi)) ; /* get workspace */ t = cnz + cnz/5 + 2*n ; /* add elbow room to C */ if (!P || !W || !cs_sprealloc (C, t)) return (cs_idone (P, C, W, 0)) ; len = W ; nv = W + (n+1) ; next = W + 2*(n+1) ; head = W + 3*(n+1) ; elen = W + 4*(n+1) ; degree = W + 5*(n+1) ; w = W + 6*(n+1) ; hhead = W + 7*(n+1) ; last = P ; /* use P as workspace for last */ /* --- Initialize quotient graph ---------------------------------------- */ for (k = 0 ; k < n ; k++) len [k] = Cp [k+1] - Cp [k] ; len [n] = 0 ; nzmax = C->nzmax ; Ci = C->i ; for (i = 0 ; i <= n ; i++) { head [i] = -1 ; /* degree list i is empty */ last [i] = -1 ; next [i] = -1 ; hhead [i] = -1 ; /* hash list i is empty */ nv [i] = 1 ; /* node i is just one node */ w [i] = 1 ; /* node i is alive */ elen [i] = 0 ; /* Ek of node i is empty */ degree [i] = len [i] ; /* degree of node i */ } mark = cs_wclear (0, 0, w, n) ; /* clear w */ elen [n] = -2 ; /* n is a dead element */ Cp [n] = -1 ; /* n is a root of assembly tree */ w [n] = 0 ; /* n is a dead element */ /* --- Initialize degree lists ------------------------------------------ */ for (i = 0 ; i < n ; i++) { d = degree [i] ; if (d == 0) /* node i is empty */ { elen [i] = -2 ; /* element i is dead */ nel++ ; Cp [i] = -1 ; /* i is a root of assembly tree */ w [i] = 0 ; } else if (d > dense) /* node i is dense */ { nv [i] = 0 ; /* absorb i into element n */ elen [i] = -1 ; /* node i is dead */ nel++ ; Cp [i] = CS_FLIP (n) ; nv [n]++ ; } else { if (head [d] != -1) last [head [d]] = i ; next [i] = head [d] ; /* put node i in degree list d */ head [d] = i ; } } while (nel < n) /* while (selecting pivots) do */ { /* --- Select node of minimum approximate degree -------------------- */ for (k = -1 ; mindeg < n && (k = head [mindeg]) == -1 ; mindeg++) ; if (next [k] != -1) last [next [k]] = -1 ; head [mindeg] = next [k] ; /* remove k from degree list */ elenk = elen [k] ; /* elenk = |Ek| */ nvk = nv [k] ; /* # of nodes k represents */ nel += nvk ; /* nv[k] nodes of A eliminated */ /* --- Garbage collection ------------------------------------------- */ if (elenk > 0 && cnz + mindeg >= nzmax) { for (j = 0 ; j < n ; j++) { if ((p = Cp [j]) >= 0) /* j is a live node or element */ { Cp [j] = Ci [p] ; /* save first entry of object */ Ci [p] = CS_FLIP (j) ; /* first entry is now CS_FLIP(j) */ } } for (q = 0, p = 0 ; p < cnz ; ) /* scan all of memory */ { if ((j = CS_FLIP (Ci [p++])) >= 0) /* found object j */ { Ci [q] = Cp [j] ; /* restore first entry of object */ Cp [j] = q++ ; /* new pointer to object j */ for (k3 = 0 ; k3 < len [j]-1 ; k3++) Ci [q++] = Ci [p++] ; } } cnz = q ; /* Ci [cnz...nzmax-1] now free */ } /* --- Construct new element ---------------------------------------- */ dk = 0 ; nv [k] = -nvk ; /* flag k as in Lk */ p = Cp [k] ; pk1 = (elenk == 0) ? p : cnz ; /* do in place if elen[k] == 0 */ pk2 = pk1 ; for (k1 = 1 ; k1 <= elenk + 1 ; k1++) { if (k1 > elenk) { e = k ; /* search the nodes in k */ pj = p ; /* list of nodes starts at Ci[pj]*/ ln = len [k] - elenk ; /* length of list of nodes in k */ } else { e = Ci [p++] ; /* search the nodes in e */ pj = Cp [e] ; ln = len [e] ; /* length of list of nodes in e */ } for (k2 = 1 ; k2 <= ln ; k2++) { i = Ci [pj++] ; if ((nvi = nv [i]) <= 0) continue ; /* node i dead, or seen */ dk += nvi ; /* degree[Lk] += size of node i */ nv [i] = -nvi ; /* negate nv[i] to denote i in Lk*/ Ci [pk2++] = i ; /* place i in Lk */ if (next [i] != -1) last [next [i]] = last [i] ; if (last [i] != -1) /* remove i from degree list */ { next [last [i]] = next [i] ; } else { head [degree [i]] = next [i] ; } } if (e != k) { Cp [e] = CS_FLIP (k) ; /* absorb e into k */ w [e] = 0 ; /* e is now a dead element */ } } if (elenk != 0) cnz = pk2 ; /* Ci [cnz...nzmax] is free */ degree [k] = dk ; /* external degree of k - |Lk\i| */ Cp [k] = pk1 ; /* element k is in Ci[pk1..pk2-1] */ len [k] = pk2 - pk1 ; elen [k] = -2 ; /* k is now an element */ /* --- Find set differences ----------------------------------------- */ mark = cs_wclear (mark, lemax, w, n) ; /* clear w if necessary */ for (pk = pk1 ; pk < pk2 ; pk++) /* scan 1: find |Le\Lk| */ { i = Ci [pk] ; if ((eln = elen [i]) <= 0) continue ;/* skip if elen[i] empty */ nvi = -nv [i] ; /* nv [i] was negated */ wnvi = mark - nvi ; for (p = Cp [i] ; p <= Cp [i] + eln - 1 ; p++) /* scan Ei */ { e = Ci [p] ; if (w [e] >= mark) { w [e] -= nvi ; /* decrement |Le\Lk| */ } else if (w [e] != 0) /* ensure e is a live element */ { w [e] = degree [e] + wnvi ; /* 1st time e seen in scan 1 */ } } } /* --- Degree update ------------------------------------------------ */ for (pk = pk1 ; pk < pk2 ; pk++) /* scan2: degree update */ { i = Ci [pk] ; /* consider node i in Lk */ p1 = Cp [i] ; p2 = p1 + elen [i] - 1 ; pn = p1 ; for (h = 0, d = 0, p = p1 ; p <= p2 ; p++) /* scan Ei */ { e = Ci [p] ; if (w [e] != 0) /* e is an unabsorbed element */ { dext = w [e] - mark ; /* dext = |Le\Lk| */ if (dext > 0) { d += dext ; /* sum up the set differences */ Ci [pn++] = e ; /* keep e in Ei */ h += e ; /* compute the hash of node i */ } else { Cp [e] = CS_FLIP (k) ; /* aggressive absorb. e->k */ w [e] = 0 ; /* e is a dead element */ } } } elen [i] = pn - p1 + 1 ; /* elen[i] = |Ei| */ p3 = pn ; p4 = p1 + len [i] ; for (p = p2 + 1 ; p < p4 ; p++) /* prune edges in Ai */ { j = Ci [p] ; if ((nvj = nv [j]) <= 0) continue ; /* node j dead or in Lk */ d += nvj ; /* degree(i) += |j| */ Ci [pn++] = j ; /* place j in node list of i */ h += j ; /* compute hash for node i */ } if (d == 0) /* check for mass elimination */ { Cp [i] = CS_FLIP (k) ; /* absorb i into k */ nvi = -nv [i] ; dk -= nvi ; /* |Lk| -= |i| */ nvk += nvi ; /* |k| += nv[i] */ nel += nvi ; nv [i] = 0 ; elen [i] = -1 ; /* node i is dead */ } else { degree [i] = CS_MIN (degree [i], d) ; /* update degree(i) */ Ci [pn] = Ci [p3] ; /* move first node to end */ Ci [p3] = Ci [p1] ; /* move 1st el. to end of Ei */ Ci [p1] = k ; /* add k as 1st element in of Ei */ len [i] = pn - p1 + 1 ; /* new len of adj. list of node i */ h = ((h<0) ? (-h):h) % n ; /* finalize hash of i */ next [i] = hhead [h] ; /* place i in hash bucket */ hhead [h] = i ; last [i] = h ; /* save hash of i in last[i] */ } } /* scan2 is done */ degree [k] = dk ; /* finalize |Lk| */ lemax = CS_MAX (lemax, dk) ; mark = cs_wclear (mark+lemax, lemax, w, n) ; /* clear w */ /* --- Supernode detection ------------------------------------------ */ for (pk = pk1 ; pk < pk2 ; pk++) { i = Ci [pk] ; if (nv [i] >= 0) continue ; /* skip if i is dead */ h = last [i] ; /* scan hash bucket of node i */ i = hhead [h] ; hhead [h] = -1 ; /* hash bucket will be empty */ for ( ; i != -1 && next [i] != -1 ; i = next [i], mark++) { ln = len [i] ; eln = elen [i] ; for (p = Cp [i]+1 ; p <= Cp [i] + ln-1 ; p++) w [Ci [p]] = mark; jlast = i ; for (j = next [i] ; j != -1 ; ) /* compare i with all j */ { ok = (len [j] == ln) && (elen [j] == eln) ; for (p = Cp [j] + 1 ; ok && p <= Cp [j] + ln - 1 ; p++) { if (w [Ci [p]] != mark) ok = 0 ; /* compare i and j*/ } if (ok) /* i and j are identical */ { Cp [j] = CS_FLIP (i) ; /* absorb j into i */ nv [i] += nv [j] ; nv [j] = 0 ; elen [j] = -1 ; /* node j is dead */ j = next [j] ; /* delete j from hash bucket */ next [jlast] = j ; } else { jlast = j ; /* j and i are different */ j = next [j] ; } } } } /* --- Finalize new element------------------------------------------ */ for (p = pk1, pk = pk1 ; pk < pk2 ; pk++) /* finalize Lk */ { i = Ci [pk] ; if ((nvi = -nv [i]) <= 0) continue ;/* skip if i is dead */ nv [i] = nvi ; /* restore nv[i] */ d = degree [i] + dk - nvi ; /* compute external degree(i) */ d = CS_MIN (d, n - nel - nvi) ; if (head [d] != -1) last [head [d]] = i ; next [i] = head [d] ; /* put i back in degree list */ last [i] = -1 ; head [d] = i ; mindeg = CS_MIN (mindeg, d) ; /* find new minimum degree */ degree [i] = d ; Ci [p++] = i ; /* place i in Lk */ } nv [k] = nvk ; /* # nodes absorbed into k */ if ((len [k] = p-pk1) == 0) /* length of adj list of element k*/ { Cp [k] = -1 ; /* k is a root of the tree */ w [k] = 0 ; /* k is now a dead element */ } if (elenk != 0) cnz = p ; /* free unused space in Lk */ } /* --- Postordering ----------------------------------------------------- */ for (i = 0 ; i < n ; i++) Cp [i] = CS_FLIP (Cp [i]) ;/* fix assembly tree */ for (j = 0 ; j <= n ; j++) head [j] = -1 ; for (j = n ; j >= 0 ; j--) /* place unordered nodes in lists */ { if (nv [j] > 0) continue ; /* skip if j is an element */ next [j] = head [Cp [j]] ; /* place j in list of its parent */ head [Cp [j]] = j ; } for (e = n ; e >= 0 ; e--) /* place elements in lists */ { if (nv [e] <= 0) continue ; /* skip unless e is an element */ if (Cp [e] != -1) { next [e] = head [Cp [e]] ; /* place e in list of its parent */ head [Cp [e]] = e ; } } for (k = 0, i = 0 ; i <= n ; i++) /* postorder the assembly tree */ { if (Cp [i] == -1) k = cs_tdfs (i, k, head, next, P, w) ; } return (cs_idone (P, C, W, 1)) ; }