EXODUS/packages/seacas/libraries/chaco/coarsen/maxmatch4.c

/*
 * Copyright(C) 1999-2020, 2022, 2022 National Technology & Engineering Solutions
 * of Sandia, LLC (NTESS).  Under the terms of Contract DE-NA0003525 with
 * NTESS, the U.S. Government retains certain rights in this software.
 *
 * See packages/seacas/LICENSE for details
 */

#include "defs.h"    // for TRUE, FALSE
#include "smalloc.h" // for sfree, smalloc
#include "structs.h" // for vtx_data

/* Find a maximal matching in a graph using Luby's algorithm.  Assign a */
/* random value to each edge and include an edge in the matching if it  */
/* has a higher value than all neighboring edges that aren't disallowed. */
/* (Use float instead of double values to save space.) */
/* THIS ROUTINE IS SLOWER THAN MAXMATCH3, AND SO PROBABLY OBSOLETE. */

int maxmatch4(struct vtx_data **graph,  /* array of vtx data for graph */
              int               nvtxs,  /* number of vertices in graph */
              int               nedges, /* number of edges in graph */
              int              *mflag,  /* flag indicating vtx selected or not */
              int               using_ewgts)
{
  extern int HEAVY_MATCH; /* try for heavy edges in matching? */
  int       *iptr;        /* loops through integer arrays */
  float     *edgevals;    /* random values for all edges */
  float     *evptr;       /* loops through edgevals */
  double     maxval;      /* largest edge value for a vertex */
  int        neighbor;    /* neighbor of a vertex */
  int        nmerged;     /* number of edges in matching */
  int        change;      /* any new edges in matching? */
  int       *start;       /* start of edgevals list for each vertex */
  int        i, j, k;     /* loop counters */

  double drandom(void);

  /* Allocate and initialize space. */
  evptr = edgevals = smalloc(2 * nedges * sizeof(float));

  start    = smalloc((nvtxs + 2) * sizeof(int));
  start[1] = 0;
  for (i = 1; i <= nvtxs; i++) {
    start[i + 1] = start[i] + graph[i]->nedges - 1;
  }

  /* Assign a random value to each edge. */
  if (!using_ewgts || !HEAVY_MATCH) { /* All edges are equal. */
    for (i = 1; i <= nvtxs; i++) {
      for (j = 1; j < graph[i]->nedges; j++) {
        neighbor = graph[i]->edges[j];
        if (neighbor > i) {
          *evptr = (float)drandom();
        }
        else { /* Look up already-generated value. */
          for (k = 1; graph[neighbor]->edges[k] != i; k++) {
            ;
          }
          *evptr = edgevals[start[neighbor] + k - 1];
        }
        evptr++;
      }
    }
  }
  else { /* Prefer heavy weight edges. */
    for (i = 1; i <= nvtxs; i++) {
      for (j = 1; j < graph[i]->nedges; j++) {
        neighbor = graph[i]->edges[j];
        if (neighbor > i) {
          *evptr = graph[i]->ewgts[j] * drandom();
        }
        else { /* Look up already-generated value. */
          for (k = 1; graph[neighbor]->edges[k] != i; k++) {
            ;
          }
          *evptr = edgevals[start[neighbor] + k - 1];
        }
        evptr++;
      }
    }
  }

  for (iptr = mflag, i = nvtxs; i; i--) {
    *(++iptr) = -(nvtxs + 1);
  }
  nmerged = 0;
  change  = TRUE;
  while (change) {
    change = FALSE;

    for (i = 1; i <= nvtxs; i++) { /* Find largest valued edge of each vtx */
      if (mflag[i] < 0) {
        maxval = 0.0;
        k      = -1;
        evptr  = &(edgevals[start[i]]);
        for (j = 1; j < graph[i]->nedges; j++) {
          if (*evptr > maxval && mflag[graph[i]->edges[j]] < 0) {
            maxval = *evptr;
            k      = j;
          }
          evptr++;
        }
        if (k == -1) {
          mflag[i] = 0; /* No neighbors are alive. */
        }
        else {
          mflag[i] = -graph[i]->edges[k];
        }
      }
    }

    /* If vtxs agree on largest valued edge, add to independent set. */
    for (i = 1; i <= nvtxs; i++) {
      if (-mflag[i] > i) {
        if (-mflag[-mflag[i]] == i) { /* Add edge to independent set. */
          nmerged++;
          mflag[i]        = -mflag[i];
          mflag[mflag[i]] = i;
          change          = TRUE;
        }
      }
    }
  }

  /* Maximal independent set is indicated by corresponding pairs */
  /* of positive values in the mflag array. */
  for (i = 1; i <= nvtxs; i++) {
    if (mflag[i] < 0) {
      mflag[i] = 0;
    }
  }
  sfree(start);
  sfree(edgevals);
  return (nmerged);
}