The OPTNET Procedure

This example looks at an application of cycle detection to help create a kidney donor exchange. Suppose someone needs a kidney transplant and a family member is willing to donate one. If the donor and recipient are incompatible (because of blood types, tissue mismatch, and so on), the transplant cannot happen. Now suppose two donor-recipient pairs A and B are in this situation, but donor A is compatible with recipient B and donor B is compatible with recipient A. Then two transplants can take place in a two-way swap, shown graphically in Output 2.2.1. More generally, an -way swap can be performed involving donors and recipients (Willingham 2009).

To model this problem, define a directed graph as follows. Each node is an incompatible donor-recipient pair. Link exists if the donor from node is compatible with the recipient from node . The link weight is a measure of the quality of the match. By introducing dummy links with weight 0, you can also include altruistic donors with no recipients, or recipients without donors. The idea is to find a maximum weight node-disjoint union of directed cycles. You want the union to be node-disjoint so that no kidney is donated more than once, and you want cycles so that the donor from node gives up a kidney if and only if the recipient from node receives a kidney.

Without any other constraints, the problem could be solved as a linear assignment problem, as described in the section Linear Assignment (Matching). But doing so would allow arbitrarily long cycles in the solution. Because of practical considerations (such as travel) and to mitigate risk, each cycle must have no more than links. The kidney exchange problem is to find a maximum weight node-disjoint union of short directed cycles.

One way to solve this problem is to explicitly generate all cycles of at most length and then solve a set packing problem. You can use PROC OPTNET to generate the cycles and then PROC OPTMODEL (see SAS/OR User's Guide: Mathematical Programming) to read the PROC OPTNET output, formulate the set packing problem, call the mixed integer linear programming solver, and output the optimal solution.

The following DATA step sets up the problem, first creating a random graph on nodes with link probability and Uniform(0,1) weight:

/* create random graph on n nodes with arc probability p 
   and uniform(0,1) weight */
%let n = 100;
%let p = 0.02;
data LinkSetIn;
   do from = 0 to &n - 1;
      do to = 0 to &n - 1;
         if from eq to then continue;
         else if ranuni(1) < &p then do;
            weight = ranuni(2);
            output;
         end;
      end;
   end;
run;

The following statements use PROC OPTNET to generate all cycles with length greater than or equal to 2 and less than or equal to 10:

/* generate all cycles with 2 <= length <= max_length */
%let max_length = 10;
proc optnet
   loglevel        = moderate
   graph_direction = directed
   data_links      = LinkSetIn;
   cycle
      minLength    = 2
      maxLength    = &max_length
      out          = Cycles
      mode         = all_cycles;
run;
%put &_OROPTNET_;
%put &_OROPTNET_CYCLE_;

PROC OPTNET finds 224 cycles of the appropriate length, as shown in Output 2.2.2.

From the resulting data set Cycles, use the following DATA step to convert the cycles into one observation per arc:

/* convert Cycles into one observation per arc */
data Cycles0(keep=c i j);
   set Cycles;
   retain last;
   c    = cycle;
   i    = last;
   j    = node;
   last = j;
   if order ne 1 then output;
run;

Given the set of cycles, you can now formulate a mixed integer linear program (MILP) to maximize the total cycle weight. Let define the set of cycles of appropriate length, define the set of nodes in cycle , define the set of links in cycle , and denote the link weight for link . Define a binary decision variable . Set to 1 if cycle is used in the solution; otherwise, set it to 0. Then, the following MILP defines the problem that you want to solve to maximize the quality of the kidney exchange:

The constraint (incomp_pair) ensures that each node (incompatible pair) in the graph is intersected at most once. That is, a donor can donate a kidney only once. You can use PROC OPTMODEL to solve this mixed integer linear programming problem as follows:

/* solve set packing problem to find maximum weight node-disjoint union 
   of short directed cycles */
proc optmodel;
   /* declare index sets and parameters, and read data */
   set <num,num> ARCS;
   num weight {ARCS};
   read data LinkSetIn into ARCS=[from to] weight;
   set <num,num,num> TRIPLES;
   read data Cycles0 into TRIPLES=[c i j];
   set CYCLES = setof {<c,i,j> in TRIPLES} c;
   set ARCS_c {c in CYCLES} = setof {<(c),i,j> in TRIPLES} <i,j>;
   set NODES_c {c in CYCLES} = union {<i,j> in ARCS_c[c]} {i,j};
   set NODES = union {c in CYCLES} NODES_c[c];
   num cycle_weight {c in CYCLES} = sum {<i,j> in ARCS_c[c]} weight[i,j];

   /* UseCycle[c] = 1 if cycle c is used, 0 otherwise */
   var UseCycle {CYCLES} binary;

   /* declare objective */
   max TotalWeight 
      = sum {c in CYCLES} cycle_weight[c] * UseCycle[c];

   /* each node appears in at most one cycle */
   con node_packing {i in NODES}:
      sum {c in CYCLES: i in NODES_c[c]} UseCycle[c] <= 1;

   /* call solver */
   solve with milp;

   /* output optimal solution */
   create data Solution from 
      [c]={c in CYCLES: UseCycle[c].sol > 0.5} cycle_weight;
quit;
%put &_OROPTMODEL_;

PROC OPTMODEL solves the problem by using the mixed integer linear programming solver. As shown in Output 2.2.3, it was able to find a total weight (quality level) of 26.02.

The data set Solution, shown in Output 2.2.4, now contains the cycles that define the best exchange and their associated weight (quality).