Originally posted by: Oblivion

I have observed that using dvar interval instead of dvar int can lead to duplicated solutions and I don't understand why. Has anybody some explanation for this ?

• Consider the Program 1 that tries to fix the beginning of a one unit length task within a window of 3 unit. This version uses a dvar int variable​

using CP;

execute {
   cp.param.searchType="DepthFirst";
   cp.param.logVerbosity="Quiet";
   cp.param.workers=1;
}

dvar int b in 0..2; // task beginning

constraints {
        bt >= 0; // at least one constraint involving the variable
                 // otherwise it doesn't backtrack
}

include "allSolutions.mod"; // A main bloc for enumerating solutions

// --- PostProcessing
execute {
  writeln("b = ",b);
}

This program has 3 solutions : 

----------- Solution 1-------------
 b = 0
----------- Solution 2-------------
 b = 1
----------- Solution 3-------------
 b = 2

No (more) solution
# solutions : 3

• Consider now the Program 2 that does the same but using a dvar interval variable : 

using CP;

execute {
   cp.param.searchType="DepthFirst";
   cp.param.logVerbosity="Quiet";
   cp.param.workers=1;
}

dvar interval t in 0..3 size 1;

constraints {
        startOf(t) >= 0;     // at least one constraint for backtrack
}

include "allSolutions.mod";

// --- Post processing

execute {
  writeln("t = ",t);
}

Now we get 4 solutions

----------- Solution 1-------------
t =  <1 0 1 1>
----------- Solution 2-------------
t =  <1 0 1 1>
----------- Solution 3-------------
t =  <1 1 2 1>
----------- Solution 4-------------
t =  <1 2 3 1>

No (more) solution
Nb Solutions : 4

Why is the first solution duplicated ?

A third version with both variables and an additional constraint  startOf(t) == b  also gives 4 solutions.

Originally posted by: rdumeur

Dear Oblivion,

First in order to reduce the number of duplicate solutions when using interval var, you may try use setting cp parameters:

SearchType = "DepthFirst"

Workers = 1

But even with those, CPO search with interval var may still produce duplicate solutions.

Cheers,