[gecode-users] odering information for unary scheduling constraints

[Kish Shen]

Hi,

I am looking at the unary scheduling constraints, as I have just added 
them to the ECLiPSe gecode interface. ECLiPSe has both unary and 
cumulative constraints, which were added some time ago (more than 10 
years ago now, I think), and for the unary constraint, there is a 
version with ordering booleans, where each pair of task has a 0/1 domain 
variable (BoolVar for gecode) associated with it, which specify the 
order of the pair of task (if i is before j or j before i for pair i,j).
This seems to be very useful, because it can used to specify ordering if 
required, or be used in search to try the different orderings.

I guess for gecode, you would need to post extra reified constraints 
specifying the ordering? Does it make sense to move this into the unary 
constraint?

I was also looking at the global constraint catalog, to see if they have 
a unary constraint with ordering. I found disj, which specify ordering 
in a different way: for each task, there is a set variable that 
gives/specify the tasks that happen earlier, and a variable position 
that gives the absolute order of the task. The description then mention 
that the algorithm for shaving the position attribute is implemented in 
gecode. Is this in the unary constraint? Is it used internally? I can't 
see any ordering information that can be accessed from the outside in 
the description of the unary constraint, or have I missed something?

Thanks and cheers,

Kish

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