asprin

A general framework for qualitative and quantitative optimization in answer set programming.

Description

asprin is a general framework for optimization in ASP that allows:

• computing optimal stable models of logic programs with preferences, and
• defining new preference types in a very easy way. Some preference types (subset, pareto...) are already defined in asprin's library, but many more can be defined simply writing a logic program.

For a formal description of asprin, please read our paper (bibtex).

Starting with version 3, asprin is documented in the Potassco guide. Older versions are documented in the Potassco guide on Sourceforge.

Usage

$ asprin [number_of_models] [options] [files]

By default, asprin loads its library asprin_lib.lp. This may be disabled with option --no-asprin-lib.

Option --help prints help.

Options --approximation=weak and --approximation=heuristic activate solving modes different than the basic ones, and are often faster than it.

Option --meta=query can be used to compute optimal models that contain the atom query.

Options --meta=simple or --meta=combine should be used to compute many optimal models using non stratified preference programs (in asprin's library this can only happen with CP nets, see below).

Option --on-opt-heur can be used to enumerate diverse (or similar) optimal stable models. For example, try with --on-opt-heur=+,p,1,false --on-opt-heur=-,p,1,true.

Option --improve-limit can be used to enumerate close to optimal stable models. For example, try with --improve-limit 2,1000.

Building

The easiest way to obtain asprin is using Anaconda. Packages are available in the Potassco channel. First install either Anaconda or Miniconda and then run: conda install -c potassco asprin.

asprin can also be installed with pip via pip install asprin. For a local installation, add option --user. In this case, setting environment variable PYTHONUSERBASE to dir before running pip, asprin will be installed in dir/bin/asprin.

If that does not work, you can always download the sources from here in some directory dir, and run asprin with python dir/asprin/asprin/asprin.py.

System tests may be run with asprin --test and asprin --test --all.

asprin has been tested with Python 2.7.13 and 3.5.3, using clingo 5.3.0.

asprin uses the ply library, version 3.11, which is bundled in asprin/src/spec_parser/ply, and was retrieved from http://www.dabeaz.com/ply/.

Examples

$ cat examples/example1.lp
dom(1..3).
1 { a(X) : dom(X) }.
#show a/1.

#preference(p,subset) { 
  a(X)
}.
#optimize(p).


$ asprin examples/example1.lp 0
asprin version 3.0.0
Reading from examples/example1.lp
Solving...
Answer: 1
a(3)
OPTIMUM FOUND
Answer: 2
a(2)
OPTIMUM FOUND
Answer: 3
a(1)
OPTIMUM FOUND

Models       : 3
  Optimum    : yes
  Optimal    : 3

$ cat examples/example2.lp
%
% base program
%

dom(1..3).
1 { a(X) : dom(X) } 2.
1 { b(X) : dom(X) } 2.
#show a/1.
#show b/1.

%
% basic preference statements
%

#preference(p(1),subset){
  a(X)
}.

#preference(p(2),less(weight)){
  X :: b(X)
}.

#preference(p(3),aso){
  a(X) >> not a(X) || b(X)
}.

#preference(p(4),poset){
  a(X);
  b(X);
  a(X) >> b(X)
}.

%
% composite preference statements
%

#preference(q,pareto){
  **p(X)
}.

#preference(r,neg){
  **q
}.

%
% optimize statement
%

#optimize(r).

$ asprin examples/example2.lp 
asprin version 3.0.0
Reading from examples/example2.lp
Solving...
Answer: 1
a(3) b(1)
OPTIMUM FOUND

Models       : 1+
  Optimum    : yes

CP nets

asprin preference library implements the preference type cp, that stands for CP nets.

CP nets where introduced in the following paper:

• Craig Boutilier, Ronen I. Brafman, Carmel Domshlak, Holger H. Hoos, David Poole: CP-nets: A Tool for Representing and Reasoning with Conditional Ceteris Paribus Preference Statements. J. Artif. Intell. Res. 21: 135-191 (2004)

Propositional preference elements of type cp have one of the following forms:

1. a >> not a || { l1; ...; ln }, or
2. not a >> a || { l1; ...; ln }

where a is an atom and l1, ..., ln are literals.

The semantics is defined using the notion of improving flips. Let A be the set of atoms appearing in a cp preference statement, and let X and Y be two subsets of A. There is an improving flip from X to Y if there is some preference element such that X and Y satisfy all li's, and either the element has the form (1) and Y is the union of X and {a}, or the element has the form (2) and Y is X minus {a}. Then, for any two subsets W and Z of A, W is better than Z if there is a sequence of improving flips from W to Z. A CP net is consistent if there is no set X such that X is better than X. This definition for subsets of A is extended to the stable models of a logic program. A stable model X is better than Y if the intersection of X and A is better than the intersection of Y and A. Note that this implies that the ceteris-paribus assumption only applies to the atoms A appearing in the preference statement.

We provide various encoding and solving techniques for CP nets, that can be applied depending on the structure of the CP net. For tree-like CP nets, see example cp_tree.lp. For acyclic CP nets, see example cp_acyclic.lp. For general CP nets, see example cp_general.lp.

asprin implementation of CP nets is correct only for consistent CP nets. Note that tree-like and acyclic CP nets are always consistent, but this does not hold in general.

Contributors

• Javier Romero