Qualitative and quantitative optimization in answer set programming

## Project description

# 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](http://www.cs.uni-potsdam.de/wv/pdfformat/brderosc15a.pdf) ([bibtex](http://www.cs.uni-potsdam.de/wv/bibtex/brderosc15a.bib)).

Starting with version 3, `asprin` is documented in the [Potassco guide](https://github.com/potassco/guide/releases/).

Older versions are documented in the [Potassco guide on Sourceforge](https://sourceforge.net/projects/potassco/files/guide/).

## Usage

```bash

$ 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

<!--- TO BE CHANGED -->

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](https://pip.pypa.io) 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`.

<!--- TO BE CHANGED -->

If that does not work,

you can always download the sources from

[here](https://github.com/potassco/asprin/releases/download/v3.1.0/asprin-3.1.0.tar.gz) 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](https://github.com/potassco/asprin/tree/master/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 `X` and `Y` be two interpretations of a logic program.

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, `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 interpretation `X` such that `X` is better than `X`.

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](https://github.com/potassco/asprin/blob/master/asprin/examples/cp_tree.lp).

For acyclic CP nets, see example [cp_acyclic.lp](https://github.com/potassco/asprin/blob/master/asprin/examples/cp_acyclic.lp).

For general CP nets, see example [cp_general.lp](https://github.com/potassco/asprin/blob/master/asprin/examples/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

> 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](http://www.cs.uni-potsdam.de/wv/pdfformat/brderosc15a.pdf) ([bibtex](http://www.cs.uni-potsdam.de/wv/bibtex/brderosc15a.bib)).

Starting with version 3, `asprin` is documented in the [Potassco guide](https://github.com/potassco/guide/releases/).

Older versions are documented in the [Potassco guide on Sourceforge](https://sourceforge.net/projects/potassco/files/guide/).

## Usage

```bash

$ 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

<!--- TO BE CHANGED -->

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](https://pip.pypa.io) 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`.

<!--- TO BE CHANGED -->

If that does not work,

you can always download the sources from

[here](https://github.com/potassco/asprin/releases/download/v3.1.0/asprin-3.1.0.tar.gz) 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](https://github.com/potassco/asprin/tree/master/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 `X` and `Y` be two interpretations of a logic program.

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, `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 interpretation `X` such that `X` is better than `X`.

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](https://github.com/potassco/asprin/blob/master/asprin/examples/cp_tree.lp).

For acyclic CP nets, see example [cp_acyclic.lp](https://github.com/potassco/asprin/blob/master/asprin/examples/cp_acyclic.lp).

For general CP nets, see example [cp_general.lp](https://github.com/potassco/asprin/blob/master/asprin/examples/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

## Project details

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