= Hydraulic System Planning =

<<TableOfContents>>

== Problem Description ==

A simplified version of the hydraulic system on a space shuttle consists of a directed graph, G, such that:

    * Nodes of this graph are labeled as tanks,jets, or junctions.
    * Every link between two nodes is labeled by a valve.
    * There are no paths in G between any two tanks.
    * For every jet there always is a path in G from a tank to this jet.

Tanks can be full or empty. Valves can be open or closed. Some of the valves can be stuck in the close position. A state of G is specified by the set of full tanks, the set of open valves, and the set of valves stuck in the closed position.

A node of G is called pressurized in state S if it is a full tank or if there exists a path from some full tank of G to this node such that all the valves on the edges of this path are open.

We assume that in a state S a shuttle controller can open a valve V2 corresponding to a directed link <N1,N2> only if N1 is pressurized and not stuck.

Problem: Given a graph G together with an initial state and a number of jets j1 ... jn, a shuttle controller needs to find a shortest sequential plan to pressurize j1,...jn. Write a program which automates this process.

== Predicates ==

 * '''Input''': {{{tank/1, jet/1, junction/1, valve/1, link/3, numValves/1, full/1, stuck/1, goal/1 }}}

 * '''Output''': {{{switchon/2}}}

== Input format ==

The graph should be described by the collection of ground atoms:

    * {{{tank(t)}}}: t is a tank
    * {{{jet(j)}}}: j is a jet
    * {{{junction(p)}}}: p is a junction
    * {{{valve(v)}}}: v is a valve
    * {{{link(n1, n2, v)}}}: v is the valve on the pipe connecting node n1 and n2.
    * {{{numValves(n)}}}: n is the total number of valves in the graph

The state description uses atoms:

    * {{{full(t)}}} iff tank t is full. A tank is empty if it is not mentioned to be full in the input.
    * {{{stuck(v)}}}: valve v is stuck

The goal(s) to achieve:

    * {{{goal(j)}}}: jet j needs to be pressurized

== Output format ==

A sequence of atoms of the form

    * {{{switchon(v, t)}}}.

means an action to open valve v at time step t, where t is an integer. The switches should occur at consecutive time steps beginning from 0. (Note that, in contrast to the original description, the order of atoms in the output does not matter.) 

== Example(s) ==

INPUT
{{{
tank(t111). tank(t112). tank(t113).
jet(j1). jet(j2). jet(j3).
junction(p1). junction(p2). junction(p3). junction(p4). junction(p5). junction(p6).
valve(v1).  valve(v2).  valve(v3).  valve(v4).  valve(v5).
valve(v6).  valve(v7).  valve(v8).  valve(v9).  valve(v10).
valve(v11). valve(v12). valve(v13).
numValves(13).
link(t111, p1, v1).  link(p1, p2, v2).  link(p2, j1, v3).
link(t112, p3, v4).  link(p4, p3, v5).  link(p4, j2, v6).
link(t113, p5, v7).  link(p5, p6, v8).  link(p6, j3, v9).
link(p1, p3, v10). link(p3, p5, v11).
link(p4, p2, v12). link(p6, p4, v13). 
full(t111). leaking(v2). goal(j1).
}}}

Output: {{{switchon(v1,0). switchon(v3,6). switchon(v8,3). switchon(v10,1). switchon(v11,2). switchon(v12,5). switchon(v13,4).}}}

== Note ==
With respect to the version of this problem as of the 2nd ASP Competition, there can be more than one goal per instance.

== Author(s) ==
 * Author: Francesco Calimeri 	
   * Affiliation: University of Calabria, Italy
 * Author: Maria Carmela Santoro 	
   * Affiliation: University of Calabria, Italy