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README

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+VHPOP 2.0
+=========
+
+VHPOP is a versatile heuristic partial order planner loosely based on
+UCPOP with the following main features:
+
+  * Can plan with either ground or lifted actions.
+
+  * Can enforce joint parameter domain constraints when using lifted
+    actions.
+
+  * Has several different plan ranking heuristics to choose from that
+    can be combined into complex plan ranking functions.
+
+  * Efficiently implements all common flaw selection strategies,
+    including LCFR and ZLIFO, and many novel ones, and allows flaw
+    selection strategies to be specified using a concise notation.
+
+  * Several flaw selection strategies can be used simultaneously.
+
+  * Implements A*, IDA*, and hill climbing search.
+
+  * Fully supports levels 1 and 3 of PDDL+.
+
+For installation instructions, see `INSTALL'.
+
+For copyright information and distribution restrictions, see `COPYING'.
+
+
+Search Algorithms
+-----------------
+
+You can select the search algorithms to use with the -s flag.  The
+options are as follows:
+
+  A for A*.
+  IDA for IDA*.
+  HC for hill climbing.
+
+
+Plan Selection
+--------------
+
+The plan ranking function is specified with the -h flag.  A plan
+ranking function is a sequence of simple heuristic plan ranking
+functions in decreasing order of significance.  For example,
+"S+OC/LIFO" uses the number of steps plus the number of open
+conditions of a plan as primary rank, and selects plans in LIFO order
+in case the primary rank of several plans are the same.  Some simple
+plan ranking functions use a weight, which can be specified with the
+-w flag.  VHPOP implements the following simple plan ranking
+functions:
+
+  LIFO gives priority to plans created later.
+  FIFO gives priority to plans created earlier.
+  OC gives priority to plans with few open conditions.
+  UC gives priority to plans with few threatened links.
+  BUC gives priority to plans with no threatened links.
+  S+OC uses h(p) = |S(p)| + w*|OC(p)| for plan p.
+  UCPOP uses h(p) = |S(p)| + w(|OC(p)| + |UC(p)|) for plan p.
+  ADD_COST uses the additive cost heuristic.
+  ADD_WORK uses the additive work heuristic.
+  ADD uses h(p) = |S(p)| + w*ADD_COST for plan p.
+  ADDR is like ADD, but tries to take reuse into account.
+  ADDR_COST uses the ADDR cost heuristic.
+  ADDR_WORK uses the ADDR work heuristic.
+  MAX uses the max heuristic.
+  MAX_COST uses the max cost heuristic.
+  MAX_WORK uses the max work heuristic.
+  MAXR is like MAX, but tries to take reuse into account.
+  MAXR_COST uses the MAXR cost heuristic.
+  MAXR_WORK uses the MAXR work heuristic.
+
+
+Flaw Selection
+--------------
+
+A flaw selection strategy is an ordered list of selection criteria.
+Each selection criterion is of the form
+
+  {flaw types}[max refinements]ordering criterion,
+
+and applies to flaws of the given types which can be resolved in at
+most "max refinements" ways.  The limit on number of refinements is
+optional and can be left out.  The ordering criterion is used to order
+flaws that the selection criterion applies to.  LIFO order is used if
+the ordering criterion cannot be used to distinguish two or more
+flaws.
+
+The different flaw types are "o" (open condition), "t" (static open
+condition), "l" (local open condition), "u" (unsafe open condition),
+"n" (non-separable threat), and "s" (separable threat).  The different
+ordering criteria are "LIFO", "FIFO", "R" (random), "LR" (least
+refinements first), "MR" (opposite of LR), "New" (open conditions that
+can be resolved with new step first), "Reuse" (opposite of New),
+"MC_h" (most cost with heuristic h), "LC_h" (opposite of MC_h), "MW_h"
+(most work with heuristic h), "LW_h" (opposite of MW_h).
+
+Flaws are matched with selection criteria, and it is required for
+completeness that every flaw matches at least one selection criterion
+in a flaw selection strategy.  The flaw that matches the earliest
+selection criterion, and is ordered before any other flaws matching
+the same criterion (according to the ordering criterion), is the flaw
+that gets selected by the flaw selection strategy.  Note that we do
+not always need to test all flaws.  If, for example, the first
+selection criterion is {n,s}LIFO, and we have found a threat, the we
+do not need to consider any other flaws for selection.
+
+Most common flaw selection strategies are predefined, along with
+several novel ones:
+
+  UCPOP		{n,s}LIFO/{o}LIFO
+  UCPOP-LC	{n,s}LIFO/{o}LR
+  DSep-LIFO	{n}LIFO/{o}LIFO/{s}LIFO
+  DSep-FIFO	{n}LIFO/{o}FIFO/{s}LIFO
+  DSep-LC	{n}LIFO/{o}LR/{s}LIFO
+  DUnf-LIFO	{n,s}0LIFO/{n,s}1LIFO/{o}LIFO/{n,s}LIFO
+  DUnf-FIFO	{n,s}0LIFO/{n,s}1LIFO/{o}FIFO/{n,s}LIFO
+  DUnf-LC	{n,s}0LIFO/{n,s}1LIFO/{o}LR/{n,s}LIFO
+  DUnf-Gen	{n,s,o}0LIFO/{n,s,o}1LIFO/{n,s,o}LIFO
+  DRes-LIFO	{n,s}0LIFO/{o}LIFO/{n,s}LIFO
+  DRes-FIFO	{n,s}0LIFO/{o}FIFO/{n,s}LIFO
+  DRes-LC	{n,s}0LIFO/{o}LR/{n,s}LIFO
+  DEnd-LIFO	{o}LIFO/{n,s}LIFO
+  DEnd-FIFO	{o}FIFO/{n,s}LIFO
+  DEnd-LC	{o}LR/{n,s}LIFO
+  LCFR		{n,s,o}LR
+  LCFR-DSep	{n,o}LR/{s}LR
+  ZLIFO		{n}LIFO/{o}0LIFO/{o}1NEW/{o}LIFO/{s}LIFO
+  ZLIFO*	{o}0LIFO/{n,s}LIFO/{o}1NEW/{o}LIFO
+  Static	{t}LIFO/{n,s}LIFO/{o}LIFO
+  LCFR-Loc	{n,s,l}LR
+  LCFR-Conf	{n,s,u}LR/{o}LR
+  LCFR-Loc-Conf	{n,s,u}LR/{l}LR
+  MC		{n,s}LR/{o}MC_add
+  MC-Loc	{n,s}LR/{l}MC_add
+  MC-Loc-Conf	{n,s}LR/{u}MC_add/{l}MC_add
+  MW		{n,s}LR/{o}MW_add
+  MW-Loc	{n,s}LR/{l}MW_add
+  MW-Loc-Conf	{n,s}LR/{u}MW_add/{l}MW_add
+
+Several flaw selection strategies can be used simultaneously in a
+round-robin scheme.  For example,
+
+  ./vhpop -f LCFR -l 10000 -f MW -f unlimited <domain> <problem>
+
+specifies that both LCFR and MW should be used.  The search limit with
+LCFR is 10,000 generated search nodes, and with MW the number of
+search nodes generated is only limited by the physical memory of the
+machine.  The use of multiple flaw selection strategies is similar to
+running multiple instances of VHPOP concurrently.  A separate search
+queue is kept for each flaw selection strategy.  Flaw selection
+strategies are switched after 1,000; 2,000; 4,000; 8,000;
+etc. generated search nodes or when the search limit is reached for a
+strategy.  The first plan, if any, found is returned as the solution
+regardless of which flaw selection strategy was used.
+
+
+Bugs and Missing Features
+-------------------------
+
+* Universally quantified preconditions are not supported when using
+  lifted actions.
+
+
+Plans for Future Improvements
+-----------------------------
+
+* Improve memory efficiency.
+
+* Improve speed of reachability analysis.
+
+* Improve handling of variable bindings.
+
+* Allow secondary ordering criterion for flaw selection strategies.