Global Grammar Constraints Toby Walsh National ICT Australia and University of New South Wales Joint work with Claude-Guy Quimper To be presented at CP06

Global grammar constraints ● Often easy to specify a global constraint – ALLDIFFERENT([X1,..Xn]) iff Xi=/=Xj for i<j ● Difficult to build an efficient and effective propagator – Especially if we want global reasoning

Global grammar constraints ● Promising direction initiated by Beldiceanu, Carlsson, Pesant and Petit is to specify constraints via automata/grammar – Sequence of variables = string in some formal language – Satisfying assignment = string accepted by the grammar/automata

REGULAR constraint ● REGULAR(A,[X1,..Xn]) holds iff – X1.. Xn is a string accepted by the deterministic finite automaton A – Proposed by Pesant at CP 2004 – GAC algorithm using dynamic programming – However, DP is not needed since simple ternary encoding is just as efficient and effective – Encoding similar to that used by Beldiceanu et al for their automata with counters

REGULAR constraint ● Deterministic finite automaton (DFA) – – Q is finite set of states – Sigma is alphabet (from which strings formed) – T is transition function: Q x Sigma -> Q – q0 is starting state – F subseteq Q are accepting states ● DFAs accept precisely regular languages

REGULAR constraint ● Many global constraints are instances of REGULAR – AMONG – CONTIGUITY – LEX – PRECEDENCE – STRETCH –.. ● Domain consistency can be enforced in O(ndQ) time using dynamic programming

REGULAR constraint ● REGULAR constraint can be encoded into ternary constraints ● Introduce Qi+1 – state of the DFA after the ith transition ● Then post sequence of constraints – C(Xi,Qi,Qi+1) iff DFA goes from state Qi to Qi+1 on symbol Xi

REGULAR constraint ● REGULAR constraint can be encoded into ternary constraints ● Constraint graph is Berge-acyclic – Constraints only overlap on one variable – Enforcing GAC on ternary constraints achieves GAC on REGULAR in O(ndQ) time

REGULAR constraint ● REGULAR constraint can be encoded into ternary constraints ● Constraint graph is Berge-acyclic – Constraints only overlap on one variable – Enforcing GAC on ternary constraints achieves GAC on REGULAR in O(ndQ) time ● Encoding provides access to states of automata – Can be useful for expressing problems – E.g. minimizing number of times we are in a particular state

REGULAR constraint ● STRETCH([X1,..Xn]) holds iff – Any stretch of consecutive values is between shortest(v) and longest(v) length – Any change (v1,v2) is in some permitted set, P – For example, you can only have 3 consecutive night shifts and a night shift must be followed by a day off

REGULAR constraint ● STRETCH([X1,..Xn]) holds iff – Any stretch of consecutive values is between shortest(v) and longest(v) length – Any change (v1,v2) is in some permitted set, P ● DFA – Qi is – Q0= – T(,a)= if q+1<=longest(a) – T(,b)= if (a,b) in P and q>=shortest(a) – All states are accepting

NFA constraint ● Automaton does not need to be deterministic ● Non-deterministic finite automaton (NFA) still only accept regular languages – But may require exponentially fewer states – Important as O(ndQ) running time for propagator – E.g. 0* (1|2)^k 2 (1|2)* 2 (1|2)^k 0* – Where 0=closed, 1=production, 2=maintenance ● Can use the same ternary encoding

Soft REGULAR constraint ● May wish to be “near” to a regular string ● Near could be – Hamming distance – Edit distance ● SoftREGULAR(A,[X1,..Xn],N) holds iff – X1..Xn is at distance N from a string accepted by the finite automaton A – Can encode this into a sequence of 5-ary constraints

Soft REGULAR constraint ● SoftREGULAR(A,[X1,..Xn],N) – Consider Hamming distance (edit distance similar though a little more complex) – Qi+1 is state of automaton after the ith transition – Di+1 is Hamming distance up to the ith variable – Post sequence of constraints ● C(Xi,Qi,Qi+1,Di,Di+1) where ● Di+1=Di if T(Xi,Qi)=Qi+1 else Di+1=1+Di

Soft REGULAR constraint ● SoftREGULAR(A,[X1,..Xn],N) – To propagate – Dynamic programming ● Pass support along sequence – Just post the 5-ary constraints ● Accept less than GAC – Tuple up the variables

Cyclic forms of REGULAR ● REGULAR+(A,[X1,..,Xn]) – X1.. XnX1 is accepted by A – Can convert into REGULAR by increasing states by factor of d where d is number of initial symbols – qi => (qi,initial value) – T(qi,a)=qj => T((qi,b),a)=(qj,b) – Thereby pass along value taken by X1 so it can be checked on last transition

Cyclic forms of REGULAR ● REGULARo(A,[X1,..,Xn]) – Xi.. X1+(i+n-1)mod n is accepted by A for each 1<=i<=n – Can decompose into n instances of the REGULAR constraint – However, this hinders propagation ● Suppose A accepts just alternating sequences of 0 and 1 ● Xi in {0,1} and REGULARo(A,[X1,X2.X3]) – Unfortunately enforcing GAC on REGULARo is NP- hard

Cyclic forms of REGULAR ● REGULARo(A,[X1,..,Xn]) – Reduction from Hamiltonian cycle – Consider polynomial sized automaton A1 that accepts any sequence in which the 1st character is never repeated – Consider polynomial sized automaton A2 that accepts any walk in a graph ● T(a,b)=b iff (a,b) in edges of graph – Consider polynomial sized automaton A1 intersect A2 – This accepts only those strings corresponding to Hamiltonian cycles

Other generalizations of REGULAR ● REGULAR FIX(A,[X1,..Xn],[B1,..Bm]) iff – REGULAR(A,[X1,..Xn]) and Bi=1 iff exists j. Xj=I – Certain values must occur within the sequence – For example, there must be a maintenance shift – Unfortunately NP-hard to enforce GAC on this

Other generalizations of REGULAR ● REGULAR FIX(A,[X1,..Xn],[B1,..Bm]) – Simple reduction from Hamiltonian path – Automaton A accepts any walk on a graph – n=m and Bi=1 for all i

Chomsky hierarchy ● Regular languages ● Context-free languages ● Context-sensitive languages ●..

Chomsky hierarchy ● Regular languages – GAC propagator in O(ndQ) time ● Conext-free languages – GAC propagator in O(n^3) time and O(n^2) space – Asymptotically optimal as same as parsing! ● Conext-sensitive languages – Checking if a string is in the language PSPACE- complete – Undecidable to know if empty string in grammar and thus to detect domain wipeout and enforce GAC!

Context-free grammars ● Possible applications – Hierarchy configuration – Bioinformatics – Natual language parsing – … ● CFG(G,[X1,…Xn]) holds iff – X1.. Xn is a string accepted by the context free grammar G

Context-free grammars ● CFG(G,[X1,…Xn]) – Consider a block stacking example – S -> NP | P | PN | NPN – N -> n | nN – P -> aa | bb | aPa | bPb – These rules give n* w rev(w) n* where w is (a|b)* – Not expressible using a regular language ● Chomsky normal form – Non-terminal -> Terminal – Non-terminal -> Non-terminal Non-terminal

CFG propagator ● Adapt CYK parser ● Works on Chomsky normal form – Non-terminal -> Terminal – Non-terminal -> Non-terminal Non-terminal ● Using dynamic programming to compute supports ● Bottom up – Enforces GAC in Theta(n^3) time Simultaneously and independently proposed by Sellmannn [CP06]

CFG propagator ● Adapt Earley chart parser – Carry support information ● Works on grammar in any form ● More top down – Better on tightly restricted grammars ● Enforces GAC in O(n^3) time – Best case is better as not Theta(n^3)

Conclusions ● Global grammar constraints – Specify wide range of global constraints – Provide efficient and effective propagators automatically – Nice marriage of formal language theory and constraint programming!