Knowledge compilation is a technique for compiling a knowledge base into a target data structure on which many desired operations can be performed efficiently.
Principal Author: Bahare Fatemi
Collaborators: Wenyi Wang, Mehrdad Ghomi
Knowledge compilation has been emerging recently as a new direction of research with regard to the computational intractability of general propositional reasoning. The aim of knowledge compilation is to compile a models/theories into target data structures supporting tractable reasoning. In this page, we provides a brief description of knowledge compilation technique and a series of target languages that are computationally tractable for several reasoning tasks. We use real world examples to provide a clear insight of the technique used by knowledge compilation. We also provide a comparison of the target languages in terms of supporting tractable reasoning as well as succinctness.
More general than
Long before knowledge compilation emerged as a research field, compilation to languages like BDD and PI were used for efficient query answering. Knowledge compilation emerged as a research field with the introduction of the NNF (negation normal form) language and with representing that the previous languages are subsets of the NNF.
Imagine a library with thousands of books without any order. Suppose you are searching for a specific book. Searching through all the books one by one to find a desired book takes a considerable amount of time.
Now suppose the books are in alphabetical order. Searching for a book takes much less time.
Imagine that you are a librarian and you are in charge of helping people find their books. Assume that searching the whole library for a specific book takes 5 minutes and as a librarian, you have to do the search 5 million times. If you decide to sort the books of the library, that takes 300 minutes. But after sorting the books, searching through the library for a book will cost one minute. If the books are not sorted, you have to take 1 million times * 5 minutes each time = 5000000 minutes and if the books are sorted alphabetically, you have to take 300 minutes + 1 million times * 1 hour each = 1000300 hours, which is a huge save of time.
Now let's consider a similar problem in the field of data structure and algorithm. Suppose we have the information about students in computer science department of UBC, and the query is to find a specific student record. Sorting students will help the department to find the specific student easier. Before sorting, finding a specific student record has O(n) time complexity where n is the number of records in the dataset. If we sort the students' records and store them in a binary tree, binary search algorithm helps us to find a specific record in O(log(n)). Querying on the data is like searching through the library and students' records are like the books.
In both of the above examples, we have a data and some desired queries. We spend some time to convert the original data to a target structure, on which the queries can be answered more efficiently. Knowledge compilation plays the same role in artificial intelligence field. In artificial intelligence there are lots of real world models and we have to query on these models. Knowledge compilation compiles these real world models to a target data structure in which operations are less costly as output. So the function of the knowledge compilation is to compile the input model to a target data structure. Figure 1 shows a high level description of knowledge compilation.
Knowledge compilation has been emerging recently as a new direction of research with the computational intractability of general propositional reasoning. Knowledge compilation splits the reasoning process into two phases: off-line compilation phase and on-line query-answering phase. The main motivation for knowledge compilation is to put as much load as possible in off-line phase to make the reasoning and querying more efficient. The other motivation is to make the reasoning system simpler in on-line phase.
As an example of knowledge compilation, we can think of the input model as Boolean logic. Basic operations in Boolean logic are conditioning, conjoining, disjoining, and so on, as we can see in figure 2. Using knowledge compilation, one can compile Boolean logic theory into a data structure called DNNF (decomposable negation normal form) , on which operations are less costly. For example conditioning can be done in linear time complexity using DNNF, while it has exponential time complexity in CNF (Conjunctive Normal Form), which is the prevalent language of Boolean logic theories.
The NNF language and its subsets
Negation normal form (NNF) is a rooted, directed acyclic graph where each leaf is labeled with true, false, and each internal leaf is labeled with or .
Variables of a node in the NNF structure can be exploited by .
Many of the target data structures used by knowledge compilation for logical theories are subsets of NNF. These subsets correspond to NNFs with specific properties. Here are such properties.
For all and-node in an NNF, with its children . If for this NNF satisfies the decomposability property .
For all or-node in an NNF, with its children . If for this NNF satisfies the determinism property .
For all or-node in an NNF, with its children . If for this NNF satisfies the smoothness property .
A decision node in an NNF, is a node labeled with true, false, or is an or-node having the form , in which X is a variable and and are decision nodes .
Subsets of NNF language
There are different kinds of languages which are subsets of NNF language. Some of them are widely studied in computer science. This section introduces them by their acronym in table 1 taken from .
DNNF is a NNF with decomposability property, BDD is an NNF whose root is a decision node, and OBDD is defined as well-known ordered binary decision diagrams . Other languages are easy to recognize by their acronyms.
|NNF||Negation Normal Form|
|DNNF||Decomposable Negation Normal Form|
|d-NNF||Deterministic Negation Normal Form|
|s-NNF||Smooth Negation Normal Form|
|d-DNNF||Deterministic Decomposable Negation Normal Form|
|sd-DNNF||Smooth Deterministic Decomposable Negation Normal Form|
|BDD||Binary Decision Diagram|
|OBDD||Ordered Binary Decision Diagram|
There are some important queries for logical theories which have different time complexities in different languages. There is a list of some important queries and a brief description of them in table 2 taken from .
|CO||polytime consistency check|
|VA||polytime validity check|
|CE||polytime clausal entailment check|
|IM||polytime implicant check|
|EQ||polytime equivalence check|
|SE||polytime sentential entailment check|
|CT||polytime model counting|
|ME||polytime model enumeration|
Table 3 taken from  shows subsets of the NNF language and queries they support in polytime. ✔ means satisfaction of that query on the corresponding language, ✘ means not satisfaction of that query on the corresponding language, and ? means it is an open problem.
There are algorithms for compiling a Boolean logic theory into the above languages. A detail analysis of these algorithms is out of this page's scope. Interested readers can refer to c2d compiler and DSHARP.
Imposing different properties like decomposability, smoothness, and determinism to NNFs results in more polynomial operation. On the other side adding different properties makes the data structure exponentially larger. for example there is a succinctness ordering of target compilation language between DNNF and OBDD which is taken from :
 Darwiche, Adnan. "Decomposable negation normal form." Journal of the ACM (JACM) 48.4 (2001): 608-647.
 Darwiche, Adnan. "On the tractable counting of theory models and its application to belief revision and truth maintenance." arXiv preprint cs/0003044 (2000).
 Darwiche, Adnan, and Pierre Marquis. "A knowledge compilation map." Journal of Artificial Intelligence Research 17.1 (2002): 229-264.
 Bryant, Randal E. "Graph-based algorithms for boolean function manipulation." Computers, IEEE Transactions on 100.8 (1986): 677-691.
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