Dynamic Programming via Static Incrementalization
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Dynamic programming is an important algorithm design technique. It is used for problems whose solutions involve recursively solving subproblems that share subsubproblems. While a straightforward recursive program solves common subsubproblems repeatedly, a dynamic programming algorithm solves every subsubproblem just once, saves the result, and reuses it when the subsubproblem is encountered again. This can reduce the time complexity from exponential to polynomial. This paper describes a systematic method for transforming programs written as straightforward recursions into programs that use dynamic programming. The method extends the original program to cache all possibly computed values, incrementalizes the extended program with respect to an input increment to use and maintain all cached results, prunes out cached results that are not used in the incremental computation, and uses the resulting incremental program to form an optimized new program. Incrementalization statically exploits semantics of both control structures and data structures and maintains as invariants equalities characterizing cached results. It provides the basis of a general method for achieving drastic program speedups. Compared with previous methods that perform memoization or tabulation, the method based on incrementalization is more powerful and systematic. It has been implemented in a prototype system CACHET and applied to numerous problems and succeeded on all of them.
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