Intensified iterative deepening A* with application to job shop scheduling

We propose a novel, exact any-time search strategy that combines iterative deepening $$\text{ A}$$* ($$\text{ IDA}$$*) with depth-first search and we consider the job shop scheduling problem with makespan minimization as a test bed. The combination of these search strategies is done so that limited depth-first searches are issued from some of the states distributed along the frontier reached by $$\text{ IDA}$$* in each iteration. In this way, a proper equilibrium between intensification and diversification search effort is achieved while the algorithm keeps the capability of obtaining tight lower bounds. To evaluate the proposed strategy and to compare it with other methods, we have conducted an experimental study involving a number of conventional benchmarks with instances of various sizes. The results of these experiments show that the proposed algorithm takes less time than other methods in reaching optimal solutions for small and medium-size instances, and that it is quite competitive in reaching good solutions and good lower bounds for the instances that cannot be optimally solved.