An External Partial Permutations Memory for Ant Colony Optimization

A novel external memory implementation based on the use of partially complete sequences of solution components from above-average quality individuals over a number of previous iterations is introduced. Elements of such variable-size partial permutation sequences are taken from randomly selected positions of parental individuals and stored in an external memory called the partial permutation memory. Partial permutation sequences are associated with lifetimes together with their parent solutions’ fitness values that are used in retrieving and updating the contents of the memory. When a solution is to be constructed, a partial permutation sequence is retrieved from the memory based on its age and associated fitness value, and the remaining components of the partial solution is completed with an ant colony optimization algorithm. Resulting solutions are also used to update some elements within the memory. The implemented algorithm is used for the solution of a difficult combinatorial optimization problem, namely the quadratic assignment problem, for which significant performance achievements are provided in terms of convergence speed and solution quality.