This master’s thesis addresses the challenge of assortment planning in retail under uncertain demand and operational constraints. It develops a hybrid methodology that integrates SARIMAX time-series forecasting with the Discrete Functional Particle Method (DFPM) for optimisation, enabling both strategic (long-term) and tactical (monthly) decision support. Key elements include the preprocessing of real sales and inventory data, construction of an Inventory Efficiency Ratio as a risk metric, eigenvalue-guided tuning of DFPM parameters, and blending of mathematically optimal weights with historical baselines. The framework is implemented in Python and structured into theoretical foundations, practical application, and performance analysis. The work contributes a flexible, data-driven approach to improve assortment decisions in dynamically changing retail environments.