Mistral AI migrates 40,000 lines of Fortran 77 to C++ for energy operator

Mistral AI assisted a European energy operator in migrating 40,000 lines of a physics-intensive reservoir simulator from Fortran 77 to C++. The legacy code lacked a test suite and centralized documentation, and the migration required architectural refactoring beyond simple syntax translation, such as converting global COMMON blocks to object-oriented C++ structures. To ensure accuracy, a parity harness was built to verify numerical equality between the legacy and migrated code, including final results and critical intermediate points.