PhD Thesis - staffweb - University of Greenwich

APPENDIX 2 : Journal Paper "Case Study : An Incremental Approach to Re-Engineeringa Legacy FORTRAN C.F.D. Code in C++", Ewer J., Knight B. and Cowell D.,Advances in Engineering Software, Vol. 22, pp 153-168, 1995perform some function or uses many intermediate stages for calculating some property. Inthe example code fragment (Figure 1) the routines HCSOLV and LINRLX demonstrate thedifferent levels of abstraction mixed together. HCSOLV actually takes flow properties,physical properties and previous conditions to calculate the new values of enthalpy forevery cell of the domain. The HCSOLV routine was only called once in the whole code.Conversely LINRLX merely added a fraction of the difference between an old and newvalue of some property and was called from many places within the code. It was concludedthat HCSOLV was a highly abstracted routine whereas the less abstracted LINRLX was autility routine that should be moved down into HCSOLV. The simple assignmentstatements were inappropriately located and should be moved into adjacent routines.Legacy code fragmentIF ( IHEAT ) THENRELAXA = SRELAX(5)RMETHD = VRMETH(5)MITERS = MAXITR(5)FALSET = VFALST(5)CALL HCSOLV(3, RELAXA, ... )CALL SYSRES( ... )SOLERR(5) = RESIDURELAXA = VRELAX(5)CALL LINRLX( ... )VARERR(5) = RESIDURELAXA = VRELAX(8)RMETHD = VRMETH(8)CALL CSOLVT( ... )IF ( ERRINF .EQ. 0 ) STOPVARERR(8) = RESIDUENDIFis problematic becauseLiteral values used and simple assignmentsprior to calling a complex numericalcalculation routine.Highly abstracted routine call.Less abstracted utility routine call.Utility routine call.More low level simple assignments.Call to highly abstract routine.Figure 1 :Mixed levels of abstraction.- Inflexible memory usage. FORTRAN-77 array sizes must be statically declared at compiletime. This limits the flexibility of the code for running problems of different sizes. Thisoften forces a re-compilation of the system for the specific problem to be run. This wasparticularly true of the un-structured mesh code where a few cells with a high number offaces greatly increased the storage requirements of all cells.FORTRAN codeSOLTYP(1) = 5K = K + 0.37777 * (...)is problematic because:The SOLTYP array holds the solver types but the index1 and solver type 5 could relate to anything. Theintended meanings must be found elsewhere in the code.Explicit values used in numerical algorithmsgive no indication of the purpose of the values used.Figure 2 :Use of literal constants.- Excessive use of literal constants. Much of the meaning of CWNN was hidden by the useof literal constants. Future code developers would have little understanding of what theseAppendix 11.2 Page 142-7 7