IMPORTANT, MUST BE READ... : The Spaghetti Factor -- H5N1 Software Complexity Metric Proposal
Title : The Spaghetti Factor -- H5N1 Software Complexity Metric Proposal
The Spaghetti Factor -- H5N1 Software Complexity Metric Proposal
IMPORTANT, MUST BE READ...
I've had to review code that has spaghetti-level complexity inwards command menses (too high cyclomatic complexity). And I've had to review code that has spaghetti-level complexity its information menses (too many global variables mixed together into a unmarried computation). And I've had to review procedures that precisely instruct on for page later page alongside no terminate inwards sight. But the materials that volition actually brand your encephalon wound is code that has all of these problems.
There are many complexity metrics out there. But I haven't seen a 1 that direct tries to assistance repose 3 cardinal points of complexity into a unmarried intuitive number: code complexity, information complexity, in addition to module size. So hither is a proposal that could assistance drive improvement inwards a lot of the terrible embedded command code I've seen:
The Spaghetti Factor metric (SF):
SF = SCC + (Globals*5) + (SLOC/20)
SCC = Strict Cyclomatic Complexity
Globals = # of read/write global variables
SLOC = # source lines of non-comment code (e.g., C statements)
Scoring:
5-10 - This is the sweetness spot for most code except elementary helper functions
fifteen - Don't instruct inwards a higher house this for most modules
xx - Look closely; review to run into if refactoring makes sense
thirty - Refactor the design
l - Untestable; throw the module away in addition to laid the design
75 - Unmaintainable; throw the module away; throw the pattern away; commencement over
100 - Nightmare; in all likelihood you lot bespeak to throw the whole subsystem away in addition to re-architect it
Notation:
SCC is Strict Cyclomatic Complexity (sometimes called CC2). This is a variant of McCabe Cyclomatic complexity (MCC). In full general terms, MCC is based on the divulge of branches inwards the program. SCC additionally considers complexity based on the divulge of atmospheric condition inside each branch. SCC is an approximation of how many bear witness cases it takes to exercise all the paths through code including all the unlike ways at that topographic point are to trigger each branch. In other words, it is an justice of how much piece of occupation it is to do unit of measurement testing. Think of it every bit an approximation to the seek required for MC/DC testing. But inwards exercise it is also a mensurate of how hard it is to empathize the code. The thought is to continue SCC depression plenty that it is viable to empathize in addition to bear witness paths through the code.
Globals is the divulge of read/write global variables accessed past times the module. This does non include "const" values, nor file static variables. In an ideal footing you lot receive got zip or near-zero global variables. If you lot receive got inherent global state, you lot should encapsulated that inwards a nation object alongside appropriate access functions to enforce well-disciplined writes. Referencing an unstructured pile of dozens or hundreds of global variables tin brand software hard to test, in addition to tin brand subsystem testing almost impossible. Partly that is due to the bear witness scaffolding required, but partly that is precisely due to the seek of chasing downwards all the globals in addition to trying to figure out what they do both inbound in addition to outbound. Moreover, likewise many globals tin instruct far nearly impossible to chase downwards bugs or empathize the effects of changing 1 component division of the code on the repose of the code. An of import destination of this component division of the metric is to discourage exercise of many disjoint global variables to implicitly transcend information closed to from routine to routine instead of passing parameters along alongside business office calls.
SLOC is the divulge of non-comment "Source Lines of Code." For C programs, this is the divulge of programming statements. Typical guidelines are a maximum 100-225 maximum lines of code for a module, alongside most modules beingness smaller than that.
As an illustration calculation, if you lot receive got 100 lines of code alongside an SCC of ix in addition to 1 global reference, your marking volition last SF = ix + (1*5) + (100/20) = 19. A marking of nineteen is on the upper border of beingness OK. If you lot receive got a distribution of complexity across modules, you'd desire most of them to last a chip lower inwards complexity than this illustration calculation.
Discussion:
The guideline values are taken primarily from MCC, which typically has a guideline of 10 for most modules, fifteen every bit a commons bound, in addition to thirty every bit limit. To trace of piece of occupation organisation human relationship for globals in addition to length, based on my experience, I've changed that to fifteen for most modules, xx every bit a soft bound in addition to thirty every bit a hard limit. You mightiness wishing to suit the threshold in addition to multipliers based on your organisation in addition to experience. In item it is slow to brand a instance that these limits aren't strict plenty for life-critical software, in addition to a instance tin last made for beingness a piddling to a greater extent than relaxed inwards throw-away GUI management code. But I retrieve this is a proficient starting signal for most every-day embedded software that is written past times a human (as opposed to auto-generated code).
The biggest exception is unremarkably what to do virtually switch statements. If you lot exempt them you lot tin terminate upward alongside multiple switches inwards 1 module, or multiple switch/if/switch/if layered nesting. (Neither is a pretty sight.) I retrieve it is justifiable to exempt modules that receive got ONLY a switch in addition to conditional logic to do sanity checking on the switch value. But, because thirty is a pretty generous limit, you're alone going to run into this rarely. Generally the alone legitimate argue to receive got a switch bigger than that is for something similar processing a message type for a communication protocol. So I retrieve you lot should non blanket exempt switch statements, but rather include them inwards an overall case-by-case sign-off past times technology scientific discipline management every bit to which few exceptions are justifiable.
Some mightiness brand the observation that this metric discourages extensive fault checking. That's a unlike topic, in addition to certainly the intent is NOT to discourage fault checking. But the elementary respond is that fault checking has to last tested in addition to understood, thence you lot can't precisely ignore that component division of the complexity. One way to handgrip that province of affairs is to seat fault checking into a subroutine or wrapper business office to instruct that complexity out of the way, in addition to thence receive got that wrapper telephone telephone the actual business office that does the work. Another way is to intermission your overall code downwards into smaller pieces thence that each slice is elementary plenty for you lot to empathize in addition to bear witness both the functionality in addition to the fault checking.
Finally, whatsoever metric tin last gamed, in addition to that is for sure truthful of elementary metrics similar this one. A proficient metric marking doesn't necessarily hateful your code is fantastic. Additionally, this metric does non consider everything that's important, such every bit the full divulge of globals across your code base. On the other hand, if you lot marking poorly on this metric, most probable your code is inwards bespeak of improvement.
What I recommend is that you lot exercise this metric every bit a way to position code that is needlessly complex. It is the rare slice of code indeed that unavoidably needs to receive got a high marking on this complexity metric. And if all your code has a proficient score, that agency it should last that much easier to do peer review in addition to unit of measurement testing to ensure that other aspects of the code are inwards proficient shape.
References:
- Blog post on cyclomatic complexity
- Jack Ganssle on how to compute MCC
- Description of MCC variants, including SCC (CC2)
- Blog post on global variables
- Blog post on modularity
H5N1 NIST newspaper on applying metrics is here: http://www.mccabe.com/pdf/mccabe-nist235r.pdf including an interesting give-and-take of the pitfalls of treatment switch statements inside a complexity framework.
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