Thinking about edge cases

Software developers use the edge case liberally in conversations. I have the impression it's used without much thought and mostly means: "I don't want to care about this now". Even when it is used more thoughtfully edge case comes with some assumptions that do not hold. We are better off without the term.

What does edge case mean? Let's start with the "case" part: it's some state your software can be in. Okay, that was easy. Now to the "edge" part: this is a bit more fuzzy as software generally doesn't come with edges. I think, it means two things: something at the boundary of the input type domain and a thing of low probability.

If have a 1-dimensional type that is finite from values [x,z] then you can expect that there should be more problems on "the edges" so close to x and z. An example could be [0, max integer].

This makes all sense. The problem is there aren't many of those in your software, even 1-dimensional types have more boundaries. The most lowly type int has a lot of edges to look at [min int, -1, 0, -1, max int]. Once you convert from int32 to int64 you get the union of the edges of both types and this is only lowly int.

If you have a decimal type then edges are harder to define: there are no min/max values (for most implementations) but you get CPU caching related "edges". Your number could be spread over multiple pages. Functionally this is the same behavior but now you could run into performance problems.

This again was only for lowly integer types. The next level of complexity is floating point types: resolution of numbers, min, max, NAN, INF. A whole new set of problems.

Okay what's the mean of all this? Typically, I don't know your types well you’re programming with. You also cannot know the interesting cases.

The probability meaning is even trickier. In the general case you don't know the input value distribution so you have to assume that all values are used. This is the safe and easy assumption. Pragmatically, you can assume that some values aren't actually used in your context as you also control the caller. In this case you can redefine the domain of input values. This allows to lower your software development cost.

What doesn't hold is that "edge cases" are different and allow anything other than to answer: “This is either correct or incorrect”. This would mean we also allow undefined results which are either undefined correct or undefined incorrect.

What's your caller doing if the result is undefined and the range of input values is also undefined that produces those values? Accept occasional disaster. If you want to avoid you have to avoid undefined behavior.

Okay, what can I do?

Step 0: Is your software important enough? Correctness won't come for free. Define how much you want to invest into correctness. If the cost of an invalid result is lower than a correct implementation stop.

Step 1: Define the allowed input values. If you do not give an answer for a given input you cannot make an error.

Step 2a: As a testing strategy: start with what you know and write explicit test.

If you're lucky you know your types really well. This is half realistically for integer and I wouldn't bet that I can write correct code for float, string or timestamps.

Step 2b: For all other types try to exhaustively test. Drop the concept of an edge case. If I don't know the type I'm not able to define the interesting cases. Look into something like Smallcheck for exhaustively testing "small" values and Quickcheck and fuzzing for randomized testing.

Step 3: Never use the word edge case again.

The positive result of this humble world view is that you'll learn. As an exercise you can write a simple function something that works with 2 float values, 2 dates with different time zones or a string. Look at all the problems once you start testing values you haven't thought about. The floating point, Unicode and calendar implementations have enough juice to make your life interesting. If you dear to look.

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