Generating Random String of Numbers and Letters Using Go's "testing/quick" Package

Question

I've been breaking my head over this for a few days now and can't seem to be able to figure it out. Perhaps it's glaringly obvious, but I don't seem to be able to spot it. I've read up on all the basics of unicode, UTF-8, UTF-16, normalisation, etc, but to no avail. Hopefully somebody's able to help me out here...

I'm using Go's Value function from the testing/quick package to generate random values for the fields in my data structs, in order to implement the Generator interface for the structs in question. Specifically, given a Metadata struct, I've defined the implementation as follows:

func (m *Metadata) Generate(r *rand.Rand, size int) (value reflect.Value) {
    value = reflect.ValueOf(m).Elem()
    for i := 0; i < value.NumField(); i++ {
        if t, ok := quick.Value(value.Field(i).Type(), r); ok {
            value.Field(i).Set(t)
        }
    }
    return
}

Now, in doing so, I'll end up with both the receiver and the return value being set with random generated values of the appropriate type (strings, ints, etc. in the receiver and reflect.Value in the returned reflect.Value).

Now, the implementation for the Value function states that it will return something of type []rune converted to type string. As far as I know, this should allow me to then use the functions in the runes, unicode and norm packages to define a filter which filters out everything which is not part of 'Latin', 'Letter' or 'Number'. I defined the following filter which uses a transform to filter out letters which are not in those character rangetables (as defined in the unicode package):

func runefilter(in reflect.Value) (out reflect.Value) {
    out = in // Make sure you return something
    if in.Kind() == reflect.String {
        instr := in.String()
        t := transform.Chain(norm.NFD, runes.Remove(runes.NotIn(rangetable.Merge(unicode.Letter, unicode.Latin, unicode.Number))), norm.NFC)
        outstr, _, _ := transform.String(t, instr)
        out = reflect.ValueOf(outstr)
    }
    return
}

Now, I think I've tried just about anything, but I keep ending up with a series of strings which are far from the Latin range, e.g.:

𥗉똿穊

𢷽嚶
秓䝏小𪖹䮋
𪿝ท솲
𡉪䂾

ʋ𥅮ᦸ
堮𡹯憨𥗼𧵕ꥆ
𢝌𐑮𧍛併怃𥊇
鯮
𣏲𝐒
⓿ꐠ槹𬠂黟
𢼭踁퓺𪇖
俇𣄃𔘧

𢝶
𝖸쩈𤫐𢬿詢𬄙

𫱘𨆟𑊙
欓

So, can anybody explain what I'm overlooking here and how I could instead define a transformer which removes/replaces non-letter/number/latin characters so that I can use the Value function as intended (but with a smaller subset of 'random' characters)?

Thanks!

Answer 1

The answer to my own question is, it seems, a combination of the answers provided in the comments by @nj_ and @jimb and the answer provided by @benjaminkadish.

In short, the answer boils down to:

1. "Not such a great idea as you thought it was", or "Bit of an ill-posed question"
2. "You were using the union of 'Letter', 'Latin' and 'Number' (Letter || Number || Latin), instead of the intersection of 'Latin' with the union of 'Letter' and 'Number' ((Letter || Number) && Latin))

Now for the longer version...

The idea behind me using the testing/quick package is that I wanted random data for (fuzzy) testing of my code. In the past, I've always written the code for doing things like that myself, again and again. This meant a lot of the same code across different projects. Now, I could of course written my own package for it, but it turns out that, even better than that, there's actually a standard package which does just about exactly what I want.

Now, it turns out the package does exactly what I want very well. The codepoints in the strings which it generates are actually random and not just restricted to what we're accustomed to using in everyday life. Now, this is of course exactly the thing which you want in doing fuzzy testing in order to test the code with values outside the usual assumptions.

In practice, that means I'm running into two problems:

1. There's some limits on what I would consider reasonable input for a string. Meaning that, in testing the processing of a Name field or a URL field, I can reasonably assume there's not going to be a value like 'James Mc⌢' (let alone 'James Mc🙁') or 'www.🕸site.com', but just 'James McFrown' and 'www.website.com'. Hence, I can't expect a reasonable system to be able to support it. Of course, things shouldn't completely break down, but it also can't be expected to handle the former examples without any problems.
2. When I filter the generated string on values which one might consider reasonable, the chance of ending up with a valid string is very small. The set of possible characters in the set used by the testing/quick is just so large (0x10FFFF) and the set of reasonable characters so small, you end up with empty strings most of the time.

So, what do we need to take away from this?

So, whilst I hoped to use the standard testing/quick package to replace my often repeated code to generate random data for fuzzy testing, it does this so well that it provides data outside the range of what I would consider reasonable for the code to be able to handle. It seems that the choice, in the end, is to:

1. Either be able to actually handle all fuzzy options, meaning that if somebody's name is 'Arnold 💰💰' ('Arnold Moneybags'), it shouldn't go arse over end. Or...
2. Use custom/derived types with their own Generator. This means you're going to have to use the derived type instead of the basic type throughout the code. (Comparable to defining a string as wchar_t instead of char in C++ and working with those by default.). Or...
3. Don't use testing/quick for fuzzy testing, because as soon as you run into a generated string value, you can (and should) get a very random string.

As always, further comments are of course welcome, as it's quite possible I overlooked something.

Answer 2

Confusingly the Generate interface needs a function using the type not a the pointer to the type. You want your type signature to look like

func (m Metadata) Generate(r *rand.Rand, size int) (value reflect.Value)

You can play with this here. Note: the most important thing to do in that playground is to switch the type of the generate function from m Metadata to m *Metadata and see that Hi Mom! never prints.

In addition, I think you would be better served using your own type and writing a generate method for that type using a list of all of the characters you want to use. For example:

type LatinString string
const latin = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz01233456789"

and then use the generator

func (l LatinString) Generate(rand *rand.Rand, size int) reflect.Value {
    var buffer bytes.Buffer
    for i := 0; i < size; i++ {
        buffer.WriteString(string(latin[rand.Intn(len(latin))]))
    }
    s := LatinString(buffer.String())
    return reflect.ValueOf(s)
}

playground

Edit: also this library is pretty cool, thanks for showing it to me