Golang’s quick testing package is great for generating random input for your property based test. Sometimes you want to control how the input values are generated. Here’s how you can create a custom generator for the input parameters to your property based test.

Lets assume you want to test the function IsValid(string) with many random inputs …. but …. the input may only contain characters in range a-z, A-Z, and 0-9. Here’s how to do it:

func randomAlphaNumericString(output []reflect.Value, rnd *rand.Rand) {
    alphabet := []rune("abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ0123456789")
    size := rnd.Intn(8192)
    v := reflect.New(reflect.TypeOf("")).Elem()
    v.SetString(randStringOfLen(rnd, size, alphabet))
    output[0] = v
}

//Generates a string of len n containing random characters from alphabet
func randStringOfLen(rnd *rand.Rand, n int, alphabet[]rune) string {
    b := make([]rune, n)
    for i := range b {
        b[i] = alphabet[rnd.Intn(len(alphabet))]
    }
    return string(b)
}

func TestMyMethod(t *testing.T) {
    propertyTest := func(input string) bool { return true == IsValue(input) }
    c := quick.Config{MaxCount: 1000, Values: randomAlphaNumericString}
    if err := quick.Check(propertyTest , &c); err != nil {
        t.Error(err)
    }
}

Lets break it down into steps:

Firstly we define our propertyTest to check all input strings are valid. So far nothing special. This function takes a single string input parameter.

The quick.Config struct has the Values member. This member lets you supply a function to generate whatever parameters propertyTest needs. In our case the randomAlphaNumericString function does that job.

The randomAlphaNumericString function generates a suitable random string and it stores it as a reflect.Value in the output array at the position where the propertyTest expects to receive a string parameter.

In a previous prost, I explained property based testing. In this post we’ll see a simple example using golang’s built-in quick package.

Let assume we’re building the same bank transfer code as described in the dotnet FsCheck earlier post.

Here’s the golang equivalent of the test:

package goquickcheckexample

import (
	"testing"
	"testing/quick"
)

func TestProperties(t *testing.T) {
	bank := BuggyBank{}
	properties := func(StartBalanceA int, StartBalanceB int, TransferAmount int) bool {
		bank.BalanceOfAccountA = StartBalanceA
		bank.BalanceOfAccountB = StartBalanceB
		err := bank.Transfer(TransferAmount)
		if err != nil {
			//Transfer failed
			balancesChanged := (bank.BalanceOfAccountA != StartBalanceA) || (bank.BalanceOfAccountB != StartBalanceB)
			if balancesChanged {
				t.Log("Balances changed on failed transfer")
			}
			return !balancesChanged
		}
		//Transfer succeeded
		balanceAIsNonNegative := bank.BalanceOfAccountA >= 0
		balanceAChanged := bank.BalanceOfAccountA != StartBalanceA
		balanceBchanged := bank.BalanceOfAccountB != StartBalanceB
		if !balanceAIsNonNegative {
			t.Log("Balance of A ended negative")
		}
		if !balanceAChanged {
			t.Log("Balance of A did not change")
		}
		if !balanceBchanged {
			t.Log("Balance of B did not change")
		}
		return balanceAIsNonNegative && balanceAChanged && balanceBchanged
	}

	c := quick.Config{MaxCount: 1000000}
	if err := quick.Check(properties, &c); err != nil {
		t.Error(err)
	}
}

Note: If you want all test runs to use the same set of random numbers then use: c := quick.Config{MaxCount: 1000000, Rand: rand.New(rand.NewSource(0))}

When I ran the test, it detected a defect: Transfers succeed even when the source account’s balance is insufficient:

bank_test.go:28: Balance of A ended negative
bank_test.go:41: #2: failed on input 6319534437456565100, -3125004238116898490, 8226184717426742479

After fixing that bug, it detected a defect: The code allowed transferring negative amounts:

bank_test.go:34: Balance of A ended negative
bank_test.go:47: #22: failed on input 5995030153294015290, -7891513722668943486, -3464545538278061921

While analyzing this defect we notice yet another one: This code is not safe against integer overflow.