call ChallengeCode without VerifyCode

README.md

@@ -56,12 +56,12 @@ func main() {
 		Length: 128,
 	}
 
-	fmt.Println(p.VerifyCode())
+	fmt.Println(p.VerifyCode()) // This is optional
 	// Output: 45d7820e694481f399e7fb9c444f0cb63301a7254d1401443835d9af2c9a6a5ec5b243c3470feb945336025964ef05c8d2f0e44baf76762ba6136914
 
 	fmt.Println(p.ChallengeCode())
 	// Output: iQoF8w9kq5RnuMdisRXypyOoMCF7FGz-ro7dwHjC28U
 }
 ```
 
-> Note: You have to call `p.VerifyCode()` before `p.ChallengeCode()` to generate a random string and its hash
+> Note: Calling `p.VerifyCode()` optional, but calling it after `p.ChallengeCode()` will reset `pkce.RandomString`

pkce.go

@@ -47,8 +47,15 @@ func (p *Pkce) VerifyCode() (string, error) {
 // ChallengeCode returns a challenge code as mentioned in https://tools.ietf.org/html/rfc7636#section-4.2.
 // The code is based on
 // 	code_challenge = BASE64URL-ENCODE(SHA256(ASCII(code_verifier)))
-func (p *Pkce) ChallengeCode() string {
+func (p *Pkce) ChallengeCode() (string, error) {
+	if p.RandomString == "" {
+		code, err := p.VerifyCode()
+		if err != nil {
+			return "", err
+		}
+		p.RandomString = code
+	}
 	hash := sha256.Sum256([]byte(p.RandomString))
 	toBase64 := base64.RawURLEncoding.EncodeToString(hash[:])
-	return toBase64
+	return toBase64, nil
 }

pkce_test.go

@@ -13,7 +13,30 @@ func TestPkce_ChallengeCode(t *testing.T) {
 		RandomString: "45d7820e694481f399e7fb9c444f0cb63301a7254d1401443835d9af2c9a6a5ec5b243c3470feb945336025964ef05c8d2f0e44baf76762ba6136914",
 	}
 
-	assert.Equal(t, "iQoF8w9kq5RnuMdisRXypyOoMCF7FGz-ro7dwHjC28U", p.ChallengeCode())
+	code, err := p.ChallengeCode()
+	if assert.Nil(t, err) {
+		assert.Equal(t, "iQoF8w9kq5RnuMdisRXypyOoMCF7FGz-ro7dwHjC28U", code)
+	}
+}
+
+func TestPkce_ChallengeCode2(t *testing.T) {
+	p := Pkce{
+		Length: 50,
+	}
+
+	code, err := p.ChallengeCode()
+	if assert.Nil(t, err) {
+		assert.IsType(t, "", code)
+	}
+}
+
+func TestPkce_ChallengeCode3(t *testing.T) {
+	p := Pkce{}
+
+	_, err := p.ChallengeCode()
+	if assert.Error(t, err) {
+		assert.Equal(t, errors.New("length should be greater than 0"), err)
+	}
 }
 
 func TestPkce_VerifyCode(t *testing.T) {
@@ -103,7 +126,11 @@ func ExamplePkce_ChallengeCode() {
 	p := Pkce{
 		RandomString: "45d7820e694481f399e7fb9c444f0cb63301a7254d1401443835d9af2c9a6a5ec5b243c3470feb945336025964ef05c8d2f0e44baf76762ba6136914",
 	}
-	fmt.Println(p.ChallengeCode())
+	code, err := p.ChallengeCode()
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(code)
 	// Output: iQoF8w9kq5RnuMdisRXypyOoMCF7FGz-ro7dwHjC28U
 }
 
@@ -115,5 +142,9 @@ func ExamplePkce_ChallengeCode_generatedString() {
 	if err != nil {
 		panic(err)
 	}
-	fmt.Println(code, p.ChallengeCode())
+	challengeCode, err := p.ChallengeCode()
+	if err != nil {
+		panic(err)
+	}
+	fmt.Println(code, challengeCode)
 }