Refactor random number generation and add version-specific implementations

Removed error handling from `randNum` in `random_test.go` and introduced new `randNum` implementations for different Go versions (1.19, 1.20-1.21, 1.22+). This ensures compatibility with different versions of Go and utilizes the appropriate version-specific random number generation methods. We are using math/rand instead crypto/rand. For our needs this should be sufficient.

random.go

@@ -7,8 +7,6 @@ package mail
 import (
 	"crypto/rand"
 	"encoding/binary"
-	"fmt"
-	"math/big"
 	"strings"
 )
 
@@ -52,23 +50,3 @@ func randomStringSecure(length int) (string, error) {
 
 	return randString.String(), nil
 }
-
-// randNum returns a random number with a maximum value of length
-func randNum(length int) (int, error) {
-	if length <= 0 {
-		return 0, fmt.Errorf("provided number is <= 0: %d", length)
-	}
-	length64 := big.NewInt(int64(length))
-	if !length64.IsUint64() {
-		return 0, fmt.Errorf("big.NewInt() generation returned negative value: %d", length64)
-	}
-	randNum64, err := rand.Int(rand.Reader, length64)
-	if err != nil {
-		return 0, err
-	}
-	randomNum := int(randNum64.Int64())
-	if randomNum < 0 {
-		return 0, fmt.Errorf("generated random number does not fit as int64: %d", randNum64)
-	}
-	return randomNum, nil
-}

random_119.go

@@ -0,0 +1,22 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+//go:build go1.19 && !go1.20
+// +build go1.19,!go1.20
+
+package mail
+
+import (
+	"math/rand"
+	"time"
+)
+
+// randNum returns a random number with a maximum value of length
+func randNum(maxval int) int {
+	if maxval <= 0 {
+		return 0
+	}
+	rand.Seed(time.Now().UnixNano())
+	return rand.Intn(maxval)
+}

random_121.go

@@ -0,0 +1,20 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+//go:build go1.20 && !go1.22
+// +build go1.20,!go1.22
+
+package mail
+
+import (
+	"math/rand"
+)
+
+// randNum returns a random number with a maximum value of length
+func randNum(maxval int) int {
+	if maxval <= 0 {
+		return 0
+	}
+	return rand.Intn(maxval)
+}

random_122.go

@@ -0,0 +1,22 @@
+// SPDX-FileCopyrightText: 2022-2023 The go-mail Authors
+//
+// SPDX-License-Identifier: MIT
+
+//go:build go1.22
+// +build go1.22
+
+package mail
+
+import (
+	"math/rand/v2"
+)
+
+// randNum returns a random number with a maximum value of maxval.
+// go-mail compiled with Go 1.22+ will make use of the novel math/rand/v2 interface
+// Older versions of Go will use math/rand
+func randNum(maxval int) int {
+	if maxval <= 0 {
+		return 0
+	}
+	return rand.IntN(maxval)
+}

random_test.go

@@ -55,13 +55,7 @@ func TestRandomNum(t *testing.T) {
 
 	for _, tc := range tt {
 		t.Run(tc.testName, func(t *testing.T) {
-			rn, err := randNum(tc.max)
+			rn := randNum(tc.max)
-			if err != nil {
-				t.Errorf("random number generation failed: %s", err)
-			}
-			if rn < 0 {
-				t.Errorf("random number generation failed: %d is smaller than zero", rn)
-			}
 			if rn > tc.max {
 				t.Errorf("random number generation failed: %d is bigger than given value %d", rn, tc.max)
 			}