Fix thread safety

diacritics.go

@@ -9,13 +9,6 @@ import (
 	"golang.org/x/text/unicode/norm"
 )
 
-var transformChain = transform.Chain(
-	runes.Map(mapDecomposeUnavailable),
-	norm.NFD,
-	runes.Remove(runes.In(unicode.Mn)),
-	norm.NFC,
-)
-
 var unavailableMapping = map[rune]rune{
 	'\u0181': 'B',
 	'\u1d81': 'd',
@@ -193,6 +186,13 @@ var unavailableMapping = map[rune]rune{
 
 // Normalize removes diacritical characters and replaces them with their ASCII representation
 func Normalize(input string) string {
+	var transformChain = transform.Chain(
+		runes.Map(mapDecomposeUnavailable),
+		norm.NFD,
+		runes.Remove(runes.In(unicode.Mn)),
+		norm.NFC,
+	)
+
 	input = strings.Replace(input, "\u00df", "ss", -1) // ß to ss handling
 	result, _, _ := transform.String(transformChain, input)
 	return result

diacritics_test.go

@@ -1,6 +1,7 @@
 package godiacritics
 
 import (
+	"sync"
 	"testing"
 
 	"github.com/stretchr/testify/assert"
@@ -27,3 +28,19 @@ func TestRemoval(t *testing.T) {
 		})
 	}
 }
+
+func TestRemovalParallel(t *testing.T) {
+	wg := sync.WaitGroup{}
+
+	for i := 0; i < 1000; i++ {
+		wg.Add(1)
+
+		go func() {
+			actual := Normalize("Ä möre ȼꝋmpleᶍ ⱸxamplé")
+			assert.Equal(t, "A more complex example", actual)
+			wg.Done()
+		}()
+	}
+
+	wg.Wait()
+}