fix idn-hostname validation

Author: OptimumCode

json-schema-validator/src/commonMain/kotlin/io/github/optimumcode/json/schema/internal/formats/IdnHostnameFormatValidator.kt

@@ -48,7 +48,7 @@ internal object IdnHostnameFormatValidator : AbstractStringFormatValidator() {
     if (value.isEmpty()) {
       return FormatValidator.Invalid()
     }
-    if (value.length == 1 && isLabelSeparator(value[0])) {
+    if (isLabelSeparator(value[0]) || isLabelSeparator(value[value.lastIndex])) {
       return FormatValidator.Invalid()
     }
 
@@ -113,6 +113,11 @@ internal object IdnHostnameFormatValidator : AbstractStringFormatValidator() {
       return false
     }
 
+    if (unicode.isEmpty()) {
+      // empty labels are not valid
+      return false
+    }
+
     // https://datatracker.ietf.org/doc/html/rfc5891#section-4.2.3.1
     if (unicode[0] == '-' || unicode.codePointBefore(unicode.length) == '-'.code) {
       // cannot start or end with hyphen

json-schema-validator/src/commonTest/kotlin/io/github/optimumcode/json/schema/assertions/general/format/JsonSchemaIdnHostnameFormatValidationTest.kt

@@ -33,6 +33,9 @@ class JsonSchemaIdnHostnameFormatValidationTest : FunSpec() {
         listOf(
           TestCase("", "empty value"),
           TestCase(".", "single separator"),
+          TestCase(".example", "leading separator"),
+          TestCase("example.", "trailing separator"),
+          TestCase("example..com", "two separators in a row"),
           TestCase("\u3002", "single separator U+3002"),
           TestCase("\uFF0E", "single separator U+FF0E"),
           TestCase("\uFF61", "single separator U+FF61"),