improve validation logic for inline attachments

Author: J0nasMayr

src/main/java/it/aboutbits/springboot/emailservice/lib/application/EmailParameter.java

@@ -11,9 +11,11 @@
 
 import java.io.InputStream;
 import java.time.OffsetDateTime;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Objects;
 import java.util.Set;
+import java.util.regex.Pattern;
 
 @Builder
 @NullMarked
@@ -48,12 +50,43 @@ public record Email(
             @Valid
             Set<Attachment> attachments
     ) {
+        // the "cid:" scheme is case-insensitive, the contentId itself is not
+        private static final Pattern CID_REFERENCE_PATTERN = Pattern.compile(
+                "cid:([^\\s\"'<>]+)",
+                Pattern.CASE_INSENSITIVE
+        );
+
         @AssertTrue(message = "each inline attachment contentId must be referenced in the htmlBody as cid:contentId")
         public boolean isInlineAttachmentsValid() {
+            var contentIds = inlineContentIds();
+            if (contentIds.isEmpty()) {
+                return true;
+            }
+
+            if (htmlBody == null) {
+                return false;
+            }
+
+            var referencedContentIds = new HashSet<String>();
+            var matcher = CID_REFERENCE_PATTERN.matcher(htmlBody);
+            while (matcher.find()) {
+                referencedContentIds.add(matcher.group(1));
+            }
+
+            return referencedContentIds.containsAll(contentIds);
+        }
+
+        @AssertTrue(message = "each inline attachment contentId must be unique")
+        public boolean isInlineAttachmentContentIdsUnique() {
+            var contentIds = inlineContentIds();
+            return contentIds.size() == new HashSet<>(contentIds).size();
+        }
+
+        private List<String> inlineContentIds() {
             return attachments.stream()
                     .map(Attachment::contentId)
                     .filter(Objects::nonNull)
-                    .allMatch(contentId -> htmlBody.contains("cid:" + contentId));
+                    .toList();
         }
 
         @Builder