Add utilities to detect and replace broken links.

application/src/main/java/org/togetherjava/tjbot/features/utils/LinkDetection.java

@@ -1,5 +1,11 @@
 package org.togetherjava.tjbot.features.utils;
 
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
+import java.util.concurrent.CompletableFuture;
+
 import com.linkedin.urls.Url;
 import com.linkedin.urls.detection.UrlDetector;
 import com.linkedin.urls.detection.UrlDetectorOptions;
@@ -57,6 +63,54 @@ public static List<String> extractLinks(String content, Set<LinkFilter> filter)
     public static boolean containsLink(String content) {
         return !(new UrlDetector(content, UrlDetectorOptions.BRACKET_MATCH).detect().isEmpty());
     }
+    public static CompletableFuture<Boolean> isLinkBroken(String url) {
+        HttpClient client = HttpClient.newHttpClient();
+
+        HttpRequest request = HttpRequest.newBuilder(URI.create(url))
+                .method("HEAD", HttpRequest.BodyPublishers.noBody())
+                .build();
+
+        return client.sendAsync(request, HttpResponse.BodyHandlers.discarding())
+                .thenApply(response -> response.statusCode() >= 400)
+                .exceptionally(ignored -> true);
+    }
+    public static CompletableFuture<String> replaceDeadLinks(
+            String text,
+            String replacement
+    ) {
+        Set<LinkFilter> filters = Set.of(
+                LinkFilter.SUPPRESSED,
+                LinkFilter.NON_HTTP_SCHEME
+        );
+
+        List<String> links = extractLinks(text, filters);
+
+        if (links.isEmpty()) {
+            return CompletableFuture.completedFuture(text);
+        }
+
+        StringBuilder result = new StringBuilder(text);
+
+        List<CompletableFuture<Void>> checks = links.stream()
+                .map(link ->
+                        isLinkBroken(link).thenAccept(isDead -> {
+                            if (isDead) {
+                                int index = result.indexOf(link);
+                                if (index != -1) {
+                                    result.replace(
+                                            index,
+                                            index + link.length(),
+                                            replacement
+                                    );
+                                }
+                            }
+                        })
+                )
+                .toList();
+
+        return CompletableFuture.allOf(checks.toArray(new CompletableFuture[0]))
+                .thenApply(v -> result.toString());
+    }
 
     private static Optional<String> toLink(Url url, Set<LinkFilter> filter) {
         String raw = url.getOriginalUrl();
@@ -76,7 +130,6 @@ private static Optional<String> toLink(Url url, Set<LinkFilter> filter) {
             // Remove trailing punctuation
             link = link.substring(0, link.length() - 1);
         }
-
         return Optional.of(link);
     }