Add utilities to detect and replace broken links.

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

@@ -4,14 +4,24 @@
 import com.linkedin.urls.detection.UrlDetector;
 import com.linkedin.urls.detection.UrlDetectorOptions;
 
+import java.net.URI;
+import java.net.http.HttpClient;
+import java.net.http.HttpRequest;
+import java.net.http.HttpResponse;
 import java.util.List;
+import java.util.Objects;
 import java.util.Optional;
 import java.util.Set;
+import java.util.concurrent.CompletableFuture;
 
 /**
  * Utility class to detect links.
  */
 public class LinkDetection {
+    private static final HttpClient HTTP_CLIENT = HttpClient.newHttpClient();
+
+    private static final Set<LinkFilter> DEFAULT_FILTERS =
+            Set.of(LinkFilter.SUPPRESSED, LinkFilter.NON_HTTP_SCHEME);
 
     /**
      * Possible ways to filter a link.
@@ -58,6 +68,56 @@ public static boolean containsLink(String content) {
         return !(new UrlDetector(content, UrlDetectorOptions.BRACKET_MATCH).detect().isEmpty());
     }
 
+    public static CompletableFuture<Boolean> isLinkBroken(String url) {
+        HttpRequest headRequest = HttpRequest.newBuilder(URI.create(url))
+            .method("HEAD", HttpRequest.BodyPublishers.noBody())
+            .build();
+
+        return HTTP_CLIENT.sendAsync(headRequest, HttpResponse.BodyHandlers.discarding())
+            .thenApply(response -> {
+                int status = response.statusCode();
+                return status < 200 || status >= 400;
+            })
+            .exceptionally(ignored -> true)
+            .thenCompose(result -> {
+                if (!Boolean.TRUE.equals(result)) {
+                    return CompletableFuture.completedFuture(false);
+                }
+                HttpRequest getRequest = HttpRequest.newBuilder(URI.create(url)).GET().build();
+                return HTTP_CLIENT.sendAsync(getRequest, HttpResponse.BodyHandlers.discarding())
+                    .thenApply(resp -> resp.statusCode() >= 400)
+                    .exceptionally(ignored -> true); // still never null
+            });
+    }
+
+    public static CompletableFuture<String> replaceDeadLinks(String text, String replacement) {
+        List<String> links = extractLinks(text, DEFAULT_FILTERS);
+
+        if (links.isEmpty()) {
+            return CompletableFuture.completedFuture(text);
+        }
+
+        List<CompletableFuture<String>> deadLinkFutures = links.stream()
+            .distinct()
+            .map(link -> isLinkBroken(link)
+                .thenApply(isBroken -> Boolean.TRUE.equals(isBroken) ? link : null))
+
+            .toList();
+
+        return CompletableFuture.allOf(deadLinkFutures.toArray(new CompletableFuture[0]))
+            .thenApply(ignored -> deadLinkFutures.stream()
+                .map(CompletableFuture::join)
+                .filter(Objects::nonNull)
+                .toList())
+            .thenApply(deadLinks -> {
+                String result = text;
+                for (String deadLink : deadLinks) {
+                    result = result.replace(deadLink, replacement);
+                }
+                return result;
+            });
+    }
+
     private static Optional<String> toLink(Url url, Set<LinkFilter> filter) {
         String raw = url.getOriginalUrl();
         if (filter.contains(LinkFilter.SUPPRESSED) && raw.contains(">")) {
@@ -76,7 +136,6 @@ private static Optional<String> toLink(Url url, Set<LinkFilter> filter) {
             // Remove trailing punctuation
             link = link.substring(0, link.length() - 1);
         }
-
         return Optional.of(link);
     }