JNIKiwixReader ctor taking a file descriptor

... and a corresponding unit test

include/reader.h

@@ -55,7 +55,8 @@ class Reader
    *                    unsplitted path as if the file were not splitted
    *                    (.zim extesion).
    */
-  Reader(const string zimFilePath);
+  explicit Reader(const string zimFilePath);
+  explicit Reader(int fd);
   ~Reader() = default;
 
   /**

src/reader.cpp

@@ -86,6 +86,15 @@ Reader::Reader(const string zimFilePath)
   srand(time(nullptr));
 }
 
+Reader::Reader(int fd)
+  :  zimArchive(new zim::Archive(fd)),
+     zimFilePath("")
+{
+
+  /* initialize random seed: */
+  srand(time(nullptr));
+}
+
 zim::Archive* Reader::getZimArchive() const
 {
   return zimArchive.get();

src/wrapper/java/kiwixreader.cpp

@@ -45,6 +45,35 @@ JNIEXPORT jlong JNICALL Java_org_kiwix_kiwixlib_JNIKiwixReader_getNativeReader(
   }
 }
 
+namespace
+{
+
+int jni2fd(const jobject& fdObj, JNIEnv* env)
+{
+  jclass class_fdesc = env->FindClass("java/io/FileDescriptor");
+  jfieldID field_fd = env->GetFieldID(class_fdesc, "fd", "I");
+  return env->GetIntField(fdObj, field_fd);
+}
+
+} // unnamed namespace
+
+JNIEXPORT jlong JNICALL Java_org_kiwix_kiwixlib_JNIKiwixReader_getNativeReaderByFD(
+    JNIEnv* env, jobject obj, jobject fdObj)
+{
+  int fd = jni2fd(fdObj, env);
+
+  LOG("Attempting to create reader with fd: %d", fd);
+  Lock l;
+  try {
+    kiwix::Reader* reader = new kiwix::Reader(fd);
+    return reinterpret_cast<jlong>(new Handle<kiwix::Reader>(reader));
+  } catch (std::exception& e) {
+    LOG("Error opening ZIM file");
+    LOG(e.what());
+    return 0;
+  }
+}
+
 JNIEXPORT void JNICALL
 Java_org_kiwix_kiwixlib_JNIKiwixReader_dispose(JNIEnv* env, jobject obj)
 {

src/wrapper/java/org/kiwix/kiwixlib/JNIKiwixReader.java

@@ -25,6 +25,7 @@ import org.kiwix.kiwixlib.JNIKiwixString;
 import org.kiwix.kiwixlib.JNIKiwixInt;
 import org.kiwix.kiwixlib.JNIKiwixSearcher;
 import org.kiwix.kiwixlib.Pair;
+import java.io.FileDescriptor;
 
 public class JNIKiwixReader
 {
@@ -151,11 +152,21 @@ public class JNIKiwixReader
         throw new JNIKiwixException("Cannot open zimfile "+filename);
     }
   }
+
+  public JNIKiwixReader(FileDescriptor fd) throws JNIKiwixException
+  {
+    nativeHandle = getNativeReaderByFD(fd);
+    if (nativeHandle == 0) {
+        throw new JNIKiwixException("Cannot open zimfile by fd "+fd.toString());
+    }
+  }
+
   public JNIKiwixReader() {
 
   }
   public native void dispose();
 
   private native long getNativeReader(String filename);
+  private native long getNativeReaderByFD(FileDescriptor fd);
   private long nativeHandle;
 }

src/wrapper/java/org/kiwix/testing/test.java

@@ -41,6 +41,24 @@ throws JNIKiwixException, IOException
 
 }
 
+@Test
+public void testReaderByFd()
+throws JNIKiwixException, IOException
+{
+  FileInputStream fis = new FileInputStream("small.zim");
+  JNIKiwixReader reader = new JNIKiwixReader(fis.getFD());
+  assertEquals("Test ZIM file", reader.getTitle());
+  assertEquals(45, reader.getFileSize()); // The file size is in KiB
+  assertEquals("A/main.html", reader.getMainPage());
+  String s = getFileContent("small_zimfile_data/main.html");
+  byte[] c = reader.getContent(new JNIKiwixString("A/main.html"),
+                                new JNIKiwixString(),
+                                new JNIKiwixString(),
+                                new JNIKiwixInt());
+  assertEquals(s, new String(c));
+
+}
+
 @Test
 public void testLibrary()
 throws IOException