Add methods to serialize cursors

lib/src/main/java/org/automerge/AutomergeSys.java

@@ -336,6 +336,12 @@ public static native long lookupCursorIndexInDoc(DocPointer doc, ObjectId obj, C
 	public static native long lookupCursorIndexInTx(TransactionPointer tx, ObjectId obj, Cursor cursor,
 			Optional<ChangeHash[]> heads);
 
+    public static native String cursorToString(Cursor cursor);
+
+    public static native Cursor cursorFromString(String encoded);
+
+    public static native Cursor cursorFromBytes(byte[] encoded);
+
 	public static native Optional<ObjectType> getObjectTypeInDoc(DocPointer doc, ObjectId obj);
 
 	public static native Optional<ObjectType> getObjectTypeInTx(TransactionPointer tx, ObjectId obj);

lib/src/main/java/org/automerge/Cursor.java

@@ -2,4 +2,21 @@
 
 public class Cursor {
 	private byte[] raw;
+
+    public static Cursor fromBytes(byte[] encoded) {
+        return AutomergeSys.cursorFromBytes(encoded);
+    }
+
+    public static Cursor fromString(String encoded) {
+        return AutomergeSys.cursorFromString(encoded);
+    }
+
+    @Override
+    public String toString() {
+        return AutomergeSys.cursorToString(this);
+    }
+
+    public byte[] toBytes() {
+        return raw.clone();
+    }
 }

lib/src/test/java/org/automerge/TestCursor.java

@@ -58,4 +58,28 @@ public void testCursorInTx() {
 			tx.commit();
 		}
 	}
+
+    @Test
+    public void testToFromString() {
+        Cursor cursor = doc.makeCursor(text, 3);
+        String encoded = cursor.toString();
+        Cursor decoded = Cursor.fromString(encoded);
+        Assertions.assertEquals(doc.lookupCursorIndex(text, decoded), 3);
+
+        Assertions.assertThrows(IllegalArgumentException.class, () -> {
+            Cursor.fromString("invalid");
+        });
+    }
+
+    @Test
+    public void testToFromBytes() {
+        Cursor cursor = doc.makeCursor(text, 3);
+        byte[] encoded = cursor.toBytes();
+        Cursor decoded = Cursor.fromBytes(encoded);
+        Assertions.assertEquals(doc.lookupCursorIndex(text, decoded), 3);
+
+        Assertions.assertThrows(IllegalArgumentException.class, () -> {
+            Cursor.fromBytes(new byte[] { 0x01, 0x01 });
+        });
+    }
 }

rust/src/cursor.rs

@@ -5,8 +5,6 @@ use jni::{
     JNIEnv,
 };
 
-use crate::AUTOMERGE_EXCEPTION;
-
 #[derive(Debug)]
 pub struct Cursor(automerge::Cursor);
 
@@ -85,12 +83,39 @@ pub unsafe extern "C" fn cursorFromString(
 ) -> jobject {
     let s = env.get_string(JString::from_raw(s)).unwrap();
     let Ok(s) = s.to_str() else {
-        env.throw_new(AUTOMERGE_EXCEPTION, "invalid cursor string")
-            .unwrap();
+        env.throw_new(
+            "java/lang/IllegalArgumentException",
+            "invalid cursor string",
+        )
+        .unwrap();
         return JObject::null().into_raw();
     };
     let Ok(cursor) = automerge::Cursor::try_from(s) else {
-        env.throw_new(AUTOMERGE_EXCEPTION, "invalid cursor string")
+        env.throw_new(
+            "java/lang/IllegalArgumentException",
+            "invalid cursor string",
+        )
+        .unwrap();
+        return JObject::null().into_raw();
+    };
+    Cursor::from(cursor).into_raw(&env).unwrap()
+}
+
+#[no_mangle]
+#[jni_fn]
+pub unsafe extern "C" fn cursorFromBytes(
+    env: jni::JNIEnv,
+    _class: jni::objects::JClass,
+    bytes: jni::sys::jbyteArray,
+) -> jobject {
+    let bytes = env
+        .get_byte_array_elements(bytes, jni::objects::ReleaseMode::NoCopyBack)
+        .unwrap();
+    let bytes =
+        std::slice::from_raw_parts(bytes.as_ptr() as *const u8, bytes.size().unwrap() as usize);
+    let Ok(cursor) = automerge::Cursor::try_from(bytes) else {
+        // throw IllegalArgumentException
+        env.throw_new("java/lang/IllegalArgumentException", "invalid cursor bytes")
             .unwrap();
         return JObject::null().into_raw();
     };