Fix FressianWriteHandler serialization bug

src/main/java/com/github/rschmitt/dynamicobject/FressianWriteHandler.java

@@ -1,5 +1,6 @@
 package com.github.rschmitt.dynamicobject;
 
+import clojure.lang.PersistentHashMap;
 import org.fressian.CachedObject;
 import org.fressian.Writer;
 import org.fressian.handlers.WriteHandler;
@@ -73,7 +74,14 @@ private TransformedMap(
             Function<Object, Object> keysTransformation,
             BiFunction<Object, Object, Object> valuesTransformation
         ) {
-            this.backingMap = backingMap;
+            if (backingMap.size() == 8) {
+                // If the backing map has 8 fields, Clojure Fressian deserializes it to a PersistentHashMap
+                // which means serializing -> deserializing -> serializing will yield a different value from the
+                // first serialization unless we serialize it as a PersistentHashMap here.
+                this.backingMap = (Map) PersistentHashMap.create(backingMap);
+            } else {
+                this.backingMap = backingMap;
+            }
             this.keysTransformation = keysTransformation;
             this.valuesTransformation = valuesTransformation;
         }

src/test/java/com/github/rschmitt/dynamicobject/FressianTest.java

@@ -1,5 +1,6 @@
 package com.github.rschmitt.dynamicobject;
 
+import static org.junit.jupiter.api.Assertions.assertArrayEquals;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertTrue;
 
@@ -51,6 +52,30 @@ public void formatCompatibilityTest() throws Exception {
         assertEquals(SAMPLE_VALUE, deserialized);
     }
 
+    @Test
+    public void repeatedSerializationYieldsSameResultWith8Fields() throws Exception {
+        // This only surfaces with 8 fields, because Clojure Fressian deserializes maps with 8 keys
+        // as a PersistentHashMap, when it should deserialize it as a PersistentArrayMap.
+
+        DynamicObject.registerTag(DOWith8Fields.class, "doWith8Fields");
+        DOWith8Fields initialDO = DynamicObject.newInstance(DOWith8Fields.class)
+                .withField1("a")
+                .withField2("b")
+                .withField3("c")
+                .withField4("d")
+                .withField5("e")
+                .withField6("f")
+                .withField7("g")
+                .withField8("h");
+
+        byte[] serialized = DynamicObject.toFressianByteArray(initialDO);
+        DOWith8Fields deserialized = DynamicObject.fromFressianByteArray(serialized);
+
+        byte[] reserialized = DynamicObject.toFressianByteArray(deserialized);
+
+        assertArrayEquals(serialized, reserialized);
+    }
+
     @Test
     public void cachedKeys_canBeRoundTripped() throws Exception {
         String cachedValue = "cached value";
@@ -93,4 +118,15 @@ public interface BinarySerialized extends DynamicObject<BinarySerialized> {
         @Key(":null") BinarySerialized withNull(Object nil);
         @Cached @Key(":cached") BinarySerialized withCached(String cached);
     }
+
+    public interface DOWith8Fields extends DynamicObject<DOWith8Fields> {
+        @Key(":field1") DOWith8Fields withField1(String field1);
+        @Key(":field2") DOWith8Fields withField2(String field2);
+        @Key(":field3") DOWith8Fields withField3(String field3);
+        @Key(":field4") DOWith8Fields withField4(String field4);
+        @Key(":field5") DOWith8Fields withField5(String field5);
+        @Key(":field6") DOWith8Fields withField6(String field6);
+        @Key(":field7") DOWith8Fields withField7(String field7);
+        @Key(":field8") DOWith8Fields withField8(String field8);
+    }
 }