Addresses cognitect/transit-clj#47 by interposing a transformer on ta…

…g resolution when present. This handles the case where a transformer may convert an object into a form that the marhsaling phase may not be able to handle. This bifurcation between the classifying tag and the marshaled form was specifically causing issues where symbol map keys with metadata did not marshal into a stringable form but was classified as stringable earlier in the write phase.

src/main/java/com/cognitect/transit/impl/AbstractEmitter.java

@@ -26,6 +26,7 @@ protected AbstractEmitter(WriteHandlerMap writeHandlerMap, WriteHandler defaultW
 
     protected AbstractEmitter(WriteHandlerMap writeHandlerMap, WriteHandler defaultWriteHandler, Function<Object,Object> transform) {
         this.writeHandlerMap = writeHandlerMap;
+        this.writeHandlerMap.transform = transform;
         this.defaultWriteHandler = defaultWriteHandler;
         this.transform = transform;
     }

src/main/java/com/cognitect/transit/impl/TagProvider.java

@@ -5,4 +5,5 @@
 
 public interface TagProvider {
     public String getTag(Object o);
+    public String getTagAfterPossibleTransform(Object o);
 }

src/main/java/com/cognitect/transit/impl/WriteHandlerMap.java

@@ -19,6 +19,7 @@
 import java.math.BigDecimal;
 import java.math.BigInteger;
 import java.util.*;
+import java.util.function.Function;
 
 public class WriteHandlerMap implements TagProvider, Map<Class, WriteHandler<?, ?>> {
 
@@ -71,6 +72,7 @@ private static Map<Class, WriteHandler<?,?>> defaultHandlers() {
 
     private final Map<Class, WriteHandler<?, ?>> handlers;
     private WriteHandlerMap verboseHandlerMap;
+    Function<Object, Object> transform = null;
 
     public WriteHandlerMap() {
         this(null);
@@ -233,4 +235,12 @@ public String getTag(Object o) {
         if (h == null) return null;
         return h.tag(o);
     }
+
+    @Override
+    public String getTagAfterPossibleTransform(Object o) {
+        if (transform != null)
+            return this.getTag(transform.apply(o));
+        else
+            return this.getTag(o);
+    }
 }

src/main/java/com/cognitect/transit/impl/WriteHandlers.java

@@ -104,7 +104,7 @@ private boolean stringableKeys(Map<Object, Object> m) {
             Iterator<Object> i = m.keySet().iterator();
             while(i.hasNext()) {
                 Object key = i.next();
-                String tag = tagProvider.getTag(key);
+                String tag = tagProvider.getTagAfterPossibleTransform(key);
 
                 if(tag != null && tag.length() > 1)
                     return false;

src/main/java/com/cognitect/transit/impl/WriterFactory.java

@@ -19,7 +19,7 @@ public class WriterFactory {
 
     private static final Map<Map<Class, WriteHandler<?,?>>, WriteHandlerMap> handlerCache = new Cache<Map<Class, WriteHandler<?,?>>, WriteHandlerMap>();
 
-    private static WriteHandlerMap handlerMap(Map<Class, WriteHandler<?, ?>> customHandlers) {
+    private static WriteHandlerMap buildWriteHandlerMap(Map<Class, WriteHandler<?, ?>> customHandlers) {
         if (customHandlers instanceof WriteHandlerMap)
             return new WriteHandlerMap(customHandlers);
 
@@ -35,7 +35,7 @@ private static WriteHandlerMap handlerMap(Map<Class, WriteHandler<?, ?>> customH
     }
 
     private static WriteHandlerMap verboseHandlerMap(Map<Class, WriteHandler<?, ?>> customHandlers) {
-        return handlerMap(customHandlers).verboseWriteHandlerMap();
+        return buildWriteHandlerMap(customHandlers).verboseWriteHandlerMap();
     }
 
     public static <T> Writer<T> getJsonInstance(final OutputStream out, Map<Class, WriteHandler<?,?>> customHandlers,  WriteHandler<?, ?> defaultWriteHandler, boolean verboseMode) throws IOException {
@@ -50,7 +50,7 @@ public static <T> Writer<T> getJsonInstance(final OutputStream out, Map<Class, W
         if (verboseMode) {
             emitter = new JsonVerboseEmitter(gen, verboseHandlerMap(customHandlers), defaultWriteHandler, transform);
         } else {
-            emitter = new JsonEmitter(gen, handlerMap(customHandlers), defaultWriteHandler, transform);
+            emitter = new JsonEmitter(gen, buildWriteHandlerMap(customHandlers), defaultWriteHandler, transform);
         }
 
         final WriteCache writeCache = new WriteCache(!verboseMode);
@@ -76,7 +76,7 @@ public static <T> Writer<T> getMsgpackInstance(final OutputStream out, Map<Class
 
         Packer packer = new MessagePack().createPacker(out);
 
-        final MsgpackEmitter emitter = new MsgpackEmitter(packer, handlerMap(customHandlers), defaultWriteHandler, transform);
+        final MsgpackEmitter emitter = new MsgpackEmitter(packer, buildWriteHandlerMap(customHandlers), defaultWriteHandler, transform);
 
         final WriteCache writeCache = new WriteCache(true);