Add `JsonMerge()`.

Closes #15.

src/Json.c

@@ -1433,3 +1433,32 @@ finish:
     va_end(argp);
     return val;
 }
+
+void
+JsonMerge(HashMap *obj1, HashMap *obj2)
+{
+    char *key;
+    JsonValue *val2;
+
+    while (HashMapIterate(obj2, &key, (void **) &val2))
+    {
+        JsonValue *val1 = HashMapGet(obj1, key);
+
+        if (val1)
+        {
+            if (JsonValueType(val1) == JsonValueType(val2) &&
+                JsonValueType(val1) == JSON_OBJECT)
+            {
+                JsonMerge(JsonValueAsObject(val1), JsonValueAsObject(val2));
+            }
+            else
+            {
+                JsonValueFree(HashMapSet(obj1, key, JsonValueDuplicate(val2)));
+            }
+        }
+        else
+        {
+            HashMapSet(obj1, key, JsonValueDuplicate(val2));
+        }
+    }
+}

src/include/Json.h

@@ -320,4 +320,13 @@ extern JsonValue * JsonGet(HashMap *, size_t,...);
  */
 extern JsonValue * JsonSet(HashMap *, JsonValue *, size_t,...);
 
+/**
+ * Recursively merge two JSON objects. The second object is merged
+ * on top of the first; any keys present in the first object that are
+ * also present in the second object are replaced with those in the
+ * second object, and any keys present in the second object that are
+ * not present in the first object are copied to the first object.
+ */
+extern void JsonMerge(HashMap *, HashMap *);
+
 #endif                             /* CYTOPLASM_JSON_H */