Wrap arrays in List wrappers instead of creating copies in Adapter#to…

…List

This is a neat performance improvement with no real cost. Instead of creating a new list and adding all elements to it, a new list wrapper is created that accesses the underlying array object directly.

I haven't figured out if it's possible to do the same for the object-to-list case.

jmespath-gson/src/main/java/io/burt/jmespath/gson/GsonRuntime.java

@@ -11,6 +11,7 @@
 import io.burt.jmespath.JmesPathType;
 import io.burt.jmespath.RuntimeConfiguration;
 
+import java.util.AbstractList;
 import java.util.ArrayList;
 import java.util.Collection;
 import java.util.Collections;
@@ -34,24 +35,37 @@ public JsonElement parseString(String str) {
     return parser.parse(str);
   }
 
+  private static class JsonArrayListWrapper extends AbstractList<JsonElement> {
+    private final JsonArray array;
+
+    JsonArrayListWrapper(JsonArray array) {
+      this.array = array;
+    }
+
+    @Override
+    public JsonElement get(int index) {
+      return array.get(index);
+    }
+
+    @Override
+    public int size() {
+      return array.size();
+    }
+  }
+
   @Override
   public List<JsonElement> toList(JsonElement value) {
-    List<JsonElement> list;
-    if (value.isJsonObject()) {
-      list = new ArrayList<>(value.getAsJsonObject().size());
+    if (value.isJsonArray()) {
+      return new JsonArrayListWrapper(value.getAsJsonArray());
+    } else if (value.isJsonObject()) {
+      List<JsonElement> list = new ArrayList<>(value.getAsJsonObject().size());
       for(Map.Entry<String, JsonElement> entry : value.getAsJsonObject().entrySet()) {
         list.add(entry.getValue());
       }
       return list;
+    } else {
+      return Collections.emptyList();
     }
-    if (value.isJsonArray()) {
-      list = new ArrayList<>(value.getAsJsonArray().size());
-      for (JsonElement e : value.getAsJsonArray()) {
-        list.add(e);
-      }
-      return list;
-    }
-    return Collections.emptyList();
   }
 
   @Override

jmespath-jackson/src/main/java/io/burt/jmespath/jackson/JacksonRuntime.java

@@ -1,5 +1,6 @@
 package io.burt.jmespath.jackson;
 
+import java.util.AbstractList;
 import java.util.List;
 import java.util.ArrayList;
 import java.util.Collection;
@@ -43,9 +44,29 @@ public JsonNode parseString(String string) {
     }
   }
 
+  private static class ArrayNodeListWrapper extends AbstractList<JsonNode> {
+    private final ArrayNode array;
+
+    ArrayNodeListWrapper(ArrayNode array) {
+      this.array = array;
+    }
+
+    @Override
+    public JsonNode get(int index) {
+      return array.get(index);
+    }
+
+    @Override
+    public int size() {
+      return array.size();
+    }
+  }
+
   @Override
   public List<JsonNode> toList(JsonNode value) {
-    if (value.isArray() || value.isObject()) {
+    if (value.isArray()) {
+      return new ArrayNodeListWrapper((ArrayNode) value);
+    } else if (value.isObject()) {
       List<JsonNode> elements = new ArrayList<>(value.size());
       for (JsonNode element : value) {
         elements.add(element);