The extension mapper, that maps from http headers

Signed-off-by: Fabio José <fabiojose@gmail.com>

api/src/main/java/io/cloudevents/v02/http/BinaryFormatExtensionMapperImpl.java

@@ -0,0 +1,62 @@
+/**
+ * Copyright 2019 The CloudEvents Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package io.cloudevents.v02.http;
+
+import java.util.List;
+import java.util.Locale;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.Objects;
+import java.util.AbstractMap.SimpleEntry;
+import java.util.stream.Collectors;
+
+import io.cloudevents.fun.BinaryFormatExtensionMapper;
+import io.cloudevents.v02.ContextAttributes;
+
+/**
+ * 
+ * @author fabiojose
+ *
+ */
+public class BinaryFormatExtensionMapperImpl implements
+	BinaryFormatExtensionMapper {
+	
+	private static final List<String> RESERVED_HEADERS = 
+		ContextAttributes.VALUES.stream()
+			.map(attribute -> BinaryFormatAttributeMapperImpl
+					.HEADER_PREFIX + attribute)
+			.collect(Collectors.toList());
+	static {
+		RESERVED_HEADERS.add("content-type");
+	};
+
+	@Override
+	public Map<String, String> map(Map<String, Object> headers) {
+		Objects.requireNonNull(headers);
+		
+		// remove all reserved words and then remaining may be extensions
+		return 
+		headers.entrySet()
+			.stream()
+			.map(header -> new SimpleEntry<>(header.getKey()
+					.toLowerCase(Locale.US), header.getValue().toString()))
+			.filter(header -> {
+				return !RESERVED_HEADERS.contains(header.getKey());
+			})
+			.collect(Collectors.toMap(Entry::getKey, Entry::getValue));
+	}
+
+}