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/*
 * Copyright 2002-2018 the original author or 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 org.springframework.web.reactive.resource;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.ListIterator;

import reactor.core.publisher.Mono;

import org.springframework.core.io.Resource;
import org.springframework.lang.Nullable;
import org.springframework.util.Assert;
import org.springframework.web.server.ServerWebExchange;

/**
 * Default immutable implementation of {@link ResourceTransformerChain}.
 *
 * @author Rossen Stoyanchev
 * @since 5.0
 */
class DefaultResourceTransformerChain implements ResourceTransformerChain {

	private final ResourceResolverChain resolverChain;

	@Nullable
	private final ResourceTransformer transformer;

	@Nullable
	private final ResourceTransformerChain nextChain;


	public DefaultResourceTransformerChain(
			ResourceResolverChain resolverChain, @Nullable List transformers) {

		Assert.notNull(resolverChain, "ResourceResolverChain is required");
		this.resolverChain = resolverChain;
		transformers = (transformers != null ? transformers : Collections.emptyList());
		DefaultResourceTransformerChain chain = initTransformerChain(resolverChain, new ArrayList<>(transformers));
		this.transformer = chain.transformer;
		this.nextChain = chain.nextChain;
	}

	private DefaultResourceTransformerChain initTransformerChain(ResourceResolverChain resolverChain,
			ArrayList transformers) {

		DefaultResourceTransformerChain chain = new DefaultResourceTransformerChain(resolverChain, null, null);
		ListIterator it = transformers.listIterator(transformers.size());
		while (it.hasPrevious()) {
			chain = new DefaultResourceTransformerChain(resolverChain, it.previous(), chain);
		}
		return chain;
	}

	public DefaultResourceTransformerChain(ResourceResolverChain resolverChain,
			@Nullable ResourceTransformer transformer, @Nullable ResourceTransformerChain chain) {

		Assert.isTrue((transformer == null && chain == null) || (transformer != null && chain != null),
				"Both transformer and transformer chain must be null, or neither is");
		this.resolverChain = resolverChain;
		this.transformer = transformer;
		this.nextChain = chain;
	}


	@Override
	public ResourceResolverChain getResolverChain() {
		return this.resolverChain;
	}

	@Override
	public Mono transform(ServerWebExchange exchange, Resource resource) {
		return (this.transformer != null && this.nextChain != null ?
				this.transformer.transform(exchange, resource, this.nextChain) :
				Mono.just(resource));
	}

}