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package org.apache.juneau.encoders;

import java.util.*;
import java.util.concurrent.*;

import org.apache.juneau.collections.*;
import org.apache.juneau.http.header.*;

/**
 * Represents the group of {@link Encoder encoders} keyed by codings.
 *
 * <h5 class='topic'>Description</h5>
 *
 * Maintains a set of encoders and the codings that they can handle.
 *
 * <p>
 * The {@link #getEncoderMatch(String)} and {@link #getEncoder(String)} methods are then used to find appropriate
 * encoders for specific <c>Accept-Encoding</c> and <c>Content-Encoding</c> header values.
 *
 * <h5 class='topic'>Match ordering</h5>
 *
 * Encoders are matched against <c>Accept-Encoding</c> strings in the order they exist in this group.
 *
 * <p>
 * Adding new entries will cause the entries to be prepended to the group.
 * This allows for previous encoders to be overridden through subsequent calls.
 *
 * <p>
 * For example, calling <code>groupBuilder.append(E1.<jk>class</jk>,E2.<jk>class</jk>).append(E3.<jk>class</jk>,
 * E4.<jk>class</jk>)</code> will result in the order <c>E3, E4, E1, E2</c>.
 *
 * <h5 class='section'>Example:</h5>
 * <p class='bcode w800'>
 *    <jc>// Create an encoder group with support for gzip compression.</jc>
 *    EncoderGroup g = EncoderGroup.<jsm>create</jsm>().append(GzipEncoder.<jk>class</jk>).build();
 *
 *    <jc>// Should return "gzip"</jc>
 *    String matchedCoding = g.findMatch(<js>"compress;q=1.0, gzip;q=0.8, identity;q=0.5, *;q=0"</js>);
 *
 *    <jc>// Get the encoder</jc>
 *    IEncoder encoder = g.getEncoder(matchedCoding);
 * </p>
 */
public final class EncoderGroup {

   /**
    * A default encoder group consisting of identity and G-Zip encoding.
    */
   public static final EncoderGroup DEFAULT = create().append(IdentityEncoder.class, GzipEncoder.class).build();

   // Maps Accept-Encoding headers to matching encoders.
   private final ConcurrentHashMap<String,EncoderMatch> cache = new ConcurrentHashMap<>();

   private final List<String> encodings;
   private final Encoder[] encodingsEncoders;
   private final List<Encoder> encoders;

   /**
    * Instantiates a new clean-slate {@link EncoderGroupBuilder} object.
    *
    * <p>
    * This is equivalent to simply calling <code><jk>new</jk> EncoderGroupBuilder()</code>.
    *
    * @return A new {@link EncoderGroupBuilder} object.
    */
   public static EncoderGroupBuilder create() {
      return new EncoderGroupBuilder();
   }

   /**
    * Returns a builder that's a copy of the settings on this encoder group.
    *
    * @return A new {@link EncoderGroupBuilder} initialized to this group.
    */
   public EncoderGroupBuilder builder() {
      return new EncoderGroupBuilder(this);
   }

   /**
    * Constructor
    *
    * @param encoders The encoders to add to this group.
    */
   public EncoderGroup(Encoder[] encoders) {
      this.encoders = AList.unmodifiable(encoders);

      AList<String> lc = AList.of();
      AList<Encoder> l = AList.of();
      for (Encoder e : encoders) {
         for (String c: e.getCodings()) {
            lc.add(c);
            l.add(e);
         }
      }

      this.encodings = lc.unmodifiable();
      this.encodingsEncoders = l.asArrayOf(Encoder.class);
   }

   /**
    * Returns the coding string for the matching encoder that can handle the specified <c>Accept-Encoding</c>
    * or <c>Content-Encoding</c> header value.
    *
    * <p>
    * Returns <jk>null</jk> if no encoders can handle it.
    *
    * <p>
    * This method is fully compliant with the RFC2616/14.3 and 14.11 specifications.
    *
    * @param acceptEncoding The <c>Accept-Encoding</c> or <c>Content-Encoding</c> value.
    * @return The coding value (e.g. <js>"gzip"</js>).
    */
   public EncoderMatch getEncoderMatch(String acceptEncoding) {
      EncoderMatch em = cache.get(acceptEncoding);
      if (em != null)
         return em;

      AcceptEncoding ae = AcceptEncoding.of(acceptEncoding);
      int match = ae.match(encodings);

      if (match >= 0) {
         em = new EncoderMatch(encodings.get(match), encodingsEncoders[match]);
         cache.putIfAbsent(acceptEncoding, em);
      }

      return cache.get(acceptEncoding);
   }

   /**
    * Returns the encoder registered with the specified coding (e.g. <js>"gzip"</js>).
    *
    * @param encoding The coding string.
    * @return The encoder, or <jk>null</jk> if encoder isn't registered with that coding.
    */
   public Encoder getEncoder(String encoding) {
      EncoderMatch em = getEncoderMatch(encoding);
      return (em == null ? null : em.getEncoder());
   }

   /**
    * Returns the set of codings supported by all encoders in this group.
    *
    * @return An unmodifiable list of codings supported by all encoders in this group.  Never <jk>null</jk>.
    */
   public List<String> getSupportedEncodings() {
      return encodings;
   }

   /**
    * Returns the encoders in this group.
    *
    * @return An unmodifiable list of encoders in this group.
    */
   public List<Encoder> getEncoders() {
      return encoders;
   }
}