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package org.apache.juneau.http;

import static org.apache.juneau.http.Constants.*;

import org.apache.juneau.internal.*;

/**
 * Represents a parsed <l>Accept-Language</l> HTTP request header.
 * 
 * <p>
 * List of acceptable human languages for response.
 * 
 * <h5 class='figure'>Example</h5>
 * <p class='bcode'>
 *    Accept-Language: en-US
 * </p>
 * 
 * <h5 class='topic'>RFC2616 Specification</h5>
 * 
 * The Accept-Language request-header field is similar to Accept, but restricts the set of natural languages that are
 * preferred as a response to the request.
 * Language tags are defined in section 3.10.
 * 
 * <p class='bcode'>
 *    Accept-Language = "Accept-Language" ":"
 *                      1#( language-range [ ";" "q" "=" qvalue ] )
 *    language-range  = ( ( 1*8ALPHA *( "-" 1*8ALPHA ) ) | "*" )
 * </p>
 * 
 * <p>
 * Each language-range MAY be given an associated quality value which represents an estimate of the user's preference
 * for the languages specified by that range.
 * The quality value defaults to "q=1".
 * For example...
 * <p class='bcode'>
 *    Accept-Language: da, en-gb;q=0.8, en;q=0.7
 * </p>
 * <p>
 * ...would mean: "I prefer Danish, but will accept British English and other types of English."
 * 
 * <p>
 * A language-range matches a language-tag if it exactly equals the tag, or if it exactly equals a prefix of the tag
 * such that the first tag character following the prefix is "-".
 * 
 * <p>
 * The special range "*", if present in the Accept-Language field, matches every tag not matched by any other range
 * present in the Accept-Language field.
 * 
 * <p>
 * Note: This use of a prefix matching rule does not imply that language tags are assigned to languages in such a way
 * that it is always true that if a user understands a language with a certain
 * tag, then this user will also understand all languages with tags for which this tag is a prefix.
 * The prefix rule simply allows the use of prefix tags if this is the case.
 * 
 * <p>
 * The language quality factor assigned to a language-tag by the Accept-Language field is the quality value of the
 * longest language- range in the field that matches the language-tag.
 * 
 * <p>
 * If no language- range in the field matches the tag, the language quality factor assigned is 0.
 * 
 * <p>
 * If no Accept-Language header is present in the request, the server SHOULD assume that all languages are equally
 * acceptable.
 * 
 * <p>
 * If an Accept-Language header is present, then all languages which are assigned a quality factor greater than 0 are
 * acceptable.
 * 
 * <p>
 * It might be contrary to the privacy expectations of the user to send an Accept-Language header with the complete
 * linguistic preferences of the user in every request.
 * For a discussion of this issue, see section 15.1.4.
 * 
 * <p>
 * As intelligibility is highly dependent on the individual user, it is recommended that client applications make the
 * choice of linguistic preference available to the user.
 * If the choice is not made available, then the Accept-Language header field MUST NOT be given in the request.
 * 
 * <p>
 * Note: When making the choice of linguistic preference available to the user, we remind implementors of the fact that
 * users are not familiar with the details of language matching as described above, and should provide appropriate
 * guidance.
 * As an example, users might assume that on selecting "en-gb", they will be served any kind of English document if
 * British English is not available.
 * A user agent might suggest in such a case to add "en" to get the best matching behavior.
 * 
 * <h5 class='section'>See Also:</h5>
 * <ul class='doctree'>
 *    <li class='extlink'><a class='doclink' href='https://www.w3.org/Protocols/rfc2616/rfc2616.html'>Hypertext Transfer Protocol -- HTTP/1.1</a>
 * </ul>
 */
public final class AcceptLanguage extends HeaderRangeArray {

   private static final Cache<String,AcceptLanguage> cache = new Cache<>(NOCACHE, CACHE_MAX_SIZE);

   /**
    * Returns a parsed <code>Accept-Language</code> header.
    * 
    * @param value The <code>Accept-Language</code> header string.
    * @return The parsed <code>Accept-Language</code> header, or <jk>null</jk> if the string was null.
    */
   public static AcceptLanguage forString(String value) {
      if (value == null)
         return null;
      AcceptLanguage a = cache.get(value);
      if (a == null)
         a = cache.put(value, new AcceptLanguage(value));
      return a;
   }

   private AcceptLanguage(String raw) {
      super(raw);
   }
}