/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.juneau.http.classic.header;

import static org.apache.juneau.commons.utils.CollectionUtils.*;
import static org.apache.juneau.commons.utils.PredicateUtils.*;
import static org.apache.juneau.commons.utils.Shorts.*;
import static org.apache.juneau.commons.utils.StringUtils.*;

import java.util.*;
import java.util.function.*;
import java.util.stream.*;

import org.apache.http.*;
import org.apache.http.Header;
import org.apache.http.util.*;
import org.apache.juneau.commons.svl.*;
import org.apache.juneau.commons.utils.*;
import org.apache.juneau.http.HttpHeaders;
import org.apache.juneau.http.UnmodifiableBean;

/**
 * A simple list of HTTP headers with various convenience methods.
 *
 * <h5 class='figure'>Example</h5>
 * <p class='bjava'>
 *    HeaderList <jv>headers</jv> = HeaderList
 *       .<jsm>create</jsm>()
 *       .append(Accept.<jsm>of</jsm>(<js>"text/xml"</js>))
 *       .append(<js>"Content-Type"</js>, ()-&gt;<jsm>getDynamicContentTypeFromSomewhere</jsm>());
 * </p>
 *
 * <p>
 * Convenience creators are provided for creating lists with minimal code:
 * <p class='bjava'>
 *    HeaderList <jv>headers</jv> = HeaderList.<jsm>of</jsm>(Accept.<jsf>TEXT_XML</jsf>, ContentType.<jsf>TEXT_XML</jsf>);
 * </p>
 *
 * <p>
 * Static methods are provided on {@link HttpHeaders} to further simplify creation of header lists.
 * <p class='bjava'>
 *    <jk>import static</jk> org.apache.juneau.http.HttpHeaders.*;
 *
 *    HeaderList <jv>headers</jv> = <jsm>headerList</jsm>(<jsm>accept</jsm>(<js>"text/xml"</js>), <jsm>contentType</jsm>(<js>"text/xml"</js>));
 * </p>
 *
 * <h5 class='section'>See Also:</h5><ul>
 *    <li class='link'><a class="doclink" href="https://juneau.apache.org/docs/topics/JuneauRestCommon">juneau-rest-common</a>
 * </ul>
 */
@SuppressWarnings({
   "java:S110", // Inheritance depth acceptable for this class hierarchy
   "java:S127", // set(Header)/set(List) remove duplicate-named headers while iterating and decrement the cached size bound (j/j2) to match
   "java:S1192", // Duplicated literals (argument/property names) read more clearly inline than as constants
   "java:S2160" // equals() inherited from ArrayList; list equality is element-based, which is correct for header lists
})
public class HeaderList extends ArrayList<Header> {

   /** Represents no header list in annotations. */
   public static final class Void extends HeaderList {
      private static final long serialVersionUID = 1L;
   }

   private static final long serialVersionUID = 1L;

   /**
    * Instantiates a new list.
    *
    * @return A new list.
    */
   public static HeaderList create() {
      return new HeaderList();
   }

   /**
    * Creates a new {@link HeaderList} initialized with the specified headers.
    *
    * @param headers
    *    The headers to add to the list.
    *    <br>Can be <jk>null</jk>.
    *    <br><jk>null</jk> entries are ignored.
    * @return A new unmodifiable instance, never <jk>null</jk>.
    */
   public static HeaderList of(Header...headers) {
      return new HeaderList().append(headers);
   }

   /**
    * Creates a new {@link HeaderList} initialized with the specified headers.
    *
    * @param headers
    *    The headers to add to the list.
    *    <br>Can be <jk>null</jk>.
    *    <br><jk>null</jk> entries are ignored.
    * @return A new unmodifiable instance, never <jk>null</jk>.
    */
   public static HeaderList of(List<Header> headers) {
      return new HeaderList().append(headers);
   }

   /**
    * Creates a new {@link HeaderList} initialized with the specified name/value pairs.
    *
    * <h5 class='figure'>Example</h5>
    * <p class='bjava'>
    *    HeaderList <jv>headers</jv> = HeaderList.<jsm>ofPairs</jsm>(<js>"Accept"</js>, <js>"text/xml"</js>, <js>"Content-Type"</js>, <js>"text/xml"</js>);
    * </p>
    *
    * @param pairs
    *    Initial list of pairs.
    *    <br>Must be an even number of parameters representing key/value pairs.
    * @throws RuntimeException If odd number of parameters were specified.
    * @return A new instance.
    */
   public static HeaderList ofPairs(String...pairs) {
      var x = new HeaderList();
      if (pairs == null)
         pairs = new String[0];
      req(pairs.length % 2 == 0, "Odd number of parameters passed into HeaderList.ofPairs()");
      for (var i = 0; i < pairs.length; i += 2)
         x.add(BasicHeader.of(pairs[i], pairs[i + 1]));
      return x;
   }

   private transient VarResolver varResolver;
   boolean caseSensitive;

   /**
    * Constructor.
    */
   public HeaderList() {
      super();
   }

   /**
    * Copy constructor.
    *
    * @param copyFrom The bean to copy.
    */
   protected HeaderList(HeaderList copyFrom) {
      super(copyFrom);
      caseSensitive = copyFrom.caseSensitive;
   }

   /**
    * Adds the specified header to the end of the headers in this list.
    *
    * @param value The header to add.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList append(Header value) {
      if (nn(value))
         add(value);
      return this;
   }

   /**
    * Adds the specified headers to the end of the headers in this list.
    *
    * @param values The headers to add.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList append(Header...values) {
      if (nn(values))
         for (var value : values)
            if (nn(value))
               append(value);
      return this;
   }

   /**
    * Adds the specified headers to the end of the headers in this list.
    *
    * @param values The headers to add.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList append(List<Header> values) {
      if (nn(values))
         values.forEach(this::append);
      return this;
   }

   /**
    * Appends the specified header to the end of this list.
    *
    * <p>
    * The header is added as a {@link BasicHeader}.
    *
    * @param name The header name.
    * @param value The header value.
    * @return This object.
    */
   public HeaderList append(String name, Object value) {
      return append(createPart(name, value));
   }

   /**
    * Appends the specified header to the end of this list using a value supplier.
    *
    * <p>
    * The header is added as a {@link BasicHeader}.
    *
    * <p>
    * Value is re-evaluated on each call to {@link BasicHeader#getValue()}.
    *
    * @param name The header name.
    * @param value The header value supplier.
    * @return This object.
    */
   public HeaderList append(String name, Supplier<?> value) {
      return append(createPart(name, value));
   }

   /**
    * Specifies that the headers in this list should be treated as case-sensitive.
    *
    * <p>
    * The default behavior is case-insensitive.
    *
    * @param value The new value for this setting.
    * @return This object.
    */
   public HeaderList caseSensitive(boolean value) {
      caseSensitive = value;
      return this;
   }

   /**
    * Tests if headers with the given name are contained within this list.
    *
    * <p>
    * Header name comparison is case insensitive.
    *
    * @param name The header name.
    * @return <jk>true</jk> if at least one header with the name is present.
    */
   public boolean contains(String name) {
      return stream().anyMatch(x -> eq(x.getName(), name));
   }

   /**
    * Makes a copy of this list.
    *
    * @return A new copy of this list.
    */
   public HeaderList copy() {
      return new HeaderList(this);
   }

   /**
    * Performs an action on all matching headers in this list.
    *
    * <p>
    * This is the preferred method for iterating over headers as it does not involve
    * creation or copy of lists/arrays.
    *
    * @param filter A predicate to apply to each element to determine if it should be included.  Can be <jk>null</jk> (if null, all headers are matched).
    * @param action An action to perform on each element.
    * @return This object.
    */
   public HeaderList forEach(Predicate<Header> filter, Consumer<Header> action) {
      forEach(x -> consumeIf(filter, action, x));
      return this;
   }

   /**
    * Performs an action on all headers with the specified name in this list.
    *
    * <p>
    * This is the preferred method for iterating over headers as it does not involve
    * creation or copy of lists/arrays.
    *
    * @param name The header name.
    * @param action An action to perform on each element.
    * @return This object.
    */
   public HeaderList forEach(String name, Consumer<Header> action) {
      return forEach(x -> eq(name, x.getName()), action);
   }

   /**
    * Performs an action on the values for all matching headers in this list.
    *
    * @param filter A predicate to apply to each element to determine if it should be included.  Can be <jk>null</jk> (if null, all headers are matched).
    * @param action An action to perform on each element.
    * @return This object.
    */
   public HeaderList forEachValue(Predicate<Header> filter, Consumer<String> action) {
      return forEach(filter, x -> action.accept(x.getValue()));
   }

   /**
    * Performs an action on the values of all matching headers in this list.
    *
    * @param name The header name.
    * @param action An action to perform on each element.
    * @return This object.
    */
   public HeaderList forEachValue(String name, Consumer<String> action) {
      return forEach(name, x -> action.accept(x.getValue()));
   }

   /**
    * Gets a header representing all of the header values with the given name.
    *
    * <p>
    * Same as {@link #get(String, Class)} but the header name is pulled from the {@link org.apache.juneau.http.Header#name()} or
    *    {@link org.apache.juneau.http.Header#value()} annotations.
    *
    * <h5 class='figure'>Example</h5>
    * <p class='bjava'>
    *    Age <jv>age</jv> = headerList.get(Age.<jk>class</jk>);
    * </p>
    *
    * @param <T> The return type.
    * @param type The header implementation class.  Must not be <jk>null</jk> or a {@link IllegalArgumentException} is thrown.
    * @return A header with a condensed value, or {@link Optional#empty()} if no headers by the given name are present.
    */
   public <T> Optional<T> get(Class<T> type) {
      reqnn("type", type);

      String name = HeaderBeanMeta.of(type).getSchema().getName();
      req(nn(name), "Header name could not be found on bean type '%s'", cn(type));

      return get(name, type);
   }

   /**
    * Gets a header representing all of the header values with the given name.
    *
    * <p>
    * If more that one header with the given name exists the values will be combined with <js>", "</js> as per
    * <a href='https://tools.ietf.org/html/rfc2616#section-4.2'>RFC 2616 Section 4.2</a>.
    *
    * @param name The header name.
    * @return A header with a condensed value, or {@link Optional#empty()} if no headers by the given name are present
    */
   public Optional<Header> get(String name) {

      Header first = null;
      List<Header> rest = null;
      for (var x : this) {
         if (eq(x.getName(), name)) {
            if (first == null)
               first = x;
            else {
               if (rest == null)
                  rest = list();
               rest.add(x);
            }
         }
      }

      if (first == null)
         return oe();

      if (rest == null)
         return o(first);

      var sb = new CharArrayBuffer(128);
      sb.append(first.getValue());
      for (var element : rest) {
         sb.append(", ");
         sb.append(element.getValue());
      }

      return o(new BasicHeader(name, sb.toString()));
   }

   /**
    * Gets a header representing all of the header values with the given name.
    *
    * <p>
    * If more that one header with the given name exists the values will be combined with <js>", "</js> as per
    * <a href='https://tools.ietf.org/html/rfc2616#section-4.2'>RFC 2616 Section 4.2</a>.
    *
    * <p>
    * The implementation class must have a public constructor taking in one of the following argument lists:
    * <ul>
    *    <li><c>X(String <jv>value</jv>)</c>
    *    <li><c>X(Object <jv>value</jv>)</c>
    *    <li><c>X(String <jv>name</jv>, String <jv>value</jv>)</c>
    *    <li><c>X(String <jv>name</jv>, Object <jv>value</jv>)</c>
    * </ul>
    *
    * <h5 class='figure'>Example</h5>
    * <p class='bjava'>
    *    BasicIntegerHeader <jv>age</jv> = headerList.get(<js>"Age"</js>, BasicIntegerHeader.<jk>class</jk>);
    * </p>
    *
    * @param <T> The header implementation class.
    * @param name The header name.
    * @param type The header implementation class.
    * @return A header with a condensed value, or {@link Optional#empty()} if no headers by the given name are present.
    */
   public <T> Optional<T> get(String name, Class<T> type) {

      Header first = null;
      List<Header> rest = null;
      for (var x : this) {
         if (eq(x.getName(), name)) {
            if (first == null)
               first = x;
            else {
               if (rest == null)
                  rest = list();
               rest.add(x);
            }
         }
      }

      if (first == null)
         return oe();

      if (rest == null)
         return o(HeaderBeanMeta.of(type).construct(name, first.getValue()));

      var sb = new CharArrayBuffer(128);
      sb.append(first.getValue());
      for (var element : rest) {
         sb.append(", ");
         sb.append(element.getValue());
      }

      return o(HeaderBeanMeta.of(type).construct(name, sb.toString()));
   }

   /**
    * Gets all of the headers.
    *
    * <p>
    * The returned array maintains the relative order in which the headers were added.
    * Each call creates a new array not backed by this list.
    *
    * <p>
    * As a general rule, it's more efficient to use the other methods with consumers to
    * get headers.
    *
    * @return An array containing all headers, never <jk>null</jk>.
    */
   public Header[] getAll() { return stream().toArray(Header[]::new); }

   /**
    * Gets all of the headers with the given name.
    *
    * <p>
    * The returned array maintains the relative order in which the headers were added.
    * Header name comparison is case insensitive.
    * Headers with null values are ignored.
    * Each call creates a new array not backed by this list.
    *
    * <p>
    * As a general rule, it's more efficient to use the other methods with consumers to
    * get headers.
    *
    * @param name The header name.
    *
    * @return An array containing all matching headers, never <jk>null</jk>.
    */
   public Header[] getAll(String name) {
      return stream().filter(x -> eq(x.getName(), name)).toArray(Header[]::new);
   }

   /**
    * Gets the first header with the given name.
    *
    * <p>
    * Header name comparison is case insensitive.
    *
    * @param name The header name.
    * @return The first matching header, or {@link Optional#empty()} if not found.
    */
   public Optional<Header> getFirst(String name) {
      for (var i = 0; i < size(); i++) {
         var x = get(i);
         if (eq(x.getName(), name))
            return o(x);
      }
      return oe();
   }

   /**
    * Gets the last header with the given name.
    *
    * <p>
    * Header name comparison is case insensitive.
    *
    * @param name The header name.
    * @return The last matching header, or {@link Optional#empty()} if not found.
    */
   public Optional<Header> getLast(String name) {
      for (var i = size() - 1; i >= 0; i--) {
         var x = get(i);
         if (eq(x.getName(), name))
            return o(x);
      }
      return oe();
   }

   /**
    * Returns all the string values for all headers with the specified name.
    *
    * @param name The header name.
    * @return An array containing all values.  Never <jk>null</jk>.
    */
   public String[] getValues(String name) {
      return stream().filter(x -> eq(x.getName(), name)).map(Header::getValue).toArray(String[]::new);
   }

   /**
    * Returns an iterator over this list of headers.
    *
    * @return A new iterator over this list of headers.
    */
   public HeaderIterator headerIterator() {
      return new BasicHeaderIterator(toArray(new Header[0]), null, caseSensitive);
   }

   /**
    * Returns an iterator over the headers with a given name in this list.
    *
    * @param name The name of the headers over which to iterate, or <jk>null</jk> for all headers
    *
    * @return A new iterator over the matching headers in this list.
    */
   public HeaderIterator headerIterator(String name) {
      return new BasicHeaderIterator(getAll(name), name, caseSensitive);
   }

   /**
    * Adds the specified header to the beginning of the headers in this list.
    *
    * @param value The header to add.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList prepend(Header value) {
      if (nn(value))
         add(0, value);
      return this;
   }

   /**
    * Adds the specified headers to the beginning of the headers in this list.
    *
    * @param values The headers to add.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList prepend(Header...values) {
      if (nn(values))
         prepend(l(values));
      return this;
   }

   /**
    * Adds the specified headers to the beginning of the headers in this list.
    *
    * @param values The headers to add.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList prepend(List<Header> values) {
      if (nn(values))
         addAll(0, values);
      return this;
   }

   /**
    * Appends the specified header to the beginning of this list.
    *
    * <p>
    * The header is added as a {@link BasicHeader}.
    *
    * @param name The header name.
    * @param value The header value.
    * @return This object.
    */
   public HeaderList prepend(String name, Object value) {
      return prepend(createPart(name, value));
   }

   /**
    * Appends the specified header to the beginning of this list using a value supplier.
    *
    * <p>
    * The header is added as a {@link BasicHeader}.
    *
    * <p>
    * Value is re-evaluated on each call to {@link BasicHeader#getValue()}.
    *
    * @param name The header name.
    * @param value The header value supplier.
    * @return This object.
    */
   public HeaderList prepend(String name, Supplier<?> value) {
      return prepend(createPart(name, value));
   }

   /**
    * Removes the specified header from this list.
    *
    * @param value The header to remove.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList remove(Header value) {
      if (nn(value))
         removeIf(x -> eq(x.getName(), value.getName()) && eq(x.getValue(), value.getValue()));
      return this;
   }

   /**
    * Removes the specified headers from this list.
    *
    * @param values The headers to remove.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList remove(Header...values) {
      for (var value : values)
         remove(value);
      return this;
   }

   /**
    * Removes the specified headers from this list.
    *
    * @param values The headers to remove.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList remove(List<Header> values) {
      if (nn(values))
         values.forEach(this::remove);
      return this;
   }

   /**
    * Removes the header with the specified name from this list.
    *
    * @param name The header name.
    * @return This object.
    */
   public HeaderList remove(String name) {
      removeIf(x -> eq(x.getName(), name));
      return this;
   }

   /**
    * Removes the header with the specified name from this list.
    *
    * @param names The header name.
    * @return This object.
    */
   public HeaderList remove(String...names) {
      if (nn(names))
         for (var name : names)
            remove(name);
      return this;
   }

   /**
    * Removes all headers from this list.
    *
    * @return This object.
    */
   public HeaderList removeAll() {
      clear();
      return this;
   }

   /**
    * Allows header values to contain SVL variables.
    *
    * <p>
    * Resolves variables in header values when using the following methods:
    * <ul>
    *    <li class='jm'>{@link #append(String, Object) append(String,Object)}
    *    <li class='jm'>{@link #append(String, Supplier) append(String,Supplier&lt;?&gt;)}
    *    <li class='jm'>{@link #prepend(String, Object) prepend(String,Object)}
    *    <li class='jm'>{@link #prepend(String, Supplier) prepend(String,Supplier&lt;?&gt;)}
    *    <li class='jm'>{@link #set(String, Object) set(String,Object)}
    *    <li class='jm'>{@link #set(String, Supplier) set(String,Supplier&lt;?&gt;)}
    * </ul>
    *
    * <p>
    * Uses {@link VarResolver#DEFAULT} to resolve variables.
    *
    * @return This object.
    */
   public HeaderList resolving() {
      return resolving(VarResolver.DEFAULT);
   }

   /**
    * Allows header values to contain SVL variables.
    *
    * <p>
    * Resolves variables in header values when using the following methods:
    * <ul>
    *    <li class='jm'>{@link #append(String, Object) append(String,Object)}
    *    <li class='jm'>{@link #append(String, Supplier) append(String,Supplier&lt;?&gt;)}
    *    <li class='jm'>{@link #prepend(String, Object) prepend(String,Object)}
    *    <li class='jm'>{@link #prepend(String, Supplier) prepend(String,Supplier&lt;?&gt;)}
    *    <li class='jm'>{@link #set(String, Object) set(String,Object)}
    *    <li class='jm'>{@link #set(String, Supplier) set(String,Supplier&lt;?&gt;)}
    * </ul>
    *
    * @param varResolver The variable resolver to use for resolving variables.  Can be <jk>null</jk> to disable variable resolution.
    * @return This object.
    */
   public HeaderList resolving(VarResolver varResolver) {
      this.varResolver = varResolver;
      return this;
   }

   /**
    * Adds or replaces the header(s) with the same name.
    *
    * <p>
    * If no header with the same name is found the given header is added to the end of the list.
    *
    * @param value The headers to replace.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList set(Header value) {
      if (nn(value)) {
         var replaced = false;
         for (int i = 0, j = size(); i < j; i++) {
            var x = get(i);
            if (eq(x.getName(), value.getName())) {
               if (replaced) {
                  remove(i);
                  j--;
               } else {
                  set(i, value);
                  replaced = true;
               }
            }
         }

         if (! replaced)
            add(value);
      }

      return this;
   }

   /**
    * Adds or replaces the header(s) with the same name.
    *
    * <p>
    * If no header with the same name is found the given header is added to the end of the list.
    *
    * @param values The headers to replace.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   public HeaderList set(Header...values) {
      if (nn(values))
         set(l(values));
      return this;
   }

   /**
    * Replaces the first occurrence of the headers with the same name.
    *
    * <p>
    * If no header with the same name is found the given header is added to the end of the list.
    *
    * @param values The headers to replace.  <jk>null</jk> values are ignored.
    * @return This object.
    */
   
   @SuppressWarnings({
      "java:S3776" // Cognitive complexity acceptable for this specific logic
   })
   public HeaderList set(List<Header> values) {

      if (nn(values)) {
         for (var h : values) {
            if (nn(h)) {
               for (int i2 = 0, j2 = size(); i2 < j2; i2++) {
                  var x = get(i2);
                  if (eq(x.getName(), h.getName())) {
                     remove(i2);
                     j2--;
                  }
               }
            }
         }

         for (var x : values) {
            if (nn(x)) {
               add(x);
            }
         }
      }

      return this;
   }

   /**
    * Replaces the first occurrence of the headers with the same name.
    *
    * @param name The header name.
    * @param value The header value.
    * @return This object.
    */
   public HeaderList set(String name, Object value) {
      return set(createPart(name, value));
   }

   /**
    * Replaces the first occurrence of the headers with the same name.
    *
    * @param name The header name.
    * @param value The header value.
    * @return This object.
    */
   public HeaderList set(String name, Supplier<?> value) {
      return set(createPart(name, value));
   }

   /**
    * Makes a copy of this list of headers and adds a collection of default headers.
    *
    * <p>
    * Default headers are set if they're not already in the list.
    *
    * @param headers The list of default headers.
    * @return A new list, or the same list if the headers were empty.
    */
   public HeaderList setDefault(Header...headers) {
      if (nn(headers))
         setDefault(l(headers));
      return this;
   }

   /**
    * Adds a collection of default headers.
    *
    * <p>
    * Default headers are set if they're not already in the list.
    *
    * @param headers The list of default headers.
    * @return This object.
    */
   public HeaderList setDefault(List<Header> headers) {
      if (nn(headers))
         headers.stream().filter(x -> nn(x) && ! contains(x.getName())).forEach(this::set);
      return this;
   }

   /**
    * Replaces the first occurrence of the headers with the same name.
    *
    * @param name The header name.
    * @param value The header value.
    * @return This object.
    */
   public HeaderList setDefault(String name, Object value) {
      return setDefault(createPart(name, value));
   }

   /**
    * Replaces the first occurrence of the headers with the same name.
    *
    * @param name The header name.
    * @param value The header value.
    * @return This object.
    */
   public HeaderList setDefault(String name, Supplier<?> value) {
      return setDefault(createPart(name, value));
   }

   /**
    * Returns an unmodifiable snapshot of this list.
    *
    * <p>
    * Idempotency: if this list is already an {@link Unmodifiable} snapshot, it is returned as-is.
    *
    * @return An unmodifiable snapshot of this list, or this list if it is already unmodifiable.
    */
   public HeaderList unmodifiable() {
      return this instanceof Unmodifiable ? this : new Unmodifiable(this);
   }

   /**
    * Returns <jk>true</jk> if this list is an unmodifiable snapshot.
    *
    * @return <jk>true</jk> if this list is an {@link Unmodifiable} snapshot.
    */
   public boolean isUnmodifiable() {
      return this instanceof UnmodifiableBean;
   }

   /**
    * Returns a stream of the headers in this list with the specified name.
    *
    * <p>
    * This does not involve a copy of the underlying array of <c>Header</c> objects so should perform well.
    *
    * @param name The header name.
    * @return This object.
    */
   public Stream<Header> stream(String name) {
      return stream().filter(x -> eq(name, x.getName()));
   }

   @Override /* Overridden from Object */
   public String toString() {
      return "[" + join(this, ", ") + "]";
   }

   /**
    * Creates a new header out of the specified name/value pair.
    *
    * @param name The header name.
    * @param value The header value.
    * @return A new header.
    */
   private Header createPart(String name, Object value) {
      var isResolving = nn(varResolver);

      if (value instanceof Supplier<?> value2) {
         return isResolving ? new BasicHeader(name, resolver(value2)) : new BasicHeader(name, value2);
      }
      return isResolving ? new BasicHeader(name, resolver(value)) : new BasicHeader(name, value);
   }

   private boolean eq(String s1, String s2) {
      return Shorts.eq(! caseSensitive, s1, s2);  // NOAI
   }

   private Supplier<Object> resolver(Object input) {
      return () -> varResolver.resolve(s(unwrap(input)));
   }

   private static Object unwrap(Object o) {
      while (o instanceof Supplier<?> s)
         o = s.get();
      return o;
   }

   /**
    * An unmodifiable snapshot of a {@link HeaderList}.
    *
    * <p>
    * This is the "collection-mutator-override" variant of the immutability paradigm.  Because {@link HeaderList}
    * extends {@link ArrayList}, a single {@code modify()} funnel cannot intercept the inherited {@code ArrayList}
    * mutators, so this subclass snapshot-copies the elements in its constructor and overrides the entire
    * collection-mutator surface — its own fluent mutators plus the inherited {@code ArrayList} mutators (including the
    * {@code iterator()} / {@code listIterator()} mutators) — to throw {@link UnsupportedOperationException}.  Read
    * operations and non-mutating iteration continue to work.
    */
   public static class Unmodifiable extends HeaderList implements UnmodifiableBean {

      private static final long serialVersionUID = 1L;

      /**
       * Constructor.
       *
       * @param copyFrom The list to snapshot-copy.  Must not be <jk>null</jk> or a {@link NullPointerException} is thrown.
       */
      Unmodifiable(HeaderList copyFrom) {
         super(copyFrom);
      }

      // --- Fluent mutators -----------------------------------------------------------------------------------------

      @Override public HeaderList append(Header value) { throw uoex(RO); }
      @Override public HeaderList append(Header...values) { throw uoex(RO); }
      @Override public HeaderList append(List<Header> values) { throw uoex(RO); }
      @Override public HeaderList append(String name, Object value) { throw uoex(RO); }
      @Override public HeaderList append(String name, Supplier<?> value) { throw uoex(RO); }
      @Override public HeaderList caseSensitive(boolean value) { throw uoex(RO); }
      @Override public HeaderList prepend(Header value) { throw uoex(RO); }
      @Override public HeaderList prepend(Header...values) { throw uoex(RO); }
      @Override public HeaderList prepend(List<Header> values) { throw uoex(RO); }
      @Override public HeaderList prepend(String name, Object value) { throw uoex(RO); }
      @Override public HeaderList prepend(String name, Supplier<?> value) { throw uoex(RO); }
      @Override public HeaderList remove(Header value) { throw uoex(RO); }
      @Override public HeaderList remove(Header...values) { throw uoex(RO); }
      @Override public HeaderList remove(List<Header> values) { throw uoex(RO); }
      @Override public HeaderList remove(String name) { throw uoex(RO); }
      @Override public HeaderList remove(String...names) { throw uoex(RO); }
      @Override public HeaderList removeAll() { throw uoex(RO); }
      @Override public HeaderList resolving() { throw uoex(RO); }
      @Override public HeaderList resolving(VarResolver varResolver) { throw uoex(RO); }
      @Override public HeaderList set(Header value) { throw uoex(RO); }
      @Override public HeaderList set(Header...values) { throw uoex(RO); }
      @Override public HeaderList set(List<Header> values) { throw uoex(RO); }
      @Override public HeaderList set(String name, Object value) { throw uoex(RO); }
      @Override public HeaderList set(String name, Supplier<?> value) { throw uoex(RO); }
      @Override public HeaderList setDefault(Header...headers) { throw uoex(RO); }
      @Override public HeaderList setDefault(List<Header> headers) { throw uoex(RO); }
      @Override public HeaderList setDefault(String name, Object value) { throw uoex(RO); }
      @Override public HeaderList setDefault(String name, Supplier<?> value) { throw uoex(RO); }

      // --- Inherited ArrayList collection mutators -----------------------------------------------------------------

      @Override public boolean add(Header e) { throw uoex(RO); }
      @Override public void add(int index, Header element) { throw uoex(RO); }
      @Override public boolean addAll(Collection<? extends Header> c) { throw uoex(RO); }
      @Override public boolean addAll(int index, Collection<? extends Header> c) { throw uoex(RO); }
      @Override public void clear() { throw uoex(RO); }
      @Override public boolean remove(Object o) { throw uoex(RO); }
      @Override public Header remove(int index) { throw uoex(RO); }
      @Override public boolean removeAll(Collection<?> c) { throw uoex(RO); }
      @Override public boolean removeIf(Predicate<? super Header> filter) { throw uoex(RO); }
      @Override public void replaceAll(UnaryOperator<Header> operator) { throw uoex(RO); }
      @Override public boolean retainAll(Collection<?> c) { throw uoex(RO); }
      @Override public Header set(int index, Header element) { throw uoex(RO); }
      @Override public void sort(Comparator<? super Header> c) { throw uoex(RO); }

      @Override public Iterator<Header> iterator() {
         var i = super.iterator();
         return new Iterator<>() {
            @Override public boolean hasNext() { return i.hasNext(); }
            @Override public Header next() { return i.next(); }
            @Override public void remove() { throw uoex(RO); }
         };
      }

      @Override public ListIterator<Header> listIterator() { return listIterator(0); }

      @Override public ListIterator<Header> listIterator(int index) {
         var i = super.listIterator(index);
         return new ListIterator<>() {
            @Override public boolean hasNext() { return i.hasNext(); }
            @Override public Header next() { return i.next(); }
            @Override public boolean hasPrevious() { return i.hasPrevious(); }
            @Override public Header previous() { return i.previous(); }
            @Override public int nextIndex() { return i.nextIndex(); }
            @Override public int previousIndex() { return i.previousIndex(); }
            @Override public void add(Header e) { throw uoex(RO); }
            @Override public void set(Header e) { throw uoex(RO); }
            @Override public void remove() { throw uoex(RO); }
         };
      }
   }

   private static final String RO = "Bean is unmodifiable.";
}