/*
 *  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.tomcat.util.http;

import java.io.IOException;
import java.io.PrintWriter;
import java.io.StringWriter;
import java.util.Collections;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;

import org.apache.tomcat.util.buf.MessageBytes;
import org.apache.tomcat.util.buf.StringUtils;
import org.apache.tomcat.util.res.StringManager;

/**
 * Memory-efficient repository for Mime Headers. When the object is recycled, it will keep the allocated headers[] and
 * all the MimeHeaderField - no GC is generated.
 * <p>
 * For input headers it is possible to use the MessageByte for Fields - so no GC will be generated.
 * <p>
 * The only garbage is generated when using the String for header names/values - this can't be avoided when the servlet
 * calls header methods, but is easy to avoid inside tomcat. The goal is to use _only_ MessageByte-based Fields, and
 * reduce to 0 the memory overhead of tomcat.
 * <p>
 * This class is used to contain standard internet message headers, used for SMTP (RFC822) and HTTP (RFC2068) messages
 * as well as for MIME (RFC 2045) applications such as transferring typed data and grouping related items in multipart
 * message bodies.
 * <p>
 * Message headers, as specified in RFC822, include a field name and a field body. Order has no semantic significance,
 * and several fields with the same name may exist. However, most fields do not (and should not) exist more than once in
 * a header.
 * <p>
 * Many kinds of field body must conform to a specified syntax, including the standard parenthesized comment syntax.
 * This class supports only two simple syntaxes, for dates and integers.
 * <p>
 * When processing headers, care must be taken to handle the case of multiple same-name fields correctly. The values of
 * such fields are only available as strings. They may be accessed by index (treating the header as an array of fields),
 * or by name (returning an array of string values).
 * <p>
 * Headers are first parsed and stored in the order they are received. This is based on the fact that most servlets will
 * not directly access all headers, and most headers are single-valued. (the alternative - a hash or similar data
 * structure - will add an overhead that is not needed in most cases)
 * <p>
 * Apache seems to be using a similar method for storing and manipulating headers.
 *
 * @author dac@eng.sun.com
 * @author James Todd [gonzo@eng.sun.com]
 * @author Costin Manolache
 * @author kevin seguin
 */
public class MimeHeaders {

    /**
     * Initial size - should be == average number of headers per request
     */
    public static final int DEFAULT_HEADER_SIZE = 8;

    private static final StringManager sm = StringManager.getManager("org.apache.tomcat.util.http");

    /**
     * The header fields.
     */
    private MimeHeaderField[] headers = new MimeHeaderField[DEFAULT_HEADER_SIZE];

    /**
     * The current number of header fields.
     */
    private int count;

    /**
     * The limit on the number of header fields.
     */
    private int limit = -1;

    /**
     * Creates a new MimeHeaders object using a default buffer size.
     */
    public MimeHeaders() {
        // NO-OP
    }

    /**
     * Set limit on the number of header fields.
     *
     * @param limit The new limit
     */
    public void setLimit(int limit) {
        this.limit = limit;
        if (limit > 0 && headers.length > limit && count < limit) {
            // shrink header list array
            MimeHeaderField[] tmp = new MimeHeaderField[limit];
            System.arraycopy(headers, 0, tmp, 0, count);
            headers = tmp;
        }
    }

    /**
     * Clears all header fields.
     */
    public void recycle() {
        for (int i = 0; i < count; i++) {
            headers[i].recycle();
        }
        count = 0;
    }

    @Override
    public String toString() {
        StringWriter sw = new StringWriter();
        PrintWriter pw = new PrintWriter(sw);
        pw.println("=== MimeHeaders ===");
        Enumeration<String> e = names();
        while (e.hasMoreElements()) {
            String n = e.nextElement();
            Enumeration<String> ev = values(n);
            while (ev.hasMoreElements()) {
                pw.print(n);
                pw.print(" = ");
                pw.println(ev.nextElement());
            }
        }
        return sw.toString();
    }


    public Map<String,String> toMap() {
        if (count == 0) {
            return Collections.emptyMap();
        }
        Map<String,String> result = new HashMap<>();

        for (int i = 0; i < count; i++) {
            String name = headers[i].getName().toStringType();
            String value = headers[i].getValue().toStringType();
            result.merge(name, value, StringUtils::join);
        }
        return result;
    }


    public void filter(Set<String> allowedHeaders) {
        int j = -1;
        for (int i = 0; i < count; i++) {
            String name = headers[i].getName().toStringType();
            if (allowedHeaders.contains(name)) {
                ++j;
                if (j != i) {
                    headers[j] = headers[i];
                }
            }
        }
        count = ++j;
    }


    public void duplicate(MimeHeaders source) throws IOException {
        for (int i = 0; i < source.size(); i++) {
            MimeHeaderField mhf = createHeader();
            mhf.getName().duplicate(source.getName(i));
            mhf.getValue().duplicate(source.getValue(i));
        }
    }


    // -------------------- Idx access to headers ----------

    /**
     * @return the current number of header fields.
     */
    public int size() {
        return count;
    }

    /**
     * @param n The header index
     *
     * @return the Nth header name, or null if there is no such header. This may be used to iterate through all header
     *             fields.
     */
    public MessageBytes getName(int n) {
        return n >= 0 && n < count ? headers[n].getName() : null;
    }

    /**
     * @param n The header index
     *
     * @return the Nth header value, or null if there is no such header. This may be used to iterate through all header
     *             fields.
     */
    public MessageBytes getValue(int n) {
        return n >= 0 && n < count ? headers[n].getValue() : null;
    }

    /**
     * Find the index of a header with the given name.
     *
     * @param name     The header name
     * @param starting Index on which to start looking
     *
     * @return the header index
     */
    public int findHeader(String name, int starting) {
        // We can use a hash - but it's not clear how much
        // benefit you can get - there is an overhead
        // and the number of headers is small (4-5 ?)
        // Another problem is that we'll pay the overhead
        // of constructing the hashtable

        // A custom search tree may be better
        for (int i = starting; i < count; i++) {
            if (headers[i].getName().equalsIgnoreCase(name)) {
                return i;
            }
        }
        return -1;
    }

    // -------------------- --------------------

    /**
     * Returns an enumeration of strings representing the header field names. Field names may appear multiple times in
     * this enumeration, indicating that multiple fields with that name exist in this header.
     *
     * @return the enumeration
     */
    public Enumeration<String> names() {
        return new NamesEnumerator(this);
    }

    public Enumeration<String> values(String name) {
        return new ValuesEnumerator(this, name);
    }

    // -------------------- Adding headers --------------------


    /**
     * Adds a partially constructed field to the header. This field has not had its name or value initialized.
     */
    private MimeHeaderField createHeader() {
        if (limit > -1 && count >= limit) {
            throw new IllegalStateException(sm.getString("headers.maxCountFail", Integer.valueOf(limit)));
        }
        MimeHeaderField mh;
        int len = headers.length;
        if (count >= len) {
            // expand header list array
            int newLength = count * 2;
            if (limit > 0 && newLength > limit) {
                newLength = limit;
            }
            MimeHeaderField[] tmp = new MimeHeaderField[newLength];
            System.arraycopy(headers, 0, tmp, 0, len);
            headers = tmp;
        }
        if ((mh = headers[count]) == null) {
            headers[count] = mh = new MimeHeaderField();
        }
        count++;
        return mh;
    }

    /**
     * Create a new named header , return the MessageBytes container for the new value
     *
     * @param name The header name
     *
     * @return the message bytes container for the value
     */
    public MessageBytes addValue(String name) {
        MimeHeaderField mh = createHeader();
        mh.getName().setString(name);
        return mh.getValue();
    }

    /**
     * Create a new named header using un-translated byte[]. The conversion to chars can be delayed until encoding is
     * known.
     *
     * @param b      The header name bytes
     * @param startN Offset
     * @param len    Length
     *
     * @return the message bytes container for the value
     */
    public MessageBytes addValue(byte[] b, int startN, int len) {
        MimeHeaderField mhf = createHeader();
        mhf.getName().setBytes(b, startN, len);
        return mhf.getValue();
    }

    /**
     * Allow "set" operations, which removes all current values for this header.
     *
     * @param name The header name
     *
     * @return the message bytes container for the value
     */
    public MessageBytes setValue(String name) {
        for (int i = 0; i < count; i++) {
            if (headers[i].getName().equalsIgnoreCase(name)) {
                for (int j = i + 1; j < count; j++) {
                    if (headers[j].getName().equalsIgnoreCase(name)) {
                        removeHeader(j--);
                    }
                }
                return headers[i].getValue();
            }
        }
        MimeHeaderField mh = createHeader();
        mh.getName().setString(name);
        return mh.getValue();
    }

    // -------------------- Getting headers --------------------

    /**
     * Finds and returns a header field with the given name. If no such field exists, null is returned. If more than one
     * such field is in the header, an arbitrary one is returned.
     *
     * @param name The header name
     *
     * @return the value
     */
    public MessageBytes getValue(String name) {
        for (int i = 0; i < count; i++) {
            if (headers[i].getName().equalsIgnoreCase(name)) {
                return headers[i].getValue();
            }
        }
        return null;
    }

    /**
     * Finds and returns a unique header field with the given name. If no such field exists, null is returned. If the
     * specified header field is not unique then an {@link IllegalArgumentException} is thrown.
     *
     * @param name The header name
     *
     * @return the value if unique
     *
     * @throws IllegalArgumentException if the header has multiple values
     */
    public MessageBytes getUniqueValue(String name) {
        MessageBytes result = null;
        for (int i = 0; i < count; i++) {
            if (headers[i].getName().equalsIgnoreCase(name)) {
                if (result == null) {
                    result = headers[i].getValue();
                } else {
                    throw new IllegalArgumentException();
                }
            }
        }
        return result;
    }

    public String getHeader(String name) {
        MessageBytes mh = getValue(name);
        return mh != null ? mh.toStringType() : null;
    }

    // -------------------- Removing --------------------

    /**
     * Removes a header field with the specified name. Does nothing if such a field could not be found.
     *
     * @param name the name of the header field to be removed
     */
    public void removeHeader(String name) {
        for (int i = 0; i < count; i++) {
            if (headers[i].getName().equalsIgnoreCase(name)) {
                removeHeader(i--);
            }
        }
    }

    /**
     * Reset, move to the end and then reduce count by 1.
     *
     * @param idx the index of the header to remove.
     */
    public void removeHeader(int idx) {
        // Implementation note. This method must not change the order of the
        // remaining headers because, if there are multiple header values for
        // the same name, the order of those headers is significant. It is
        // simpler to retain order for all values than try to determine if there
        // are multiple header values for the same name.

        // Clear the header to remove
        MimeHeaderField mh = headers[idx];
        mh.recycle();

        // Move the remaining headers
        System.arraycopy(headers, idx + 1, headers, idx, count - idx - 1);

        // Place the removed header at the end
        headers[count - 1] = mh;

        // Reduce the count
        count--;
    }

}

/**
 * Enumerate the distinct header names. Each nextElement() is O(n) ( a comparison is done with all previous elements ).
 * This is less frequent than add() - we want to keep add O(1).
 */
class NamesEnumerator implements Enumeration<String> {
    private int pos;
    private final int size;
    private String next;
    private final MimeHeaders headers;

    NamesEnumerator(MimeHeaders headers) {
        this.headers = headers;
        pos = 0;
        size = headers.size();
        findNext();
    }

    private void findNext() {
        next = null;
        for (; pos < size; pos++) {
            next = headers.getName(pos).toStringType();
            for (int j = 0; j < pos; j++) {
                if (headers.getName(j).equalsIgnoreCase(next)) {
                    // duplicate.
                    next = null;
                    break;
                }
            }
            if (next != null) {
                // it's not a duplicate
                break;
            }
        }
        // next time findNext is called it will try the
        // next element
        pos++;
    }

    @Override
    public boolean hasMoreElements() {
        return next != null;
    }

    @Override
    public String nextElement() {
        String current = next;
        findNext();
        return current;
    }
}

/**
 * Enumerate the values for a (possibly ) multiple value element.
 */
class ValuesEnumerator implements Enumeration<String> {
    private int pos;
    private final int size;
    private MessageBytes next;
    private final MimeHeaders headers;
    private final String name;

    ValuesEnumerator(MimeHeaders headers, String name) {
        this.name = name;
        this.headers = headers;
        pos = 0;
        size = headers.size();
        findNext();
    }

    private void findNext() {
        next = null;
        for (; pos < size; pos++) {
            MessageBytes n1 = headers.getName(pos);
            if (n1.equalsIgnoreCase(name)) {
                next = headers.getValue(pos);
                break;
            }
        }
        pos++;
    }

    @Override
    public boolean hasMoreElements() {
        return next != null;
    }

    @Override
    public String nextElement() {
        MessageBytes current = next;
        findNext();
        return current.toStringType();
    }
}

class MimeHeaderField {

    private final MessageBytes nameB = MessageBytes.newInstance();
    private final MessageBytes valueB = MessageBytes.newInstance();

    /**
     * Creates a new, uninitialized header field.
     */
    MimeHeaderField() {
        // NO-OP
    }

    public void recycle() {
        nameB.recycle();
        valueB.recycle();
    }

    public MessageBytes getName() {
        return nameB;
    }

    public MessageBytes getValue() {
        return valueB;
    }

    @Override
    public String toString() {
        return nameB + ": " + valueB;
    }
}