/*
 * Copyright 2015 The Netty Project
 *
 * The Netty Project 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.
 */

/*
 * Copyright 2014 Twitter, Inc.
 *
 * 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 io.netty.handler.codec.http2;

import static io.netty.handler.codec.http2.Http2CodecUtil.MAX_HEADER_TABLE_SIZE;
import static io.netty.handler.codec.http2.Http2CodecUtil.MIN_HEADER_TABLE_SIZE;

final class HpackDynamicTable {

    // a circular queue of header fields
    HpackHeaderField[] hpackHeaderFields;
    int head;
    int tail;
    private long size;
    private long capacity = -1; // ensure setCapacity creates the array

    
Creates a new dynamic table with the specified initial capacity.
/**
     * Creates a new dynamic table with the specified initial capacity.
     */
    HpackDynamicTable(long initialCapacity) {
        setCapacity(initialCapacity);
    }

    
Return the number of header fields in the dynamic table.
/**
     * Return the number of header fields in the dynamic table.
     */
    public int length() {
        int length;
        if (head < tail) {
            length = hpackHeaderFields.length - tail + head;
        } else {
            length = head - tail;
        }
        return length;
    }

    
Return the current size of the dynamic table. This is the sum of the size of the entries.
/**
     * Return the current size of the dynamic table. This is the sum of the size of the entries.
     */
    public long size() {
        return size;
    }

    
Return the maximum allowable size of the dynamic table.
/**
     * Return the maximum allowable size of the dynamic table.
     */
    public long capacity() {
        return capacity;
    }

    
Return the header field at the given index. The first and newest entry is always at index 1,
and the oldest entry is at the index length().
/**
     * Return the header field at the given index. The first and newest entry is always at index 1,
     * and the oldest entry is at the index length().
     */
    public HpackHeaderField getEntry(int index) {
        if (index <= 0 || index > length()) {
            throw new IndexOutOfBoundsException();
        }
        int i = head - index;
        if (i < 0) {
            return hpackHeaderFields[i + hpackHeaderFields.length];
        } else {
            return hpackHeaderFields[i];
        }
    }

    
Add the header field to the dynamic table. Entries are evicted from the dynamic table until
the size of the table and the new header field is less than or equal to the table's capacity.
If the size of the new entry is larger than the table's capacity, the dynamic table will be
cleared.
/**
     * Add the header field to the dynamic table. Entries are evicted from the dynamic table until
     * the size of the table and the new header field is less than or equal to the table's capacity.
     * If the size of the new entry is larger than the table's capacity, the dynamic table will be
     * cleared.
     */
    public void add(HpackHeaderField header) {
        int headerSize = header.size();
        if (headerSize > capacity) {
            clear();
            return;
        }
        while (capacity - size < headerSize) {
            remove();
        }
        hpackHeaderFields[head++] = header;
        size += header.size();
        if (head == hpackHeaderFields.length) {
            head = 0;
        }
    }

    
Remove and return the oldest header field from the dynamic table.
/**
     * Remove and return the oldest header field from the dynamic table.
     */
    public HpackHeaderField remove() {
        HpackHeaderField removed = hpackHeaderFields[tail];
        if (removed == null) {
            return null;
        }
        size -= removed.size();
        hpackHeaderFields[tail++] = null;
        if (tail == hpackHeaderFields.length) {
            tail = 0;
        }
        return removed;
    }

    
Remove all entries from the dynamic table.
/**
     * Remove all entries from the dynamic table.
     */
    public void clear() {
        while (tail != head) {
            hpackHeaderFields[tail++] = null;
            if (tail == hpackHeaderFields.length) {
                tail = 0;
            }
        }
        head = 0;
        tail = 0;
        size = 0;
    }

    
Set the maximum size of the dynamic table. Entries are evicted from the dynamic table until
the size of the table is less than or equal to the maximum size.
/**
     * Set the maximum size of the dynamic table. Entries are evicted from the dynamic table until
     * the size of the table is less than or equal to the maximum size.
     */
    public void setCapacity(long capacity) {
        if (capacity < MIN_HEADER_TABLE_SIZE || capacity > MAX_HEADER_TABLE_SIZE) {
            throw new IllegalArgumentException("capacity is invalid: " + capacity);
        }
        // initially capacity will be -1 so init won't return here
        if (this.capacity == capacity) {
            return;
        }
        this.capacity = capacity;

        if (capacity == 0) {
            clear();
        } else {
            // initially size will be 0 so remove won't be called
            while (size > capacity) {
                remove();
            }
        }

        int maxEntries = (int) (capacity / HpackHeaderField.HEADER_ENTRY_OVERHEAD);
        if (capacity % HpackHeaderField.HEADER_ENTRY_OVERHEAD != 0) {
            maxEntries++;
        }

        // check if capacity change requires us to reallocate the array
        if (hpackHeaderFields != null && hpackHeaderFields.length == maxEntries) {
            return;
        }

        HpackHeaderField[] tmp = new HpackHeaderField[maxEntries];

        // initially length will be 0 so there will be no copy
        int len = length();
        int cursor = tail;
        for (int i = 0; i < len; i++) {
            HpackHeaderField entry = hpackHeaderFields[cursor++];
            tmp[i] = entry;
            if (cursor == hpackHeaderFields.length) {
                cursor = 0;
            }
        }

        tail = 0;
        head = tail + len;
        hpackHeaderFields = tmp;
    }
}