/*
 * JBoss, Home of Professional Open Source.
 * Copyright 2014 Red Hat, Inc., and individual contributors
 * as indicated by the @author tags.
 *
 * 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.undertow.protocols.http2;

import java.nio.ByteBuffer;

import io.undertow.UndertowMessages;
import io.undertow.util.HttpString;

import static io.undertow.protocols.http2.Hpack.HeaderField;

A decoder for HPACK.
Author:
Stuart Douglas/**
 * A decoder for HPACK.
 *
 * @author Stuart Douglas
 */
public class HpackDecoder {

    private static final int DEFAULT_RING_BUFFER_SIZE = 10;

    
The object that receives the headers that are emitted from this decoder
/**
     * The object that receives the headers that are emitted from this decoder
     */
    private HeaderEmitter headerEmitter;

    
The header table
/**
     * The header table
     */
    private HeaderField[] headerTable;

    
The current HEAD position of the header table. We use a ring buffer type
construct as it would be silly to actually shuffle the items around in the
array.
/**
     * The current HEAD position of the header table. We use a ring buffer type
     * construct as it would be silly to actually shuffle the items around in the
     * array.
     */
    private int firstSlotPosition = 0;

    
The current table size by index (aka the number of index positions that are filled up)
/**
     * The current table size by index (aka the number of index positions that are filled up)
     */
    private int filledTableSlots = 0;

    
the current calculates memory size, as per the HPACK algorithm
/**
     * the current calculates memory size, as per the HPACK algorithm
     */
    private int currentMemorySize = 0;

    
The current memory size, as specified by the client
/**
     * The current memory size, as specified by the client
     */
    private int specifiedMemorySize;

    
The maximum allowed memory size, as specified by us. If the client tries to increase beyond this amount it is an error
/**
     * The maximum allowed memory size, as specified by us. If the client tries to increase beyond this amount it is an error
     */
    private final int maxAllowedMemorySize;

    private boolean first = true;

    private final StringBuilder stringBuilder = new StringBuilder();

    public HpackDecoder(int maxAllowedMemorySize) {
        this.specifiedMemorySize = Math.min(Hpack.DEFAULT_TABLE_SIZE, maxAllowedMemorySize);
        this.maxAllowedMemorySize = maxAllowedMemorySize;
        headerTable = new HeaderField[DEFAULT_RING_BUFFER_SIZE];
    }

    public HpackDecoder() {
        this(Hpack.DEFAULT_TABLE_SIZE);
    }

    
Decodes the provided frame data. If this method leaves data in the buffer then
this buffer should be compacted so this data is preserved, unless there is no
more data in which case this should be considered a protocol error.
Params:
buffer – The buffer/**
     * Decodes the provided frame data. If this method leaves data in the buffer then
     * this buffer should be compacted so this data is preserved, unless there is no
     * more data in which case this should be considered a protocol error.
     *
     * @param buffer The buffer
     */
    public void decode(ByteBuffer buffer, boolean moreData) throws HpackException {
        while (buffer.hasRemaining()) {
            int originalPos = buffer.position();
            byte b = buffer.get();
            if ((b & 0b10000000) != 0) {
                first = false;
                //if the first bit is set it is an indexed header field
                buffer.position(buffer.position() - 1); //unget the byte
                int index = Hpack.decodeInteger(buffer, 7); //prefix is 7
                if (index == -1) {
                    if(!moreData) {
                        throw UndertowMessages.MESSAGES.hpackFailed();
                    }
                    buffer.position(originalPos);
                    return;
                } else if(index == 0) {
                    throw UndertowMessages.MESSAGES.zeroNotValidHeaderTableIndex();
                }
                handleIndex(index);
            } else if ((b & 0b01000000) != 0) {
                first = false;
                //Literal Header Field with Incremental Indexing
                HttpString headerName = readHeaderName(buffer, 6);
                if (headerName == null) {
                    if(!moreData) {
                        throw UndertowMessages.MESSAGES.hpackFailed();
                    }
                    buffer.position(originalPos);
                    return;
                }
                String headerValue = readHpackString(buffer);
                if (headerValue == null) {
                    if(!moreData) {
                        throw UndertowMessages.MESSAGES.hpackFailed();
                    }
                    buffer.position(originalPos);
                    return;
                }
                headerEmitter.emitHeader(headerName, headerValue, false);
                addEntryToHeaderTable(new HeaderField(headerName, headerValue));
            } else if ((b & 0b11110000) == 0) {
                first = false;
                //Literal Header Field without Indexing
                HttpString headerName = readHeaderName(buffer, 4);
                if (headerName == null) {
                    if(!moreData) {
                        throw UndertowMessages.MESSAGES.hpackFailed();
                    }
                    buffer.position(originalPos);
                    return;
                }
                String headerValue = readHpackString(buffer);
                if (headerValue == null) {
                    if(!moreData) {
                        throw UndertowMessages.MESSAGES.hpackFailed();
                    }
                    buffer.position(originalPos);
                    return;
                }
                headerEmitter.emitHeader(headerName, headerValue, false);
            } else if ((b & 0b11110000) == 0b00010000) {
                first = false;
                //Literal Header Field never indexed
                HttpString headerName = readHeaderName(buffer, 4);
                if (headerName == null) {
                    buffer.position(originalPos);
                    return;
                }
                String headerValue = readHpackString(buffer);
                if (headerValue == null) {
                    if(!moreData) {
                        throw UndertowMessages.MESSAGES.hpackFailed();
                    }
                    buffer.position(originalPos);
                    return;
                }
                headerEmitter.emitHeader(headerName, headerValue, true);
            } else if ((b & 0b11100000) == 0b00100000) {
                if(!first) {
                    throw new HpackException();
                }
                //context update max table size change
                if (!handleMaxMemorySizeChange(buffer, originalPos)) {
                    return;
                }
            } else {
                throw UndertowMessages.MESSAGES.invalidHpackEncoding(b);
            }
        }
        if(!moreData) {
            first = true;
        }
    }

    private boolean handleMaxMemorySizeChange(ByteBuffer buffer, int originalPos) throws HpackException {
        buffer.position(buffer.position() - 1); //unget the byte
        int size = Hpack.decodeInteger(buffer, 5);
        if (size == -1) {
            buffer.position(originalPos);
            return false;
        }
        if(size > maxAllowedMemorySize) {
            throw new HpackException(Http2Channel.ERROR_PROTOCOL_ERROR);
        }
        specifiedMemorySize = size;
        if (currentMemorySize > specifiedMemorySize) {
            int newTableSlots = filledTableSlots;
            int tableLength = headerTable.length;
            int newSize = currentMemorySize;
            while (newSize > specifiedMemorySize) {
                int clearIndex = firstSlotPosition;
                firstSlotPosition++;
                if (firstSlotPosition == tableLength) {
                    firstSlotPosition = 0;
                }
                HeaderField oldData = headerTable[clearIndex];
                headerTable[clearIndex] = null;
                newSize -= oldData.size;
                newTableSlots--;
            }
            this.filledTableSlots = newTableSlots;
            currentMemorySize = newSize;
        }
        return true;
    }

    private HttpString readHeaderName(ByteBuffer buffer, int prefixLength) throws HpackException {
        buffer.position(buffer.position() - 1); //unget the byte
        int index = Hpack.decodeInteger(buffer, prefixLength);
        if (index == -1) {
            return null;
        } else if (index != 0) {
            return handleIndexedHeaderName(index);
        } else {
            String string = readHpackString(buffer);
            if (string == null) {
                return null;
            }
            return new HttpString(string);
        }
    }

    private String readHpackString(ByteBuffer buffer) throws HpackException {
        if (!buffer.hasRemaining()) {
            return null;
        }
        byte data = buffer.get(buffer.position());

        int length = Hpack.decodeInteger(buffer, 7);
        if (buffer.remaining() < length || length == -1) {
            return null;
        }
        boolean huffman = (data & 0b10000000) != 0;
        if (huffman) {
            return readHuffmanString(length, buffer);
        }
        for (int i = 0; i < length; ++i) {
            stringBuilder.append((char) buffer.get());
        }
        String ret = stringBuilder.toString();
        stringBuilder.setLength(0);
        return ret;
    }

    private String readHuffmanString(int length, ByteBuffer buffer) throws HpackException {
        HPackHuffman.decode(buffer, length, stringBuilder);
        String ret = stringBuilder.toString();
        stringBuilder.setLength(0);
        return ret;
    }

    private HttpString handleIndexedHeaderName(int index) throws HpackException {
        if (index <= Hpack.STATIC_TABLE_LENGTH) {
            return Hpack.STATIC_TABLE[index].name;
        } else {
            if (index >= Hpack.STATIC_TABLE_LENGTH + filledTableSlots) {
                throw new HpackException();
            }
            int adjustedIndex = getRealIndex(index - Hpack.STATIC_TABLE_LENGTH);
            HeaderField res = headerTable[adjustedIndex];
            if (res == null) {
                throw new HpackException();
            }
            return res.name;
        }
    }

    
Handle an indexed header representation
Params:
index – The index
Throws:
HpackException – /**
     * Handle an indexed header representation
     *
     * @param index The index
     * @throws HpackException
     */
    private void handleIndex(int index) throws HpackException {
        if (index <= Hpack.STATIC_TABLE_LENGTH) {
            addStaticTableEntry(index);
        } else {
            int adjustedIndex = getRealIndex(index - Hpack.STATIC_TABLE_LENGTH);
            HeaderField headerField = headerTable[adjustedIndex];
            headerEmitter.emitHeader(headerField.name, headerField.value, false);
        }
    }

    
because we use a ring buffer type construct, and don't actually shuffle
items in the array, we need to figure out he real index to use.

package private for unit tests
Params:
index – The index from the hpack
Returns:
the real index into the array/**
     * because we use a ring buffer type construct, and don't actually shuffle
     * items in the array, we need to figure out he real index to use.
     * <p/>
     * package private for unit tests
     *
     * @param index The index from the hpack
     * @return the real index into the array
     */
    int getRealIndex(int index) throws HpackException {
        //the index is one based, but our table is zero based, hence -1
        //also because of our ring buffer setup the indexes are reversed
        //index = 1 is at position firstSlotPosition + filledSlots
        int newIndex = (firstSlotPosition + (filledTableSlots - index)) % headerTable.length;
        if(newIndex < 0) {
            throw UndertowMessages.MESSAGES.invalidHpackIndex(index);
        }
        return newIndex;
    }

    private void addStaticTableEntry(int index) throws HpackException {
        //adds an entry from the static table.
        HeaderField entry = Hpack.STATIC_TABLE[index];
        headerEmitter.emitHeader(entry.name, entry.value == null ? "" : entry.value, false);
    }

    private void addEntryToHeaderTable(HeaderField entry) {
        if (entry.size > specifiedMemorySize) {
            //it is to big to fit, so we just completely clear the table.
            while (filledTableSlots > 0) {
                headerTable[firstSlotPosition] = null;
                firstSlotPosition++;
                if (firstSlotPosition == headerTable.length) {
                    firstSlotPosition = 0;
                }
                filledTableSlots--;
            }
            currentMemorySize = 0;
            return;
        }
        resizeIfRequired();
        int newTableSlots = filledTableSlots + 1;
        int tableLength = headerTable.length;
        int index = (firstSlotPosition + filledTableSlots) % tableLength;
        headerTable[index] = entry;
        int newSize = currentMemorySize + entry.size;
        while (newSize > specifiedMemorySize) {
            int clearIndex = firstSlotPosition;
            firstSlotPosition++;
            if (firstSlotPosition == tableLength) {
                firstSlotPosition = 0;
            }
            HeaderField oldData = headerTable[clearIndex];
            headerTable[clearIndex] = null;
            newSize -= oldData.size;
            newTableSlots--;
        }
        this.filledTableSlots = newTableSlots;
        currentMemorySize = newSize;
    }

    private void resizeIfRequired() {
        if(filledTableSlots == headerTable.length) {
            HeaderField[] newArray = new HeaderField[headerTable.length + 10]; //we only grow slowly
            for(int i = 0; i < headerTable.length; ++i) {
                newArray[i] = headerTable[(firstSlotPosition + i) % headerTable.length];
            }
            firstSlotPosition = 0;
            headerTable = newArray;
        }
    }


    public interface HeaderEmitter {

        void emitHeader(HttpString name, String value, boolean neverIndex) throws HpackException;
    }


    public HeaderEmitter getHeaderEmitter() {
        return headerEmitter;
    }

    public void setHeaderEmitter(HeaderEmitter headerEmitter) {
        this.headerEmitter = headerEmitter;
    }

    //package private fields for unit tests

    int getFirstSlotPosition() {
        return firstSlotPosition;
    }

    HeaderField[] getHeaderTable() {
        return headerTable;
    }

    int getFilledTableSlots() {
        return filledTableSlots;
    }

    int getCurrentMemorySize() {
        return currentMemorySize;
    }

    int getSpecifiedMemorySize() {
        return specifiedMemorySize;
    }
}