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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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package jdk.incubator.http.internal.hpack;

import jdk.incubator.http.internal.hpack.HPACK.Logger;
import jdk.internal.vm.annotation.Stable;

import java.io.IOException;
import java.nio.ByteBuffer;
import java.util.concurrent.atomic.AtomicLong;

import static jdk.incubator.http.internal.hpack.HPACK.Logger.Level.EXTRA;
import static jdk.incubator.http.internal.hpack.HPACK.Logger.Level.NORMAL;
import static java.lang.String.format;
import static java.util.Objects.requireNonNull;

Decodes headers from their binary representation.
 Typical lifecycle looks like this:
 new Decoder (setMaxCapacity? decode)* 
API Note:

 The design intentions behind Decoder were to facilitate flexible and
incremental style of processing.
 Decoder does not require a complete header block in a single ByteBuffer. The header block can be spread across many buffers of any size and decoded one-by-one the way it makes most sense for the user. This way also allows not to limit the size of the header block. 
 Headers are delivered to the callback as soon as they become decoded. Using the callback also gives the user a freedom to decide how headers are processed. The callback does not limit the number of headers decoded during single decoding operation.
Since:
9/**
 * Decodes headers from their binary representation.
 *
 * <p> Typical lifecycle looks like this:
 *
 * <p> {@link #Decoder(int) new Decoder}
 * ({@link #setMaxCapacity(int) setMaxCapacity}?
 * {@link #decode(ByteBuffer, boolean, DecodingCallback) decode})*
 *
 * @apiNote
 *
 * <p> The design intentions behind Decoder were to facilitate flexible and
 * incremental style of processing.
 *
 * <p> {@code Decoder} does not require a complete header block in a single
 * {@code ByteBuffer}. The header block can be spread across many buffers of any
 * size and decoded one-by-one the way it makes most sense for the user. This
 * way also allows not to limit the size of the header block.
 *
 * <p> Headers are delivered to the {@linkplain DecodingCallback callback} as
 * soon as they become decoded. Using the callback also gives the user a freedom
 * to decide how headers are processed. The callback does not limit the number
 * of headers decoded during single decoding operation.
 *
 * @since 9
 */
public final class Decoder {

    private final Logger logger;
    private static final AtomicLong DECODERS_IDS = new AtomicLong();

    @Stable
    private static final State[] states = new State[256];

    static {
        // To be able to do a quick lookup, each of 256 possibilities are mapped
        // to corresponding states.
        //
        // We can safely do this since patterns 1, 01, 001, 0001, 0000 are
        // Huffman prefixes and therefore are inherently not ambiguous.
        //
        // I do it mainly for better debugging (to not go each time step by step
        // through if...else tree). As for performance win for the decoding, I
        // believe is negligible.
        for (int i = 0; i < states.length; i++) {
            if ((i & 0b1000_0000) == 0b1000_0000) {
                states[i] = State.INDEXED;
            } else if ((i & 0b1100_0000) == 0b0100_0000) {
                states[i] = State.LITERAL_WITH_INDEXING;
            } else if ((i & 0b1110_0000) == 0b0010_0000) {
                states[i] = State.SIZE_UPDATE;
            } else if ((i & 0b1111_0000) == 0b0001_0000) {
                states[i] = State.LITERAL_NEVER_INDEXED;
            } else if ((i & 0b1111_0000) == 0b0000_0000) {
                states[i] = State.LITERAL;
            } else {
                throw new InternalError(String.valueOf(i));
            }
        }
    }

    private final long id;
    private final HeaderTable table;

    private State state = State.READY;
    private final IntegerReader integerReader;
    private final StringReader stringReader;
    private final StringBuilder name;
    private final StringBuilder value;
    private int intValue;
    private boolean firstValueRead;
    private boolean firstValueIndex;
    private boolean nameHuffmanEncoded;
    private boolean valueHuffmanEncoded;
    private int capacity;

    
Constructs a Decoder with the specified initial capacity of the header table. 
 The value has to be agreed between decoder and encoder out-of-band,
e.g. by a protocol that uses HPACK
(see 4.2. Maximum Table Size).
Params:
capacity – 
        a non-negative integer
Throws:
IllegalArgumentException – 
        if capacity is negative/**
     * Constructs a {@code Decoder} with the specified initial capacity of the
     * header table.
     *
     * <p> The value has to be agreed between decoder and encoder out-of-band,
     * e.g. by a protocol that uses HPACK
     * (see <a href="https://tools.ietf.org/html/rfc7541#section-4.2">4.2. Maximum Table Size</a>).
     *
     * @param capacity
     *         a non-negative integer
     *
     * @throws IllegalArgumentException
     *         if capacity is negative
     */
    public Decoder(int capacity) {
        id = DECODERS_IDS.incrementAndGet();
        logger = HPACK.getLogger().subLogger("Decoder#" + id);
        if (logger.isLoggable(NORMAL)) {
            logger.log(NORMAL, () -> format("new decoder with maximum table size %s",
                                            capacity));
        }
        if (logger.isLoggable(NORMAL)) {
            /* To correlate with logging outside HPACK, knowing
               hashCode/toString is important */
            logger.log(NORMAL, () -> {
                String hashCode = Integer.toHexString(
                        System.identityHashCode(this));
                return format("toString='%s', identityHashCode=%s",
                              toString(), hashCode);
            });
        }
        setMaxCapacity0(capacity);
        table = new HeaderTable(capacity, logger.subLogger("HeaderTable"));
        integerReader = new IntegerReader();
        stringReader = new StringReader();
        name = new StringBuilder(512);
        value = new StringBuilder(1024);
    }

    
Sets a maximum capacity of the header table.
 The value has to be agreed between decoder and encoder out-of-band,
e.g. by a protocol that uses HPACK
(see 4.2. Maximum Table Size).
Params:
capacity – 
        a non-negative integer
Throws:
IllegalArgumentException – 
        if capacity is negative/**
     * Sets a maximum capacity of the header table.
     *
     * <p> The value has to be agreed between decoder and encoder out-of-band,
     * e.g. by a protocol that uses HPACK
     * (see <a href="https://tools.ietf.org/html/rfc7541#section-4.2">4.2. Maximum Table Size</a>).
     *
     * @param capacity
     *         a non-negative integer
     *
     * @throws IllegalArgumentException
     *         if capacity is negative
     */
    public void setMaxCapacity(int capacity) {
        if (logger.isLoggable(NORMAL)) {
            logger.log(NORMAL, () -> format("setting maximum table size to %s",
                                            capacity));
        }
        setMaxCapacity0(capacity);
    }

    private void setMaxCapacity0(int capacity) {
        if (capacity < 0) {
            throw new IllegalArgumentException("capacity >= 0: " + capacity);
        }
        // FIXME: await capacity update if less than what was prior to it
        this.capacity = capacity;
    }

    
Decodes a header block from the given buffer to the given callback.
 Suppose a header block is represented by a sequence of ByteBuffers in the form of Iterator<ByteBuffer>. And the consumer of decoded headers is represented by the callback. Then to decode the header block, the following approach might be used: 

while (buffers.hasNext()) {
    ByteBuffer input = buffers.next();
    decoder.decode(input, callback, !buffers.hasNext());
 }

 The decoder reads as much as possible of the header block from the
given buffer, starting at the buffer's position, and increments its
position to reflect the bytes read. The buffer's mark and limit will not
be modified.
 Once the method is invoked with endOfHeaderBlock == true, the current header block is deemed ended, and inconsistencies, if any, are reported immediately by throwing an IOException. 
 Each callback method is called only after the implementation has
processed the corresponding bytes. If the bytes revealed a decoding
error, the callback method is not called.
 In addition to exceptions thrown directly by the method, any exceptions thrown from the callback will bubble up. 
Params:
headerBlock – 
        the chunk of the header block, may be empty
endOfHeaderBlock – 
        true if the chunk is the final (or the only one) in the sequence
consumer – 
        the callback
Throws:
IOException – 
        in case of a decoding error
NullPointerException – 
        if either headerBlock or consumer are null
API Note:
The method asks for endOfHeaderBlock flag instead of returning it for two reasons. The first one is that the user of the decoder always knows which chunk is the last. The second one is to throw the most detailed exception possible, which might be useful for diagnosing issues.
Implementation Note:
This implementation is not atomic in respect to decoding
errors. In other words, if the decoding operation has thrown a decoding
error, the decoder is no longer usable./**
     * Decodes a header block from the given buffer to the given callback.
     *
     * <p> Suppose a header block is represented by a sequence of
     * {@code ByteBuffer}s in the form of {@code Iterator<ByteBuffer>}. And the
     * consumer of decoded headers is represented by the callback. Then to
     * decode the header block, the following approach might be used:
     *
     * <pre>{@code
     * while (buffers.hasNext()) {
     *     ByteBuffer input = buffers.next();
     *     decoder.decode(input, callback, !buffers.hasNext());
     * }
     * }</pre>
     *
     * <p> The decoder reads as much as possible of the header block from the
     * given buffer, starting at the buffer's position, and increments its
     * position to reflect the bytes read. The buffer's mark and limit will not
     * be modified.
     *
     * <p> Once the method is invoked with {@code endOfHeaderBlock == true}, the
     * current header block is deemed ended, and inconsistencies, if any, are
     * reported immediately by throwing an {@code IOException}.
     *
     * <p> Each callback method is called only after the implementation has
     * processed the corresponding bytes. If the bytes revealed a decoding
     * error, the callback method is not called.
     *
     * <p> In addition to exceptions thrown directly by the method, any
     * exceptions thrown from the {@code callback} will bubble up.
     *
     * @apiNote The method asks for {@code endOfHeaderBlock} flag instead of
     * returning it for two reasons. The first one is that the user of the
     * decoder always knows which chunk is the last. The second one is to throw
     * the most detailed exception possible, which might be useful for
     * diagnosing issues.
     *
     * @implNote This implementation is not atomic in respect to decoding
     * errors. In other words, if the decoding operation has thrown a decoding
     * error, the decoder is no longer usable.
     *
     * @param headerBlock
     *         the chunk of the header block, may be empty
     * @param endOfHeaderBlock
     *         true if the chunk is the final (or the only one) in the sequence
     *
     * @param consumer
     *         the callback
     * @throws IOException
     *         in case of a decoding error
     * @throws NullPointerException
     *         if either headerBlock or consumer are null
     */
    public void decode(ByteBuffer headerBlock,
                       boolean endOfHeaderBlock,
                       DecodingCallback consumer) throws IOException {
        requireNonNull(headerBlock, "headerBlock");
        requireNonNull(consumer, "consumer");
        if (logger.isLoggable(NORMAL)) {
            logger.log(NORMAL, () -> format("reading %s, end of header block? %s",
                                            headerBlock, endOfHeaderBlock));
        }
        while (headerBlock.hasRemaining()) {
            proceed(headerBlock, consumer);
        }
        if (endOfHeaderBlock && state != State.READY) {
            logger.log(NORMAL, () -> format("unexpected end of %s representation",
                                            state));
            throw new IOException("Unexpected end of header block");
        }
    }

    private void proceed(ByteBuffer input, DecodingCallback action)
            throws IOException {
        switch (state) {
            case READY:
                resumeReady(input);
                break;
            case INDEXED:
                resumeIndexed(input, action);
                break;
            case LITERAL:
                resumeLiteral(input, action);
                break;
            case LITERAL_WITH_INDEXING:
                resumeLiteralWithIndexing(input, action);
                break;
            case LITERAL_NEVER_INDEXED:
                resumeLiteralNeverIndexed(input, action);
                break;
            case SIZE_UPDATE:
                resumeSizeUpdate(input, action);
                break;
            default:
                throw new InternalError("Unexpected decoder state: " + state);
        }
    }

    private void resumeReady(ByteBuffer input) {
        int b = input.get(input.position()) & 0xff; // absolute read
        State s = states[b];
        if (logger.isLoggable(EXTRA)) {
            logger.log(EXTRA, () -> format("next binary representation %s (first byte 0x%02x)",
                                           s, b));
        }
        switch (s) {
            case INDEXED:
                integerReader.configure(7);
                state = State.INDEXED;
                firstValueIndex = true;
                break;
            case LITERAL:
                state = State.LITERAL;
                firstValueIndex = (b & 0b0000_1111) != 0;
                if (firstValueIndex) {
                    integerReader.configure(4);
                }
                break;
            case LITERAL_WITH_INDEXING:
                state = State.LITERAL_WITH_INDEXING;
                firstValueIndex = (b & 0b0011_1111) != 0;
                if (firstValueIndex) {
                    integerReader.configure(6);
                }
                break;
            case LITERAL_NEVER_INDEXED:
                state = State.LITERAL_NEVER_INDEXED;
                firstValueIndex = (b & 0b0000_1111) != 0;
                if (firstValueIndex) {
                    integerReader.configure(4);
                }
                break;
            case SIZE_UPDATE:
                integerReader.configure(5);
                state = State.SIZE_UPDATE;
                firstValueIndex = true;
                break;
            default:
                throw new InternalError(String.valueOf(s));
        }
        if (!firstValueIndex) {
            input.get(); // advance, next stop: "String Literal"
        }
    }

    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 1 |        Index (7+)         |
    //            +---+---------------------------+
    //
    private void resumeIndexed(ByteBuffer input, DecodingCallback action)
            throws IOException {
        if (!integerReader.read(input)) {
            return;
        }
        intValue = integerReader.get();
        integerReader.reset();
        if (logger.isLoggable(NORMAL)) {
            logger.log(NORMAL, () -> format("indexed %s", intValue));
        }
        try {
            HeaderTable.HeaderField f = getHeaderFieldAt(intValue);
            action.onIndexed(intValue, f.name, f.value);
        } finally {
            state = State.READY;
        }
    }

    private HeaderTable.HeaderField getHeaderFieldAt(int index)
            throws IOException
    {
        HeaderTable.HeaderField f;
        try {
            f = table.get(index);
        } catch (IndexOutOfBoundsException e) {
            throw new IOException("header fields table index", e);
        }
        return f;
    }

    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 0 | 0 | 0 | 0 |  Index (4+)   |
    //            +---+---+-----------------------+
    //            | H |     Value Length (7+)     |
    //            +---+---------------------------+
    //            | Value String (Length octets)  |
    //            +-------------------------------+
    //
    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 0 | 0 | 0 | 0 |       0       |
    //            +---+---+-----------------------+
    //            | H |     Name Length (7+)      |
    //            +---+---------------------------+
    //            |  Name String (Length octets)  |
    //            +---+---------------------------+
    //            | H |     Value Length (7+)     |
    //            +---+---------------------------+
    //            | Value String (Length octets)  |
    //            +-------------------------------+
    //
    private void resumeLiteral(ByteBuffer input, DecodingCallback action)
            throws IOException {
        if (!completeReading(input)) {
            return;
        }
        try {
            if (firstValueIndex) {
                if (logger.isLoggable(NORMAL)) {
                    logger.log(NORMAL, () -> format("literal without indexing ('%s', '%s')",
                                                    intValue, value));
                }
                HeaderTable.HeaderField f = getHeaderFieldAt(intValue);
                action.onLiteral(intValue, f.name, value, valueHuffmanEncoded);
            } else {
                if (logger.isLoggable(NORMAL)) {
                    logger.log(NORMAL, () -> format("literal without indexing ('%s', '%s')",
                                                    name, value));
                }
                action.onLiteral(name, nameHuffmanEncoded, value, valueHuffmanEncoded);
            }
        } finally {
            cleanUpAfterReading();
        }
    }

    //
    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 0 | 1 |      Index (6+)       |
    //            +---+---+-----------------------+
    //            | H |     Value Length (7+)     |
    //            +---+---------------------------+
    //            | Value String (Length octets)  |
    //            +-------------------------------+
    //
    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 0 | 1 |           0           |
    //            +---+---+-----------------------+
    //            | H |     Name Length (7+)      |
    //            +---+---------------------------+
    //            |  Name String (Length octets)  |
    //            +---+---------------------------+
    //            | H |     Value Length (7+)     |
    //            +---+---------------------------+
    //            | Value String (Length octets)  |
    //            +-------------------------------+
    //
    private void resumeLiteralWithIndexing(ByteBuffer input,
                                           DecodingCallback action)
            throws IOException {
        if (!completeReading(input)) {
            return;
        }
        try {
            //
            // 1. (name, value) will be stored in the table as strings
            // 2. Most likely the callback will also create strings from them
            // ------------------------------------------------------------------------
            //    Let's create those string beforehand (and only once!) to benefit everyone
            //
            String n;
            String v = value.toString();
            if (firstValueIndex) {
                if (logger.isLoggable(NORMAL)) {
                    logger.log(NORMAL, () -> format("literal with incremental indexing ('%s', '%s')",
                                                    intValue, value));
                }
                HeaderTable.HeaderField f = getHeaderFieldAt(intValue);
                n = f.name;
                action.onLiteralWithIndexing(intValue, n, v, valueHuffmanEncoded);
            } else {
                n = name.toString();
                if (logger.isLoggable(NORMAL)) {
                    logger.log(NORMAL, () -> format("literal with incremental indexing ('%s', '%s')",
                                                    n, value));
                }
                action.onLiteralWithIndexing(n, nameHuffmanEncoded, v, valueHuffmanEncoded);
            }
            table.put(n, v);
        } finally {
            cleanUpAfterReading();
        }
    }

    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 0 | 0 | 0 | 1 |  Index (4+)   |
    //            +---+---+-----------------------+
    //            | H |     Value Length (7+)     |
    //            +---+---------------------------+
    //            | Value String (Length octets)  |
    //            +-------------------------------+
    //
    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 0 | 0 | 0 | 1 |       0       |
    //            +---+---+-----------------------+
    //            | H |     Name Length (7+)      |
    //            +---+---------------------------+
    //            |  Name String (Length octets)  |
    //            +---+---------------------------+
    //            | H |     Value Length (7+)     |
    //            +---+---------------------------+
    //            | Value String (Length octets)  |
    //            +-------------------------------+
    //
    private void resumeLiteralNeverIndexed(ByteBuffer input,
                                           DecodingCallback action)
            throws IOException {
        if (!completeReading(input)) {
            return;
        }
        try {
            if (firstValueIndex) {
                if (logger.isLoggable(NORMAL)) {
                    logger.log(NORMAL, () -> format("literal never indexed ('%s', '%s')",
                                                    intValue, value));
                }
                HeaderTable.HeaderField f = getHeaderFieldAt(intValue);
                action.onLiteralNeverIndexed(intValue, f.name, value, valueHuffmanEncoded);
            } else {
                if (logger.isLoggable(NORMAL)) {
                    logger.log(NORMAL, () -> format("literal never indexed ('%s', '%s')",
                                                    name, value));
                }
                action.onLiteralNeverIndexed(name, nameHuffmanEncoded, value, valueHuffmanEncoded);
            }
        } finally {
            cleanUpAfterReading();
        }
    }

    //              0   1   2   3   4   5   6   7
    //            +---+---+---+---+---+---+---+---+
    //            | 0 | 0 | 1 |   Max size (5+)   |
    //            +---+---------------------------+
    //
    private void resumeSizeUpdate(ByteBuffer input,
                                  DecodingCallback action) throws IOException {
        if (!integerReader.read(input)) {
            return;
        }
        intValue = integerReader.get();
        if (logger.isLoggable(NORMAL)) {
            logger.log(NORMAL, () -> format("dynamic table size update %s",
                                            intValue));
        }
        assert intValue >= 0;
        if (intValue > capacity) {
            throw new IOException(
                    format("Received capacity exceeds expected: capacity=%s, expected=%s",
                           intValue, capacity));
        }
        integerReader.reset();
        try {
            action.onSizeUpdate(intValue);
            table.setMaxSize(intValue);
        } finally {
            state = State.READY;
        }
    }

    private boolean completeReading(ByteBuffer input) throws IOException {
        if (!firstValueRead) {
            if (firstValueIndex) {
                if (!integerReader.read(input)) {
                    return false;
                }
                intValue = integerReader.get();
                integerReader.reset();
            } else {
                if (!stringReader.read(input, name)) {
                    return false;
                }
                nameHuffmanEncoded = stringReader.isHuffmanEncoded();
                stringReader.reset();
            }
            firstValueRead = true;
            return false;
        } else {
            if (!stringReader.read(input, value)) {
                return false;
            }
        }
        valueHuffmanEncoded = stringReader.isHuffmanEncoded();
        stringReader.reset();
        return true;
    }

    private void cleanUpAfterReading() {
        name.setLength(0);
        value.setLength(0);
        firstValueRead = false;
        state = State.READY;
    }

    private enum State {
        READY,
        INDEXED,
        LITERAL_NEVER_INDEXED,
        LITERAL,
        LITERAL_WITH_INDEXING,
        SIZE_UPDATE
    }

    HeaderTable getTable() {
        return table;
    }
}