internal/framed/Http2.java

/*
 * Copyright (C) 2013 Square, 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 com.squareup.okhttp.internal.framed;

import com.squareup.okhttp.Protocol;
import java.io.IOException;
import java.util.List;
import java.util.logging.Logger;
import okio.Buffer;
import okio.BufferedSink;
import okio.BufferedSource;
import okio.ByteString;
import okio.Source;
import okio.Timeout;

import static com.squareup.okhttp.internal.framed.Http2.FrameLogger.formatHeader;
import static java.lang.String.format;
import static java.util.logging.Level.FINE;
import static okio.ByteString.EMPTY;

/**
 * Read and write HTTP/2 frames.
 * <p>
 * This implementation assumes we do not send an increased
 * {@link Settings#getMaxFrameSize frame size setting} to the peer. Hence, we
 * expect all frames to have a max length of {@link #INITIAL_MAX_FRAME_SIZE}.
 * <p>http://tools.ietf.org/html/draft-ietf-httpbis-http2-17
 */
public final class Http2 implements Variant {
  private static final Logger logger = Logger.getLogger(FrameLogger.class.getName());

  @Override public Protocol getProtocol() {
    return Protocol.HTTP_2;
  }

  private static final ByteString CONNECTION_PREFACE
      = ByteString.encodeUtf8("PRI * HTTP/2.0\r\n\r\nSM\r\n\r\n");

  /** The initial max frame size, applied independently writing to, or reading from the peer. */
  static final int INITIAL_MAX_FRAME_SIZE = 0x4000; // 16384

  static final byte TYPE_DATA = 0x0;
  static final byte TYPE_HEADERS = 0x1;
  static final byte TYPE_PRIORITY = 0x2;
  static final byte TYPE_RST_STREAM = 0x3;
  static final byte TYPE_SETTINGS = 0x4;
  static final byte TYPE_PUSH_PROMISE = 0x5;
  static final byte TYPE_PING = 0x6;
  static final byte TYPE_GOAWAY = 0x7;
  static final byte TYPE_WINDOW_UPDATE = 0x8;
  static final byte TYPE_CONTINUATION = 0x9;

  static final byte FLAG_NONE = 0x0;
  static final byte FLAG_ACK = 0x1; // Used for settings and ping.
  static final byte FLAG_END_STREAM = 0x1; // Used for headers and data.
  static final byte FLAG_END_HEADERS = 0x4; // Used for headers and continuation.
  static final byte FLAG_END_PUSH_PROMISE = 0x4;
  static final byte FLAG_PADDED = 0x8; // Used for headers and data.
  static final byte FLAG_PRIORITY = 0x20; // Used for headers.
  static final byte FLAG_COMPRESSED = 0x20; // Used for data.

  /**
   * Creates a frame reader with max header table size of 4096 and data frame
   * compression disabled.
   */
  @Override public FrameReader newReader(BufferedSource source, boolean client) {
    return new Reader(source, 4096, client);
  }

  @Override public FrameWriter newWriter(BufferedSink sink, boolean client) {
    return new Writer(sink, client);
  }

  static final class Reader implements FrameReader {
    private final BufferedSource source;
    private final ContinuationSource continuation;
    private final boolean client;

    // Visible for testing.
    final Hpack.Reader hpackReader;

    Reader(BufferedSource source, int headerTableSize, boolean client) {
      this.source = source;
      this.client = client;
      this.continuation = new ContinuationSource(this.source);
      this.hpackReader = new Hpack.Reader(headerTableSize, continuation);
    }

    @Override public void readConnectionPreface() throws IOException {
      if (client) return; // Nothing to read; servers doesn't send a connection preface!
      ByteString connectionPreface = source.readByteString(CONNECTION_PREFACE.size());
      if (logger.isLoggable(FINE)) logger.fine(format("<< CONNECTION %s", connectionPreface.hex()));
      if (!CONNECTION_PREFACE.equals(connectionPreface)) {
        throw ioException("Expected a connection header but was %s", connectionPreface.utf8());
      }
    }

    @Override public boolean nextFrame(Handler handler) throws IOException {
      try {
        source.require(9); // Frame header size
      } catch (IOException e) {
        return false; // This might be a normal socket close.
      }

      /*  0                   1                   2                   3
       *  0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       * +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       * |                 Length (24)                   |
       * +---------------+---------------+---------------+
       * |   Type (8)    |   Flags (8)   |
       * +-+-+-----------+---------------+-------------------------------+
       * |R|                 Stream Identifier (31)                      |
       * +=+=============================================================+
       * |                   Frame Payload (0...)                      ...
       * +---------------------------------------------------------------+
       */
      int length = readMedium(source);
      if (length < 0 || length > INITIAL_MAX_FRAME_SIZE) {
        throw ioException("FRAME_SIZE_ERROR: %s", length);
      }
      byte type = (byte) (source.readByte() & 0xff);
      byte flags = (byte) (source.readByte() & 0xff);
      int streamId = (source.readInt() & 0x7fffffff); // Ignore reserved bit.
      if (logger.isLoggable(FINE)) logger.fine(formatHeader(true, streamId, length, type, flags));

      switch (type) {
        case TYPE_DATA:
          readData(handler, length, flags, streamId);
          break;

        case TYPE_HEADERS:
          readHeaders(handler, length, flags, streamId);
          break;

        case TYPE_PRIORITY:
          readPriority(handler, length, flags, streamId);
          break;

        case TYPE_RST_STREAM:
          readRstStream(handler, length, flags, streamId);
          break;

        case TYPE_SETTINGS:
          readSettings(handler, length, flags, streamId);
          break;

        case TYPE_PUSH_PROMISE:
          readPushPromise(handler, length, flags, streamId);
          break;

        case TYPE_PING:
          readPing(handler, length, flags, streamId);
          break;

        case TYPE_GOAWAY:
          readGoAway(handler, length, flags, streamId);
          break;

        case TYPE_WINDOW_UPDATE:
          readWindowUpdate(handler, length, flags, streamId);
          break;

        default:
          // Implementations MUST discard frames that have unknown or unsupported types.
          source.skip(length);
      }
      return true;
    }

    private void readHeaders(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (streamId == 0) throw ioException("PROTOCOL_ERROR: TYPE_HEADERS streamId == 0");

      boolean endStream = (flags & FLAG_END_STREAM) != 0;

      short padding = (flags & FLAG_PADDED) != 0 ? (short) (source.readByte() & 0xff) : 0;

      if ((flags & FLAG_PRIORITY) != 0) {
        readPriority(handler, streamId);
        length -= 5; // account for above read.
      }

      length = lengthWithoutPadding(length, flags, padding);

      List<Header> headerBlock = readHeaderBlock(length, padding, flags, streamId);

      handler.headers(false, endStream, streamId, -1, headerBlock, HeadersMode.HTTP_20_HEADERS);
    }

    private List<Header> readHeaderBlock(int length, short padding, byte flags, int streamId)
        throws IOException {
      continuation.length = continuation.left = length;
      continuation.padding = padding;
      continuation.flags = flags;
      continuation.streamId = streamId;

      // TODO: Concat multi-value headers with 0x0, except COOKIE, which uses 0x3B, 0x20.
      // http://tools.ietf.org/html/draft-ietf-httpbis-http2-17#section-8.1.2.5
      hpackReader.readHeaders();
      return hpackReader.getAndResetHeaderList();
    }

    private void readData(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      // TODO: checkState open or half-closed (local) or raise STREAM_CLOSED
      boolean inFinished = (flags & FLAG_END_STREAM) != 0;
      boolean gzipped = (flags & FLAG_COMPRESSED) != 0;
      if (gzipped) {
        throw ioException("PROTOCOL_ERROR: FLAG_COMPRESSED without SETTINGS_COMPRESS_DATA");
      }

      short padding = (flags & FLAG_PADDED) != 0 ? (short) (source.readByte() & 0xff) : 0;
      length = lengthWithoutPadding(length, flags, padding);

      handler.data(inFinished, streamId, source, length);
      source.skip(padding);
    }

    private void readPriority(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (length != 5) throw ioException("TYPE_PRIORITY length: %d != 5", length);
      if (streamId == 0) throw ioException("TYPE_PRIORITY streamId == 0");
      readPriority(handler, streamId);
    }

    private void readPriority(Handler handler, int streamId) throws IOException {
      int w1 = source.readInt();
      boolean exclusive = (w1 & 0x80000000) != 0;
      int streamDependency = (w1 & 0x7fffffff);
      int weight = (source.readByte() & 0xff) + 1;
      handler.priority(streamId, streamDependency, weight, exclusive);
    }

    private void readRstStream(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (length != 4) throw ioException("TYPE_RST_STREAM length: %d != 4", length);
      if (streamId == 0) throw ioException("TYPE_RST_STREAM streamId == 0");
      int errorCodeInt = source.readInt();
      ErrorCode errorCode = ErrorCode.fromHttp2(errorCodeInt);
      if (errorCode == null) {
        throw ioException("TYPE_RST_STREAM unexpected error code: %d", errorCodeInt);
      }
      handler.rstStream(streamId, errorCode);
    }

    private void readSettings(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (streamId != 0) throw ioException("TYPE_SETTINGS streamId != 0");
      if ((flags & FLAG_ACK) != 0) {
        if (length != 0) throw ioException("FRAME_SIZE_ERROR ack frame should be empty!");
        handler.ackSettings();
        return;
      }

      if (length % 6 != 0) throw ioException("TYPE_SETTINGS length %% 6 != 0: %s", length);
      Settings settings = new Settings();
      for (int i = 0; i < length; i += 6) {
        short id = source.readShort();
        int value = source.readInt();

        switch (id) {
          case 1: // SETTINGS_HEADER_TABLE_SIZE
            break;
          case 2: // SETTINGS_ENABLE_PUSH
            if (value != 0 && value != 1) {
              throw ioException("PROTOCOL_ERROR SETTINGS_ENABLE_PUSH != 0 or 1");
            }
            break;
          case 3: // SETTINGS_MAX_CONCURRENT_STREAMS
            id = 4; // Renumbered in draft 10.
            break;
          case 4: // SETTINGS_INITIAL_WINDOW_SIZE
            id = 7; // Renumbered in draft 10.
            if (value < 0) {
              throw ioException("PROTOCOL_ERROR SETTINGS_INITIAL_WINDOW_SIZE > 2^31 - 1");
            }
            break;
          case 5: // SETTINGS_MAX_FRAME_SIZE
            if (value < INITIAL_MAX_FRAME_SIZE || value > 16777215) {
              throw ioException("PROTOCOL_ERROR SETTINGS_MAX_FRAME_SIZE: %s", value);
            }
            break;
          case 6: // SETTINGS_MAX_HEADER_LIST_SIZE
            break; // Advisory only, so ignored.
          default:
            throw ioException("PROTOCOL_ERROR invalid settings id: %s", id);
        }
        settings.set(id, 0, value);
      }
      handler.settings(false, settings);
      if (settings.getHeaderTableSize() >= 0) {
        hpackReader.headerTableSizeSetting(settings.getHeaderTableSize());
      }
    }

    private void readPushPromise(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (streamId == 0) {
        throw ioException("PROTOCOL_ERROR: TYPE_PUSH_PROMISE streamId == 0");
      }
      short padding = (flags & FLAG_PADDED) != 0 ? (short) (source.readByte() & 0xff) : 0;
      int promisedStreamId = source.readInt() & 0x7fffffff;
      length -= 4; // account for above read.
      length = lengthWithoutPadding(length, flags, padding);
      List<Header> headerBlock = readHeaderBlock(length, padding, flags, streamId);
      handler.pushPromise(streamId, promisedStreamId, headerBlock);
    }

    private void readPing(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (length != 8) throw ioException("TYPE_PING length != 8: %s", length);
      if (streamId != 0) throw ioException("TYPE_PING streamId != 0");
      int payload1 = source.readInt();
      int payload2 = source.readInt();
      boolean ack = (flags & FLAG_ACK) != 0;
      handler.ping(ack, payload1, payload2);
    }

    private void readGoAway(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (length < 8) throw ioException("TYPE_GOAWAY length < 8: %s", length);
      if (streamId != 0) throw ioException("TYPE_GOAWAY streamId != 0");
      int lastStreamId = source.readInt();
      int errorCodeInt = source.readInt();
      int opaqueDataLength = length - 8;
      ErrorCode errorCode = ErrorCode.fromHttp2(errorCodeInt);
      if (errorCode == null) {
        throw ioException("TYPE_GOAWAY unexpected error code: %d", errorCodeInt);
      }
      ByteString debugData = EMPTY;
      if (opaqueDataLength > 0) { // Must read debug data in order to not corrupt the connection.
        debugData = source.readByteString(opaqueDataLength);
      }
      handler.goAway(lastStreamId, errorCode, debugData);
    }

    private void readWindowUpdate(Handler handler, int length, byte flags, int streamId)
        throws IOException {
      if (length != 4) throw ioException("TYPE_WINDOW_UPDATE length !=4: %s", length);
      long increment = (source.readInt() & 0x7fffffffL);
      if (increment == 0) throw ioException("windowSizeIncrement was 0", increment);
      handler.windowUpdate(streamId, increment);
    }

    @Override public void close() throws IOException {
      source.close();
    }
  }

  static final class Writer implements FrameWriter {
    private final BufferedSink sink;
    private final boolean client;
    private final Buffer hpackBuffer;
    private final Hpack.Writer hpackWriter;
    private int maxFrameSize;
    private boolean closed;

    Writer(BufferedSink sink, boolean client) {
      this.sink = sink;
      this.client = client;
      this.hpackBuffer = new Buffer();
      this.hpackWriter = new Hpack.Writer(hpackBuffer);
      this.maxFrameSize = INITIAL_MAX_FRAME_SIZE;
    }

    @Override public synchronized void flush() throws IOException {
      if (closed) throw new IOException("closed");
      sink.flush();
    }

    @Override public synchronized void ackSettings(Settings peerSettings) throws IOException {
      if (closed) throw new IOException("closed");
      this.maxFrameSize = peerSettings.getMaxFrameSize(maxFrameSize);
      int length = 0;
      byte type = TYPE_SETTINGS;
      byte flags = FLAG_ACK;
      int streamId = 0;
      frameHeader(streamId, length, type, flags);
      sink.flush();
    }

    @Override public synchronized void connectionPreface() throws IOException {
      if (closed) throw new IOException("closed");
      if (!client) return; // Nothing to write; servers don't send connection headers!
      if (logger.isLoggable(FINE)) {
        logger.fine(format(">> CONNECTION %s", CONNECTION_PREFACE.hex()));
      }
      sink.write(CONNECTION_PREFACE.toByteArray());
      sink.flush();
    }

    @Override public synchronized void synStream(boolean outFinished, boolean inFinished,
        int streamId, int associatedStreamId, List<Header> headerBlock)
        throws IOException {
      if (inFinished) throw new UnsupportedOperationException();
      if (closed) throw new IOException("closed");
      headers(outFinished, streamId, headerBlock);
    }

    @Override public synchronized void synReply(boolean outFinished, int streamId,
        List<Header> headerBlock) throws IOException {
      if (closed) throw new IOException("closed");
      headers(outFinished, streamId, headerBlock);
    }

    @Override public synchronized void headers(int streamId, List<Header> headerBlock)
        throws IOException {
      if (closed) throw new IOException("closed");
      headers(false, streamId, headerBlock);
    }

    @Override public synchronized void pushPromise(int streamId, int promisedStreamId,
        List<Header> requestHeaders) throws IOException {
      if (closed) throw new IOException("closed");
      hpackWriter.writeHeaders(requestHeaders);

      long byteCount = hpackBuffer.size();
      int length = (int) Math.min(maxFrameSize - 4, byteCount);
      byte type = TYPE_PUSH_PROMISE;
      byte flags = byteCount == length ? FLAG_END_HEADERS : 0;
      frameHeader(streamId, length + 4, type, flags);
      sink.writeInt(promisedStreamId & 0x7fffffff);
      sink.write(hpackBuffer, length);

      if (byteCount > length) writeContinuationFrames(streamId, byteCount - length);
    }

    void headers(boolean outFinished, int streamId, List<Header> headerBlock) throws IOException {
      if (closed) throw new IOException("closed");
      hpackWriter.writeHeaders(headerBlock);

      long byteCount = hpackBuffer.size();
      int length = (int) Math.min(maxFrameSize, byteCount);
      byte type = TYPE_HEADERS;
      byte flags = byteCount == length ? FLAG_END_HEADERS : 0;
      if (outFinished) flags |= FLAG_END_STREAM;
      frameHeader(streamId, length, type, flags);
      sink.write(hpackBuffer, length);

      if (byteCount > length) writeContinuationFrames(streamId, byteCount - length);
    }

    private void writeContinuationFrames(int streamId, long byteCount) throws IOException {
      while (byteCount > 0) {
        int length = (int) Math.min(maxFrameSize, byteCount);
        byteCount -= length;
        frameHeader(streamId, length, TYPE_CONTINUATION, byteCount == 0 ? FLAG_END_HEADERS : 0);
        sink.write(hpackBuffer, length);
      }
    }

    @Override public synchronized void rstStream(int streamId, ErrorCode errorCode)
        throws IOException {
      if (closed) throw new IOException("closed");
      if (errorCode.httpCode == -1) throw new IllegalArgumentException();

      int length = 4;
      byte type = TYPE_RST_STREAM;
      byte flags = FLAG_NONE;
      frameHeader(streamId, length, type, flags);
      sink.writeInt(errorCode.httpCode);
      sink.flush();
    }

    @Override public int maxDataLength() {
      return maxFrameSize;
    }

    @Override public synchronized void data(boolean outFinished, int streamId, Buffer source,
        int byteCount) throws IOException {
      if (closed) throw new IOException("closed");
      byte flags = FLAG_NONE;
      if (outFinished) flags |= FLAG_END_STREAM;
      dataFrame(streamId, flags, source, byteCount);
    }

    void dataFrame(int streamId, byte flags, Buffer buffer, int byteCount) throws IOException {
      byte type = TYPE_DATA;
      frameHeader(streamId, byteCount, type, flags);
      if (byteCount > 0) {
        sink.write(buffer, byteCount);
      }
    }

    @Override public synchronized void settings(Settings settings) throws IOException {
      if (closed) throw new IOException("closed");
      int length = settings.size() * 6;
      byte type = TYPE_SETTINGS;
      byte flags = FLAG_NONE;
      int streamId = 0;
      frameHeader(streamId, length, type, flags);
      for (int i = 0; i < Settings.COUNT; i++) {
        if (!settings.isSet(i)) continue;
        int id = i;
        if (id == 4) id = 3; // SETTINGS_MAX_CONCURRENT_STREAMS renumbered.
        else if (id == 7) id = 4; // SETTINGS_INITIAL_WINDOW_SIZE renumbered.
        sink.writeShort(id);
        sink.writeInt(settings.get(i));
      }
      sink.flush();
    }

    @Override public synchronized void ping(boolean ack, int payload1, int payload2)
        throws IOException {
      if (closed) throw new IOException("closed");
      int length = 8;
      byte type = TYPE_PING;
      byte flags = ack ? FLAG_ACK : FLAG_NONE;
      int streamId = 0;
      frameHeader(streamId, length, type, flags);
      sink.writeInt(payload1);
      sink.writeInt(payload2);
      sink.flush();
    }

    @Override public synchronized void goAway(int lastGoodStreamId, ErrorCode errorCode,
        byte[] debugData) throws IOException {
      if (closed) throw new IOException("closed");
      if (errorCode.httpCode == -1) throw illegalArgument("errorCode.httpCode == -1");
      int length = 8 + debugData.length;
      byte type = TYPE_GOAWAY;
      byte flags = FLAG_NONE;
      int streamId = 0;
      frameHeader(streamId, length, type, flags);
      sink.writeInt(lastGoodStreamId);
      sink.writeInt(errorCode.httpCode);
      if (debugData.length > 0) {
        sink.write(debugData);
      }
      sink.flush();
    }

    @Override public synchronized void windowUpdate(int streamId, long windowSizeIncrement)
        throws IOException {
      if (closed) throw new IOException("closed");
      if (windowSizeIncrement == 0 || windowSizeIncrement > 0x7fffffffL) {
        throw illegalArgument("windowSizeIncrement == 0 || windowSizeIncrement > 0x7fffffffL: %s",
            windowSizeIncrement);
      }
      int length = 4;
      byte type = TYPE_WINDOW_UPDATE;
      byte flags = FLAG_NONE;
      frameHeader(streamId, length, type, flags);
      sink.writeInt((int) windowSizeIncrement);
      sink.flush();
    }

    @Override public synchronized void close() throws IOException {
      closed = true;
      sink.close();
    }

    void frameHeader(int streamId, int length, byte type, byte flags) throws IOException {
      if (logger.isLoggable(FINE)) logger.fine(formatHeader(false, streamId, length, type, flags));
      if (length > maxFrameSize) {
        throw illegalArgument("FRAME_SIZE_ERROR length > %d: %d", maxFrameSize, length);
      }
      if ((streamId & 0x80000000) != 0) throw illegalArgument("reserved bit set: %s", streamId);
      writeMedium(sink, length);
      sink.writeByte(type & 0xff);
      sink.writeByte(flags & 0xff);
      sink.writeInt(streamId & 0x7fffffff);
    }
  }

  private static IllegalArgumentException illegalArgument(String message, Object... args) {
    throw new IllegalArgumentException(format(message, args));
  }

  private static IOException ioException(String message, Object... args) throws IOException {
    throw new IOException(format(message, args));
  }

  /**
   * Decompression of the header block occurs above the framing layer. This
   * class lazily reads continuation frames as they are needed by {@link
   * Hpack.Reader#readHeaders()}.
   */
  static final class ContinuationSource implements Source {
    private final BufferedSource source;

    int length;
    byte flags;
    int streamId;

    int left;
    short padding;

    public ContinuationSource(BufferedSource source) {
      this.source = source;
    }

    @Override public long read(Buffer sink, long byteCount) throws IOException {
      while (left == 0) {
        source.skip(padding);
        padding = 0;
        if ((flags & FLAG_END_HEADERS) != 0) return -1;
        readContinuationHeader();
        // TODO: test case for empty continuation header?
      }

      long read = source.read(sink, Math.min(byteCount, left));
      if (read == -1) return -1;
      left -= read;
      return read;
    }

    @Override public Timeout timeout() {
      return source.timeout();
    }

    @Override public void close() throws IOException {
    }

    private void readContinuationHeader() throws IOException {
      int previousStreamId = streamId;

      length = left = readMedium(source);
      byte type = (byte) (source.readByte() & 0xff);
      flags = (byte) (source.readByte() & 0xff);
      if (logger.isLoggable(FINE)) logger.fine(formatHeader(true, streamId, length, type, flags));
      streamId = (source.readInt() & 0x7fffffff);
      if (type != TYPE_CONTINUATION) throw ioException("%s != TYPE_CONTINUATION", type);
      if (streamId != previousStreamId) throw ioException("TYPE_CONTINUATION streamId changed");
    }
  }

  private static int lengthWithoutPadding(int length, byte flags, short padding)
      throws IOException {
    if ((flags & FLAG_PADDED) != 0) length--; // Account for reading the padding length.
    if (padding > length) {
      throw ioException("PROTOCOL_ERROR padding %s > remaining length %s", padding, length);
    }
    return (short) (length - padding);
  }

  /**
   * Logs a human-readable representation of HTTP/2 frame headers.
   *
   * <p>The format is:
   *
   * <pre>
   *   direction streamID length type flags
   * </pre>
   * Where direction is {@code <<} for inbound and {@code >>} for outbound.
   *
   * <p> For example, the following would indicate a HEAD request sent from
   * the client.
   * <pre>
   * {@code
   *   << 0x0000000f    12 HEADERS       END_HEADERS|END_STREAM
   * }
   * </pre>
   */
  static final class FrameLogger {

    static String formatHeader(boolean inbound, int streamId, int length, byte type, byte flags) {
      String formattedType = type < TYPES.length ? TYPES[type] : format("0x%02x", type);
      String formattedFlags = formatFlags(type, flags);
      return format("%s 0x%08x %5d %-13s %s", inbound ? "<<" : ">>", streamId, length,
          formattedType, formattedFlags);
    }

    /**
     * Looks up valid string representing flags from the table. Invalid
     * combinations are represented in binary.
     */
    // Visible for testing.
    static String formatFlags(byte type, byte flags) {
      if (flags == 0) return "";
      switch (type) { // Special case types that have 0 or 1 flag.
        case TYPE_SETTINGS:
        case TYPE_PING:
          return flags == FLAG_ACK ? "ACK" : BINARY[flags];
        case TYPE_PRIORITY:
        case TYPE_RST_STREAM:
        case TYPE_GOAWAY:
        case TYPE_WINDOW_UPDATE:
          return BINARY[flags];
      }
      String result = flags < FLAGS.length ? FLAGS[flags] : BINARY[flags];
      // Special case types that have overlap flag values.
      if (type == TYPE_PUSH_PROMISE && (flags & FLAG_END_PUSH_PROMISE) != 0) {
        return result.replace("HEADERS", "PUSH_PROMISE"); // TODO: Avoid allocation.
      } else if (type == TYPE_DATA && (flags & FLAG_COMPRESSED) != 0) {
        return result.replace("PRIORITY", "COMPRESSED"); // TODO: Avoid allocation.
      }
      return result;
    }

    /** Lookup table for valid frame types. */
    private static final String[] TYPES = new String[] {
        "DATA",
        "HEADERS",
        "PRIORITY",
        "RST_STREAM",
        "SETTINGS",
        "PUSH_PROMISE",
        "PING",
        "GOAWAY",
        "WINDOW_UPDATE",
        "CONTINUATION"
    };

    /**
     * Lookup table for valid flags for DATA, HEADERS, CONTINUATION. Invalid
     * combinations are represented in binary.
     */
    private static final String[] FLAGS = new String[0x40]; // Highest bit flag is 0x20.
    private static final String[] BINARY = new String[256];

    static {
      for (int i = 0; i < BINARY.length; i++) {
        BINARY[i] = format("%8s", Integer.toBinaryString(i)).replace(' ', '0');
      }

      FLAGS[FLAG_NONE] = "";
      FLAGS[FLAG_END_STREAM] = "END_STREAM";

      int[] prefixFlags = new int[] {FLAG_END_STREAM};

      FLAGS[FLAG_PADDED] = "PADDED";
      for (int prefixFlag : prefixFlags) {
         FLAGS[prefixFlag | FLAG_PADDED] = FLAGS[prefixFlag] + "|PADDED";
      }

      FLAGS[FLAG_END_HEADERS] = "END_HEADERS"; // Same as END_PUSH_PROMISE.
      FLAGS[FLAG_PRIORITY] = "PRIORITY"; // Same as FLAG_COMPRESSED.
      FLAGS[FLAG_END_HEADERS | FLAG_PRIORITY] = "END_HEADERS|PRIORITY"; // Only valid on HEADERS.
      int[] frameFlags =
          new int[] {FLAG_END_HEADERS, FLAG_PRIORITY, FLAG_END_HEADERS | FLAG_PRIORITY};

      for (int frameFlag : frameFlags) {
        for (int prefixFlag : prefixFlags) {
          FLAGS[prefixFlag | frameFlag] = FLAGS[prefixFlag] + '|' + FLAGS[frameFlag];
          FLAGS[prefixFlag | frameFlag | FLAG_PADDED] =
              FLAGS[prefixFlag] + '|' + FLAGS[frameFlag] + "|PADDED";
        }
      }

      for (int i = 0; i < FLAGS.length; i++) { // Fill in holes with binary representation.
        if (FLAGS[i] == null) FLAGS[i] = BINARY[i];
      }
    }
  }

  private static int readMedium(BufferedSource source) throws IOException {
    return (source.readByte() & 0xff) << 16
        |  (source.readByte() & 0xff) <<  8
        |  (source.readByte() & 0xff);
  }

  private static void writeMedium(BufferedSink sink, int i) throws IOException {
    sink.writeByte((i >>> 16) & 0xff);
    sink.writeByte((i >>>  8) & 0xff);
    sink.writeByte(i          & 0xff);
  }
}