quic/core/quic_data_writer.cc

// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#include "quic/core/quic_data_writer.h"

#include <algorithm>
#include <limits>

#include "absl/strings/string_view.h"
#include "quic/core/crypto/quic_random.h"
#include "quic/core/quic_constants.h"
#include "quic/platform/api/quic_bug_tracker.h"
#include "quic/platform/api/quic_flags.h"
#include "common/quiche_endian.h"

namespace quic {

QuicDataWriter::QuicDataWriter(size_t size, char* buffer)
    : quiche::QuicheDataWriter(size, buffer) {}

QuicDataWriter::QuicDataWriter(size_t size,
                               char* buffer,
                               quiche::Endianness endianness)
    : quiche::QuicheDataWriter(size, buffer, endianness) {}

QuicDataWriter::~QuicDataWriter() {}

bool QuicDataWriter::WriteUFloat16(uint64_t value) {
  uint16_t result;
  if (value < (UINT64_C(1) << kUFloat16MantissaEffectiveBits)) {
    // Fast path: either the value is denormalized, or has exponent zero.
    // Both cases are represented by the value itself.
    result = static_cast<uint16_t>(value);
  } else if (value >= kUFloat16MaxValue) {
    // Value is out of range; clamp it to the maximum representable.
    result = std::numeric_limits<uint16_t>::max();
  } else {
    // The highest bit is between position 13 and 42 (zero-based), which
    // corresponds to exponent 1-30. In the output, mantissa is from 0 to 10,
    // hidden bit is 11 and exponent is 11 to 15. Shift the highest bit to 11
    // and count the shifts.
    uint16_t exponent = 0;
    for (uint16_t offset = 16; offset > 0; offset /= 2) {
      // Right-shift the value until the highest bit is in position 11.
      // For offset of 16, 8, 4, 2 and 1 (binary search over 1-30),
      // shift if the bit is at or above 11 + offset.
      if (value >= (UINT64_C(1) << (kUFloat16MantissaBits + offset))) {
        exponent += offset;
        value >>= offset;
      }
    }

    QUICHE_DCHECK_GE(exponent, 1);
    QUICHE_DCHECK_LE(exponent, kUFloat16MaxExponent);
    QUICHE_DCHECK_GE(value, UINT64_C(1) << kUFloat16MantissaBits);
    QUICHE_DCHECK_LT(value, UINT64_C(1) << kUFloat16MantissaEffectiveBits);

    // Hidden bit (position 11) is set. We should remove it and increment the
    // exponent. Equivalently, we just add it to the exponent.
    // This hides the bit.
    result = static_cast<uint16_t>(value + (exponent << kUFloat16MantissaBits));
  }

  if (endianness() == quiche::NETWORK_BYTE_ORDER) {
    result = quiche::QuicheEndian::HostToNet16(result);
  }
  return WriteBytes(&result, sizeof(result));
}

bool QuicDataWriter::WriteConnectionId(QuicConnectionId connection_id) {
  if (connection_id.IsEmpty()) {
    return true;
  }
  return WriteBytes(connection_id.data(), connection_id.length());
}

bool QuicDataWriter::WriteLengthPrefixedConnectionId(
    QuicConnectionId connection_id) {
  return WriteUInt8(connection_id.length()) && WriteConnectionId(connection_id);
}

bool QuicDataWriter::WriteRandomBytes(QuicRandom* random, size_t length) {
  char* dest = BeginWrite(length);
  if (!dest) {
    return false;
  }

  random->RandBytes(dest, length);
  IncreaseLength(length);
  return true;
}

bool QuicDataWriter::WriteInsecureRandomBytes(QuicRandom* random,
                                              size_t length) {
  char* dest = BeginWrite(length);
  if (!dest) {
    return false;
  }

  random->InsecureRandBytes(dest, length);
  IncreaseLength(length);
  return true;
}

// Converts a uint64_t into an IETF/Quic formatted Variable Length
// Integer. IETF Variable Length Integers have 62 significant bits, so
// the value to write must be in the range of 0..(2^62)-1.
//
// Performance notes
//
// Measurements and experiments showed that unrolling the four cases
// like this and dereferencing next_ as we do (*(next_+n)) gains about
// 10% over making a loop and dereferencing it as *(next_++)
//
// Using a register for next didn't help.
//
// Branches are ordered to increase the likelihood of the first being
// taken.
//
// Low-level optimization is useful here because this function will be
// called frequently, leading to outsize benefits.
bool QuicDataWriter::WriteVarInt62(uint64_t value) {
  QUICHE_DCHECK_EQ(endianness(), quiche::NETWORK_BYTE_ORDER);

  size_t remaining_bytes = remaining();
  char* next = buffer() + length();

  if ((value & kVarInt62ErrorMask) == 0) {
    // We know the high 2 bits are 0 so |value| is legal.
    // We can do the encoding.
    if ((value & kVarInt62Mask8Bytes) != 0) {
      // Someplace in the high-4 bytes is a 1-bit. Do an 8-byte
      // encoding.
      if (remaining_bytes >= 8) {
        *(next + 0) = ((value >> 56) & 0x3f) + 0xc0;
        *(next + 1) = (value >> 48) & 0xff;
        *(next + 2) = (value >> 40) & 0xff;
        *(next + 3) = (value >> 32) & 0xff;
        *(next + 4) = (value >> 24) & 0xff;
        *(next + 5) = (value >> 16) & 0xff;
        *(next + 6) = (value >> 8) & 0xff;
        *(next + 7) = value & 0xff;
        IncreaseLength(8);
        return true;
      }
      return false;
    }
    // The high-order-4 bytes are all 0, check for a 1, 2, or 4-byte
    // encoding
    if ((value & kVarInt62Mask4Bytes) != 0) {
      // The encoding will not fit into 2 bytes, Do a 4-byte
      // encoding.
      if (remaining_bytes >= 4) {
        *(next + 0) = ((value >> 24) & 0x3f) + 0x80;
        *(next + 1) = (value >> 16) & 0xff;
        *(next + 2) = (value >> 8) & 0xff;
        *(next + 3) = value & 0xff;
        IncreaseLength(4);
        return true;
      }
      return false;
    }
    // The high-order bits are all 0. Check to see if the number
    // can be encoded as one or two bytes. One byte encoding has
    // only 6 significant bits (bits 0xffffffff ffffffc0 are all 0).
    // Two byte encoding has more than 6, but 14 or less significant
    // bits (bits 0xffffffff ffffc000 are 0 and 0x00000000 00003fc0
    // are not 0)
    if ((value & kVarInt62Mask2Bytes) != 0) {
      // Do 2-byte encoding
      if (remaining_bytes >= 2) {
        *(next + 0) = ((value >> 8) & 0x3f) + 0x40;
        *(next + 1) = (value)&0xff;
        IncreaseLength(2);
        return true;
      }
      return false;
    }
    if (remaining_bytes >= 1) {
      // Do 1-byte encoding
      *next = (value & 0x3f);
      IncreaseLength(1);
      return true;
    }
    return false;
  }
  // Can not encode, high 2 bits not 0
  return false;
}

bool QuicDataWriter::WriteVarInt62(
    uint64_t value,
    QuicVariableLengthIntegerLength write_length) {
  QUICHE_DCHECK_EQ(endianness(), quiche::NETWORK_BYTE_ORDER);

  size_t remaining_bytes = remaining();
  if (remaining_bytes < write_length) {
    return false;
  }

  const QuicVariableLengthIntegerLength min_length = GetVarInt62Len(value);
  if (write_length < min_length) {
    QUIC_BUG(quic_bug_10347_1) << "Cannot write value " << value
                               << " with write_length " << write_length;
    return false;
  }
  if (write_length == min_length) {
    return WriteVarInt62(value);
  }

  if (write_length == VARIABLE_LENGTH_INTEGER_LENGTH_2) {
    return WriteUInt8(0b01000000) && WriteUInt8(value);
  }
  if (write_length == VARIABLE_LENGTH_INTEGER_LENGTH_4) {
    return WriteUInt8(0b10000000) && WriteUInt8(0) && WriteUInt16(value);
  }
  if (write_length == VARIABLE_LENGTH_INTEGER_LENGTH_8) {
    return WriteUInt8(0b11000000) && WriteUInt8(0) && WriteUInt16(0) &&
           WriteUInt32(value);
  }

  QUIC_BUG(quic_bug_10347_2)
      << "Invalid write_length " << static_cast<int>(write_length);
  return false;
}

// static
QuicVariableLengthIntegerLength QuicDataWriter::GetVarInt62Len(uint64_t value) {
  if ((value & kVarInt62ErrorMask) != 0) {
    QUIC_BUG(quic_bug_10347_3) << "Attempted to encode a value, " << value
                               << ", that is too big for VarInt62";
    return VARIABLE_LENGTH_INTEGER_LENGTH_0;
  }
  if ((value & kVarInt62Mask8Bytes) != 0) {
    return VARIABLE_LENGTH_INTEGER_LENGTH_8;
  }
  if ((value & kVarInt62Mask4Bytes) != 0) {
    return VARIABLE_LENGTH_INTEGER_LENGTH_4;
  }
  if ((value & kVarInt62Mask2Bytes) != 0) {
    return VARIABLE_LENGTH_INTEGER_LENGTH_2;
  }
  return VARIABLE_LENGTH_INTEGER_LENGTH_1;
}

bool QuicDataWriter::WriteStringPieceVarInt62(
    const absl::string_view& string_piece) {
  if (!WriteVarInt62(string_piece.size())) {
    return false;
  }
  if (!string_piece.empty()) {
    if (!WriteBytes(string_piece.data(), string_piece.size())) {
      return false;
    }
  }
  return true;
}

}  // namespace quic