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/*
* Copyright (c) 2018 The WebRTC project authors. All Rights Reserved.
*
* Use of this source code is governed by a BSD-style license
* that can be found in the LICENSE file in the root of the source
* tree. An additional intellectual property rights grant can be found
* in the file PATENTS. All contributing project authors may
* be found in the AUTHORS file in the root of the source tree.
*/
#include "logging/rtc_event_log/encoder/delta_encoding.h"
#include <algorithm>
#include <cstddef>
#include <cstdint>
#include <limits>
#include <memory>
#include <optional>
#include <string>
#include <vector>
#include "absl/memory/memory.h"
#include "absl/strings/string_view.h"
#include "logging/rtc_event_log/encoder/bit_writer.h"
#include "logging/rtc_event_log/encoder/var_int.h"
#include "rtc_base/bitstream_reader.h"
#include "rtc_base/checks.h"
#include "rtc_base/logging.h"
namespace webrtc {
namespace {
// TODO(eladalon): Only build the decoder in tools and unit tests.
bool g_force_unsigned_for_testing = false;
bool g_force_signed_for_testing = false;
size_t BitsToBytes(size_t bits) {
return (bits / 8) + (bits % 8 > 0 ? 1 : 0);
}
// TODO(eladalon): Replace by something more efficient.
uint64_t UnsignedBitWidth(uint64_t input, bool zero_val_as_zero_width = false) {
if (zero_val_as_zero_width && input == 0) {
return 0;
}
uint64_t width = 0;
do { // input == 0 -> width == 1
width += 1;
input >>= 1;
} while (input != 0);
return width;
}
uint64_t SignedBitWidth(uint64_t max_pos_magnitude,
uint64_t max_neg_magnitude) {
const uint64_t bitwidth_pos = UnsignedBitWidth(max_pos_magnitude, true);
const uint64_t bitwidth_neg =
(max_neg_magnitude > 0) ? UnsignedBitWidth(max_neg_magnitude - 1, true)
: 0;
return 1 + std::max(bitwidth_pos, bitwidth_neg);
}
// Return the maximum integer of a given bit width.
// Examples:
// MaxUnsignedValueOfBitWidth(1) = 0x01
// MaxUnsignedValueOfBitWidth(6) = 0x3f
// MaxUnsignedValueOfBitWidth(8) = 0xff
// MaxUnsignedValueOfBitWidth(32) = 0xffffffff
uint64_t MaxUnsignedValueOfBitWidth(uint64_t bit_width) {
RTC_DCHECK_GE(bit_width, 1);
RTC_DCHECK_LE(bit_width, 64);
return (bit_width == 64) ? std::numeric_limits<uint64_t>::max()
: ((static_cast<uint64_t>(1) << bit_width) - 1);
}
// Computes the delta between `previous` and `current`, under the assumption
// that wrap-around occurs after `width` is exceeded.
uint64_t UnsignedDelta(uint64_t previous, uint64_t current, uint64_t bit_mask) {
return (current - previous) & bit_mask;
}
// Determines the encoding type (e.g. fixed-size encoding).
// Given an encoding type, may also distinguish between some variants of it
// (e.g. which fields of the fixed-size encoding are explicitly mentioned by
// the header, and which are implicitly assumed to hold certain default values).
enum class EncodingType {
kFixedSizeUnsignedDeltasNoEarlyWrapNoOpt = 0,
kFixedSizeSignedDeltasEarlyWrapAndOptSupported = 1,
kReserved1 = 2,
kReserved2 = 3,
kNumberOfEncodingTypes // Keep last
};
// The width of each field in the encoding header. Note that this is the
// width in case the field exists; not all fields occur in all encoding types.
constexpr size_t kBitsInHeaderForEncodingType = 2;
constexpr size_t kBitsInHeaderForDeltaWidthBits = 6;
constexpr size_t kBitsInHeaderForSignedDeltas = 1;
constexpr size_t kBitsInHeaderForValuesOptional = 1;
constexpr size_t kBitsInHeaderForValueWidthBits = 6;
static_assert(static_cast<size_t>(EncodingType::kNumberOfEncodingTypes) <=
1 << kBitsInHeaderForEncodingType,
"Not all encoding types fit.");
// Default values for when the encoding header does not specify explicitly.
constexpr bool kDefaultSignedDeltas = false;
constexpr bool kDefaultValuesOptional = false;
constexpr uint64_t kDefaultValueWidthBits = 64;
// Parameters for fixed-size delta-encoding/decoding.
// These are tailored for the sequence which will be encoded (e.g. widths).
class FixedLengthEncodingParameters final {
public:
static bool ValidParameters(uint64_t delta_width_bits,
bool signed_deltas,
bool values_optional,
uint64_t value_width_bits) {
return (1 <= delta_width_bits && delta_width_bits <= 64 &&
1 <= value_width_bits && value_width_bits <= 64 &&
delta_width_bits <= value_width_bits);
}
FixedLengthEncodingParameters(uint64_t delta_width_bits,
bool signed_deltas,
bool values_optional,
uint64_t value_width_bits)
: delta_width_bits_(delta_width_bits),
signed_deltas_(signed_deltas),
values_optional_(values_optional),
value_width_bits_(value_width_bits),
delta_mask_(MaxUnsignedValueOfBitWidth(delta_width_bits_)),
value_mask_(MaxUnsignedValueOfBitWidth(value_width_bits_)) {
RTC_DCHECK(ValidParameters(delta_width_bits, signed_deltas, values_optional,
value_width_bits));
}
// Number of bits necessary to hold the widest(*) of the deltas between the
// values in the sequence.
// (*) - Widest might not be the largest, if signed deltas are used.
uint64_t delta_width_bits() const { return delta_width_bits_; }
// Whether deltas are signed.
bool signed_deltas() const { return signed_deltas_; }
// Whether the values of the sequence are optional. That is, it may be
// that some of them do not have a value (not even a sentinel value indicating
// invalidity).
bool values_optional() const { return values_optional_; }
// Number of bits necessary to hold the largest value in the sequence.
uint64_t value_width_bits() const { return value_width_bits_; }
// Masks where only the bits relevant to the deltas/values are turned on.
uint64_t delta_mask() const { return delta_mask_; }
uint64_t value_mask() const { return value_mask_; }
void SetSignedDeltas(bool signed_deltas) { signed_deltas_ = signed_deltas; }
void SetDeltaWidthBits(uint64_t delta_width_bits) {
delta_width_bits_ = delta_width_bits;
delta_mask_ = MaxUnsignedValueOfBitWidth(delta_width_bits);
}
private:
uint64_t delta_width_bits_; // Normally const, but mutable in tests.
bool signed_deltas_; // Normally const, but mutable in tests.
const bool values_optional_;
const uint64_t value_width_bits_;
uint64_t delta_mask_; // Normally const, but mutable in tests.
const uint64_t value_mask_;
};
// Performs delta-encoding of a single (non-empty) sequence of values, using
// an encoding where all deltas are encoded using the same number of bits.
// (With the exception of optional elements; those are encoded as a bit vector
// with one bit per element, plus a fixed number of bits for every element that
// has a value.)
class FixedLengthDeltaEncoder final {
public:
// See webrtc::EncodeDeltas() for general details.
// This function return a bit pattern that would allow the decoder to
// determine whether it was produced by FixedLengthDeltaEncoder, and can
// therefore be decoded by FixedLengthDeltaDecoder, or whether it was produced
// by a different encoder.
static std::string EncodeDeltas(
std::optional<uint64_t> base,
const std::vector<std::optional<uint64_t>>& values);
FixedLengthDeltaEncoder(const FixedLengthDeltaEncoder&) = delete;
FixedLengthDeltaEncoder& operator=(const FixedLengthDeltaEncoder&) = delete;
private:
// Calculate min/max values of unsigned/signed deltas, given the bit width
// of all the values in the series.
static void CalculateMinAndMaxDeltas(
std::optional<uint64_t> base,
const std::vector<std::optional<uint64_t>>& values,
uint64_t bit_width,
uint64_t* max_unsigned_delta,
uint64_t* max_pos_signed_delta,
uint64_t* min_neg_signed_delta);
// No effect outside of unit tests.
// In unit tests, may lead to forcing signed/unsigned deltas, etc.
static void ConsiderTestOverrides(FixedLengthEncodingParameters* params,
uint64_t delta_width_bits_signed,
uint64_t delta_width_bits_unsigned);
// FixedLengthDeltaEncoder objects are to be created by EncodeDeltas() and
// released by it before it returns. They're mostly a convenient way to
// avoid having to pass a lot of state between different functions.
// Therefore, it was deemed acceptable to let them have a reference to
// `values`, whose lifetime must exceed the lifetime of `this`.
FixedLengthDeltaEncoder(const FixedLengthEncodingParameters& params,
std::optional<uint64_t> base,
const std::vector<std::optional<uint64_t>>& values,
size_t existent_values_count);
// Perform delta-encoding using the parameters given to the ctor on the
// sequence of values given to the ctor.
std::string Encode();
// Exact lengths.
size_t OutputLengthBytes(size_t existent_values_count) const;
size_t HeaderLengthBits() const;
size_t EncodedDeltasLengthBits(size_t existent_values_count) const;
// Encode the compression parameters into the stream.
void EncodeHeader();
// Encode a given delta into the stream.
void EncodeDelta(uint64_t previous, uint64_t current);
void EncodeUnsignedDelta(uint64_t previous, uint64_t current);
void EncodeSignedDelta(uint64_t previous, uint64_t current);
// The parameters according to which encoding will be done (width of
// fields, whether signed deltas should be used, etc.)
const FixedLengthEncodingParameters params_;
// The encoding scheme assumes that at least one value is transmitted OOB,
// so that the first value can be encoded as a delta from that OOB value,
// which is `base_`.
const std::optional<uint64_t> base_;
// The values to be encoded.
// Note: This is a non-owning reference. See comment above ctor for details.
const std::vector<std::optional<uint64_t>>& values_;
// Buffer into which encoded values will be written.
// This is created dynmically as a way to enforce that the rest of the
// ctor has finished running when this is constructed, so that the lower
// bound on the buffer size would be guaranteed correct.
std::unique_ptr<BitWriter> writer_;
};
// TODO(eladalon): Reduce the number of passes.
std::string FixedLengthDeltaEncoder::EncodeDeltas(
std::optional<uint64_t> base,
const std::vector<std::optional<uint64_t>>& values) {
RTC_DCHECK(!values.empty());
// As a special case, if all of the elements are identical to the base,
// (including, for optional fields, about their existence/non-existence),
// the empty string is used to signal that.
if (std::all_of(
values.cbegin(), values.cend(),
[base](std::optional<uint64_t> val) { return val == base; })) {
return std::string();
}
bool non_decreasing = true;
uint64_t max_value_including_base = base.value_or(0u);
size_t existent_values_count = 0;
{
uint64_t previous = base.value_or(0u);
for (size_t i = 0; i < values.size(); ++i) {
if (!values[i].has_value()) {
continue;
}
++existent_values_count;
non_decreasing &= (previous <= values[i].value());
max_value_including_base =
std::max(max_value_including_base, values[i].value());
previous = values[i].value();
}
}
// If the sequence is non-decreasing, it may be assumed to have width = 64;
// there's no reason to encode the actual max width in the encoding header.
const uint64_t value_width_bits =
non_decreasing ? 64 : UnsignedBitWidth(max_value_including_base);
uint64_t max_unsigned_delta;
uint64_t max_pos_signed_delta;
uint64_t min_neg_signed_delta;
CalculateMinAndMaxDeltas(base, values, value_width_bits, &max_unsigned_delta,
&max_pos_signed_delta, &min_neg_signed_delta);
const uint64_t delta_width_bits_unsigned =
UnsignedBitWidth(max_unsigned_delta);
const uint64_t delta_width_bits_signed =
SignedBitWidth(max_pos_signed_delta, min_neg_signed_delta);
// Note: Preference for unsigned if the two have the same width (efficiency).
const bool signed_deltas =
delta_width_bits_signed < delta_width_bits_unsigned;
const uint64_t delta_width_bits =
signed_deltas ? delta_width_bits_signed : delta_width_bits_unsigned;
const bool values_optional =
!base.has_value() || (existent_values_count < values.size());
FixedLengthEncodingParameters params(delta_width_bits, signed_deltas,
values_optional, value_width_bits);
// No effect in production.
ConsiderTestOverrides(¶ms, delta_width_bits_signed,
delta_width_bits_unsigned);
FixedLengthDeltaEncoder encoder(params, base, values, existent_values_count);
return encoder.Encode();
}
void FixedLengthDeltaEncoder::CalculateMinAndMaxDeltas(
std::optional<uint64_t> base,
const std::vector<std::optional<uint64_t>>& values,
uint64_t bit_width,
uint64_t* max_unsigned_delta_out,
uint64_t* max_pos_signed_delta_out,
uint64_t* min_neg_signed_delta_out) {
RTC_DCHECK(!values.empty());
RTC_DCHECK(max_unsigned_delta_out);
RTC_DCHECK(max_pos_signed_delta_out);
RTC_DCHECK(min_neg_signed_delta_out);
const uint64_t bit_mask = MaxUnsignedValueOfBitWidth(bit_width);
uint64_t max_unsigned_delta = 0;
uint64_t max_pos_signed_delta = 0;
uint64_t min_neg_signed_delta = 0;
std::optional<uint64_t> prev = base;
for (size_t i = 0; i < values.size(); ++i) {
if (!values[i].has_value()) {
continue;
}
if (!prev.has_value()) {
// If the base is non-existent, the first existent value is encoded as
// a varint, rather than as a delta.
RTC_DCHECK(!base.has_value());
prev = values[i];
continue;
}
const uint64_t current = values[i].value();
const uint64_t forward_delta = UnsignedDelta(*prev, current, bit_mask);
const uint64_t backward_delta = UnsignedDelta(current, *prev, bit_mask);
max_unsigned_delta = std::max(max_unsigned_delta, forward_delta);
if (forward_delta < backward_delta) {
max_pos_signed_delta = std::max(max_pos_signed_delta, forward_delta);
} else {
min_neg_signed_delta = std::max(min_neg_signed_delta, backward_delta);
}
prev = current;
}
*max_unsigned_delta_out = max_unsigned_delta;
*max_pos_signed_delta_out = max_pos_signed_delta;
*min_neg_signed_delta_out = min_neg_signed_delta;
}
void FixedLengthDeltaEncoder::ConsiderTestOverrides(
FixedLengthEncodingParameters* params,
uint64_t delta_width_bits_signed,
uint64_t delta_width_bits_unsigned) {
if (g_force_unsigned_for_testing) {
params->SetDeltaWidthBits(delta_width_bits_unsigned);
params->SetSignedDeltas(false);
} else if (g_force_signed_for_testing) {
params->SetDeltaWidthBits(delta_width_bits_signed);
params->SetSignedDeltas(true);
} else {
// Unchanged.
}
}
FixedLengthDeltaEncoder::FixedLengthDeltaEncoder(
const FixedLengthEncodingParameters& params,
std::optional<uint64_t> base,
const std::vector<std::optional<uint64_t>>& values,
size_t existent_values_count)
: params_(params), base_(base), values_(values) {
RTC_DCHECK(!values_.empty());
writer_ =
std::make_unique<BitWriter>(OutputLengthBytes(existent_values_count));
}
std::string FixedLengthDeltaEncoder::Encode() {
EncodeHeader();
if (params_.values_optional()) {
// Encode which values exist and which don't.
for (std::optional<uint64_t> value : values_) {
writer_->WriteBits(value.has_value() ? 1u : 0u, 1);
}
}
std::optional<uint64_t> previous = base_;
for (std::optional<uint64_t> value : values_) {
if (!value.has_value()) {
RTC_DCHECK(params_.values_optional());
continue;
}
if (!previous.has_value()) {
// If the base is non-existent, the first existent value is encoded as
// a varint, rather than as a delta.
RTC_DCHECK(!base_.has_value());
writer_->WriteBits(EncodeVarInt(value.value()));
} else {
EncodeDelta(previous.value(), value.value());
}
previous = value;
}
return writer_->GetString();
}
size_t FixedLengthDeltaEncoder::OutputLengthBytes(
size_t existent_values_count) const {
return BitsToBytes(HeaderLengthBits() +
EncodedDeltasLengthBits(existent_values_count));
}
size_t FixedLengthDeltaEncoder::HeaderLengthBits() const {
if (params_.signed_deltas() == kDefaultSignedDeltas &&
params_.values_optional() == kDefaultValuesOptional &&
params_.value_width_bits() == kDefaultValueWidthBits) {
return kBitsInHeaderForEncodingType + kBitsInHeaderForDeltaWidthBits;
} else {
return kBitsInHeaderForEncodingType + kBitsInHeaderForDeltaWidthBits +
kBitsInHeaderForSignedDeltas + kBitsInHeaderForValuesOptional +
kBitsInHeaderForValueWidthBits;
}
}
size_t FixedLengthDeltaEncoder::EncodedDeltasLengthBits(
size_t existent_values_count) const {
if (!params_.values_optional()) {
return values_.size() * params_.delta_width_bits();
} else {
RTC_DCHECK_EQ(std::count_if(values_.begin(), values_.end(),
[](std::optional<uint64_t> val) {
return val.has_value();
}),
existent_values_count);
// One bit for each delta, to indicate if the value exists, and delta_width
// for each existent value, to indicate the delta itself.
// If base_ is non-existent, the first value (if any) is encoded as a varint
// rather than as a delta.
const size_t existence_bitmap_size_bits = 1 * values_.size();
const bool first_value_is_varint =
!base_.has_value() && existent_values_count >= 1;
const size_t first_value_varint_size_bits = 8 * kMaxVarIntLengthBytes;
const size_t deltas_count = existent_values_count - first_value_is_varint;
const size_t deltas_size_bits = deltas_count * params_.delta_width_bits();
return existence_bitmap_size_bits + first_value_varint_size_bits +
deltas_size_bits;
}
}
void FixedLengthDeltaEncoder::EncodeHeader() {
RTC_DCHECK(writer_);
const EncodingType encoding_type =
(params_.value_width_bits() == kDefaultValueWidthBits &&
params_.signed_deltas() == kDefaultSignedDeltas &&
params_.values_optional() == kDefaultValuesOptional)
? EncodingType::kFixedSizeUnsignedDeltasNoEarlyWrapNoOpt
: EncodingType::kFixedSizeSignedDeltasEarlyWrapAndOptSupported;
writer_->WriteBits(static_cast<uint64_t>(encoding_type),
kBitsInHeaderForEncodingType);
// Note: Since it's meaningless for a field to be of width 0, when it comes
// to fields that relate widths, we encode width 1 as 0, width 2 as 1,
writer_->WriteBits(params_.delta_width_bits() - 1,
kBitsInHeaderForDeltaWidthBits);
if (encoding_type == EncodingType::kFixedSizeUnsignedDeltasNoEarlyWrapNoOpt) {
return;
}
writer_->WriteBits(static_cast<uint64_t>(params_.signed_deltas()),
kBitsInHeaderForSignedDeltas);
writer_->WriteBits(static_cast<uint64_t>(params_.values_optional()),
kBitsInHeaderForValuesOptional);
writer_->WriteBits(params_.value_width_bits() - 1,
kBitsInHeaderForValueWidthBits);
}
void FixedLengthDeltaEncoder::EncodeDelta(uint64_t previous, uint64_t current) {
if (params_.signed_deltas()) {
EncodeSignedDelta(previous, current);
} else {
EncodeUnsignedDelta(previous, current);
}
}
void FixedLengthDeltaEncoder::EncodeUnsignedDelta(uint64_t previous,
uint64_t current) {
RTC_DCHECK(writer_);
const uint64_t delta = UnsignedDelta(previous, current, params_.value_mask());
writer_->WriteBits(delta, params_.delta_width_bits());
}
void FixedLengthDeltaEncoder::EncodeSignedDelta(uint64_t previous,
uint64_t current) {
RTC_DCHECK(writer_);
const uint64_t forward_delta =
UnsignedDelta(previous, current, params_.value_mask());
const uint64_t backward_delta =
UnsignedDelta(current, previous, params_.value_mask());
uint64_t delta;
if (forward_delta <= backward_delta) {
delta = forward_delta;
} else {
// Compute the unsigned representation of a negative delta.
// This is the two's complement representation of this negative value,
// when deltas are of width params_.delta_mask().
RTC_DCHECK_GE(params_.delta_mask(), backward_delta);
RTC_DCHECK_LT(params_.delta_mask() - backward_delta, params_.delta_mask());
delta = params_.delta_mask() - backward_delta + 1;
RTC_DCHECK_LE(delta, params_.delta_mask());
}
writer_->WriteBits(delta, params_.delta_width_bits());
}
// Perform decoding of a a delta-encoded stream, extracting the original
// sequence of values.
class FixedLengthDeltaDecoder final {
public:
// Checks whether FixedLengthDeltaDecoder is a suitable decoder for this
// bitstream. Note that this does NOT imply that stream is valid, and will
// be decoded successfully. It DOES imply that all other decoder classes
// will fail to decode this input, though.
static bool IsSuitableDecoderFor(absl::string_view input);
// Assuming that `input` is the result of fixed-size delta-encoding
// that took place with the same value to `base` and over `num_of_deltas`
// original values, this will return the sequence of original values.
// If an error occurs (can happen if `input` is corrupt), an empty
// vector will be returned.
static std::vector<std::optional<uint64_t>> DecodeDeltas(
absl::string_view input,
std::optional<uint64_t> base,
size_t num_of_deltas);
FixedLengthDeltaDecoder(const FixedLengthDeltaDecoder&) = delete;
FixedLengthDeltaDecoder& operator=(const FixedLengthDeltaDecoder&) = delete;
private:
// Reads the encoding header in `input` and returns a FixedLengthDeltaDecoder
// with the corresponding configuration, that can be used to decode the
// values in `input`.
// If the encoding header is corrupt (contains an illegal configuration),
// nullptr will be returned.
// When a valid FixedLengthDeltaDecoder is returned, this does not mean that
// the entire stream is free of error. Rather, only the encoding header is
// examined and guaranteed.
static std::unique_ptr<FixedLengthDeltaDecoder> Create(
absl::string_view input,
std::optional<uint64_t> base,
size_t num_of_deltas);
// FixedLengthDeltaDecoder objects are to be created by DecodeDeltas() and
// released by it before it returns. They're mostly a convenient way to
// avoid having to pass a lot of state between different functions.
// Therefore, it was deemed acceptable that `reader` does not own the buffer
// it reads, meaning the lifetime of `this` must not exceed the lifetime
// of `reader`'s underlying buffer.
FixedLengthDeltaDecoder(BitstreamReader reader,
const FixedLengthEncodingParameters& params,
std::optional<uint64_t> base,
size_t num_of_deltas);
// Perform the decoding using the parameters given to the ctor.
std::vector<std::optional<uint64_t>> Decode();
// Add `delta` to `base` to produce the next value in a sequence.
// The delta is applied as signed/unsigned depending on the parameters
// given to the ctor. Wrap-around is taken into account according to the
// values' width, as specified by the aforementioned encoding parameters.
uint64_t ApplyDelta(uint64_t base, uint64_t delta) const;
// Helpers for ApplyDelta().
uint64_t ApplyUnsignedDelta(uint64_t base, uint64_t delta) const;
uint64_t ApplySignedDelta(uint64_t base, uint64_t delta) const;
// Reader of the input stream to be decoded. Does not own that buffer.
// See comment above ctor for details.
BitstreamReader reader_;
// The parameters according to which encoding will be done (width of
// fields, whether signed deltas should be used, etc.)
const FixedLengthEncodingParameters params_;
// The encoding scheme assumes that at least one value is transmitted OOB,
// so that the first value can be encoded as a delta from that OOB value,
// which is `base_`.
const std::optional<uint64_t> base_;
// The number of values to be known to be decoded.
const size_t num_of_deltas_;
};
bool FixedLengthDeltaDecoder::IsSuitableDecoderFor(absl::string_view input) {
BitstreamReader reader(input);
uint64_t encoding_type_bits = reader.ReadBits(kBitsInHeaderForEncodingType);
if (!reader.Ok()) {
return false;
}
const auto encoding_type = static_cast<EncodingType>(encoding_type_bits);
return encoding_type ==
EncodingType::kFixedSizeUnsignedDeltasNoEarlyWrapNoOpt ||
encoding_type ==
EncodingType::kFixedSizeSignedDeltasEarlyWrapAndOptSupported;
}
std::vector<std::optional<uint64_t>> FixedLengthDeltaDecoder::DecodeDeltas(
absl::string_view input,
std::optional<uint64_t> base,
size_t num_of_deltas) {
auto decoder = FixedLengthDeltaDecoder::Create(input, base, num_of_deltas);
if (!decoder) {
return std::vector<std::optional<uint64_t>>();
}
return decoder->Decode();
}
std::unique_ptr<FixedLengthDeltaDecoder> FixedLengthDeltaDecoder::Create(
absl::string_view input,
std::optional<uint64_t> base,
size_t num_of_deltas) {
BitstreamReader reader(input);
// Encoding type
uint32_t encoding_type_bits = reader.ReadBits(kBitsInHeaderForEncodingType);
if (!reader.Ok()) {
return nullptr;
}
const EncodingType encoding = static_cast<EncodingType>(encoding_type_bits);
if (encoding != EncodingType::kFixedSizeUnsignedDeltasNoEarlyWrapNoOpt &&
encoding !=
EncodingType::kFixedSizeSignedDeltasEarlyWrapAndOptSupported) {
RTC_LOG(LS_WARNING) << "Unrecognized encoding type.";
return nullptr;
}
// See encoding for +1's rationale.
const uint64_t delta_width_bits =
reader.ReadBits(kBitsInHeaderForDeltaWidthBits) + 1;
RTC_DCHECK_LE(delta_width_bits, 64);
// signed_deltas, values_optional, value_width_bits
bool signed_deltas;
bool values_optional;
uint64_t value_width_bits;
if (encoding == EncodingType::kFixedSizeUnsignedDeltasNoEarlyWrapNoOpt) {
signed_deltas = kDefaultSignedDeltas;
values_optional = kDefaultValuesOptional;
value_width_bits = kDefaultValueWidthBits;
} else {
signed_deltas = reader.Read<bool>();
values_optional = reader.Read<bool>();
// See encoding for +1's rationale.
value_width_bits = reader.ReadBits(kBitsInHeaderForValueWidthBits) + 1;
RTC_DCHECK_LE(value_width_bits, 64);
}
if (!reader.Ok()) {
return nullptr;
}
// Note: Because of the way the parameters are read, it is not possible
// for illegal values to be read. We check nevertheless, in case the code
// changes in the future in a way that breaks this promise.
if (!FixedLengthEncodingParameters::ValidParameters(
delta_width_bits, signed_deltas, values_optional, value_width_bits)) {
RTC_LOG(LS_WARNING) << "Corrupt log; illegal encoding parameters.";
return nullptr;
}
FixedLengthEncodingParameters params(delta_width_bits, signed_deltas,
values_optional, value_width_bits);
return absl::WrapUnique(
new FixedLengthDeltaDecoder(reader, params, base, num_of_deltas));
}
FixedLengthDeltaDecoder::FixedLengthDeltaDecoder(
BitstreamReader reader,
const FixedLengthEncodingParameters& params,
std::optional<uint64_t> base,
size_t num_of_deltas)
: reader_(reader),
params_(params),
base_(base),
num_of_deltas_(num_of_deltas) {
RTC_DCHECK(reader_.Ok());
}
std::vector<std::optional<uint64_t>> FixedLengthDeltaDecoder::Decode() {
RTC_DCHECK(reader_.Ok());
std::vector<bool> existing_values(num_of_deltas_);
if (params_.values_optional()) {
for (size_t i = 0; i < num_of_deltas_; ++i) {
existing_values[i] = reader_.Read<bool>();
}
} else {
std::fill(existing_values.begin(), existing_values.end(), true);
}
std::optional<uint64_t> previous = base_;
std::vector<std::optional<uint64_t>> values(num_of_deltas_);
for (size_t i = 0; i < num_of_deltas_; ++i) {
if (!existing_values[i]) {
RTC_DCHECK(params_.values_optional());
continue;
}
if (!previous) {
// If the base is non-existent, the first existent value is encoded as
// a varint, rather than as a delta.
RTC_DCHECK(!base_.has_value());
values[i] = DecodeVarInt(reader_);
} else {
uint64_t delta = reader_.ReadBits(params_.delta_width_bits());
values[i] = ApplyDelta(*previous, delta);
}
previous = values[i];
}
if (!reader_.Ok()) {
values = {};
}
return values;
}
uint64_t FixedLengthDeltaDecoder::ApplyDelta(uint64_t base,
uint64_t delta) const {
RTC_DCHECK_LE(base, MaxUnsignedValueOfBitWidth(params_.value_width_bits()));
RTC_DCHECK_LE(delta, MaxUnsignedValueOfBitWidth(params_.delta_width_bits()));
return params_.signed_deltas() ? ApplySignedDelta(base, delta)
: ApplyUnsignedDelta(base, delta);
}
uint64_t FixedLengthDeltaDecoder::ApplyUnsignedDelta(uint64_t base,
uint64_t delta) const {
// Note: May still be used if signed deltas used.
RTC_DCHECK_LE(base, MaxUnsignedValueOfBitWidth(params_.value_width_bits()));
RTC_DCHECK_LE(delta, MaxUnsignedValueOfBitWidth(params_.delta_width_bits()));
return (base + delta) & params_.value_mask();
}
uint64_t FixedLengthDeltaDecoder::ApplySignedDelta(uint64_t base,
uint64_t delta) const {
RTC_DCHECK(params_.signed_deltas());
RTC_DCHECK_LE(base, MaxUnsignedValueOfBitWidth(params_.value_width_bits()));
RTC_DCHECK_LE(delta, MaxUnsignedValueOfBitWidth(params_.delta_width_bits()));
const uint64_t top_bit = static_cast<uint64_t>(1)
<< (params_.delta_width_bits() - 1);
const bool positive_delta = ((delta & top_bit) == 0);
if (positive_delta) {
return ApplyUnsignedDelta(base, delta);
}
const uint64_t delta_abs = (~delta & params_.delta_mask()) + 1;
return (base - delta_abs) & params_.value_mask();
}
} // namespace
std::string EncodeDeltas(std::optional<uint64_t> base,
const std::vector<std::optional<uint64_t>>& values) {
// TODO(eladalon): Support additional encodings.
return FixedLengthDeltaEncoder::EncodeDeltas(base, values);
}
std::vector<std::optional<uint64_t>> DecodeDeltas(absl::string_view input,
std::optional<uint64_t> base,
size_t num_of_deltas) {
RTC_DCHECK_GT(num_of_deltas, 0); // Allows empty vector to indicate error.
// The empty string is a special case indicating that all values were equal
// to the base.
if (input.empty()) {
std::vector<std::optional<uint64_t>> result(num_of_deltas);
std::fill(result.begin(), result.end(), base);
return result;
}
if (FixedLengthDeltaDecoder::IsSuitableDecoderFor(input)) {
return FixedLengthDeltaDecoder::DecodeDeltas(input, base, num_of_deltas);
}
RTC_LOG(LS_WARNING) << "Could not decode delta-encoded stream.";
return std::vector<std::optional<uint64_t>>();
}
void SetFixedLengthEncoderDeltaSignednessForTesting(bool signedness) {
g_force_unsigned_for_testing = !signedness;
g_force_signed_for_testing = signedness;
}
void UnsetFixedLengthEncoderDeltaSignednessForTesting() {
g_force_unsigned_for_testing = false;
g_force_signed_for_testing = false;
}
} // namespace webrtc