1 files changed, 137 insertions, 0 deletions
diff --git a/third_party/libwebrtc/logging/rtc_event_log/events/fixed_length_encoding_parameters_v3.cc b/third_party/libwebrtc/logging/rtc_event_log/events/fixed_length_encoding_parameters_v3.cc
new file mode 100644
index 0000000000..0c93e6226d
--- /dev/null
+++ b/third_party/libwebrtc/logging/rtc_event_log/events/fixed_length_encoding_parameters_v3.cc
@@ -0,0 +1,137 @@
+/*
+ *  Copyright (c) 2021 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/events/fixed_length_encoding_parameters_v3.h"
+
+#include <algorithm>
+
+#include "absl/types/optional.h"
+#include "api/array_view.h"
+#include "logging/rtc_event_log/events/rtc_event_field_extraction.h"
+#include "rtc_base/logging.h"
+
+using webrtc_event_logging::MaxUnsignedValueOfBitWidth;
+using webrtc_event_logging::SignedBitWidth;
+using webrtc_event_logging::UnsignedBitWidth;
+using webrtc_event_logging::UnsignedDelta;
+
+namespace webrtc {
+
+FixedLengthEncodingParametersV3
+FixedLengthEncodingParametersV3::CalculateParameters(
+    uint64_t base,
+    const rtc::ArrayView<const uint64_t> values,
+    uint64_t value_bit_width,
+    bool values_optional) {
+  // As a special case, if all of the elements are identical to the base
+  // we just encode the base value with a special delta header.
+  if (std::all_of(values.cbegin(), values.cend(),
+                  [base](uint64_t val) { return val == base; })) {
+    // Delta header with signed=true and delta_bitwidth=64
+    return FixedLengthEncodingParametersV3(/*delta_bit_width=*/64,
+                                           /*signed_deltas=*/true,
+                                           values_optional, value_bit_width);
+  }
+
+  const uint64_t bit_mask = MaxUnsignedValueOfBitWidth(value_bit_width);
+
+  // Calculate the bitwidth required to encode all deltas when using a
+  // unsigned or signed represenation, respectively. For the unsigned
+  // representation, we just track the largest delta. For the signed
+  // representation, we have two possibilities for each delta; either
+  // going "forward" (i.e. current - previous) or "backwards"
+  // (i.e. previous - current) where both values are calculated with
+  // wrap around. We then track the largest positive and negative
+  // magnitude across the batch, assuming that we choose the smaller
+  // delta for each element.
+  uint64_t max_unsigned_delta = 0;
+  uint64_t max_positive_signed_delta = 0;
+  uint64_t min_negative_signed_delta = 0;
+  uint64_t prev = base;
+  for (uint64_t current : values) {
+    uint64_t positive_delta = UnsignedDelta(prev, current, bit_mask);
+    uint64_t negative_delta = UnsignedDelta(current, prev, bit_mask);
+
+    max_unsigned_delta = std::max(max_unsigned_delta, positive_delta);
+
+    if (positive_delta < negative_delta) {
+      max_positive_signed_delta =
+          std::max(max_positive_signed_delta, positive_delta);
+    } else {
+      min_negative_signed_delta =
+          std::max(min_negative_signed_delta, negative_delta);
+    }
+
+    prev = current;
+  }
+
+  // We now know the largest unsigned delta and the largest magnitudes of
+  // positive and negative signed deltas. Get the bitwidths required for
+  // each of the two encodings.
+  const uint64_t unsigned_delta_bit_width =
+      UnsignedBitWidth(max_unsigned_delta);
+  const uint64_t signed_delta_bit_width =
+      SignedBitWidth(max_positive_signed_delta, min_negative_signed_delta);
+
+  // Note: Preference for unsigned if the two have the same width (efficiency).
+  bool use_signed_deltas = signed_delta_bit_width < unsigned_delta_bit_width;
+  uint64_t delta_bit_width =
+      use_signed_deltas ? signed_delta_bit_width : unsigned_delta_bit_width;
+
+  // use_signed_deltas && delta_bit_width==64 is reserved for "all values
+  // equal".
+  RTC_DCHECK(!use_signed_deltas || delta_bit_width < 64);
+
+  RTC_DCHECK(ValidParameters(delta_bit_width, use_signed_deltas,
+                             values_optional, value_bit_width));
+  return FixedLengthEncodingParametersV3(delta_bit_width, use_signed_deltas,
+                                         values_optional, value_bit_width);
+}
+
+uint64_t FixedLengthEncodingParametersV3::DeltaHeaderAsInt() const {
+  uint64_t header = delta_bit_width_ - 1;
+  RTC_CHECK_LT(header, 1u << 6);
+  if (signed_deltas_) {
+    header += 1u << 6;
+  }
+  RTC_CHECK_LT(header, 1u << 7);
+  if (values_optional_) {
+    header += 1u << 7;
+  }
+  return header;
+}
+
+absl::optional<FixedLengthEncodingParametersV3>
+FixedLengthEncodingParametersV3::ParseDeltaHeader(uint64_t header,
+                                                  uint64_t value_bit_width) {
+  uint64_t delta_bit_width = (header & ((1u << 6) - 1)) + 1;
+  bool signed_deltas = header & (1u << 6);
+  bool values_optional = header & (1u << 7);
+
+  if (header >= (1u << 8)) {
+    RTC_LOG(LS_ERROR) << "Failed to parse delta header; unread bits remaining.";
+    return absl::nullopt;
+  }
+
+  if (!ValidParameters(delta_bit_width, signed_deltas, values_optional,
+                       value_bit_width)) {
+    RTC_LOG(LS_ERROR) << "Failed to parse delta header. Invalid combination of "
+                         "values: delta_bit_width="
+                      << delta_bit_width << " signed_deltas=" << signed_deltas
+                      << " values_optional=" << values_optional
+                      << " value_bit_width=" << value_bit_width;
+    return absl::nullopt;
+  }
+
+  return FixedLengthEncodingParametersV3(delta_bit_width, signed_deltas,
+                                         values_optional, value_bit_width);
+}
+
+}  // namespace webrtc