Repo created

TMessagesProj/jni/voip/webrtc/modules/audio_coding/neteq/histogram.cc

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+/*
+ *  Copyright (c) 2019 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 "modules/audio_coding/neteq/histogram.h"
+
+#include <algorithm>
+#include <cstdlib>
+#include <numeric>
+
+#include "absl/types/optional.h"
+#include "rtc_base/checks.h"
+#include "rtc_base/numerics/safe_conversions.h"
+
+namespace webrtc {
+
+Histogram::Histogram(size_t num_buckets,
+                     int forget_factor,
+                     absl::optional<double> start_forget_weight)
+    : buckets_(num_buckets, 0),
+      forget_factor_(0),
+      base_forget_factor_(forget_factor),
+      add_count_(0),
+      start_forget_weight_(start_forget_weight) {
+  RTC_DCHECK_LT(base_forget_factor_, 1 << 15);
+}
+
+Histogram::~Histogram() {}
+
+// Each element in the vector is first multiplied by the forgetting factor
+// `forget_factor_`. Then the vector element indicated by `iat_packets` is then
+// increased (additive) by 1 - `forget_factor_`. This way, the probability of
+// `value` is slightly increased, while the sum of the histogram remains
+// constant (=1).
+// Due to inaccuracies in the fixed-point arithmetic, the histogram may no
+// longer sum up to 1 (in Q30) after the update. To correct this, a correction
+// term is added or subtracted from the first element (or elements) of the
+// vector.
+// The forgetting factor `forget_factor_` is also updated. When the DelayManager
+// is reset, the factor is set to 0 to facilitate rapid convergence in the
+// beginning. With each update of the histogram, the factor is increased towards
+// the steady-state value `base_forget_factor_`.
+void Histogram::Add(int value) {
+  RTC_DCHECK(value >= 0);
+  RTC_DCHECK(value < static_cast<int>(buckets_.size()));
+  int vector_sum = 0;  // Sum up the vector elements as they are processed.
+  // Multiply each element in `buckets_` with `forget_factor_`.
+  for (int& bucket : buckets_) {
+    bucket = (static_cast<int64_t>(bucket) * forget_factor_) >> 15;
+    vector_sum += bucket;
+  }
+
+  // Increase the probability for the currently observed inter-arrival time
+  // by 1 - `forget_factor_`. The factor is in Q15, `buckets_` in Q30.
+  // Thus, left-shift 15 steps to obtain result in Q30.
+  buckets_[value] += (32768 - forget_factor_) << 15;
+  vector_sum += (32768 - forget_factor_) << 15;  // Add to vector sum.
+
+  // `buckets_` should sum up to 1 (in Q30), but it may not due to
+  // fixed-point rounding errors.
+  vector_sum -= 1 << 30;  // Should be zero. Compensate if not.
+  if (vector_sum != 0) {
+    // Modify a few values early in `buckets_`.
+    int flip_sign = vector_sum > 0 ? -1 : 1;
+    for (int& bucket : buckets_) {
+      // Add/subtract 1/16 of the element, but not more than `vector_sum`.
+      int correction = flip_sign * std::min(std::abs(vector_sum), bucket >> 4);
+      bucket += correction;
+      vector_sum += correction;
+      if (std::abs(vector_sum) == 0) {
+        break;
+      }
+    }
+  }
+  RTC_DCHECK(vector_sum == 0);  // Verify that the above is correct.
+
+  ++add_count_;
+
+  // Update `forget_factor_` (changes only during the first seconds after a
+  // reset). The factor converges to `base_forget_factor_`.
+  if (start_forget_weight_) {
+    if (forget_factor_ != base_forget_factor_) {
+      int old_forget_factor = forget_factor_;
+      int forget_factor =
+          (1 << 15) * (1 - start_forget_weight_.value() / (add_count_ + 1));
+      forget_factor_ =
+          std::max(0, std::min(base_forget_factor_, forget_factor));
+      // The histogram is updated recursively by forgetting the old histogram
+      // with `forget_factor_` and adding a new sample multiplied by |1 -
+      // forget_factor_|. We need to make sure that the effective weight on the
+      // new sample is no smaller than those on the old samples, i.e., to
+      // satisfy the following DCHECK.
+      RTC_DCHECK_GE((1 << 15) - forget_factor_,
+                    ((1 << 15) - old_forget_factor) * forget_factor_ >> 15);
+    }
+  } else {
+    forget_factor_ += (base_forget_factor_ - forget_factor_ + 3) >> 2;
+  }
+}
+
+int Histogram::Quantile(int probability) {
+  // Find the bucket for which the probability of observing an
+  // inter-arrival time larger than or equal to `index` is larger than or
+  // equal to `probability`. The sought probability is estimated using
+  // the histogram as the reverse cumulant PDF, i.e., the sum of elements from
+  // the end up until `index`. Now, since the sum of all elements is 1
+  // (in Q30) by definition, and since the solution is often a low value for
+  // `iat_index`, it is more efficient to start with `sum` = 1 and subtract
+  // elements from the start of the histogram.
+  int inverse_probability = (1 << 30) - probability;
+  size_t index = 0;   // Start from the beginning of `buckets_`.
+  int sum = 1 << 30;  // Assign to 1 in Q30.
+  sum -= buckets_[index];
+
+  while ((sum > inverse_probability) && (index < buckets_.size() - 1)) {
+    // Subtract the probabilities one by one until the sum is no longer greater
+    // than `inverse_probability`.
+    ++index;
+    sum -= buckets_[index];
+  }
+  return static_cast<int>(index);
+}
+
+// Set the histogram vector to an exponentially decaying distribution
+// buckets_[i] = 0.5^(i+1), i = 0, 1, 2, ...
+// buckets_ is in Q30.
+void Histogram::Reset() {
+  // Set temp_prob to (slightly more than) 1 in Q14. This ensures that the sum
+  // of buckets_ is 1.
+  uint16_t temp_prob = 0x4002;  // 16384 + 2 = 100000000000010 binary.
+  for (int& bucket : buckets_) {
+    temp_prob >>= 1;
+    bucket = temp_prob << 16;
+  }
+  forget_factor_ = 0;  // Adapt the histogram faster for the first few packets.
+  add_count_ = 0;
+}
+
+int Histogram::NumBuckets() const {
+  return buckets_.size();
+}
+
+}  // namespace webrtc