webrtc / src / ab9ed5c305ba0fc9dd5edb7625575d97ea881ba8 / . / modules / audio_coding / neteq / histogram.cc

/* | |

* 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 |