Spec-compatible algorithm for selecting video source and settings.

content/renderer/media/media_stream_constraints_util_video_source.cc

+// Copyright 2017 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 "content/renderer/media/media_stream_constraints_util_video_source.h"
+
+#include <algorithm>
+#include <cmath>
+#include <utility>
+#include <vector>
+
+#include "content/renderer/media/media_stream_video_source.h"
+#include "third_party/WebKit/public/platform/WebMediaConstraints.h"
+#include "third_party/WebKit/public/platform/WebString.h"
+
+namespace content {
+
+namespace {
+
+// Number of default settings to be used as final tie-breaking criteria for
+// settings that are equally good at satisfying constraints:
+// device ID, power-line frequency, resolution and frame rate.
+const int kNumDefaultDistanceEntries = 4;
+
+// The default resolution to be preferred as tie-breaking criterion.
+const int kDefaultResolutionArea = MediaStreamVideoSource::kDefaultWidth *
+                                   MediaStreamVideoSource::kDefaultHeight;
+
+// The minimum aspect ratio to be supported by sources.
+const double kMinSourceAspectRatio = 0.05;
+
+blink::WebString ToWebString(::mojom::FacingMode facing_mode) {
+  switch (facing_mode) {
+    case ::mojom::FacingMode::USER:
+      return blink::WebString::fromASCII("user");
+    case ::mojom::FacingMode::ENVIRONMENT:
+      return blink::WebString::fromASCII("environment");
+    case ::mojom::FacingMode::LEFT:
+      return blink::WebString::fromASCII("left");
+    case ::mojom::FacingMode::RIGHT:
+      return blink::WebString::fromASCII("right");
+    default:
+      return blink::WebString::fromASCII("");
+  }
+}
+
+template <typename ConstraintType>
+bool ConstraintHasMax(const ConstraintType& constraint) {
+  return constraint.hasMax() || constraint.hasExact();
+}
+
+template <typename ConstraintType>
+bool ConstraintHasMin(const ConstraintType& constraint) {
+  return constraint.hasMin() || constraint.hasExact();
+}
+
+template <typename ConstraintType>
+auto ConstraintMax(const ConstraintType& constraint)
+    -> decltype(constraint.max()) {
+  DCHECK(ConstraintHasMax(constraint));
+  return constraint.hasExact() ? constraint.exact() : constraint.max();
+}
+
+template <typename ConstraintType>
+auto ConstraintMin(const ConstraintType& constraint)
+    -> decltype(constraint.min()) {
+  DCHECK(ConstraintHasMin(constraint));
+  return constraint.hasExact() ? constraint.exact() : constraint.min();
+}
+
+// Generic fitness distance function between two different values.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double FitnessDistance(double value1, double value2) {
+  if (std::fabs(value1 - value2) <= blink::DoubleConstraint::kConstraintEpsilon)
+    return 0.0;
+
+  return std::fabs(value1 - value2) /
+         std::max(std::fabs(value1), std::fabs(value2));
+}
+
+// Returns a pair with the minimum and maximum aspect ratios supported by the
+// source resolution settings |source_height| and |source_width|, subject to
+// given width and height constraints.
+void GetSourceAspectRatioRange(int source_height,
+                               int source_width,
+                               const blink::LongConstraint& height_constraint,
+                               const blink::LongConstraint& width_constraint,
+                               double* min_source_aspect_ratio,
+                               double* max_source_aspect_ratio) {
+  DCHECK_GE(source_height, 1);
+  DCHECK_GE(source_width, 1);
+  long min_height = 1;
+  if (ConstraintHasMin(height_constraint))
+    min_height = std::max(min_height, ConstraintMin(height_constraint));
+
+  long max_height = source_height;
+  if (ConstraintHasMax(height_constraint))
+    max_height = std::min(max_height, ConstraintMax(height_constraint));
+
+  long min_width = 1;
+  if (ConstraintHasMin(width_constraint))
+    min_width = std::max(min_width, ConstraintMin(width_constraint));
+
+  long max_width = source_width;
+  if (ConstraintHasMax(width_constraint))
+    max_width = std::min(max_width, ConstraintMax(width_constraint));
+
+  *min_source_aspect_ratio =
+      std::max(static_cast<double>(min_width) / static_cast<double>(max_height),
+               kMinSourceAspectRatio);
+  *max_source_aspect_ratio =
+      std::max(static_cast<double>(max_width) / static_cast<double>(min_height),
+               kMinSourceAspectRatio);
+}
+
+// Returns a custom distance between a string and a string constraint.
+// Returns 0 if |value| satisfies |constraint|. HUGE_VAL otherwise.
+double StringConstraintSourceDistance(const blink::WebString& value,
+                                      const blink::StringConstraint& constraint,
+                                      const char** failed_constraint_name) {
+  if (constraint.matches(value))
+    return 0.0;
+
+  if (failed_constraint_name)
+    *failed_constraint_name = constraint.name();
+  return HUGE_VAL;
+}
+
+// Returns a custom distance function suitable for screen dimensions, given
+// a |constraint| (e.g. width or height) and a candidate value |source_value|.
+// A source can support track resolutions in the range [1, |source_value|],
+// using cropping if necessary.
+// If the source range and the constraint range are disjoint, return HUGE_VAL.
+// If the constraint has maximum, penalize sources that exceed the maximum
+// by returning FitnessDistance(|source_value|, maximum). This is intended to
+// prefer, among sources that satisfy the constraint, those that have lower
+// resource usage.
+// Otherwise, return zero.
+double ResolutionConstraintSourceDistance(
+    int source_value,
+    const blink::LongConstraint& constraint,
+    const char** failed_constraint_name) {
+  DCHECK_GE(source_value, 1);
+  bool constraint_has_max = ConstraintHasMax(constraint);
+  long constraint_max = constraint_has_max ? ConstraintMax(constraint) : -1;
+
+  // If the intersection between the source range and the constraint range is
+  // empty, return HUGE_VAL.
+  if ((constraint_has_max && constraint_max < 1) ||
+      (ConstraintHasMin(constraint) &&
+       source_value < ConstraintMin(constraint))) {
+    if (failed_constraint_name)
+      *failed_constraint_name = constraint.name();
+    return HUGE_VAL;
+  }
+
+  // If the source value exceeds the maximum requested, penalize a
+  if (constraint_has_max && source_value > constraint_max)
+    return FitnessDistance(source_value, constraint_max);
+
+  return 0.0;
+}
+
+// Returns a custom distance function suitable for frame rate, given
+// a |constraint| and a candidate value.
+// A source can support track frame rates in the interval (0.0, |source_value|],
+// using frame-rate adjustments if necessary.
+// If the source range and the constraint range are disjoint, return HUGE_VAL.
+// If the constraint has maximum, penalize source frame rates that exceed the
+// maximum by returning FitnessDistance(|source_value|, maximum). This is
+// intended to prefer, among sources that satisfy the constraint, those that
+// have lower resource usage.
+// Otherwise, return zero.
+double FrameRateConstraintSourceDistance(
+    double source_value,
+    const blink::DoubleConstraint& constraint,
+    const char** failed_constraint_name) {
+  DCHECK_GT(source_value, 0.0);
+  bool constraint_has_max = ConstraintHasMax(constraint);
+  double constraint_max = constraint_has_max ? ConstraintMax(constraint) : -1.0;
+
+  if ((constraint_has_max && constraint_max <= 0.0) ||
+      (ConstraintHasMin(constraint) &&
+       source_value < ConstraintMin(constraint) -
+                          blink::DoubleConstraint::kConstraintEpsilon)) {
+    if (failed_constraint_name)
+      *failed_constraint_name = constraint.name();
+    return HUGE_VAL;
+  }
+
+  if (constraint_has_max && source_value > constraint_max)
+    return FitnessDistance(source_value, constraint_max);
+
+  return 0.0;
+}
+
+// Returns a custom distance function suitable for aspect ratio, given
+// the values for the aspect_ratio, width and height constraints, and candidate
+// source values for width and height.
+// A source can support track resolutions that range from
+//    min_width x min_height  to  max_width x max_height
+// where
+//    min_width = max(1, width_constraint.min)
+//    min_height = max(1, height_constraint.min)
+//    max_width = min(source_width, width_constraint.max)
+//    max_height = min(source_height, height_constraint.max)
+// The aspect-ratio range supported by the source is determined by the extremes
+// of those resolutions.
+//    min_ar = min_width / max_height.
+//    max_ar = max_width / min_height.
+//
+// If the supported range [min_ar, max_ar] and the range specified by the
+// aspectRatio constraint are disjoint, return HUGE_VAL. Otherwise, return zero.
+double AspectRatioConstraintSourceDistance(
+    int source_height,
+    int source_width,
+    const blink::LongConstraint& height_constraint,
+    const blink::LongConstraint& width_constraint,
+    const blink::DoubleConstraint& aspect_ratio_constraint,
+    const char** failed_constraint_name) {
+  DCHECK_GT(source_height, 1);
+  DCHECK_GT(source_width, 1);
+
+  bool ar_constraint_has_min = ConstraintHasMin(aspect_ratio_constraint);
+  double ar_constraint_min =
+      ar_constraint_has_min ? ConstraintMin(aspect_ratio_constraint) : -1.0;
+  bool ar_constraint_has_max = ConstraintHasMax(aspect_ratio_constraint);
+  double ar_constraint_max =
+      ar_constraint_has_max ? ConstraintMax(aspect_ratio_constraint) : -1.0;
+
+  double min_source_aspect_ratio;
+  double max_source_aspect_ratio;
+  GetSourceAspectRatioRange(source_height, source_width, height_constraint,
+                            width_constraint, &min_source_aspect_ratio,
+                            &max_source_aspect_ratio);
+
+  // If the supported range and the constraint rage are disjoint, return
+  // HUGE_VAL.
+  if ((ar_constraint_has_min &&
+       max_source_aspect_ratio <
+           ar_constraint_min - blink::DoubleConstraint::kConstraintEpsilon) ||
+      (ar_constraint_has_max &&
+       min_source_aspect_ratio >
+           ar_constraint_max + blink::DoubleConstraint::kConstraintEpsilon)) {
+    if (failed_constraint_name)
+      *failed_constraint_name = aspect_ratio_constraint.name();
+    return HUGE_VAL;
+  }
+
+  return 0.0;
+}
+
+// Returns a custom distance for constraints that depend on the device
+// characteristics that have a fixed value.
+double DeviceSourceDistance(
+    const std::string& device_id,
+    ::mojom::FacingMode facing_mode,
+    const blink::WebMediaTrackConstraintSet& constraint_set,
+    const char** failed_constraint_name) {
+  return StringConstraintSourceDistance(blink::WebString::fromASCII(device_id),
+                                        constraint_set.deviceId,
+                                        failed_constraint_name) +
+         StringConstraintSourceDistance(ToWebString(facing_mode),
+                                        constraint_set.facingMode,
+                                        failed_constraint_name);
+}
+
+// Returns a custom distance for constraints that depend on a video-capture
+// format.
+double FormatSourceDistance(
+    const media::VideoCaptureFormat& format,
+    const blink::WebMediaTrackConstraintSet& constraint_set,
+    const char** failed_constraint_name) {
+  return ResolutionConstraintSourceDistance(format.frame_size.height(),
+                                            constraint_set.height,
+                                            failed_constraint_name) +
+         ResolutionConstraintSourceDistance(format.frame_size.width(),
+                                            constraint_set.width,
+                                            failed_constraint_name) +
+         AspectRatioConstraintSourceDistance(
+             format.frame_size.height(), format.frame_size.width(),
+             constraint_set.height, constraint_set.width,
+             constraint_set.aspectRatio, failed_constraint_name) +
+         FrameRateConstraintSourceDistance(format.frame_rate,
+                                           constraint_set.frameRate,
+                                           failed_constraint_name);
+}
+
+// Returns a custom distance function suitable for the googPowerLineFrequency
+// constraint, given  a |constraint| and a candidate value |source_value|.
+// The distance is HUGE_VAL if |source_value| cannot satisfy |constraint|.
+// Otherwise, the distance is zero.
+double PowerLineFrequencyConstraintSourceDistance(
+    const blink::LongConstraint& constraint,
+    media::PowerLineFrequency source_value,
+    const char** failed_constraint_name) {
+  bool constraint_has_min = ConstraintHasMin(constraint);
+  bool constraint_has_max = ConstraintHasMax(constraint);
+  long constraint_min = constraint_has_min ? ConstraintMin(constraint) : -1L;
+  long constraint_max = constraint_has_max ? ConstraintMax(constraint) : -1L;
+  long source_value_long = static_cast<long>(source_value);
+
+  if ((constraint_has_max && source_value_long > constraint_max) ||
+      (constraint_has_min && source_value_long < constraint_min)) {
+    if (failed_constraint_name)
+      *failed_constraint_name = constraint.name();
+    return HUGE_VAL;
+  }
+
+  return 0.0;
+}
+
+// Returns a custom distance between a set of candidate settings and a
+// constraint set. It is simply the sum of the distances for each individual
+// setting in |candidate|.
+// If |candidate| cannot satisfy constraint, the distance is HUGE_VAL.
+// Otherwise the distance is a finite value. Candidates with lower distance
+// satisfy |constraint_set| in a "better" way.
+double CandidateSourceDistance(
+    const VideoCaptureSourceSettings& candidate,
+    const blink::WebMediaTrackConstraintSet& constraint_set,
+    const char** failed_constraint_name) {
+  return DeviceSourceDistance(candidate.device_id(), candidate.facing_mode(),
+                              constraint_set, failed_constraint_name) +
+         FormatSourceDistance(candidate.format(), constraint_set,
+                              failed_constraint_name) +
+         PowerLineFrequencyConstraintSourceDistance(
+             constraint_set.googPowerLineFrequency,
+             candidate.power_line_frequency(), failed_constraint_name);
+}
+
+// Returns the fitness distance between |value| and |constraint|.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double StringFitnessDistance(const blink::WebString& value,
+                             const blink::StringConstraint& constraint) {
+  if (!constraint.hasIdeal())
+    return 0.0;
+
+  for (auto& ideal_value : constraint.ideal()) {
+    if (value == ideal_value)
+      return 0.0;
+  }
+
+  return 1.0;
+}
+
+// Returns the fitness distance between |value| and |constraint| for
+// resolution constraints (i.e., width and height).
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double ResolutionFitnessDistance(long value,
+                                 const blink::LongConstraint& constraint) {
+  if (!constraint.hasIdeal())
+    return 0.0;
+
+  // Source resolutions greater than ideal support the ideal value with
+  // cropping.
+  if (value >= constraint.ideal())
+    return 0.0;
+
+  return FitnessDistance(value, constraint.ideal());
+}
+
+// Returns the fitness distance between |value| and |constraint| for
+// resolution constraints (i.e., width and height), ignoring cropping.
+// This measures how well a native resolution supports the idea value.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double NativeResolutionFitnessDistance(
+    long value,
+    const blink::LongConstraint& constraint) {
+  return constraint.hasIdeal() ? FitnessDistance(value, constraint.ideal())
+                               : 0.0;
+}
+
+// Returns the fitness distance between |value| and |constraint| for the
+// frameRate constraint.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double AspectRatioFitnessDistance(
+    long source_height,
+    long source_width,
+    const blink::LongConstraint& height_constraint,
+    const blink::LongConstraint& width_constraint,
+    const blink::DoubleConstraint& aspect_ratio_constraint) {
+  DCHECK_GT(source_height, 1);
+  DCHECK_GT(source_width, 1);
+
+  if (!aspect_ratio_constraint.hasIdeal())
+    return 0.0;
+
+  double min_source_aspect_ratio;
+  double max_source_aspect_ratio;
+  GetSourceAspectRatioRange(source_height, source_width, height_constraint,
+                            width_constraint, &min_source_aspect_ratio,
+                            &max_source_aspect_ratio);
+
+  // If the supported aspect ratio range does not include the ideal aspect
+  // ratio, compute fitness using the spec formula.
+  if (max_source_aspect_ratio <
+      aspect_ratio_constraint.ideal() -
+          blink::DoubleConstraint::kConstraintEpsilon) {
+    return FitnessDistance(max_source_aspect_ratio,
+                           aspect_ratio_constraint.ideal());
+  }
+
+  if (min_source_aspect_ratio >
+      aspect_ratio_constraint.ideal() +
+          blink::DoubleConstraint::kConstraintEpsilon) {
+    return FitnessDistance(min_source_aspect_ratio,
+                           aspect_ratio_constraint.ideal());
+  }
+
+  // Otherwise, the ideal aspect ratio can be supported and the fitness is 0.
+  return 0.0;
+}
+
+// Returns the fitness distance between |value| and |constraint| for the
+// frameRate constraint.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double FrameRateFitnessDistance(double value,
+                                const blink::DoubleConstraint& constraint) {
+  if (!constraint.hasIdeal())
+    return 0.0;
+
+  // Source frame rates greater than ideal support the ideal value using
+  // frame-rate adjustment.
+  if (value >=
+      constraint.ideal() - blink::DoubleConstraint::kConstraintEpsilon) {
+    return 0.0;
+  }
+
+  return FitnessDistance(value, constraint.ideal());
+}
+
+// Returns the fitness distance between |value| and |constraint| for the
+// frameRate constraint, ignoring frame-rate adjustment.
+// It measures how well the native frame rate supports the ideal value.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double NativeFrameRateFitnessDistance(
+    double value,
+    const blink::DoubleConstraint& constraint) {
+  return constraint.hasIdeal() ? FitnessDistance(value, constraint.ideal())
+                               : 0.0;
+}
+
+// Returns the fitness distance between |value| and |constraint| for the
+// googPowerLineFrequency constraint.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double PowerLineFitnessDistance(long value,
+                                const blink::LongConstraint& constraint) {
+  if (!constraint.hasIdeal())
+    return 0.0;
+
+  // This constraint is of type long, but it behaves as an enum. Thus, values
+  // equal to ideal have fitness 0.0 and any other values have fitness 1.0.
+  if (value == constraint.ideal())
+    return 0.0;
+
+  return 1.0;
+}
+
+// Returns the fitness distance between a settings candidate and a constraint
+// set. The returned value is the sum of the fitness distances between each
+// setting in |candidate| and the corresponding constraint in |constraint_set|.
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double VideoCaptureFitnessDistance(
+    const VideoCaptureSourceSettings& candidate,
+    const blink::WebMediaTrackConstraintSet& constraint_set) {
+  DCHECK(std::isfinite(
+      CandidateSourceDistance(candidate, constraint_set, nullptr)));
+  double fitness = 0.0;
+  fitness += AspectRatioFitnessDistance(
+      candidate.GetHeight(), candidate.GetWidth(), constraint_set.height,
+      constraint_set.width, constraint_set.aspectRatio);
+  fitness +=
+      StringFitnessDistance(candidate.GetDeviceId(), constraint_set.deviceId);
+  fitness += StringFitnessDistance(candidate.GetFacingMode(),
+                                   constraint_set.facingMode);
+  fitness += FrameRateFitnessDistance(candidate.GetFrameRate(),
+                                      constraint_set.frameRate);
+  fitness += PowerLineFitnessDistance(candidate.GetPowerLineFrequency(),
+                                      constraint_set.googPowerLineFrequency);
+  fitness +=
+      ResolutionFitnessDistance(candidate.GetHeight(), constraint_set.height);
+  fitness +=
+      ResolutionFitnessDistance(candidate.GetWidth(), constraint_set.width);
+
+  return fitness;
+}
+
+// Returns the native fitness distance between a settings candidate and a
+// constraint set. The returned value is the sum of the fitness distances for
+// the native values of settings that support a range of values (i.e., width,
+// height and frame rate).
+// Based on https://w3c.github.io/mediacapture-main/#dfn-applyconstraints.
+double VideoCaptureNativeFitnessDistance(
+    const VideoCaptureSourceSettings& candidate,
+    const blink::WebMediaTrackConstraintSet& constraint_set) {
+  DCHECK(std::isfinite(
+      CandidateSourceDistance(candidate, constraint_set, nullptr)));
+  double fitness = 0.0;
+  fitness += NativeFrameRateFitnessDistance(candidate.GetFrameRate(),
+                                            constraint_set.frameRate);
+  fitness += NativeResolutionFitnessDistance(candidate.GetHeight(),
+                                             constraint_set.height);
+  fitness += NativeResolutionFitnessDistance(candidate.GetWidth(),
+                                             constraint_set.width);
+
+  return fitness;
+}
+
+using DistanceVector = std::vector<double>;
+
+// This function appends additional entries to |distance_vector| based on
+// custom distance metrics between |candidate| and some default settings.
+// These entries are to be used as the final tie breaker for candidates that
+// are equally good according to the spec and the custom distance functions
+// between candidates and constraints.
+void AppendDistanceFromDefault(const VideoCaptureSourceSettings& candidate,
+                               const VideoCaptureCapabilities& capabilities,
+                               DistanceVector* distance_vector) {
+  // Favor IDs that appear first in the enumeration.
+  // This is the final tie breaker.
+  for (size_t i = 0; i < capabilities.device_capabilities.size(); ++i) {
+    if (candidate.device_id() ==
+        capabilities.device_capabilities[i]->device_id) {
+      distance_vector->push_back(i);
+      break;
+    }
+  }
+
+  // Prefer default power-line frequency.
+  double power_line_frequency_distance =
+      candidate.power_line_frequency() ==
+              media::PowerLineFrequency::FREQUENCY_DEFAULT
+          ? 0.0
+          : HUGE_VAL;
+  distance_vector->push_back(power_line_frequency_distance);
+
+  // Prefer a resolution with area close to the default.
+  int candidate_area = candidate.format().frame_size.GetArea();
+  double resolution_distance =
+      candidate_area == kDefaultResolutionArea
+          ? 0.0
+          : FitnessDistance(candidate_area, kDefaultResolutionArea);
+  distance_vector->push_back(resolution_distance);
+
+  // Prefer a frame rate close to the default.
+  double frame_rate_distance =
+      candidate.format().frame_rate == MediaStreamVideoSource::kDefaultFrameRate
+          ? 0.0
+          : FitnessDistance(candidate.format().frame_rate,
+                            MediaStreamVideoSource::kDefaultFrameRate);
+  distance_vector->push_back(frame_rate_distance);
+}
+
+}  // namespace
+
+VideoCaptureCapabilities::VideoCaptureCapabilities() = default;
+VideoCaptureCapabilities::VideoCaptureCapabilities(
+    VideoCaptureCapabilities&& other) = default;
+VideoCaptureCapabilities::~VideoCaptureCapabilities() = default;
+VideoCaptureCapabilities& VideoCaptureCapabilities::operator=(
+    VideoCaptureCapabilities&& other) = default;
+
+VideoCaptureSourceSettings::VideoCaptureSourceSettings(
+    const VideoCaptureSourceSettings& other) = default;
+VideoCaptureSourceSettings::VideoCaptureSourceSettings(
+    VideoCaptureSourceSettings&& other) = default;
+VideoCaptureSourceSettings::~VideoCaptureSourceSettings() = default;
+VideoCaptureSourceSettings& VideoCaptureSourceSettings::operator=(
+    const VideoCaptureSourceSettings& other) = default;
+VideoCaptureSourceSettings& VideoCaptureSourceSettings::operator=(
+    VideoCaptureSourceSettings&& other) = default;
+
+VideoCaptureSourceSettings::VideoCaptureSourceSettings()
+    : facing_mode_(::mojom::FacingMode::NONE),
+      power_line_frequency_(media::PowerLineFrequency::FREQUENCY_DEFAULT) {}
+
+VideoCaptureSourceSettings::VideoCaptureSourceSettings(
+    const std::string& device_id,
+    const media::VideoCaptureFormat& format,
+    ::mojom::FacingMode facing_mode,
+    media::PowerLineFrequency power_line_frequency)
+    : device_id_(device_id),
+      format_(format),
+      facing_mode_(facing_mode),
+      power_line_frequency_(power_line_frequency) {}
+
+blink::WebString VideoCaptureSourceSettings::GetFacingMode() const {
+  return ToWebString(facing_mode_);
+}
+
+long VideoCaptureSourceSettings::GetPowerLineFrequency() const {
+  return static_cast<long>(power_line_frequency_);
+}
+
+long VideoCaptureSourceSettings::GetWidth() const {
+  return format_.frame_size.width();
+}
+
+long VideoCaptureSourceSettings::GetHeight() const {
+  return format_.frame_size.height();
+}
+
+double VideoCaptureSourceSettings::GetFrameRate() const {
+  return format_.frame_rate;
+}
+
+blink::WebString VideoCaptureSourceSettings::GetDeviceId() const {
+  return blink::WebString::fromASCII(device_id_.data());
+}
+
+VideoCaptureSourceSelectionResult::VideoCaptureSourceSelectionResult()
+    : failed_constraint_name("unknown") {}
+VideoCaptureSourceSelectionResult::VideoCaptureSourceSelectionResult(
+    const VideoCaptureSourceSelectionResult& other) = default;
+VideoCaptureSourceSelectionResult::VideoCaptureSourceSelectionResult(
+    VideoCaptureSourceSelectionResult&& other) = default;
+VideoCaptureSourceSelectionResult::~VideoCaptureSourceSelectionResult() =
+    default;
+VideoCaptureSourceSelectionResult& VideoCaptureSourceSelectionResult::operator=(
+    const VideoCaptureSourceSelectionResult& other) = default;
+VideoCaptureSourceSelectionResult& VideoCaptureSourceSelectionResult::operator=(
+    VideoCaptureSourceSelectionResult&& other) = default;
+
+VideoCaptureSourceSelectionResult SelectVideoCaptureSourceSettings(
+    const VideoCaptureCapabilities& capabilities,
+    const blink::WebMediaConstraints& constraints) {
+  // A distance vector contains:
+  // a) For each advanced constraint set, a 0/1 value indicating if the
+  //    candidate satisfies the corresponding constraint set.
+  // b) Fitness distance for the candidate based on support for the ideal values
+  //    of the basic constraint set.
+  // c) A custom distance value based on how "well" a candidate satisfies each
+  //    constraint set, including basic and advanced sets.
+  // d) Native fitness distance for the candidate based on support for the
+  //    ideal values of the basic constraint set using native values for
+  //    settings that can support a range of values.
+  // e) A custom distance value based on how close the candidate is to default
+  //    settings.
+  // Parts (a) and (b) are according to spec. Parts (c) to (e) are
+  // implementation specific and used to break ties.
+  DistanceVector best_distance(2 * constraints.advanced().size() + 3 +
+                               kNumDefaultDistanceEntries);
+  std::fill(best_distance.begin(), best_distance.end(), HUGE_VAL);
+  VideoCaptureSourceSelectionResult result;
+  const char* failed_constraint_name;
+
+  for (auto& device : capabilities.device_capabilities) {
+    double basic_device_distance =
+        DeviceSourceDistance(device->device_id, device->facing_mode,
+                             constraints.basic(), &failed_constraint_name);
+    if (!std::isfinite(basic_device_distance))
+      continue;
+
+    for (auto& format : device->formats) {
+      double basic_format_distance = FormatSourceDistance(
+          format, constraints.basic(), &failed_constraint_name);
+      if (!std::isfinite(basic_format_distance))
+        continue;
+
+      for (auto& power_line_frequency : capabilities.power_line_capabilities) {
+        double basic_power_line_frequency_distance =
+            PowerLineFrequencyConstraintSourceDistance(
+                constraints.basic().googPowerLineFrequency,
+                power_line_frequency, &failed_constraint_name);
+        if (!std::isfinite(basic_power_line_frequency_distance))
+          continue;
+
+        // The candidate satisfies the basic constraint set.
+        double candidate_basic_custom_distance =
+            basic_device_distance + basic_format_distance +
+            basic_power_line_frequency_distance;
+        DCHECK(std::isfinite(candidate_basic_custom_distance));
+
+        // Temporary vector to save custom distances for advanced constraints.
+        // Custom distances must be added to the candidate distance vector after
+        // all the spec-mandated values.
+        DistanceVector advanced_custom_distance_vector;
+        VideoCaptureSourceSettings candidate(device->device_id, format,
+                                             device->facing_mode,
+                                             power_line_frequency);
+        DistanceVector candidate_distance_vector;
+        // First criteria for valid candidates is satisfaction of advanced
+        // constraint sets.
+        for (const auto& advanced : constraints.advanced()) {
+          double custom_distance =
+              CandidateSourceDistance(candidate, advanced, nullptr);
+          advanced_custom_distance_vector.push_back(custom_distance);
+          double spec_distance = std::isfinite(custom_distance) ? 0 : 1;
+          candidate_distance_vector.push_back(spec_distance);
+        }
+
+        // Second criterion is fitness distance.
+        candidate_distance_vector.push_back(
+            VideoCaptureFitnessDistance(candidate, constraints.basic()));
+
+        // Third criteria are custom distances to constraint sets.
+        candidate_distance_vector.push_back(candidate_basic_custom_distance);
+        std::copy(advanced_custom_distance_vector.begin(),
+                  advanced_custom_distance_vector.end(),
+                  std::back_inserter(candidate_distance_vector));
+
+        // Fourth criteria is native fitness distance.
+        candidate_distance_vector.push_back(
+            VideoCaptureNativeFitnessDistance(candidate, constraints.basic()));
+
+        // Final criteria are custom distances to default settings.
+        AppendDistanceFromDefault(candidate, capabilities,
+                                  &candidate_distance_vector);
+
+        DCHECK_EQ(best_distance.size(), candidate_distance_vector.size());
+        if (candidate_distance_vector < best_distance) {
+          best_distance = candidate_distance_vector;
+          result.settings = std::move(candidate);
+          result.failed_constraint_name = nullptr;
+        }
+      }
+    }
+  }
+
+  if (!result.has_value())
+    result.failed_constraint_name = failed_constraint_name;
+
+  return result;
+}
+
+}  // namespace content