/*
 *  Copyright 2025 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 "pc/codec_vendor.h"

#include <cstddef>
#include <optional>
#include <set>
#include <span>
#include <string>
#include <utility>
#include <vector>

#include "absl/algorithm/container.h"
#include "absl/strings/match.h"
#include "absl/strings/string_view.h"
#include "api/field_trials_view.h"
#include "api/media_types.h"
#include "api/payload_type.h"
#include "api/rtc_error.h"
#include "api/rtp_parameters.h"
#include "api/rtp_transceiver_direction.h"
#include "api/sequence_checker.h"
#include "call/payload_type.h"
#include "media/base/codec.h"
#include "media/base/codec_comparators.h"
#include "media/base/codec_list.h"
#include "media/base/media_constants.h"
#include "media/base/media_engine.h"
#include "media/base/sdp_video_format_utils.h"
#include "pc/codec_configuration.h"
#include "pc/media_options.h"
#include "pc/rtp_media_utils.h"
#include "pc/session_description.h"
#include "pc/typed_codec_vendor.h"
#include "rtc_base/checks.h"
#include "rtc_base/containers/flat_map.h"
#include "rtc_base/logging.h"
#include "rtc_base/string_encode.h"
#include "rtc_base/strings/string_builder.h"
#include "rtc_base/thread.h"

#ifdef RTC_ENABLE_H265
#include "api/video_codecs/h265_profile_tier_level.h"
#endif

namespace webrtc {
namespace {

std::optional<PayloadType> PayloadTypeFromString(absl::string_view s) {
  int pt;
  if (FromString(s, &pt)) {
    return PayloadType::Create(pt);
  }
  return std::nullopt;
}

bool IsRtxCodec(const RtpCodecCapability& capability) {
  return absl::EqualsIgnoreCase(capability.name, kRtxCodecName);
}

bool IsRedCodec(const RtpCodecCapability& capability) {
  return absl::EqualsIgnoreCase(capability.name, kRedCodecName);
}

bool IsUlpfecCodec(const RtpCodecCapability& capability) {
  return absl::EqualsIgnoreCase(capability.name, kUlpfecCodecName);
}

bool IsFlexfecCodec(const RtpCodecCapability& capability) {
  return absl::EqualsIgnoreCase(capability.name, kFlexfecCodecName);
}

bool IsComfortNoiseCodec(const Codec& codec) {
  return absl::EqualsIgnoreCase(codec.name, kComfortNoiseCodecName);
}

// Wrapper for FindMatchingCodecs that uses CodecList
std::optional<Codec> FindMatchingCodec(const CodecList& codecs1,
                                       const CodecList& codecs2,
                                       const Codec& codec_to_match) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  return FindMatchingCodec(codecs1.codecs(), codecs2.codecs(), codec_to_match);
}

void StripCNCodecs(CodecList& audio_codecs) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  std::erase_if(audio_codecs.writable_codecs(),
                [](const Codec& codec) { return IsComfortNoiseCodec(codec); });
}

bool IsMediaContentOfType(const ContentInfo* content, MediaType media_type) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  if (!content || !content->media_description()) {
    return false;
  }
  return content->media_description()->type() == media_type;
}

// Returns the codecs of `current_content` if it is a media section with `mid`
// of type `media_type`, i.e. the section is not being recycled. Returns an
// empty list if there is no such section, and an error if the codecs of the
// section are inconsistent.
RTCErrorOr<CodecList> GetCurrentContentCodecs(
    const ContentInfo* current_content,
    absl::string_view mid,
    MediaType media_type) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  if (!current_content || current_content->mid() != mid ||
      !IsMediaContentOfType(current_content, media_type)) {
    return CodecList();
  }
  return CodecList::Create(current_content->media_description()->codecs());
}

// Find the codec in `codec_list` that `rtx_codec` is associated with.
const Codec* GetAssociatedCodecForRtx(const CodecList& codec_list,
                                      const Codec& rtx_codec) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  int associated_pt_int;
  if (!rtx_codec.GetParam(kCodecParamAssociatedPayloadType,
                          &associated_pt_int)) {
    RTC_LOG(LS_WARNING) << "RTX codec " << rtx_codec.id
                        << " is missing an associated payload type.";
    return nullptr;
  }

  std::optional<PayloadType> associated_pt =
      PayloadType::Create(associated_pt_int);
  if (!associated_pt) {
    RTC_LOG(LS_WARNING) << "Invalid payload type " << associated_pt_int
                        << " for RTX codec " << rtx_codec.id << ".";
    return nullptr;
  }

  // Find the associated codec for the RTX codec.
  const Codec* associated_codec =
      FindCodecById(codec_list.codecs(), *associated_pt);
  if (!associated_codec) {
    RTC_LOG(LS_WARNING) << "Couldn't find associated codec with payload type "
                        << associated_pt_int << " for RTX codec "
                        << rtx_codec.id << ".";
  }
  return associated_codec;
}

// Find the codecs in `codec_list` that `red_codec` is associated with.
// Returns a vector of pointers to the codecs on success.
const RTCErrorOr<std::vector<const Codec*>> GetAssociatedCodecsForRed(
    const CodecList& codec_list,
    const Codec& red_codec) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  std::string fmtp;
  std::vector<const Codec*> codecs;
  if (!red_codec.GetParam(kCodecParamNotInNameValueFormat, &fmtp)) {
    // Don't log for video/RED where this is normal.
    if (red_codec.type == Codec::Type::kAudio) {
      RTC_LOG(LS_WARNING)
          << "RED codec " << red_codec
          << " is missing an associated payload type parameter.";
      // There are also tests that assume this behavior for audio, so don't
      // return an error here either.
    }
    return codecs;  // empty list
  }

  std::vector<absl::string_view> redundant_payloads = split(fmtp, '/');
  if (redundant_payloads.size() < 2) {
    return codecs;  // empty
  }
  for (size_t index = 0; index < redundant_payloads.size(); ++index) {
    absl::string_view associated_pt_str = redundant_payloads[index];
    std::optional<PayloadType> associated_pt =
        PayloadTypeFromString(associated_pt_str);
    if (!associated_pt) {
      RTC_LOG(LS_WARNING) << "Couldn't convert payload type "
                          << associated_pt_str << " of RED codec " << red_codec
                          << " to a valid payload type.";
      return RTCError(RTCErrorType::INTERNAL_ERROR,
                      "RED codec with invalid payload type argument");
    }

    // Find the associated codec for the RED codec.
    const Codec* associated_codec =
        FindCodecById(codec_list.codecs(), *associated_pt);
    if (!associated_codec) {
      RTC_LOG(LS_WARNING) << "Couldn't find associated codec with payload type "
                          << associated_pt_str << " for RED codec " << red_codec
                          << ".";
      return RTCError(RTCErrorType::INTERNAL_ERROR,
                      "RED codec pointing to nonexistent PT");
    }
    codecs.push_back(associated_codec);
  }
  return codecs;
}

// Adds `codec` to `codecs` unless a codec with the same payload type is
// already present. A payload type that is used by a different codec means
// that codecs from different payload type mappings have been combined, which
// is an error in the payload type assignment rather than something an
// application can cause.
RTCError PushCodec(CodecList& codecs, const Codec& codec) {
  if (codecs.PushIfNotPresent(codec) == CodecList::PushResult::kConflict) {
    // The codec is left out of the list. Since this indicates an internal
    // error, assert in debug builds.
    // TODO: https://issues.webrtc.org/455503439 - consider reporting the
    // conflict to the caller, so that the condition is also handled in
    // release builds:
    // return RTCError(RTCErrorType::INTERNAL_ERROR, "Payload type conflict");
    RTC_DCHECK_NOTREACHED()
        << "Conflicting payload type assignment. Could not add " << codec
        << " since payload type " << codec.id
        << " is already used by a different codec.";
  }
  return RTCError::OK();
}

// Returns the codecs of `codecs` that are also in `offerable_codecs`, in the
// order they appear in `codecs`. Both lists use the payload types that the
// media engine assigned, so the codecs can be compared directly.
CodecList KeepOfferableCodecs(const CodecList& codecs,
                              const CodecList& offerable_codecs) {
  CodecList kept;
  for (const Codec& codec : codecs) {
    if (FindMatchingCodec(codecs, offerable_codecs, codec).has_value()) {
      kept.push_back(codec);
    }
  }
  return kept;
}

RTCError MergeRtxCodec(const CodecConfiguration& config,
                       const Codec& primary_codec,
                       absl::string_view mid,
                       CodecList& offered_codecs,
                       PayloadTypeSuggester& pt_suggester,
                       bool pick_from_top_of_range) {
  if (!config.resiliency.rtx) {
    return RTCError::OK();
  }
  auto rtx_it = absl::c_find_if(offered_codecs, [&](const Codec& c) {
    if (c.name != kRtxCodecName)
      return false;
    int apt;
    return c.GetParam(kCodecParamAssociatedPayloadType, &apt) &&
           apt == primary_codec.id.value();
  });
  if (rtx_it == offered_codecs.end()) {
    Codec rtx = (config.codec.type == Codec::Type::kAudio)
                    ? CreateAudioCodec({kRtxCodecName, config.codec.clockrate,
                                        config.codec.channels})
                    : CreateVideoCodec(PayloadType::NotSet(), kRtxCodecName);
    rtx.SetParam(kCodecParamAssociatedPayloadType, primary_codec.id.value());
    // Convention: RTX PT = primary PT + 1.
    // Suggester will ignore this if it is already in use or invalid.
    PayloadType preferred_id = PayloadType(primary_codec.id.value() + 1);
    if (preferred_id.Valid(/*rtcp_mux=*/true)) {
      rtx.id = preferred_id;
    }
    RTCErrorOr<PayloadType> result =
        pt_suggester.SuggestPayloadType(mid, rtx, pick_from_top_of_range);
    if (!result.ok()) {
      return result.MoveError();
    }
    rtx.id = result.value();
    offered_codecs.push_back(rtx);
  }
  return RTCError::OK();
}

RTCError MergeRedCodec(const CodecConfiguration& config,
                       const Codec& primary_codec,
                       absl::string_view mid,
                       CodecList& offered_codecs,
                       PayloadTypeSuggester& pt_suggester,
                       bool pick_from_top_of_range) {
  if (!config.resiliency.red) {
    return RTCError::OK();
  }
  auto red_it = absl::c_find_if(offered_codecs, [&](const Codec& c) {
    return c.name == kRedCodecName && c.type == config.codec.type;
  });

  Codec red;
  bool newly_created = false;
  if (red_it == offered_codecs.end()) {
    red = (config.codec.type == Codec::Type::kAudio)
              ? CreateAudioCodec({kRedCodecName, 48000, 2})
              : CreateVideoCodec(kRedCodecName);
    RTCErrorOr<PayloadType> result =
        pt_suggester.SuggestPayloadType(mid, red, pick_from_top_of_range);
    if (!result.ok()) {
      return result.MoveError();
    }
    red.id = result.value();
    newly_created = true;
  } else {
    red = *red_it;
  }

  if (config.codec.type == Codec::Type::kAudio &&
      absl::EqualsIgnoreCase(config.codec.name, kOpusCodecName)) {
    if (red.params.empty()) {
      StringBuilder param;
      // Opus RED uses Opus as both the primary payload and
      // the redundancy payload, with different timestamp offsets.
      param << primary_codec.id.value() << "/" << primary_codec.id.value();
      red.SetParam(kCodecParamNotInNameValueFormat, param.Release());
    }
  }

  if (newly_created) {
    offered_codecs.push_back(red);

    if (config.codec.type == Codec::Type::kVideo) {
      // Video RED also gets an RTX codec.
      Codec red_rtx = CreateVideoCodec(PayloadType::NotSet(), kRtxCodecName);
      red_rtx.SetParam(kCodecParamAssociatedPayloadType, red.id.value());
      RTCErrorOr<PayloadType> rtx_res =
          pt_suggester.SuggestPayloadType(mid, red_rtx, pick_from_top_of_range);
      if (rtx_res.ok()) {
        red_rtx.id = rtx_res.value();
        offered_codecs.push_back(red_rtx);
      } else {
        // If error is RESOURCE_EXHAUSTED, we ran out of PT numbers.
        // In that case, we can ignore the codec altogether.
        RTC_LOG(LS_WARNING)
            << "Error when assigning RTX codec to RED codec:" << rtx_res;
        // In debug mode, check that it's the expected error code.
        RTC_DCHECK(rtx_res.error().type() == RTCErrorType::RESOURCE_EXHAUSTED);
      }
    }
  } else if (red_it != offered_codecs.end()) {
    // Update the codec in the list if it was modified
    *red_it = red;
  }
  return RTCError::OK();
}

RTCError MergeUlpfecCodec(const CodecConfiguration& config,
                          absl::string_view mid,
                          CodecList& offered_codecs,
                          PayloadTypeSuggester& pt_suggester,
                          bool pick_from_top_of_range) {
  if (!config.resiliency.ulpfec || config.codec.type != Codec::Type::kVideo) {
    return RTCError::OK();
  }
  auto fec_it = absl::c_find_if(offered_codecs, [&](const Codec& c) {
    return c.name == kUlpfecCodecName;
  });
  if (fec_it == offered_codecs.end()) {
    Codec fec = CreateVideoCodec(kUlpfecCodecName);
    RTCErrorOr<PayloadType> result =
        pt_suggester.SuggestPayloadType(mid, fec, pick_from_top_of_range);
    if (!result.ok()) {
      return result.MoveError();
    }
    fec.id = result.value();
    offered_codecs.push_back(fec);
  }
  return RTCError::OK();
}

RTCError MergeFlexfecCodec(const CodecConfiguration& config,
                           absl::string_view mid,
                           CodecList& offered_codecs,
                           PayloadTypeSuggester& pt_suggester,
                           const FieldTrialsView& trials,
                           bool pick_from_top_of_range) {
  if (!config.resiliency.flexfec || config.codec.type != Codec::Type::kVideo) {
    return RTCError::OK();
  }
  auto fec_it = absl::c_find_if(offered_codecs, [&](const Codec& c) {
    return c.name == kFlexfecCodecName;
  });
  if (fec_it == offered_codecs.end()) {
    Codec fec = CreateVideoCodec(kFlexfecCodecName);
    RTCErrorOr<PayloadType> result =
        pt_suggester.SuggestPayloadType(mid, fec, pick_from_top_of_range);
    if (!result.ok()) {
      return result.MoveError();
    }
    fec.id = result.value();
    offered_codecs.push_back(fec);
  }
  return RTCError::OK();
}

// Adds all codecs from `configurations` to `offered_codecs` that don't
// already exist in `offered_codecs` and ensure the payload types don't
// collide.
RTCError MergeCodecsFromConfigurations(
    const std::vector<CodecConfiguration>& configurations,
    absl::string_view mid,
    CodecList& offered_codecs,
    PayloadTypeSuggester& pt_suggester,
    const FieldTrialsView& trials,
    bool pick_from_top_of_range = false) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();

  // Pass 1: Primary and RTX
  for (const CodecConfiguration& config : configurations) {
    // 1. Find or add primary codec
    auto primary_it = absl::c_find_if(offered_codecs, [&](const Codec& c) {
      return MatchesWithCodecRules(config.codec, c);
    });
    Codec primary_codec;
    if (primary_it == offered_codecs.end()) {
      primary_codec = config.codec;
      RTCErrorOr<PayloadType> result = pt_suggester.SuggestPayloadType(
          mid, primary_codec, pick_from_top_of_range);
      if (!result.ok()) {
        return result.MoveError();
      }
      primary_codec.id = result.value();
      RTCError error = PushCodec(offered_codecs, primary_codec);
      if (!error.ok()) {
        return error;
      }
    } else {
      primary_codec = *primary_it;
    }

    // 2. Handle RTX
    RTCError error = MergeRtxCodec(config, primary_codec, mid, offered_codecs,
                                   pt_suggester, pick_from_top_of_range);
    if (!error.ok()) {
      return error;
    }

    // Handle Audio RED immediately after the primary codec.
    if (config.codec.type == Codec::Type::kAudio && config.resiliency.red) {
      error = MergeRedCodec(config, primary_codec, mid, offered_codecs,
                            pt_suggester, pick_from_top_of_range);
      if (!error.ok()) {
        return error;
      }
    }
  }

  // Pass 2: RED for Video
  for (const CodecConfiguration& config : configurations) {
    if (!config.resiliency.red || config.codec.type == Codec::Type::kAudio) {
      continue;
    }
    // Find the primary codec in offered_codecs to pass to MergeRedCodec.
    auto primary_it = absl::c_find_if(offered_codecs, [&](const Codec& c) {
      return MatchesWithCodecRules(config.codec, c);
    });
    RTC_DCHECK(primary_it != offered_codecs.end());

    RTCError error = MergeRedCodec(config, *primary_it, mid, offered_codecs,
                                   pt_suggester, pick_from_top_of_range);
    if (!error.ok()) {
      return error;
    }
  }

  // Pass 3: FEC (ULPFEC + FlexFEC)
  for (const CodecConfiguration& config : configurations) {
    // 4. Handle ULPFEC
    RTCError error = MergeUlpfecCodec(config, mid, offered_codecs, pt_suggester,
                                      pick_from_top_of_range);
    if (!error.ok()) {
      return error;
    }

    // 5. Handle FlexFEC
    error = MergeFlexfecCodec(config, mid, offered_codecs, pt_suggester, trials,
                              pick_from_top_of_range);
    if (!error.ok()) {
      return error;
    }
  }

  return RTCError::OK();
}

// Adds all codecs from `reference_codecs` to `offered_codecs` that don't
// already exist in `offered_codecs` and ensure the payload types don't
// collide.
RTCError MergeCodecsLegacy(const CodecList& reference_codecs,
                           absl::string_view mid,
                           CodecList& offered_codecs,
                           PayloadTypeSuggester& pt_suggester,
                           bool pick_from_top_of_range = false) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  // Add all new codecs that are not RTX/RED codecs.
  // The two-pass splitting of the loops means preferring payload types
  // of actual codecs with respect to collisions.
  for (const Codec& reference_codec : reference_codecs) {
    if (reference_codec.GetResiliencyType() != Codec::ResiliencyType::kRtx &&
        reference_codec.GetResiliencyType() != Codec::ResiliencyType::kRed &&
        !FindMatchingCodec(reference_codecs, offered_codecs, reference_codec)) {
      Codec codec = reference_codec;
      RTCErrorOr<PayloadType> suggestion =
          pt_suggester.SuggestPayloadType(mid, codec, pick_from_top_of_range);
      if (!suggestion.ok()) {
        return suggestion.MoveError();
      }
      codec.id = suggestion.value();
      RTCError error = PushCodec(offered_codecs, codec);
      if (!error.ok()) {
        return error;
      }
    }
  }

  // Add all new RTX or RED codecs.
  for (const Codec& reference_codec : reference_codecs) {
    if (reference_codec.GetResiliencyType() == Codec::ResiliencyType::kRtx &&
        !FindMatchingCodec(reference_codecs, offered_codecs, reference_codec)) {
      Codec rtx_codec = reference_codec;
      const Codec* associated_codec =
          GetAssociatedCodecForRtx(reference_codecs, rtx_codec);
      if (!associated_codec) {
        continue;
      }
      // Find a codec in the offered list that matches the reference codec.
      // Its payload type may be different than the reference codec.
      std::optional<Codec> matching_codec = FindMatchingCodec(
          reference_codecs, offered_codecs, *associated_codec);
      if (!matching_codec) {
        RTC_LOG(LS_WARNING)
            << "Couldn't find matching " << associated_codec->name << " codec.";
        continue;
      }

      rtx_codec.SetParam(kCodecParamAssociatedPayloadType,
                         matching_codec->id.value());
      RTCErrorOr<PayloadType> suggestion = pt_suggester.SuggestPayloadType(
          mid, rtx_codec, pick_from_top_of_range);
      if (!suggestion.ok()) {
        return suggestion.MoveError();
      }
      rtx_codec.id = suggestion.value();
      RTCError error = PushCodec(offered_codecs, rtx_codec);
      if (!error.ok()) {
        return error;
      }
    } else if (reference_codec.GetResiliencyType() ==
                   Codec::ResiliencyType::kRed &&
               !FindMatchingCodec(reference_codecs, offered_codecs,
                                  reference_codec)) {
      Codec red_codec = reference_codec;
      RTCErrorOr<std::vector<const Codec*>> associated_codecs =
          GetAssociatedCodecsForRed(reference_codecs, red_codec);
      if (!associated_codecs.ok()) {
        return associated_codecs.MoveError();
      }
      if (associated_codecs.value().empty()) {
        // No parameter. Just blindly add the codec.
        // This is known to be used with video, but not with audio.
        if (red_codec.type == Codec::Type::kAudio) {
          RTC_LOG(LS_WARNING)
              << "RED audio codec with no associated codecs found: "
              << red_codec;
        }
        RTCErrorOr<PayloadType> suggestion = pt_suggester.SuggestPayloadType(
            mid, red_codec, pick_from_top_of_range);
        if (!suggestion.ok()) {
          return suggestion.MoveError();
        }
        red_codec.id = suggestion.value();
        RTCError error = PushCodec(offered_codecs, red_codec);
        if (!error.ok()) {
          return error;
        }
        continue;
      }
      if (associated_codecs.value().size() < 2) {
        RTC_LOG(LS_WARNING)
            << "RED codec with only one valid associated codec ignored: "
            << red_codec;
        continue;
      }
      StringBuilder sb;
      for (const Codec* associated_codec : associated_codecs.value()) {
        std::optional<Codec> matching_codec = FindMatchingCodec(
            reference_codecs, offered_codecs, *associated_codec);
        if (!matching_codec) {
          // This should always succeed, because the associated codecs
          // were looked up in reference_codecs, and all reference
          // codecs were added in the first loop of the function.
          RTC_LOG(LS_WARNING) << "Couldn't find matching "
                              << associated_codec->name << " codec.";
          // TODO: https://issues.webrtc.org/455503439 - consider CHECK
          RTC_DCHECK_NOTREACHED();
          return RTCError(RTCErrorType::INTERNAL_ERROR,
                          "RED payload type lookup failed");
        }
        if (sb.size() > 0) {
          sb << "/";
        }
        sb << matching_codec->id;
      }
      red_codec.SetParam(kCodecParamNotInNameValueFormat, sb.Release());
      RTCErrorOr<PayloadType> suggestion = pt_suggester.SuggestPayloadType(
          mid, red_codec, pick_from_top_of_range);
      if (!suggestion.ok()) {
        return suggestion.MoveError();
      }
      red_codec.id = suggestion.value();
      RTCError error = PushCodec(offered_codecs, red_codec);
      if (!error.ok()) {
        return error;
      }
    }
  }

  offered_codecs.CheckConsistency();
  return RTCError::OK();
}

// `codecs` is a full list of codecs with correct payload type mappings, which
// don't conflict with mappings of the other media type; `supported_codecs` is
// a list filtered for the media section`s direction but with default payload
// types.
// static
CodecList MatchCodecPreference(
    const std::vector<RtpCodecCapability>& codec_preferences,
    const CodecList& codecs,
    const CodecList& supported_codecs) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  CodecList filtered_codecs;
  bool want_rtx = false;
  bool want_red = false;

  for (const RtpCodecCapability& codec_preference : codec_preferences) {
    if (IsRtxCodec(codec_preference)) {
      want_rtx = true;
    } else if (IsRedCodec(codec_preference)) {
      want_red = true;
    }
  }
  bool red_was_added = false;
  for (const RtpCodecCapability& codec_preference : codec_preferences) {
    auto found_codec = absl::c_find_if(
        supported_codecs, [&codec_preference](const Codec& codec) {
          // We should not filter out the codec in |codec_preferences| if it
          // has a higher level than the codec in |supported_codecs|, as the
          // codec in |supported_codecs| may be only with lower level in
          // |send_codecs_| and |recv_codecs_| for the same codec.
          return IsSameRtpCodecIgnoringLevel(codec, codec_preference);
        });

    if (found_codec != supported_codecs.end()) {
      std::optional<Codec> found_codec_with_correct_pt =
          FindMatchingCodec(supported_codecs, codecs, *found_codec);
      if (found_codec_with_correct_pt) {
        // RED may already have been added if its primary codec is before RED
        // in the codec list.
        bool is_red_codec = found_codec_with_correct_pt->GetResiliencyType() ==
                            Codec::ResiliencyType::kRed;
        if (!is_red_codec || !red_was_added) {
          filtered_codecs.push_back(*found_codec_with_correct_pt);
          red_was_added = is_red_codec ? true : red_was_added;
        }
        PayloadType id = found_codec_with_correct_pt->id;
        // Search for the matching rtx or red codec.
        if (want_red || want_rtx) {
          for (const Codec& codec : codecs) {
            if (want_rtx &&
                codec.GetResiliencyType() == Codec::ResiliencyType::kRtx) {
              int apt;
              if (codec.GetParam(kCodecParamAssociatedPayloadType, &apt) &&
                  apt == id.value()) {
                filtered_codecs.push_back(codec);
                break;
              }
            } else if (want_red && codec.GetResiliencyType() ==
                                       Codec::ResiliencyType::kRed) {
              // For RED, do not insert the codec again if it was already
              // inserted. audio/red for opus gets enabled by having RED before
              // the primary codec.
              auto fmtp = codec.params.find(
                  std::string(kCodecParamNotInNameValueFormat));
              if (fmtp != codec.params.end()) {
                std::vector<absl::string_view> redundant_payloads =
                    split(fmtp->second, '/');
                int first_redundant_pt;
                if (!redundant_payloads.empty() &&
                    FromString(redundant_payloads[0], &first_redundant_pt) &&
                    first_redundant_pt == id.value()) {
                  if (!red_was_added) {
                    filtered_codecs.push_back(codec);
                    red_was_added = true;
                  }
                  break;
                }
              }
            }
          }
        }
      }
    }
  }

  return filtered_codecs;
}

void NegotiatePacketization(const Codec& local_codec,
                            const Codec& remote_codec,
                            Codec* negotiated_codec) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  negotiated_codec->packetization =
      (local_codec.packetization == remote_codec.packetization)
          ? local_codec.packetization
          : std::nullopt;
}

#ifdef RTC_ENABLE_H265
void NegotiateTxMode(const Codec& local_codec,
                     const Codec& remote_codec,
                     Codec* negotiated_codec) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  negotiated_codec->tx_mode = (local_codec.tx_mode == remote_codec.tx_mode)
                                  ? local_codec.tx_mode
                                  : std::nullopt;
}
#endif

// For offer, negotiated codec must have the same level-id as that in
// |supported_codecs| with same profile.
void NegotiateVideoCodecLevelsForOffer(
    const MediaDescriptionOptions& media_description_options,
    const CodecList& supported_codecs,
    CodecList& filtered_codecs) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  if (filtered_codecs.empty() || supported_codecs.empty()) {
    return;
  }

  // TODO(http://crbugs.com/376306259): We should handle level-idx for AV1.
  // Ideally this should be done for all codecs, but RFCs of other codecs
  // do not clear define the expected behavior for the level in the offer.
#ifdef RTC_ENABLE_H265
  if (media_description_options.type == MediaType::VIDEO) {
    flat_map<H265Profile, H265Level> supported_h265_profiles;
    // The assumption here is that H.265 codecs with the same profile and tier
    // are already with highest level for that profile in both
    // |supported_codecs| and |filtered_codecs|.
    for (const Codec& supported_codec : supported_codecs) {
      if (absl::EqualsIgnoreCase(supported_codec.name, kH265CodecName)) {
        std::optional<H265ProfileTierLevel> supported_ptl =
            ParseSdpForH265ProfileTierLevel(supported_codec.params);
        if (supported_ptl.has_value()) {
          supported_h265_profiles[supported_ptl->profile] =
              supported_ptl->level;
        }
      }
    }

    if (supported_h265_profiles.empty()) {
      return;
    }

    for (Codec& filtered_codec : filtered_codecs) {
      if (absl::EqualsIgnoreCase(filtered_codec.name, kH265CodecName)) {
        std::optional<H265ProfileTierLevel> filtered_ptl =
            ParseSdpForH265ProfileTierLevel(filtered_codec.params);
        if (filtered_ptl.has_value()) {
          auto it = supported_h265_profiles.find(filtered_ptl->profile);

          if (it != supported_h265_profiles.end() &&
              filtered_ptl->level != it->second) {
            filtered_codec.SetParam(kH265FmtpLevelId,
                                    H265LevelToString(it->second));
          }
        }
      }
    }
  }
#endif
}

RTCError NegotiateCodecs(const CodecList& local_codecs,
                         const CodecList& offered_codecs,
                         CodecList& negotiated_codecs_out,
                         bool keep_offer_order,
                         bool payload_types_in_transport) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  flat_map<PayloadType, PayloadType> pt_mapping_table;
  // Since we build the negotiated codec list one entry at a time,
  // the list will have inconsistencies during building.
  std::vector<Codec> negotiated_codecs;
  for (const Codec& ours : local_codecs) {
    std::optional<Codec> theirs =
        FindMatchingCodec(local_codecs, offered_codecs, ours);
    // Note that we intentionally only find one matching codec for each of our
    // local codecs, in case the remote offer contains duplicate codecs.
    if (theirs) {
      Codec negotiated = ours;
      NegotiatePacketization(ours, *theirs, &negotiated);
      negotiated.IntersectFeedbackParams(*theirs);
      if (negotiated.GetResiliencyType() == Codec::ResiliencyType::kRtx) {
        // We support parsing the declarative rtx-time parameter.
        const auto rtx_time_it =
            theirs->params.find(std::string(kCodecParamRtxTime));
        if (rtx_time_it != theirs->params.end()) {
          negotiated.SetParam(kCodecParamRtxTime, rtx_time_it->second);
        }
      } else if (negotiated.GetResiliencyType() ==
                 Codec::ResiliencyType::kRed) {
        const auto red_it =
            theirs->params.find(std::string(kCodecParamNotInNameValueFormat));
        if (red_it != theirs->params.end()) {
          negotiated.SetParam(kCodecParamNotInNameValueFormat, red_it->second);
        }
      }
      if (absl::EqualsIgnoreCase(ours.name, kH264CodecName)) {
        H264GenerateProfileLevelIdForAnswer(ours.params, theirs->params,
                                            &negotiated.params);
      }
#ifdef RTC_ENABLE_H265
      if (absl::EqualsIgnoreCase(ours.name, kH265CodecName)) {
        H265GenerateProfileTierLevelForAnswer(ours.params, theirs->params,
                                              &negotiated.params);
        NegotiateTxMode(ours, *theirs, &negotiated);
      }
#endif
      // Use their ID, if available.
      pt_mapping_table.insert({negotiated.id, theirs->id});
      negotiated.id = theirs->id;
      negotiated.name = theirs->name;
      negotiated_codecs.push_back(std::move(negotiated));
    }
  }
  // Fix up apt parameters that point to other PTs.
  for (Codec& negotiated : negotiated_codecs) {
    if (negotiated.GetResiliencyType() == Codec::ResiliencyType::kRtx) {
      // Change the apt value according to the pt mapping table.
      // This avoids changing to apt values that don't exist any more.
      int apt_int;
      if (!negotiated.GetParam(kCodecParamAssociatedPayloadType, &apt_int)) {
        RTC_LOG(LS_WARNING) << "No apt value";
        continue;
      }
      PayloadType apt_value(apt_int);
      if (!pt_mapping_table.contains(apt_value)) {
        if (!payload_types_in_transport) {
          RTC_LOG(LS_WARNING) << "Unmapped apt value " << apt_value;
          continue;
        }
      }
      if (pt_mapping_table.contains(apt_value)) {
        negotiated.SetParam(kCodecParamAssociatedPayloadType,
                            pt_mapping_table.at(apt_value).value());
      }
    }
  }
  if (keep_offer_order) {
    // RFC3264: Although the answerer MAY list the formats in their desired
    // order of preference, it is RECOMMENDED that unless there is a
    // specific reason, the answerer list formats in the same relative order
    // they were present in the offer.
    // This can be skipped when the transceiver has any codec preferences.
    flat_map<PayloadType, int> payload_type_preferences;
    int preference = static_cast<int>(offered_codecs.size() + 1);
    for (const Codec& codec : offered_codecs) {
      payload_type_preferences[codec.id] = preference--;
    }
    absl::c_sort(negotiated_codecs, [&payload_type_preferences](
                                        const Codec& a, const Codec& b) {
      return payload_type_preferences[a.id] > payload_type_preferences[b.id];
    });
  }
  RTCErrorOr<CodecList> result = CodecList::Create(negotiated_codecs);
  if (!result.ok()) {
    return result.MoveError();
  }
  negotiated_codecs_out = result.MoveValue();
  return RTCError::OK();
}

// If there is a RED codec without its fmtp parameter, give it the ID of the
// first OPUS codec in the codec list.
void LinkRed(std::vector<Codec>& codecs) {
  int first_opus_pt = Codec::kIdNotSet;
  for (const Codec& codec : codecs) {
    if (codec.type == Codec::Type::kAudio &&
        absl::EqualsIgnoreCase(codec.name, kOpusCodecName)) {
      first_opus_pt = codec.id.value();
      break;
    }
  }

  if (first_opus_pt != Codec::kIdNotSet) {
    for (Codec& codec : codecs) {
      if (codec.type == Codec::Type::kAudio &&
          absl::EqualsIgnoreCase(codec.name, kRedCodecName)) {
        if (codec.params.empty()) {
          StringBuilder param;
          // Opus RED uses Opus as both the primary payload and
          // the redundancy payload, with different timestamp offsets.
          param << first_opus_pt << "/" << first_opus_pt;
          codec.SetParam(kCodecParamNotInNameValueFormat, param.Release());
        }
      }
    }
  }
}

// Removes audio RED codecs whose redundancy (primary) payload types are all
// absent from the codec list. This mirrors how an RTX codec is dropped when its
// associated (apt) codec is not offered: keeping such a RED codec would emit an
// a=fmtp:<red> <pt>/<pt> that references a payload type not present in the m=
// section (a dangling primary). Happens e.g. when opus is removed from an audio
// transceiver's codec preferences while RED is kept.
void RemoveRedCodecsWithoutPrimary(std::vector<Codec>& codecs) {
  std::set<int> present_pts;
  for (const Codec& codec : codecs) {
    if (codec.id.IsSet()) {
      present_pts.insert(codec.id.value());
    }
  }
  std::erase_if(codecs, [&present_pts](const Codec& codec) {
    if (codec.type != Codec::Type::kAudio ||
        codec.GetResiliencyType() != Codec::ResiliencyType::kRed) {
      return false;
    }
    std::string fmtp;
    if (!codec.GetParam(kCodecParamNotInNameValueFormat, &fmtp)) {
      // No redundancy parameter to point at a primary which is what
      // video+red uses.
      return false;
    }
    for (absl::string_view pt_str : split(fmtp, '/')) {
      int pt;
      if (FromString(pt_str, &pt) && present_pts.contains(pt)) {
        return false;  // A wrapped primary is still offered; keep RED.
      }
    }
    return true;  // None of the wrapped primaries remain; drop RED.
  });
}

// Returns the codecs of `codecs` that can end up in an offer or answer: the
// ones that match `codec_preferences` (all if empty), without comfort noise
// unless `vad_enabled` and without RED codecs whose primary codec is missing.
CodecList GetNegotiableCodecs(
    const CodecList& codecs,
    const std::vector<RtpCodecCapability>& codec_preferences,
    MediaType type,
    bool vad_enabled) {
  CodecList negotiable_codecs =
      codec_preferences.empty()
          ? codecs
          : MatchCodecPreference(codec_preferences, codecs, codecs);
  if (type == MediaType::AUDIO && !vad_enabled) {
    StripCNCodecs(negotiable_codecs);
  }
  RemoveRedCodecsWithoutPrimary(negotiable_codecs.writable_codecs());
  return negotiable_codecs;
}

// Update the ID fields of the codec vector in the legacy path
// (payload_types_in_transport_ = false).
// If any codec has an ID with value "kIdNotSet", this is an error.
RTCError RecordCodecIdsAndLinkRed(PayloadTypeSuggester& pt_suggester,
                                  const std::string& mid,
                                  std::vector<Codec>& codecs) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  for (Codec& codec : codecs) {
    RTC_DCHECK(codec.id != PayloadType::NotSet());
    pt_suggester.AddLocalMapping(mid, codec.id, codec);
  }

  LinkRed(codecs);
  return RTCError::OK();
}

// Update the ID fields of the codec vector in the redesigned path.
// If any codec has an ID with value "kIdNotSet", use the payload type suggester
// to assign and record a payload type for it.
RTCError AssignCodecIdsAndLinkRedRefactored(
    PayloadTypeSuggester& pt_suggester,
    const std::string& mid,
    std::vector<Codec>& codecs,
    bool pick_from_top_of_range = false) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  Codec* last_codec_that_can_have_rtx = nullptr;

  for (Codec& codec : codecs) {
    if (codec.id == PayloadType::NotSet()) {
      RTCErrorOr<PayloadType> result =
          pt_suggester.SuggestPayloadType(mid, codec, pick_from_top_of_range);
      if (!result.ok()) {
        return result.error();
      }
      codec.id = result.value();
    } else {
      pt_suggester.AddLocalMapping(mid, codec.id, codec);
    }

    if (codec.GetResiliencyType() == Codec::ResiliencyType::kRtx) {
      if (last_codec_that_can_have_rtx &&
          codec.params.find(kCodecParamAssociatedPayloadType) ==
              codec.params.end()) {
        // When new codecs are added that want RTX, they're added as a pair
        // of (media codec, rtx, codec). But since they don't have assigned
        // IDs yet, the apt parameter can't be set. This logic ensures that
        // the apt parameter points to the immediately preceding codec.
        codec.SetParam(kCodecParamAssociatedPayloadType,
                       last_codec_that_can_have_rtx->id.value());
      }
    } else {
      // This is a codec that can potentially be associated with an RTX codec.
      last_codec_that_can_have_rtx = &codec;
    }
  }

  return RTCError::OK();
}

// Returns the configurations of `configs` that survive `preferences` and, for
// audio, the comfort noise setting. Codecs that the transceiver's codec
// preferences rule out never reach the offer or answer, so they must not be
// assigned a payload type either. Resiliency mechanisms are only kept if they
// too are in the preferences, for the same reason.
std::vector<CodecConfiguration> FilterCodecConfigurations(
    std::span<const CodecConfiguration> configs,
    std::span<const RtpCodecCapability> preferences,
    bool vad_enabled) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  bool want_rtx = false;
  bool want_red = false;
  bool want_ulpfec = false;
  bool want_flexfec = false;
  for (const RtpCodecCapability& preference : preferences) {
    if (IsRtxCodec(preference)) {
      want_rtx = true;
    } else if (IsRedCodec(preference)) {
      want_red = true;
    } else if (IsUlpfecCodec(preference)) {
      want_ulpfec = true;
    } else if (IsFlexfecCodec(preference)) {
      want_flexfec = true;
    }
  }

  std::vector<CodecConfiguration> filtered;
  for (const CodecConfiguration& config : configs) {
    if (!vad_enabled && IsComfortNoiseCodec(config.codec)) {
      continue;
    }
    if (!preferences.empty() &&
        // We should not filter out the codec in `preferences` if it has a
        // higher level than the codec in `configs`, as the codec in `configs`
        // may be only with lower level for the same codec.
        !absl::c_any_of(
            preferences, [&config](const RtpCodecCapability& preference) {
              return IsSameRtpCodecIgnoringLevel(config.codec, preference);
            })) {
      continue;
    }
    CodecConfiguration& kept_config = filtered.emplace_back(config);
    if (!preferences.empty()) {
      if (!want_rtx) {
        kept_config.resiliency.rtx = false;
      }
      if (!want_red) {
        kept_config.resiliency.red = false;
      }
      if (!want_ulpfec) {
        kept_config.resiliency.ulpfec = false;
      }
      if (!want_flexfec) {
        kept_config.resiliency.flexfec = false;
      }
    }
  }
  return filtered;
}

// Constructs the list of codecs that exist both in the send and receive codec
// lists. We expect that these lists are equal most of the time, with some
// codecs only in the receive configs. When there are multiple instances of the
// same codec, with different parameters, we want all the versions of the
// codec that are in the send configuration, since receive configurations are
// often more expansive.
// TODO: issues.webrtc.org/514760523 - write tests to verify outcomes.
std::vector<CodecConfiguration> IntersectCodecConfigurations(
    const std::vector<CodecConfiguration>& send_configs,
    const std::vector<CodecConfiguration>& recv_configs) {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  std::vector<CodecConfiguration> intersected;
  for (const CodecConfiguration& send_config : send_configs) {
    for (const CodecConfiguration& recv_config : recv_configs) {
      if (!MatchesWithCodecRules(send_config.codec, recv_config.codec)) {
        continue;
      }
      CodecConfiguration merged_config = recv_config;
      merged_config.codec.IntersectFeedbackParams(send_config.codec);
      NegotiatePacketization(send_config.codec, recv_config.codec,
                             &merged_config.codec);
      merged_config.resiliency.red =
          send_config.resiliency.red && recv_config.resiliency.red;
      merged_config.resiliency.ulpfec =
          send_config.resiliency.ulpfec && recv_config.resiliency.ulpfec;
      merged_config.resiliency.flexfec =
          send_config.resiliency.flexfec && recv_config.resiliency.flexfec;
      merged_config.resiliency.rtx =
          send_config.resiliency.rtx && recv_config.resiliency.rtx;
      if (absl::EqualsIgnoreCase(send_config.codec.name, kH264CodecName)) {
        H264GenerateProfileLevelIdForAnswer(send_config.codec.params,
                                            recv_config.codec.params,
                                            &merged_config.codec.params);
      }
#ifdef RTC_ENABLE_H265
      if (absl::EqualsIgnoreCase(send_config.codec.name, kH265CodecName)) {
        H265GenerateProfileTierLevelForAnswer(send_config.codec.params,
                                              recv_config.codec.params,
                                              &merged_config.codec.params);
        NegotiateTxMode(send_config.codec, recv_config.codec,
                        &merged_config.codec);
      }
#endif
      intersected.push_back(std::move(merged_config));
      break;
    }
  }
  return intersected;
}

}  // namespace

// Exposed for testing
RTCError MergeCodecsForTesting(const CodecList& reference_codecs,
                               absl::string_view mid,
                               CodecList& offered_codecs,
                               PayloadTypeSuggester& pt_suggester,
                               bool pick_from_top_of_range) {
  // This function is only available for testing the legacy path.
  return MergeCodecsLegacy(reference_codecs, mid, offered_codecs, pt_suggester,
                           pick_from_top_of_range);
}

RTCError CodecVendor::MergeAndAssignConfiguredCodecs(
    MediaType type,
    RtpTransceiverDirection direction,
    absl::string_view mid,
    CodecList& codecs_out,
    PayloadTypeSuggester& pt_suggester,
    bool pick_from_top_of_range,
    std::span<const RtpCodecCapability> codec_preferences,
    bool vad_enabled) {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  const std::vector<CodecConfiguration>& send_configs =
      (type == MediaType::AUDIO) ? audio_send_codecs_.configurations()
                                 : video_send_codecs_.configurations();
  const std::vector<CodecConfiguration>& recv_configs =
      (type == MediaType::AUDIO) ? audio_recv_codecs_.configurations()
                                 : video_recv_codecs_.configurations();
  // `intersected` owns the configurations that `configs` refers to in the
  // bidirectional case; in the other cases `configs` refers to the lists of
  // the media engine, which outlive this function.
  std::vector<CodecConfiguration> intersected;
  std::span<const CodecConfiguration> configs;
  switch (direction) {
    case RtpTransceiverDirection::kSendRecv:
    case RtpTransceiverDirection::kStopped:
    case RtpTransceiverDirection::kInactive:
      intersected = IntersectCodecConfigurations(send_configs, recv_configs);
      configs = intersected;
      break;
    case RtpTransceiverDirection::kSendOnly:
      configs = send_configs;
      break;
    case RtpTransceiverDirection::kRecvOnly:
      configs = recv_configs;
      break;
  }
  return MergeCodecsFromConfigurations(
      FilterCodecConfigurations(configs, codec_preferences, vad_enabled), mid,
      codecs_out, pt_suggester, trials_, pick_from_top_of_range);
}

RTCErrorOr<std::vector<Codec>> CodecVendor::GetNegotiatedCodecsForOffer(
    const MediaDescriptionOptions& media_description_options,
    const MediaSessionOptions& session_options,
    const ContentInfo* current_content,
    PayloadTypeSuggester& pt_suggester) {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();

  std::string mid = media_description_options.mid;
  CodecList codecs;
  CodecList supported_codecs;

  // Codecs from a previous negotiation of this media section keep their
  // payload types, so start out from them if the section is not recycled.
  RTCErrorOr<CodecList> current_codecs = GetCurrentContentCodecs(
      current_content, mid, media_description_options.type);
  if (!current_codecs.ok()) {
    return current_codecs.MoveError();
  }

  if (payload_types_in_transport_) {
    // REDESIGN path: Assume codecs from TypedCodecVendor are NotSet.
    codecs = current_codecs.MoveValue();
    for (const Codec& codec : codecs) {
      pt_suggester.AddLocalMapping(mid, codec.id, codec);
    }
    RTCError error = MergeAndAssignConfiguredCodecs(
        media_description_options.type, media_description_options.direction,
        mid, codecs, pt_suggester, /*pick_from_top_of_range=*/false,
        media_description_options.codec_preferences,
        session_options.vad_enabled);
    if (!error.ok()) {
      return error;
    }
  } else {
    // LEGACY path: Assume codecs have PTs.
    // Use MergeCodecsLegacy in order to handle PT clashes.
    RTCError error =
        MergeCodecsLegacy(current_codecs.value(), mid, codecs, pt_suggester,
                          /*pick_from_top_of_range=*/true);
    if (!error.ok()) {
      return error;
    }
    // Add our codecs that are not in the current description. Only the codecs
    // that survive the direction of the media section can end up in the offer,
    // so the ones that do not are skipped; giving them a payload type would
    // use up the dynamic payload type space for nothing.
    supported_codecs =
        media_description_options.type == MediaType::AUDIO
            ? GetAudioCodecsForOffer(media_description_options.direction)
            : GetVideoCodecsForOffer(media_description_options.direction);
    CodecList offerable_codecs;
    if (!media_description_options.codecs_to_include.empty()) {
      RTCErrorOr<CodecList> codecs_from_arg =
          CodecList::Create(media_description_options.codecs_to_include);
      if (!codecs_from_arg.ok()) {
        return codecs_from_arg.MoveError();
      }
      offerable_codecs = GetNegotiableCodecs(
          codecs_from_arg.value(), /*codec_preferences=*/{},
          media_description_options.type, session_options.vad_enabled);
    } else {
      offerable_codecs = GetNegotiableCodecs(
          supported_codecs, media_description_options.codec_preferences,
          media_description_options.type, session_options.vad_enabled);
    }
    const TypedCodecVendor& recv_codecs =
        media_description_options.type == MediaType::AUDIO ? audio_recv_codecs_
                                                           : video_recv_codecs_;
    const TypedCodecVendor& send_codecs =
        media_description_options.type == MediaType::AUDIO ? audio_send_codecs_
                                                           : video_send_codecs_;
    error = MergeCodecsLegacy(
        KeepOfferableCodecs(recv_codecs.codecs(), offerable_codecs), mid,
        codecs, pt_suggester, /*pick_from_top_of_range=*/true);
    if (!error.ok()) {
      return error;
    }
    error = MergeCodecsLegacy(
        KeepOfferableCodecs(send_codecs.codecs(), offerable_codecs), mid,
        codecs, pt_suggester, /*pick_from_top_of_range=*/true);
    if (!error.ok()) {
      return error;
    }
  }

  CodecList filtered_codecs;
  if (payload_types_in_transport_) {
    RTCError error = MergeAndAssignConfiguredCodecs(
        media_description_options.type, media_description_options.direction,
        mid, supported_codecs, pt_suggester, /*pick_from_top_of_range=*/true,
        media_description_options.codec_preferences,
        session_options.vad_enabled);
    if (!error.ok()) {
      return error;
    }
  }

  if (media_description_options.codecs_to_include.empty()) {
    if (!media_description_options.codec_preferences.empty()) {
      // Add the codecs from the current transceiver's codec preferences.
      // They override any existing codecs from previous negotiations.
      filtered_codecs =
          MatchCodecPreference(media_description_options.codec_preferences,
                               codecs, supported_codecs);
    } else {
      // Add the codecs from current content if it exists and is not rejected
      // nor recycled.
      if (current_content && !current_content->rejected &&
          current_content->mid() == mid) {
        if (!IsMediaContentOfType(current_content,
                                  media_description_options.type)) {
          // Can happen if the remote side re-uses a MID while recycling.
          return RTC_LOG_ERROR(RTCError(RTCErrorType::INTERNAL_ERROR)
                               << "Media type for content with mid='"
                               << current_content->mid()
                               << "' does not match expected type.");
        }
        const MediaContentDescription* mcd =
            current_content->media_description();
        for (const Codec& codec : mcd->codecs()) {
          // Take the codec from `codecs` rather than from the previous
          // description: the payload type suggester may have mapped the codec
          // to another payload type since, and mixing payload types from the
          // two lists can assign one payload type to two different codecs.
          std::optional<Codec> found_codec =
              FindMatchingCodec(mcd->codecs(), codecs.codecs(), codec);
          if (found_codec) {
            RTCError error = PushCodec(filtered_codecs, *found_codec);
            if (!error.ok()) {
              return error;
            }
            pt_suggester.AddLocalMapping(mid, found_codec->id, *found_codec);
          }
        }
      }
      // Add other supported codecs.
      for (const Codec& codec : supported_codecs) {
        std::optional<Codec> found_codec =
            FindMatchingCodec(supported_codecs, codecs, codec);
        if (found_codec &&
            !FindMatchingCodec(supported_codecs, filtered_codecs, codec)) {
          // Use the `found_codec` from `codecs` because it has the
          // correctly mapped payload type.
          if (media_description_options.type == MediaType::VIDEO &&
              found_codec->GetResiliencyType() == Codec::ResiliencyType::kRtx) {
            // For RTX we might need to adjust the apt parameter if we got a
            // remote offer without RTX for a codec for which we support RTX.
            // This is only done for video since we do not yet have rtx for
            // audio.
            const Codec* referenced_codec =
                GetAssociatedCodecForRtx(supported_codecs, codec);
            if (referenced_codec) {
              // Find the codec we should be referencing and point to it.
              std::optional<Codec> changed_referenced_codec = FindMatchingCodec(
                  supported_codecs, filtered_codecs, *referenced_codec);
              if (changed_referenced_codec) {
                found_codec->SetParam(kCodecParamAssociatedPayloadType,
                                      changed_referenced_codec->id.value());
              }
            }
          }
          RTCError error = PushCodec(filtered_codecs, *found_codec);
          if (!error.ok()) {
            return error;
          }
        }
      }
    }
    if (media_description_options.type == MediaType::AUDIO &&
        !session_options.vad_enabled) {
      // If application doesn't want CN codecs in offer.
      StripCNCodecs(filtered_codecs);
    } else if (media_description_options.type == MediaType::VIDEO &&
               session_options.raw_packetization_for_video) {
      for (Codec& codec : filtered_codecs) {
        if (codec.IsMediaCodec()) {
          codec.packetization = kPacketizationParamRaw;
        }
      }
    }
    NegotiateVideoCodecLevelsForOffer(media_description_options,
                                      supported_codecs, filtered_codecs);
  } else {
    // media_description_options.codecs_to_include contains codecs
    // TODO: issues.webrtc.org/360058654 - figure out if this can be deleted.
    RTCErrorOr<CodecList> codecs_from_arg =
        CodecList::Create(media_description_options.codecs_to_include);
    if (!codecs_from_arg.ok()) {
      return codecs_from_arg.MoveError();
    }
    filtered_codecs = codecs_from_arg.MoveValue();
  }
  if (payload_types_in_transport_) {
    AssignCodecIdsAndLinkRedRefactored(pt_suggester, mid,
                                       filtered_codecs.writable_codecs(),
                                       /*pick_from_top_of_range=*/false);
  } else {
    RecordCodecIdsAndLinkRed(pt_suggester, mid,
                             filtered_codecs.writable_codecs());
  }
  // Drop RED codecs left dangling after the primary they wrap (e.g. opus) was
  // filtered out, so the offer never references a non-existent payload type.
  RemoveRedCodecsWithoutPrimary(filtered_codecs.writable_codecs());
  return std::move(filtered_codecs).Finalize();
}

RTCErrorOr<Codecs> CodecVendor::GetNegotiatedCodecsForAnswer(
    const MediaDescriptionOptions& media_description_options,
    const MediaSessionOptions& session_options,
    RtpTransceiverDirection offer_rtd,
    RtpTransceiverDirection answer_rtd,
    const ContentInfo* current_content,
    std::span<const Codec> codecs_from_offer,
    PayloadTypeSuggester& pt_suggester) {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();

  CodecList codecs;
  std::string mid = media_description_options.mid;
  CodecList supported_codecs;
  // The codecs that the answer can contain, i.e. the supported codecs that
  // survive the transceiver's codec preferences and the comfort noise setting.
  // Only used in the legacy path; the redesigned path filters the codec
  // configurations instead.
  CodecList answerable_codecs;

  // Codecs from a previous negotiation of this media section keep their
  // payload types, so start out from them if the section is not recycled.
  RTCErrorOr<CodecList> current_codecs = GetCurrentContentCodecs(
      current_content, mid, media_description_options.type);
  if (!current_codecs.ok()) {
    return current_codecs.MoveError();
  }

  if (payload_types_in_transport_) {
    // REDESIGN path: Assume local codecs from TypedCodecVendor are NotSet.
    codecs = current_codecs.MoveValue();
    for (const Codec& codec : codecs) {
      pt_suggester.AddLocalMapping(mid, codec.id, codec);
    }
    // The error is only logged: when the payload type space runs out,
    // answering with fewer codecs is better than not answering at all.
    // TODO: https://issues.webrtc.org/360058654 - return the error once
    // payload type exhaustion is rare enough.
    RTCError error = MergeAndAssignConfiguredCodecs(
        media_description_options.type, RtpTransceiverDirection::kSendRecv, mid,
        codecs, pt_suggester, /*pick_from_top_of_range=*/false,
        media_description_options.codec_preferences,
        session_options.vad_enabled);
    if (!error.ok()) {
      RTC_LOG(LS_ERROR) << "Answering with fewer codecs: " << error.message();
    }
  } else {
    // LEGACY path: Assume codecs have PTs.
    MergeCodecsLegacy(current_codecs.value(), mid, codecs, pt_suggester);
    supported_codecs = media_description_options.type == MediaType::AUDIO
                           ? GetAudioCodecsForAnswer(offer_rtd, answer_rtd)
                           : GetVideoCodecsForAnswer(offer_rtd, answer_rtd);
    answerable_codecs = GetNegotiableCodecs(
        supported_codecs, media_description_options.codec_preferences,
        media_description_options.type, session_options.vad_enabled);
    const TypedCodecVendor& send_codecs =
        media_description_options.type == MediaType::AUDIO ? audio_send_codecs_
                                                           : video_send_codecs_;
    const TypedCodecVendor& recv_codecs =
        media_description_options.type == MediaType::AUDIO ? audio_recv_codecs_
                                                           : video_recv_codecs_;
    // Only the codecs that can end up in the answer are given a payload type;
    // giving one to the others would use up the dynamic payload type space for
    // nothing. The send codecs are merged before the receive codecs, as they
    // were before payload types were only given to answerable codecs.
    MergeCodecsLegacy(
        KeepOfferableCodecs(send_codecs.codecs(), answerable_codecs), mid,
        codecs, pt_suggester);
    MergeCodecsLegacy(
        KeepOfferableCodecs(recv_codecs.codecs(), answerable_codecs), mid,
        codecs, pt_suggester);
  }

  CodecList filtered_codecs;
  CodecList negotiated_codecs;
  if (media_description_options.codecs_to_include.empty()) {
    if (payload_types_in_transport_) {
      RtpTransceiverDirection direction_to_use = answer_rtd;
      if (answer_rtd == RtpTransceiverDirection::kSendRecv ||
          answer_rtd == RtpTransceiverDirection::kStopped ||
          answer_rtd == RtpTransceiverDirection::kInactive) {
        direction_to_use = RtpTransceiverDirectionReversed(offer_rtd);
      }
      // The error is only logged; see the merge above.
      RTCError error = MergeAndAssignConfiguredCodecs(
          media_description_options.type, direction_to_use, mid,
          supported_codecs, pt_suggester,
          /*pick_from_top_of_range=*/false,
          media_description_options.codec_preferences,
          session_options.vad_enabled);
      if (!error.ok()) {
        RTC_LOG(LS_ERROR) << "Answering with fewer codecs: " << error.message();
      }
    }
    if (!media_description_options.codec_preferences.empty()) {
      // Add the codecs from the current transceiver's codec preferences.
      // They override any existing codecs from previous negotiations.
      filtered_codecs =
          MatchCodecPreference(media_description_options.codec_preferences,
                               codecs, supported_codecs);
    } else {
      // Add the codecs from current content if it exists and is not rejected
      // nor recycled.
      if (current_content && !current_content->rejected &&
          current_content->mid() == mid) {
        if (!IsMediaContentOfType(current_content,
                                  media_description_options.type)) {
          // Can happen if the remote side re-uses a MID while recycling.
          return RTC_LOG_ERROR(RTCError(RTCErrorType::INTERNAL_ERROR)
                               << "Media type for content with mid='"
                               << current_content->mid()
                               << "' does not match expected type.");
        }
        const MediaContentDescription* mcd =
            current_content->media_description();
        for (const Codec& codec : mcd->codecs()) {
          if (FindMatchingCodec(mcd->codecs(), codecs.codecs(), codec)) {
            filtered_codecs.push_back(codec);
          }
        }
      }
      if (payload_types_in_transport_) {
        filtered_codecs = supported_codecs;
      } else {
        // Merge other_codecs into filtered_codecs, resolving PT conflicts.
        // Only the codecs that can end up in the answer are merged, so that
        // the others do not use up the dynamic payload type space.
        MergeCodecsLegacy(answerable_codecs, mid, filtered_codecs,
                          pt_suggester);
      }
    }

    if (media_description_options.type == MediaType::AUDIO &&
        !session_options.vad_enabled) {
      // If application doesn't want CN codecs in offer.
      StripCNCodecs(filtered_codecs);
    } else if (media_description_options.type == MediaType::VIDEO &&
               session_options.raw_packetization_for_video) {
      for (Codec& codec : filtered_codecs) {
        if (codec.IsMediaCodec()) {
          codec.packetization = kPacketizationParamRaw;
        }
      }
    }
    // An offer is external data, so needs to be checked before use.
    RTCErrorOr<CodecList> checked_codecs_from_offer =
        CodecList::Create(codecs_from_offer);
    if (!checked_codecs_from_offer.ok()) {
      return checked_codecs_from_offer.MoveError();
    }
    NegotiateCodecs(filtered_codecs, checked_codecs_from_offer.value(),
                    negotiated_codecs,
                    media_description_options.codec_preferences.empty(),
                    payload_types_in_transport_);
  } else {
    // media_description_options.codecs_to_include contains codecs
    RTCErrorOr<CodecList> codecs_from_arg =
        CodecList::Create(media_description_options.codecs_to_include);
    if (!codecs_from_arg.ok()) {
      return codecs_from_arg.MoveError();
    }
    negotiated_codecs = codecs_from_arg.MoveValue();
  }
  if (payload_types_in_transport_) {
    AssignCodecIdsAndLinkRedRefactored(pt_suggester, mid,
                                       negotiated_codecs.writable_codecs());
  } else {
    RecordCodecIdsAndLinkRed(pt_suggester, mid,
                             negotiated_codecs.writable_codecs());
  }
  return std::move(negotiated_codecs).Finalize();
}

TypedCodecVendor InitTypedCodecVendor(const MediaEngineInterface* media_engine,
                                      MediaType media_type,
                                      bool is_sender,
                                      bool rtx_enabled,
                                      const FieldTrialsView& trials) {
  return media_engine ? TypedCodecVendor(media_engine, media_type, is_sender,
                                         rtx_enabled, trials)
                      : TypedCodecVendor();
}

CodecVendor::CodecVendor(const MediaEngineInterface* media_engine,
                         bool rtx_enabled,
                         const FieldTrialsView& trials)
    : trials_(trials),
      audio_send_codecs_(InitTypedCodecVendor(media_engine,
                                              MediaType::AUDIO,
                                              /*is_sender=*/true,
                                              rtx_enabled,
                                              trials)),
      audio_recv_codecs_(InitTypedCodecVendor(media_engine,
                                              MediaType::AUDIO,
                                              /*is_sender=*/false,
                                              rtx_enabled,
                                              trials)),
      payload_types_in_transport_(
          trials.IsEnabled("WebRTC-PayloadTypesInTransport")),
      video_send_codecs_(InitTypedCodecVendor(media_engine,
                                              MediaType::VIDEO,
                                              /*is_sender=*/true,
                                              rtx_enabled,
                                              trials)),
      video_recv_codecs_(InitTypedCodecVendor(media_engine,
                                              MediaType::VIDEO,
                                              /*is_sender=*/false,
                                              rtx_enabled,
                                              trials)) {}

const CodecList& CodecVendor::audio_send_codecs() const {
  return audio_send_codecs_.codecs();
}

const CodecList& CodecVendor::audio_recv_codecs() const {
  return audio_recv_codecs_.codecs();
}

const CodecList& CodecVendor::video_send_codecs() const {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  return video_send_codecs_.codecs();
}

const CodecList& CodecVendor::video_recv_codecs() const {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  return video_recv_codecs_.codecs();
}

CodecList CodecVendor::GetVideoCodecsForOffer(
    const RtpTransceiverDirection& direction) const {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  switch (direction) {
    // If stream is inactive - generate list as if sendrecv.
    case RtpTransceiverDirection::kSendRecv:
    case RtpTransceiverDirection::kStopped:
    case RtpTransceiverDirection::kInactive:
      return video_sendrecv_codecs();
    case RtpTransceiverDirection::kSendOnly:
      return video_send_codecs_.codecs();
    case RtpTransceiverDirection::kRecvOnly:
      return video_recv_codecs_.codecs();
  }
  RTC_CHECK_NOTREACHED();
}

CodecList CodecVendor::GetVideoCodecsForAnswer(
    const RtpTransceiverDirection& offer,
    const RtpTransceiverDirection& answer) const {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  switch (answer) {
    // For inactive and sendrecv answers, generate lists as if we were to accept
    // the offer's direction. See RFC 3264 Section 6.1.
    case RtpTransceiverDirection::kSendRecv:
    case RtpTransceiverDirection::kStopped:
    case RtpTransceiverDirection::kInactive:
      return GetVideoCodecsForOffer(RtpTransceiverDirectionReversed(offer));
    case RtpTransceiverDirection::kSendOnly:
      return video_send_codecs_.codecs();
    case RtpTransceiverDirection::kRecvOnly:
      return video_recv_codecs_.codecs();
  }
  RTC_CHECK_NOTREACHED();
}

CodecList CodecVendor::GetAudioCodecsForOffer(
    const RtpTransceiverDirection& direction) const {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  switch (direction) {
    // If stream is inactive - generate list as if sendrecv.
    case RtpTransceiverDirection::kSendRecv:
    case RtpTransceiverDirection::kStopped:
    case RtpTransceiverDirection::kInactive:
      return audio_sendrecv_codecs();
    case RtpTransceiverDirection::kSendOnly:
      return audio_send_codecs_.codecs();
    case RtpTransceiverDirection::kRecvOnly:
      return audio_recv_codecs_.codecs();
  }
  RTC_CHECK_NOTREACHED();
}

CodecList CodecVendor::GetAudioCodecsForAnswer(
    const RtpTransceiverDirection& offer,
    const RtpTransceiverDirection& answer) const {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  switch (answer) {
    // For inactive and sendrecv answers, generate lists as if we were to accept
    // the offer's direction. See RFC 3264 Section 6.1.
    case RtpTransceiverDirection::kSendRecv:
    case RtpTransceiverDirection::kStopped:
    case RtpTransceiverDirection::kInactive:
      return GetAudioCodecsForOffer(RtpTransceiverDirectionReversed(offer));
    case RtpTransceiverDirection::kSendOnly:
      return audio_send_codecs_.codecs();
    case RtpTransceiverDirection::kRecvOnly:
      return audio_recv_codecs_.codecs();
  }
  RTC_CHECK_NOTREACHED();
}

CodecList CodecVendor::audio_sendrecv_codecs() const {
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  // Use NegotiateCodecs to merge our codec lists, since the operation is
  // essentially the same. Put send_codecs as the offered_codecs, which is the
  // order we'd like to follow. The reasoning is that encoding is usually more
  // expensive than decoding, and prioritizing a codec in the send list probably
  // means it's a codec we can handle efficiently.
  CodecList audio_sendrecv_codecs;
  RTCError error =
      NegotiateCodecs(audio_recv_codecs_.codecs(), audio_send_codecs_.codecs(),
                      audio_sendrecv_codecs, true, payload_types_in_transport_);
  RTC_DCHECK(error.ok());
  return audio_sendrecv_codecs;
}

CodecList CodecVendor::video_sendrecv_codecs() const {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  // Use NegotiateCodecs to merge our codec lists, since the operation is
  // essentially the same. Put send_codecs as the offered_codecs, which is the
  // order we'd like to follow. The reasoning is that encoding is usually more
  // expensive than decoding, and prioritizing a codec in the send list probably
  // means it's a codec we can handle efficiently.
  // Also for the same profile of a codec, if there are different levels in the
  // send and receive codecs, |video_sendrecv_codecs| will contain the lower
  // level of the two for that profile.
  CodecList video_sendrecv_codecs;
  RTCError error =
      NegotiateCodecs(video_recv_codecs_.codecs(), video_send_codecs_.codecs(),
                      video_sendrecv_codecs, true, payload_types_in_transport_);
  RTC_DCHECK(error.ok());
  return video_sendrecv_codecs;
}

void CodecVendor::ModifyVideoCodecs(
    const std::vector<std::pair<Codec, Codec>>& changes) {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  RTC_DCHECK(!payload_types_in_transport_);
  // For each codec in the first element that occurs in our supported codecs,
  // replace it with the codec in the second element. Exact matches only.
  for (const std::pair<Codec, Codec>& change : changes) {
    {
      CodecList send_codecs = video_send_codecs_.codecs();
      bool changed = false;
      for (Codec& codec : send_codecs.writable_codecs()) {
        if (codec == change.first) {
          codec = change.second;
          changed = true;
        }
      }
      if (changed) {
        video_send_codecs_ = TypedCodecVendor(std::move(send_codecs));
      }
    }
    {
      bool changed = false;
      CodecList recv_codecs = video_recv_codecs_.codecs();
      for (Codec& codec : recv_codecs.writable_codecs()) {
        if (codec == change.first) {
          codec = change.second;
          changed = true;
        }
      }
      if (changed) {
        video_recv_codecs_ = TypedCodecVendor(recv_codecs);
      }
    }
  }
}

void CodecVendor::SetRawPacketization(const Codec& codec) {
  RTC_DCHECK_RUN_ON(&sequence_checker_);
  RTC_DCHECK_DISALLOW_THREAD_BLOCKING_CALLS();
  RTC_DCHECK(payload_types_in_transport_);
  video_send_codecs_.SetRawPacketization(codec);
  video_recv_codecs_.SetRawPacketization(codec);
}

}  // namespace webrtc