/*
 * Copyright (C) 2019 The Android Open Source Project
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *      http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

#include "chpp/transport.h"

#include <inttypes.h>
#include <stdbool.h>
#include <stddef.h>
#include <stdint.h>
#include <string.h>

#include "chpp/app.h"
#include "chpp/clients.h"
#include "chpp/clients/discovery.h"
#include "chpp/crc.h"
#include "chpp/link.h"
#include "chpp/log.h"
#include "chpp/macros.h"
#include "chpp/memory.h"
#include "chpp/platform/platform_link.h"
#include "chpp/time.h"

/************************************************
 *  Prototypes
 ***********************************************/

static void chppSetRxState(struct ChppTransportState *context,
                           enum ChppRxState newState);
static size_t chppConsumePreamble(struct ChppTransportState *context,
                                  const uint8_t *buf, size_t len);
static size_t chppConsumeHeader(struct ChppTransportState *context,
                                const uint8_t *buf, size_t len);
static size_t chppConsumePayload(struct ChppTransportState *context,
                                 const uint8_t *buf, size_t len);
static size_t chppConsumeFooter(struct ChppTransportState *context,
                                const uint8_t *buf, size_t len);
static void chppAbortRxPacket(struct ChppTransportState *context);
#ifdef CHPP_SERVICE_ENABLED_TRANSPORT_LOOPBACK
static void chppProcessTransportLoopbackRequest(
    struct ChppTransportState *context);
#endif
#ifdef CHPP_CLIENT_ENABLED_TRANSPORT_LOOPBACK
static void chppProcessTransportLoopbackResponse(
    struct ChppTransportState *context);
#endif
static void chppSetResetComplete(struct ChppTransportState *context);
static void chppProcessResetAck(struct ChppTransportState *context);
static void chppProcessRxPacket(struct ChppTransportState *context);
static void chppProcessRxPayload(struct ChppTransportState *context);
static void chppClearRxDatagram(struct ChppTransportState *context);
static bool chppRxChecksumIsOk(const struct ChppTransportState *context);
static enum ChppTransportErrorCode chppRxHeaderCheck(
    const struct ChppTransportState *context);
static void chppRegisterRxAck(struct ChppTransportState *context);

static void chppEnqueueTxPacket(struct ChppTransportState *context,
                                uint8_t packetCode);
static size_t chppAddPreamble(uint8_t *buf);
static struct ChppTransportHeader *chppAddHeader(
    struct ChppTransportState *context);
static void chppAddPayload(struct ChppTransportState *context);
static void chppAddFooter(struct PendingTxPacket *packet);
size_t chppDequeueTxDatagram(struct ChppTransportState *context);
static void chppClearTxDatagramQueue(struct ChppTransportState *context);
static void chppTransportDoWork(struct ChppTransportState *context);
static void chppAppendToPendingTxPacket(struct PendingTxPacket *packet,
                                        const uint8_t *buf, size_t len);
static const char *chppGetPacketAttrStr(uint8_t packetCode);
static bool chppEnqueueTxDatagram(struct ChppTransportState *context,
                                  uint8_t packetCode, void *buf, size_t len);
enum ChppLinkErrorCode chppSendPendingPacket(
    struct ChppTransportState *context);

static void chppResetTransportContext(struct ChppTransportState *context);
static void chppReset(struct ChppTransportState *context,
                      enum ChppTransportPacketAttributes resetType,
                      enum ChppTransportErrorCode error);
#ifdef CHPP_CLIENT_ENABLED
struct ChppAppHeader *chppTransportGetClientRequestTimeoutResponse(
    struct ChppTransportState *context);
#endif

/************************************************
 *  Private Functions
 ***********************************************/

/**
 * Called any time the Rx state needs to be changed. Ensures that the location
 * counter among that state (rxStatus.locInState) is also reset at the same
 * time.
 *
 * @param context Maintains status for each transport layer instance.
 * @param newState Next Rx state.
 */
static void chppSetRxState(struct ChppTransportState *context,
                           enum ChppRxState newState) {
  CHPP_LOGD("Changing RX transport state from %" PRIu8 " to %" PRIu8
            " after %" PRIuSIZE " bytes",
            context->rxStatus.state, newState, context->rxStatus.locInState);
  context->rxStatus.locInState = 0;
  context->rxStatus.state = newState;
}

/**
 * Called by chppRxDataCb to find a preamble (i.e. packet start delimiter) in
 * the incoming data stream.
 * Moves the state to CHPP_STATE_HEADER as soon as it has seen a complete
 * preamble.
 * Any future backwards-incompatible versions of CHPP Transport will use a
 * different preamble.
 *
 * @param context Maintains status for each transport layer instance.
 * @param buf Input data.
 * @param len Length of input data in bytes.
 *
 * @return Length of consumed data in bytes.
 */
static size_t chppConsumePreamble(struct ChppTransportState *context,
                                  const uint8_t *buf, size_t len) {
  size_t consumed = 0;

  // TODO: Optimize loop, maybe using memchr() / memcmp() / SIMD, especially if
  // serial port calling chppRxDataCb does not implement zero filter
  while (consumed < len &&
         context->rxStatus.locInState < CHPP_PREAMBLE_LEN_BYTES) {
    size_t offset = context->rxStatus.locInState;
    if ((offset == 0 && buf[consumed] == CHPP_PREAMBLE_BYTE_FIRST) ||
        (offset == 1 && buf[consumed] == CHPP_PREAMBLE_BYTE_SECOND)) {
      // Correct byte of preamble observed
      context->rxStatus.locInState++;

    } else if (buf[consumed] == CHPP_PREAMBLE_BYTE_FIRST) {
      // Previous search failed but first byte of another preamble observed
      context->rxStatus.locInState = 1;

    } else {
      // Continue search for a valid preamble from the start
      context->rxStatus.locInState = 0;
    }

    consumed++;
  }

  // Let's see why we exited the above loop
  if (context->rxStatus.locInState == CHPP_PREAMBLE_LEN_BYTES) {
    // Complete preamble observed, move on to next state
    context->rxStatus.packetStartTimeNs = chppGetCurrentTimeNs();
    chppSetRxState(context, CHPP_STATE_HEADER);
  }

  return consumed;
}

/**
 * Called by chppRxDataCb to process the packet header from the incoming data
 * stream.
 * Moves the Rx state to CHPP_STATE_PAYLOAD afterwards.
 *
 * @param context Maintains status for each transport layer instance.
 * @param buf Input data.
 * @param len Length of input data in bytes.
 *
 * @return Length of consumed data in bytes.
 */
static size_t chppConsumeHeader(struct ChppTransportState *context,
                                const uint8_t *buf, size_t len) {
  CHPP_ASSERT(context->rxStatus.locInState <
              sizeof(struct ChppTransportHeader));
  size_t bytesToCopy = MIN(
      len, (sizeof(struct ChppTransportHeader) - context->rxStatus.locInState));
  memcpy(((uint8_t *)&context->rxHeader) + context->rxStatus.locInState, buf,
         bytesToCopy);
  context->rxStatus.locInState += bytesToCopy;

  if (context->rxStatus.locInState == sizeof(struct ChppTransportHeader)) {
    // Header fully copied. Move on

    enum ChppTransportErrorCode headerCheckResult = chppRxHeaderCheck(context);
    if (headerCheckResult != CHPP_TRANSPORT_ERROR_NONE) {
      // Header fails consistency check. NACK and return to preamble state
      chppEnqueueTxPacket(
          context, CHPP_ATTR_AND_ERROR_TO_PACKET_CODE(CHPP_TRANSPORT_ATTR_NONE,
                                                      headerCheckResult));
      chppSetRxState(context, CHPP_STATE_PREAMBLE);

    } else if (context->rxHeader.length == 0) {
      // Non-payload packet
      chppSetRxState(context, CHPP_STATE_FOOTER);

    } else {
      // Payload bearing packet
      uint8_t *tempPayload;

      if (context->rxDatagram.length == 0) {
        // Packet is a new datagram
        tempPayload = chppMalloc(context->rxHeader.length);
      } else {
        // Packet is a continuation of a fragmented datagram
        tempPayload =
            chppRealloc(context->rxDatagram.payload,
                        context->rxDatagram.length + context->rxHeader.length,
                        context->rxDatagram.length);
      }

      if (tempPayload == NULL) {
        CHPP_LOG_OOM();
        chppEnqueueTxPacket(context, CHPP_TRANSPORT_ERROR_OOM);
        chppSetRxState(context, CHPP_STATE_PREAMBLE);
      } else {
        context->rxDatagram.payload = tempPayload;
        context->rxDatagram.length += context->rxHeader.length;
        chppSetRxState(context, CHPP_STATE_PAYLOAD);
      }
    }
  }

  return bytesToCopy;
}

/**
 * Called by chppRxDataCb to copy the payload, the length of which is determined
 * by the header, from the incoming data stream.
 * Moves the Rx state to CHPP_STATE_FOOTER afterwards.
 *
 * @param context Maintains status for each transport layer instance.
 * @param buf Input data
 * @param len Length of input data in bytes
 *
 * @return Length of consumed data in bytes
 */
static size_t chppConsumePayload(struct ChppTransportState *context,
                                 const uint8_t *buf, size_t len) {
  CHPP_ASSERT(context->rxStatus.locInState < context->rxHeader.length);
  size_t bytesToCopy =
      MIN(len, (context->rxHeader.length - context->rxStatus.locInState));
  memcpy(context->rxDatagram.payload + context->rxStatus.locInDatagram, buf,
         bytesToCopy);
  context->rxStatus.locInDatagram += bytesToCopy;
  context->rxStatus.locInState += bytesToCopy;

  if (context->rxStatus.locInState == context->rxHeader.length) {
    // Entire packet payload copied. Move on
    chppSetRxState(context, CHPP_STATE_FOOTER);
  }

  return bytesToCopy;
}

/**
 * Called by chppRxDataCb to process the packet footer from the incoming data
 * stream. Checks checksum, triggering the correct response (ACK / NACK).
 * Moves the Rx state to CHPP_STATE_PREAMBLE afterwards.
 *
 * @param context Maintains status for each transport layer instance.
 * @param buf Input data.
 * @param len Length of input data in bytes.
 *
 * @return Length of consumed data in bytes.
 */
static size_t chppConsumeFooter(struct ChppTransportState *context,
                                const uint8_t *buf, size_t len) {
  CHPP_ASSERT(context->rxStatus.locInState <
              sizeof(struct ChppTransportFooter));
  size_t bytesToCopy = MIN(
      len, (sizeof(struct ChppTransportFooter) - context->rxStatus.locInState));
  memcpy(((uint8_t *)&context->rxFooter) + context->rxStatus.locInState, buf,
         bytesToCopy);

  context->rxStatus.locInState += bytesToCopy;
  if (context->rxStatus.locInState == sizeof(struct ChppTransportFooter)) {
    // Footer copied. Move on

    if (CHPP_TRANSPORT_GET_ERROR(context->rxHeader.packetCode) !=
        CHPP_TRANSPORT_ERROR_NONE) {
      CHPP_LOGE("RX packet len=%" PRIu16 " seq=%" PRIu8 " ackSeq=%" PRIu8
                " attr=0x%" PRIx8 " ERR=%" PRIu8 " flags=0x%" PRIx8,
                context->rxHeader.length, context->rxHeader.seq,
                context->rxHeader.ackSeq,
                (uint8_t)CHPP_TRANSPORT_GET_ATTR(context->rxHeader.packetCode),
                (uint8_t)CHPP_TRANSPORT_GET_ERROR(context->rxHeader.packetCode),
                context->rxHeader.flags);
    } else {
      CHPP_LOGD("RX packet len=%" PRIu16 " seq=%" PRIu8 " ackSeq=%" PRIu8
                " attr=0x%" PRIx8 " err=%" PRIu8 " flags=0x%" PRIx8,
                context->rxHeader.length, context->rxHeader.seq,
                context->rxHeader.ackSeq,
                (uint8_t)CHPP_TRANSPORT_GET_ATTR(context->rxHeader.packetCode),
                (uint8_t)CHPP_TRANSPORT_GET_ERROR(context->rxHeader.packetCode),
                context->rxHeader.flags);
    }

    if (CHPP_TRANSPORT_GET_ATTR(context->rxHeader.packetCode) ==
        CHPP_TRANSPORT_ATTR_LOOPBACK_REQUEST) {
#ifdef CHPP_SERVICE_ENABLED_TRANSPORT_LOOPBACK
      chppProcessTransportLoopbackRequest(context);
#endif

    } else if (CHPP_TRANSPORT_GET_ATTR(context->rxHeader.packetCode) ==
               CHPP_TRANSPORT_ATTR_LOOPBACK_RESPONSE) {
#ifdef CHPP_CLIENT_ENABLED_TRANSPORT_LOOPBACK
      chppProcessTransportLoopbackResponse(context);
#endif

    } else if (!chppRxChecksumIsOk(context)) {
      CHPP_LOGE("Bad checksum seq=%" PRIu8 " len=%" PRIu16,
                context->rxHeader.seq, context->rxHeader.length);
      chppAbortRxPacket(context);
      chppEnqueueTxPacket(context, CHPP_TRANSPORT_ERROR_CHECKSUM);  // NACK

    } else if (CHPP_TRANSPORT_GET_ATTR(context->rxHeader.packetCode) ==
               CHPP_TRANSPORT_ATTR_RESET) {
      CHPP_LOGI("RX RESET packet seq=%" PRIu8 " err=%" PRIu8,
                context->rxHeader.seq,
                CHPP_TRANSPORT_GET_ERROR(context->rxHeader.packetCode));
      chppMutexUnlock(&context->mutex);
      chppReset(context, CHPP_TRANSPORT_ATTR_RESET_ACK,
                CHPP_TRANSPORT_ERROR_NONE);
      chppMutexLock(&context->mutex);

    } else if (context->resetState == CHPP_RESET_STATE_PERMANENT_FAILURE) {
      // Only a reset is accepted in this state
      CHPP_LOGE("RX discarded in perm fail seq=%" PRIu8 " len=%" PRIu16,
                context->rxHeader.seq, context->rxHeader.length);
      chppAbortRxPacket(context);

    } else if (CHPP_TRANSPORT_GET_ATTR(context->rxHeader.packetCode) ==
               CHPP_TRANSPORT_ATTR_RESET_ACK) {
      CHPP_LOGI("RX RESET-ACK packet seq=%" PRIu8, context->rxHeader.seq);
      chppProcessResetAck(context);

    } else if (context->resetState == CHPP_RESET_STATE_RESETTING) {
      CHPP_LOGE("RX discarded in reset seq=%" PRIu8 " len=%" PRIu16,
                context->rxHeader.seq, context->rxHeader.length);
      chppAbortRxPacket(context);

    } else {
      chppProcessRxPacket(context);
    }

    // Done with this packet. Wait for next packet
    chppSetRxState(context, CHPP_STATE_PREAMBLE);
  }

  return bytesToCopy;
}

/**
 * Discards of an incomplete Rx packet during receive (e.g. due to a timeout or
 * bad checksum).
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppAbortRxPacket(struct ChppTransportState *context) {
  size_t undoLen = 0;
  size_t undoLoc = 0;

  switch (context->rxStatus.state) {
    case (CHPP_STATE_PREAMBLE):
    case (CHPP_STATE_HEADER): {
      break;
    }

    case (CHPP_STATE_PAYLOAD): {
      undoLen = context->rxHeader.length;
      undoLoc = context->rxStatus.locInState;
      break;
    }

    case (CHPP_STATE_FOOTER): {
      undoLen = context->rxHeader.length;
      undoLoc = context->rxHeader.length;
      break;
    }

    default: {
      CHPP_DEBUG_ASSERT(false);
    }
  }

  if (undoLen > 0) {
    // Packet has a payload we need to discard of

    CHPP_ASSERT(context->rxDatagram.length >= undoLen);
    CHPP_ASSERT(context->rxStatus.locInDatagram >= undoLoc);
    context->rxDatagram.length -= undoLen;
    context->rxStatus.locInDatagram -= undoLoc;

    if (context->rxDatagram.length == 0) {
      // Discarding this packet == discarding entire datagram
      CHPP_FREE_AND_NULLIFY(context->rxDatagram.payload);

    } else {
      // Discarding this packet == discarding part of datagram
      uint8_t *tempPayload =
          chppRealloc(context->rxDatagram.payload, context->rxDatagram.length,
                      context->rxDatagram.length + undoLen);

      if (tempPayload == NULL) {
        CHPP_LOG_OOM();
      } else {
        context->rxDatagram.payload = tempPayload;
      }
    }
  }

  chppSetRxState(context, CHPP_STATE_PREAMBLE);
}

/**
 * Processes a request that is determined to be for a transport-layer loopback.
 *
 * @param context Maintains status for each transport layer instance.
 */
#ifdef CHPP_SERVICE_ENABLED_TRANSPORT_LOOPBACK
static void chppProcessTransportLoopbackRequest(
    struct ChppTransportState *context) {
  if (context->txStatus.linkBusy) {
    CHPP_LOGE("Link busy; trans-loopback dropped");

  } else {
    context->txStatus.linkBusy = true;
    context->pendingTxPacket.length = 0;
    context->pendingTxPacket.length +=
        chppAddPreamble(&context->pendingTxPacket.payload[0]);

    struct ChppTransportHeader *txHeader =
        (struct ChppTransportHeader *)&context->pendingTxPacket
            .payload[context->pendingTxPacket.length];
    context->pendingTxPacket.length += sizeof(*txHeader);

    *txHeader = context->rxHeader;
    txHeader->packetCode = CHPP_ATTR_AND_ERROR_TO_PACKET_CODE(
        CHPP_TRANSPORT_ATTR_LOOPBACK_RESPONSE, txHeader->packetCode);

    size_t payloadLen =
        MIN(context->rxDatagram.length, CHPP_TRANSPORT_TX_MTU_BYTES);
    chppAppendToPendingTxPacket(&context->pendingTxPacket,
                                context->rxDatagram.payload, payloadLen);
    CHPP_FREE_AND_NULLIFY(context->rxDatagram.payload);
    chppClearRxDatagram(context);

    chppAddFooter(&context->pendingTxPacket);

    CHPP_LOGI("Trans-looping back len=%" PRIu16 " RX len=%" PRIuSIZE,
              txHeader->length, context->rxDatagram.length);
    enum ChppLinkErrorCode error = chppSendPendingPacket(context);

    if (error != CHPP_LINK_ERROR_NONE_QUEUED) {
      chppLinkSendDoneCb(&context->linkParams, error);
    }
  }
}
#endif

/**
 * Processes a response that is determined to be for a transport-layer loopback.
 *
 * @param context Maintains status for each transport layer instance.
 */
#ifdef CHPP_CLIENT_ENABLED_TRANSPORT_LOOPBACK
static void chppProcessTransportLoopbackResponse(
    struct ChppTransportState *context) {
  if (context->transportLoopbackData.length != context->rxDatagram.length) {
    CHPP_LOGE("RX len=%" PRIuSIZE " != TX len=%" PRIuSIZE,
              context->rxDatagram.length,
              context->transportLoopbackData.length - CHPP_PREAMBLE_LEN_BYTES -
                  sizeof(struct ChppTransportHeader) -
                  sizeof(struct ChppTransportFooter));
    context->loopbackResult = CHPP_APP_ERROR_INVALID_LENGTH;

  } else if (memcmp(context->rxDatagram.payload,
                    context->transportLoopbackData.payload,
                    context->rxDatagram.length) != 0) {
    CHPP_LOGE("RX & TX data don't match: len=%" PRIuSIZE,
              context->rxDatagram.length);
    context->loopbackResult = CHPP_APP_ERROR_INVALID_ARG;

  } else {
    context->loopbackResult = CHPP_APP_ERROR_NONE;

    CHPP_LOGD("RX successful transport-loopback (payload len=%" PRIuSIZE ")",
              context->rxDatagram.length);
  }

  context->transportLoopbackData.length = 0;
  CHPP_FREE_AND_NULLIFY(context->transportLoopbackData.payload);
  CHPP_FREE_AND_NULLIFY(context->rxDatagram.payload);
  chppClearRxDatagram(context);
}
#endif

/**
 * Method to invoke when the reset sequence is completed.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppSetResetComplete(struct ChppTransportState *context) {
  context->resetState = CHPP_RESET_STATE_NONE;
  context->resetCount = 0;
  chppConditionVariableSignal(&context->resetCondVar);
}

/**
 * An incoming reset-ack packet indicates that a reset is complete at the other
 * end of the CHPP link.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppProcessResetAck(struct ChppTransportState *context) {
  if (context->resetState == CHPP_RESET_STATE_NONE) {
    CHPP_LOGE("Unexpected reset-ack seq=%" PRIu8 " code=0x%" PRIx8,
              context->rxHeader.seq, context->rxHeader.packetCode);
    // In a reset race condition with both endpoints sending resets and
    // reset-acks, the sent resets and reset-acks will both have a sequence
    // number of 0.
    // By ignoring the received reset-ack, the next expected sequence number
    // will remain at 1 (following a reset with a sequence number of 0).
    // Therefore, no further correction is necessary (beyond ignoring the
    // received reset-ack), as the next packet (e.g. discovery) will have a
    // sequence number of 1.

    chppDatagramProcessDoneCb(context, context->rxDatagram.payload);
    chppClearRxDatagram(context);

    return;
  }

  chppSetResetComplete(context);
  context->rxStatus.receivedPacketCode = context->rxHeader.packetCode;
  context->rxStatus.expectedSeq = context->rxHeader.seq + 1;
  chppRegisterRxAck(context);

  // TODO: Configure transport layer based on (optional?) received config

  chppDatagramProcessDoneCb(context, context->rxDatagram.payload);
  chppClearRxDatagram(context);

#ifdef CHPP_CLIENT_ENABLED_DISCOVERY
  if (!context->appContext->isDiscoveryComplete) {
    chppMutexUnlock(&context->mutex);
    chppInitiateDiscovery(context->appContext);
    chppMutexLock(&context->mutex);
  } else {
    chppEnqueueTxPacket(context, CHPP_TRANSPORT_ERROR_NONE);
  }
#else
  chppEnqueueTxPacket(context, CHPP_TRANSPORT_ERROR_NONE);
#endif

  // Inform the App Layer that a reset has completed
  chppMutexUnlock(&context->mutex);
  chppAppProcessReset(context->appContext);
  chppMutexLock(&context->mutex);
}

/**
 * Process a received, checksum-validated packet.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppProcessRxPacket(struct ChppTransportState *context) {
  uint64_t now = chppGetCurrentTimeNs();
  context->rxStatus.lastGoodPacketTimeMs = (uint32_t)(now / CHPP_NSEC_PER_MSEC);
  context->rxStatus.receivedPacketCode = context->rxHeader.packetCode;
  chppRegisterRxAck(context);

  enum ChppTransportErrorCode errorCode = CHPP_TRANSPORT_ERROR_NONE;
  if (context->rxHeader.length > 0 &&
      context->rxHeader.seq != context->rxStatus.expectedSeq) {
    // Out of order payload
    errorCode = CHPP_TRANSPORT_ERROR_ORDER;
  }

  if (context->txDatagramQueue.pending > 0 ||
      errorCode == CHPP_TRANSPORT_ERROR_ORDER) {
    // There are packets to send out (could be new or retx)
    // Note: For a future ACK window > 1, makes more sense to cap the NACKs
    // to one instead of flooding with out of order NACK errors.
    chppEnqueueTxPacket(context, CHPP_ATTR_AND_ERROR_TO_PACKET_CODE(
                                     CHPP_TRANSPORT_ATTR_NONE, errorCode));
  }

  if (errorCode == CHPP_TRANSPORT_ERROR_ORDER) {
    CHPP_LOGE("Out of order RX discarded seq=%" PRIu8 " expect=%" PRIu8
              " len=%" PRIu16,
              context->rxHeader.seq, context->rxStatus.expectedSeq,
              context->rxHeader.length);
    chppAbortRxPacket(context);

  } else if (context->rxHeader.length > 0) {
    // Process payload and send ACK
    chppProcessRxPayload(context);
  }
}

/**
 * Process the payload of a validated payload-bearing packet and send out the
 * ACK.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppProcessRxPayload(struct ChppTransportState *context) {
  context->rxStatus.expectedSeq++;  // chppProcessRxPacket() already confirms
                                    // that context->rxStatus.expectedSeq ==
                                    // context->rxHeader.seq, protecting against
                                    // duplicate and out-of-order packets.

  if (context->rxHeader.flags & CHPP_TRANSPORT_FLAG_UNFINISHED_DATAGRAM) {
    // Packet is part of a larger datagram
    CHPP_LOGD("RX packet for unfinished datagram. Seq=%" PRIu8 " len=%" PRIu16
              ". Datagram len=%" PRIuSIZE ". Sending ACK=%" PRIu8,
              context->rxHeader.seq, context->rxHeader.length,
              context->rxDatagram.length, context->rxStatus.expectedSeq);

  } else {
    // End of this packet is end of a datagram

    // Send the payload to the App Layer
    // Note that it is up to the app layer to free the buffer using
    // chppDatagramProcessDoneCb() after is is done.
    chppMutexUnlock(&context->mutex);
    chppAppProcessRxDatagram(context->appContext, context->rxDatagram.payload,
                             context->rxDatagram.length);
    chppMutexLock(&context->mutex);

    CHPP_LOGD("App layer processed datagram with len=%" PRIuSIZE
              ", ending packet seq=%" PRIu8 ", len=%" PRIu16
              ". Sending ACK=%" PRIu8 " (previously sent=%" PRIu8 ")",
              context->rxDatagram.length, context->rxHeader.seq,
              context->rxHeader.length, context->rxStatus.expectedSeq,
              context->txStatus.sentAckSeq);
    chppClearRxDatagram(context);
  }

  // Send ACK because we had RX a payload-bearing packet
  chppEnqueueTxPacket(context, CHPP_TRANSPORT_ERROR_NONE);
}

/**
 * Resets the incoming datagram state, i.e. after the datagram has been
 * processed.
 * Note that this is independent from freeing the payload. It is up to the app
 * layer to inform the transport layer using chppDatagramProcessDoneCb() once it
 * is done with the buffer so it is freed.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppClearRxDatagram(struct ChppTransportState *context) {
  context->rxStatus.locInDatagram = 0;
  context->rxDatagram.length = 0;
  context->rxDatagram.payload = NULL;
}

/**
 * Validates the checksum of an incoming packet.
 *
 * @param context Maintains status for each transport layer instance.
 *
 * @return True if and only if the checksum is correct.
 */
static bool chppRxChecksumIsOk(const struct ChppTransportState *context) {
  uint32_t crc = chppCrc32(0, (const uint8_t *)&context->rxHeader,
                           sizeof(context->rxHeader));
  crc = chppCrc32(
      crc,
      &context->rxDatagram
           .payload[context->rxStatus.locInDatagram - context->rxHeader.length],
      context->rxHeader.length);

  if (context->rxFooter.checksum != crc) {
    CHPP_LOGE("RX BAD checksum: footer=0x%" PRIx32 ", calc=0x%" PRIx32
              ", len=%" PRIuSIZE,
              context->rxFooter.checksum, crc,
              (size_t)(context->rxHeader.length +
                       sizeof(struct ChppTransportHeader)));
  }

  return (context->rxFooter.checksum == crc);
}

/**
 * Performs consistency checks on received packet header to determine if it is
 * obviously corrupt / invalid / duplicate / out-of-order.
 *
 * @param context Maintains status for each transport layer instance.
 *
 * @return True if and only if header passes checks
 */
static enum ChppTransportErrorCode chppRxHeaderCheck(
    const struct ChppTransportState *context) {
  enum ChppTransportErrorCode result = CHPP_TRANSPORT_ERROR_NONE;

  if (context->rxHeader.length > CHPP_TRANSPORT_RX_MTU_BYTES) {
    result = CHPP_TRANSPORT_ERROR_HEADER;
  }

  if (result != CHPP_TRANSPORT_ERROR_NONE) {
    CHPP_LOGE("Bad header. seq=%" PRIu8 " expect=%" PRIu8 " len=%" PRIu16
              " err=%" PRIu8,
              context->rxHeader.seq, context->rxStatus.expectedSeq,
              context->rxHeader.length, result);
  }

  return result;
}

/**
 * Registers a received ACK. If an outgoing datagram is fully ACKed, it is
 * popped from the TX queue.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppRegisterRxAck(struct ChppTransportState *context) {
  uint8_t rxAckSeq = context->rxHeader.ackSeq;

  if (context->rxStatus.receivedAckSeq != rxAckSeq) {
    // A previously sent packet was actually ACKed
    // Note: For a future ACK window >1, we should loop by # of ACKed packets
    if ((uint8_t)(context->rxStatus.receivedAckSeq + 1) != rxAckSeq) {
      CHPP_LOGE("Out of order ACK: last=%" PRIu8 " rx=%" PRIu8,
                context->rxStatus.receivedAckSeq, rxAckSeq);
    } else {
      CHPP_LOGD(
          "ACK received (last registered=%" PRIu8 ", received=%" PRIu8
          "). Prior queue depth=%" PRIu8 ", front datagram=%" PRIu8
          " at loc=%" PRIuSIZE " of len=%" PRIuSIZE,
          context->rxStatus.receivedAckSeq, rxAckSeq,
          context->txDatagramQueue.pending, context->txDatagramQueue.front,
          context->txStatus.ackedLocInDatagram,
          context->txDatagramQueue.datagram[context->txDatagramQueue.front]
              .length);

      context->rxStatus.receivedAckSeq = rxAckSeq;
      if (context->txStatus.txAttempts > 1) {
        CHPP_LOGW("Seq %" PRIu8 " ACK'd after %" PRIuSIZE " reTX",
                  context->rxHeader.seq, context->txStatus.txAttempts - 1);
      }
      context->txStatus.txAttempts = 0;

      // Process and if necessary pop from Tx datagram queue
      context->txStatus.ackedLocInDatagram += CHPP_TRANSPORT_TX_MTU_BYTES;
      if (context->txStatus.ackedLocInDatagram >=
          context->txDatagramQueue.datagram[context->txDatagramQueue.front]
              .length) {
        // We are done with datagram

        context->txStatus.ackedLocInDatagram = 0;
        context->txStatus.sentLocInDatagram = 0;

        // Note: For a future ACK window >1, we need to update the queue
        // position of the datagram being sent as well (relative to the
        // front-of-queue). e.g. context->txStatus.datagramBeingSent--;

        if (chppDequeueTxDatagram(context) == 0) {
          context->txStatus.hasPacketsToSend = false;
        }
      }
    }
  }  // else {nothing was ACKed}
}

/**
 * Enqueues an outgoing packet with the specified error code. The error code
 * refers to the optional reason behind a NACK, if any. An error code of
 * CHPP_TRANSPORT_ERROR_NONE indicates that no error was reported (i.e. either
 * an ACK or an implicit NACK)
 *
 * Note that the decision as to wheather to include a payload will be taken
 * later, i.e. before the packet is being sent out from the queue. A payload is
 * expected to be included if there is one or more pending Tx datagrams and we
 * are not waiting on a pending ACK. A (repeat) payload is also included if we
 * have received a NACK.
 *
 * Further note that even for systems with an ACK window greater than one, we
 * would only need to send an ACK for the last (correct) packet, hence we only
 * need a queue length of one here.
 *
 * @param context Maintains status for each transport layer instance.
 * @param packetCode Error code and packet attributes to be sent.
 */
static void chppEnqueueTxPacket(struct ChppTransportState *context,
                                uint8_t packetCode) {
  context->txStatus.hasPacketsToSend = true;
  context->txStatus.packetCodeToSend = packetCode;

  CHPP_LOGD("chppEnqueueTxPacket called with packet code=0x%" PRIx8,
            packetCode);

  // Notifies the main CHPP Transport Layer to run chppTransportDoWork().
  chppNotifierSignal(&context->notifier, CHPP_TRANSPORT_SIGNAL_EVENT);
}

/**
 * Adds a CHPP preamble to the beginning of buf.
 *
 * @param buf The CHPP preamble will be added to buf.
 *
 * @return Size of the added preamble.
 */
static size_t chppAddPreamble(uint8_t *buf) {
  buf[0] = CHPP_PREAMBLE_BYTE_FIRST;
  buf[1] = CHPP_PREAMBLE_BYTE_SECOND;
  return CHPP_PREAMBLE_LEN_BYTES;
}

/**
 * Adds the packet header to pendingTxPacket.
 *
 * @param context Maintains status for each transport layer instance.
 *
 * @return Pointer to the added packet header.
 */
static struct ChppTransportHeader *chppAddHeader(
    struct ChppTransportState *context) {
  struct ChppTransportHeader *txHeader =
      (struct ChppTransportHeader *)&context->pendingTxPacket
          .payload[context->pendingTxPacket.length];
  context->pendingTxPacket.length += sizeof(*txHeader);

  txHeader->packetCode = context->txStatus.packetCodeToSend;
  context->txStatus.packetCodeToSend = CHPP_ATTR_AND_ERROR_TO_PACKET_CODE(
      context->txStatus.packetCodeToSend, CHPP_TRANSPORT_ERROR_NONE);

  txHeader->ackSeq = context->rxStatus.expectedSeq;
  context->txStatus.sentAckSeq = txHeader->ackSeq;

  return txHeader;
}

/**
 * Adds the packet payload to pendingTxPacket.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppAddPayload(struct ChppTransportState *context) {
  struct ChppTransportHeader *txHeader =
      (struct ChppTransportHeader *)&context->pendingTxPacket
          .payload[CHPP_PREAMBLE_LEN_BYTES];

  size_t remainingBytes =
      context->txDatagramQueue.datagram[context->txDatagramQueue.front].length -
      context->txStatus.ackedLocInDatagram;

  CHPP_LOGD("Adding payload to seq=%" PRIu8 ", remainingBytes=%" PRIuSIZE
            " of pending datagrams=%" PRIu8,
            txHeader->seq, remainingBytes, context->txDatagramQueue.pending);

  if (remainingBytes > CHPP_TRANSPORT_TX_MTU_BYTES) {
    // Send an unfinished part of a datagram
    txHeader->flags = CHPP_TRANSPORT_FLAG_UNFINISHED_DATAGRAM;
    txHeader->length = CHPP_TRANSPORT_TX_MTU_BYTES;
  } else {
    // Send final (or only) part of a datagram
    txHeader->flags = CHPP_TRANSPORT_FLAG_FINISHED_DATAGRAM;
    txHeader->length = (uint16_t)remainingBytes;
  }

  // Copy payload
  chppAppendToPendingTxPacket(
      &context->pendingTxPacket,
      context->txDatagramQueue.datagram[context->txDatagramQueue.front]
              .payload +
          context->txStatus.ackedLocInDatagram,
      txHeader->length);

  context->txStatus.sentLocInDatagram =
      context->txStatus.ackedLocInDatagram + txHeader->length;
}

/**
 * Adds a footer (containing the checksum) to a packet.
 *
 * @param packet The packet from which to calculate the checksum and append the
 * footer.
 */
static void chppAddFooter(struct PendingTxPacket *packet) {
  struct ChppTransportFooter footer;
  footer.checksum = chppCrc32(0, &packet->payload[CHPP_PREAMBLE_LEN_BYTES],
                              packet->length - CHPP_PREAMBLE_LEN_BYTES);

  CHPP_LOGD("Adding transport footer. Checksum=0x%" PRIx32 ", len: %" PRIuSIZE
            " -> %" PRIuSIZE,
            footer.checksum, packet->length, packet->length + sizeof(footer));

  chppAppendToPendingTxPacket(packet, (const uint8_t *)&footer, sizeof(footer));
}

/**
 * Dequeues the datagram at the front of the datagram tx queue, if any, and
 * frees the payload. Returns the number of remaining datagrams in the queue.
 *
 * @param context Maintains status for each transport layer instance.
 * @return Number of remaining datagrams in queue.
 */
size_t chppDequeueTxDatagram(struct ChppTransportState *context) {
  if (context->txDatagramQueue.pending == 0) {
    CHPP_LOGE("Can not dequeue empty datagram queue");

  } else {
    CHPP_LOGD("Dequeuing front datagram with index=%" PRIu8 ", len=%" PRIuSIZE
              ". Queue depth: %" PRIu8 "->%d",
              context->txDatagramQueue.front,
              context->txDatagramQueue.datagram[context->txDatagramQueue.front]
                  .length,
              context->txDatagramQueue.pending,
              context->txDatagramQueue.pending - 1);

    CHPP_FREE_AND_NULLIFY(
        context->txDatagramQueue.datagram[context->txDatagramQueue.front]
            .payload);
    context->txDatagramQueue.datagram[context->txDatagramQueue.front].length =
        0;

    context->txDatagramQueue.pending--;
    context->txDatagramQueue.front++;
    context->txDatagramQueue.front %= CHPP_TX_DATAGRAM_QUEUE_LEN;
  }

  return context->txDatagramQueue.pending;
}

/**
 * Flushes the Tx datagram queue of any pending packets.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppClearTxDatagramQueue(struct ChppTransportState *context) {
  while (context->txDatagramQueue.pending > 0) {
    chppDequeueTxDatagram(context);
  }
  context->txStatus.hasPacketsToSend = false;
}

/**
 * Sends out a pending outgoing packet based on a notification from
 * chppEnqueueTxPacket().
 *
 * A payload may or may not be included be according the following:
 * No payload: If Tx datagram queue is empty OR we are waiting on a pending ACK.
 * New payload: If there is one or more pending Tx datagrams and we are not
 * waiting on a pending ACK.
 * Repeat payload: If we haven't received an ACK yet for our previous payload,
 * i.e. we have registered an explicit or implicit NACK.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppTransportDoWork(struct ChppTransportState *context) {
  bool havePacketForLinkLayer = false;
  struct ChppTransportHeader *txHeader;
  struct ChppAppHeader *timeoutResponse = NULL;

  // Note: For a future ACK window >1, there needs to be a loop outside the lock
  chppMutexLock(&context->mutex);

  if (context->txStatus.hasPacketsToSend && !context->txStatus.linkBusy) {
    // There are pending outgoing packets and the link isn't busy
    havePacketForLinkLayer = true;
    context->txStatus.linkBusy = true;

    context->pendingTxPacket.length = 0;
    memset(&context->pendingTxPacket.payload, 0, CHPP_LINK_TX_MTU_BYTES);

    // Add preamble
    context->pendingTxPacket.length +=
        chppAddPreamble(&context->pendingTxPacket.payload[0]);

    // Add header
    txHeader = chppAddHeader(context);

    // If applicable, add payload
    if ((context->txDatagramQueue.pending > 0)) {
      // Note: For a future ACK window >1, we need to rewrite this payload
      // adding code to base the next packet on the sent location within the
      // last sent datagram, except for the case of a NACK (explicit or
      // timeout). For a NACK, we would need to base the next packet off the
      // last ACKed location.

      txHeader->seq = context->rxStatus.receivedAckSeq;
      context->txStatus.sentSeq = txHeader->seq;

      if (context->txStatus.txAttempts > CHPP_TRANSPORT_MAX_RETX &&
          context->resetState != CHPP_RESET_STATE_RESETTING) {
        CHPP_LOGE("Resetting after %d reTX", CHPP_TRANSPORT_MAX_RETX);
        havePacketForLinkLayer = false;

        chppMutexUnlock(&context->mutex);
        chppReset(context, CHPP_TRANSPORT_ATTR_RESET,
                  CHPP_TRANSPORT_ERROR_MAX_RETRIES);
        chppMutexLock(&context->mutex);

      } else {
        chppAddPayload(context);
        context->txStatus.txAttempts++;
      }

    } else {
      // No payload
      context->txStatus.hasPacketsToSend = false;
    }

    chppAddFooter(&context->pendingTxPacket);

  } else {
    CHPP_LOGW(
        "DoWork nothing to send. hasPackets=%d, linkBusy=%d, pending=%" PRIu8
        ", RX ACK=%" PRIu8 ", TX seq=%" PRIu8 ", RX state=%" PRIu8,
        context->txStatus.hasPacketsToSend, context->txStatus.linkBusy,
        context->txDatagramQueue.pending, context->rxStatus.receivedAckSeq,
        context->txStatus.sentSeq, context->rxStatus.state);
  }

  chppMutexUnlock(&context->mutex);

  if (havePacketForLinkLayer) {
    CHPP_LOGD("TX->Link: len=%" PRIuSIZE " flags=0x%" PRIx8 " code=0x%" PRIx8
              " ackSeq=%" PRIu8 " seq=%" PRIu8 " payloadLen=%" PRIu16
              " pending=%" PRIu8,
              context->pendingTxPacket.length, txHeader->flags,
              txHeader->packetCode, txHeader->ackSeq, txHeader->seq,
              txHeader->length, context->txDatagramQueue.pending);
    enum ChppLinkErrorCode error = chppSendPendingPacket(context);

    if (error != CHPP_LINK_ERROR_NONE_QUEUED) {
      // Platform implementation for platformLinkSend() is synchronous or an
      // error occurred. In either case, we should call chppLinkSendDoneCb()
      // here to release the contents of pendingTxPacket.
      chppLinkSendDoneCb(&context->linkParams, error);
    }
  }

#ifdef CHPP_CLIENT_ENABLED
  timeoutResponse = chppTransportGetClientRequestTimeoutResponse(context);
#endif
  if (timeoutResponse != NULL) {
    CHPP_LOGE("Response timeout H#%" PRIu8 " cmd=%" PRIu16 " ID=%" PRIu8,
              timeoutResponse->handle, timeoutResponse->command,
              timeoutResponse->transaction);
    chppAppProcessRxDatagram(context->appContext, (uint8_t *)timeoutResponse,
                             sizeof(struct ChppAppHeader));
  }
}

/**
 * Appends data from a buffer of length len to a PendingTxPacket, updating its
 * length.
 *
 * @param packet The PendingTxBuffer to be appended to.
 * @param buf Input data to be copied from.
 * @param len Length of input data in bytes.
 */
static void chppAppendToPendingTxPacket(struct PendingTxPacket *packet,
                                        const uint8_t *buf, size_t len) {
  CHPP_ASSERT(packet->length + len <= sizeof(packet->payload));
  memcpy(&packet->payload[packet->length], buf, len);
  packet->length += len;
}

/**
 * @return A human readable form of the packet attribution.
 */
static const char *chppGetPacketAttrStr(uint8_t packetCode) {
  switch (CHPP_TRANSPORT_GET_ATTR(packetCode)) {
    case CHPP_TRANSPORT_ATTR_RESET:
      return "(RESET)";
    case CHPP_TRANSPORT_ATTR_RESET_ACK:
      return "(RESET-ACK)";
    case CHPP_TRANSPORT_ATTR_LOOPBACK_REQUEST:
      return "(LOOP-REQ)";
    case CHPP_TRANSPORT_ATTR_LOOPBACK_RESPONSE:
      return "(LOOP-RES)";
    default:
      return "";
  }
}

/**
 * Enqueues an outgoing datagram of a specified length. The payload must have
 * been allocated by the caller using chppMalloc.
 *
 * If enqueueing is successful, the payload will be freed by this function
 * once it has been sent out.
 * If enqueueing is unsuccessful, it is up to the caller to decide when or if
 * to free the payload and/or resend it later.
 *
 * @param context Maintains status for each transport layer instance.
 * @param packetCode Error code and packet attributes to be sent.
 * @param buf Datagram payload allocated through chppMalloc. Cannot be null.
 * @param len Datagram length in bytes.
 *
 * @return True informs the sender that the datagram was successfully enqueued.
 * False informs the sender that the queue was full.
 */
static bool chppEnqueueTxDatagram(struct ChppTransportState *context,
                                  uint8_t packetCode, void *buf, size_t len) {
  bool success = false;

  if (len == 0) {
    CHPP_DEBUG_ASSERT_LOG(false, "Enqueue TX len=0!");

  } else {
    if ((len < sizeof(struct ChppAppHeader)) ||
        (CHPP_TRANSPORT_GET_ATTR(packetCode) != 0)) {
      CHPP_LOGI("Enqueue TX: code=0x%" PRIx8 "%s len=%" PRIuSIZE
                " pending=%" PRIu8,
                packetCode, chppGetPacketAttrStr(packetCode), len,
                (uint8_t)(context->txDatagramQueue.pending + 1));
    } else {
      struct ChppAppHeader *header = buf;
      CHPP_LOGI(
          "Enqueue TX: len=%" PRIuSIZE " H#%" PRIu8 " type=0x%" PRIx8
          " ID=%" PRIu8 " err=%" PRIu8 " cmd=0x%" PRIx16 " pending=%" PRIu8,
          len, header->handle, header->type, header->transaction, header->error,
          header->command, (uint8_t)(context->txDatagramQueue.pending + 1));
    }

    chppMutexLock(&context->mutex);

    if (context->txDatagramQueue.pending >= CHPP_TX_DATAGRAM_QUEUE_LEN) {
      CHPP_LOGE("Cannot enqueue TX datagram");

    } else {
      uint16_t end =
          (context->txDatagramQueue.front + context->txDatagramQueue.pending) %
          CHPP_TX_DATAGRAM_QUEUE_LEN;
      context->txDatagramQueue.datagram[end].length = len;
      context->txDatagramQueue.datagram[end].payload = buf;
      context->txDatagramQueue.pending++;

      if (context->txDatagramQueue.pending == 1) {
        // Queue was empty prior. Need to kickstart transmission.
        chppEnqueueTxPacket(context, packetCode);
      }

      success = true;
    }

    chppMutexUnlock(&context->mutex);
  }

  return success;
}

/**
 * Sends the pending outgoing packet (context->pendingTxPacket) over to the link
 * layer using chppPlatformLinkSend() and updates the last Tx packet time.
 *
 * @param context Maintains status for each transport layer instance.
 *
 * @return Result of chppPlatformLinkSend().
 */
enum ChppLinkErrorCode chppSendPendingPacket(
    struct ChppTransportState *context) {
  enum ChppLinkErrorCode error = chppPlatformLinkSend(
      &context->linkParams, context->pendingTxPacket.payload,
      context->pendingTxPacket.length);

  context->txStatus.lastTxTimeNs = chppGetCurrentTimeNs();

  return error;
}

/**
 * Resets the transport state, maintaining the link layer parameters.
 *
 * @param context Maintains status for each transport layer instance.
 */
static void chppResetTransportContext(struct ChppTransportState *context) {
  memset(&context->rxStatus, 0, sizeof(struct ChppRxStatus));
  memset(&context->rxDatagram, 0, sizeof(struct ChppDatagram));

  memset(&context->txStatus, 0, sizeof(struct ChppTxStatus));
  memset(&context->txDatagramQueue, 0, sizeof(struct ChppTxDatagramQueue));

  context->txStatus.sentSeq =
      UINT8_MAX;  // So that the seq # of the first TX packet is 0
  context->resetState = CHPP_RESET_STATE_RESETTING;
}

/**
 * Re-initializes the CHPP transport and app layer states, e.g. when receiving a
 * reset packet, and sends out a reset or reset-ack packet over the link in
 * order to reset the remote side or inform the counterpart of a reset,
 * respectively.
 *
 * If the link layer is busy, this function will reset the link as well.
 * This function retains and restores the platform-specific values of
 * transportContext.linkParams.
 *
 * @param transportContext Maintains status for each transport layer instance.
 * @param resetType Type of reset to send after resetting CHPP (reset vs.
 * reset-ack), as defined in the ChppTransportPacketAttributes struct.
 * @param error Provides the error that led to the reset.
 */
static void chppReset(struct ChppTransportState *transportContext,
                      enum ChppTransportPacketAttributes resetType,
                      enum ChppTransportErrorCode error) {
  // TODO: Configure transport layer based on (optional?) received config before
  // datagram is wiped

  chppMutexLock(&transportContext->mutex);
  struct ChppAppState *appContext = transportContext->appContext;
  transportContext->resetState = CHPP_RESET_STATE_RESETTING;

  // Reset asynchronous link layer if busy
  if (transportContext->txStatus.linkBusy == true) {
    // TODO: Give time for link layer to finish before resorting to a reset

    chppPlatformLinkReset(&transportContext->linkParams);
  }

  // Free memory allocated for any ongoing rx datagrams
  if (transportContext->rxDatagram.length > 0) {
    transportContext->rxDatagram.length = 0;
    CHPP_FREE_AND_NULLIFY(transportContext->rxDatagram.payload);
  }

  // Free memory allocated for any ongoing tx datagrams
  for (size_t i = 0; i < CHPP_TX_DATAGRAM_QUEUE_LEN; i++) {
    if (transportContext->txDatagramQueue.datagram[i].length > 0) {
      CHPP_FREE_AND_NULLIFY(
          transportContext->txDatagramQueue.datagram[i].payload);
    }
  }

  // Reset Transport Layer but restore Rx sequence number and packet code
  // (context->rxHeader is not wiped in reset)
  chppResetTransportContext(transportContext);
  transportContext->rxStatus.receivedPacketCode =
      transportContext->rxHeader.packetCode;
  transportContext->rxStatus.expectedSeq = transportContext->rxHeader.seq + 1;

  // Send reset or reset-ACK
  chppMutexUnlock(&transportContext->mutex);
  chppTransportSendReset(transportContext, resetType, error);

  // Inform the App Layer that a reset has completed
  if (resetType == CHPP_TRANSPORT_ATTR_RESET_ACK) {
    chppAppProcessReset(appContext);
  }  // else reset is sent out. Rx of reset-ack will indicate completion.
}

/**
 * Checks for a timed out client request and generates a timeout response if a
 * client request timeout has occurred.
 *
 * @param context Maintains status for each transport layer instance.
 * @return App layer response header if a timeout has occurred. Null otherwise.
 */
#ifdef CHPP_CLIENT_ENABLED
struct ChppAppHeader *chppTransportGetClientRequestTimeoutResponse(
    struct ChppTransportState *context) {
  struct ChppAppHeader *response = NULL;

  bool timeoutClientFound = false;
  uint8_t timedOutClient;
  uint16_t timedOutCmd;

  chppMutexLock(&context->mutex);

  if (context->appContext->nextRequestTimeoutNs <= chppGetCurrentTimeNs()) {
    // Determine which request has timed out

    uint64_t lowestTimeout = CHPP_TIME_MAX;
    for (uint8_t clientIdx = 0;
         clientIdx < context->appContext->registeredClientCount; clientIdx++) {
      for (uint16_t cmdIdx = 0;
           cmdIdx <
           context->appContext->registeredClients[clientIdx]->rRStateCount;
           cmdIdx++) {
        struct ChppRequestResponseState *rRState =
            &context->appContext->registeredClientStates[clientIdx]
                 ->rRStates[cmdIdx];

        if (rRState->requestState == CHPP_REQUEST_STATE_REQUEST_SENT &&
            rRState->responseTimeNs != CHPP_TIME_NONE &&
            rRState->responseTimeNs < lowestTimeout) {
          lowestTimeout = rRState->responseTimeNs;
          timedOutClient = clientIdx;
          timedOutCmd = cmdIdx;
          timeoutClientFound = true;
        }
      }
    }

    if (!timeoutClientFound) {
      CHPP_LOGE("Timeout at %" PRIu64 " but no client",
                context->appContext->nextRequestTimeoutNs / CHPP_NSEC_PER_MSEC);
      chppClientRecalculateNextTimeout(context->appContext);
    }
  }

  if (timeoutClientFound) {
    CHPP_LOGE("Client=%" PRIu8 " cmd=%" PRIu16 " timed out", timedOutClient,
              timedOutCmd);
    response = chppMalloc(sizeof(struct ChppAppHeader));
    if (response == NULL) {
      CHPP_LOG_OOM();
    } else {
      response->handle = CHPP_SERVICE_HANDLE_OF_INDEX(timedOutClient);
      response->type = CHPP_MESSAGE_TYPE_SERVICE_RESPONSE;
      response->transaction =
          context->appContext->registeredClientStates[timedOutClient]
              ->rRStates[timedOutCmd]
              .transaction;
      response->error = CHPP_APP_ERROR_TIMEOUT;
      response->command = timedOutCmd;
    }
  }

  chppMutexUnlock(&context->mutex);

  return response;
}
#endif

/************************************************
 *  Public Functions
 ***********************************************/

void chppTransportInit(struct ChppTransportState *transportContext,
                       struct ChppAppState *appContext) {
  CHPP_NOT_NULL(transportContext);
  CHPP_NOT_NULL(appContext);
  CHPP_ASSERT_LOG(!transportContext->initialized,
                  "CHPP transport already init");

  CHPP_LOGD("Initializing CHPP transport");

  chppResetTransportContext(transportContext);
  chppMutexInit(&transportContext->mutex);
  chppNotifierInit(&transportContext->notifier);
  chppConditionVariableInit(&transportContext->resetCondVar);
#ifdef CHPP_ENABLE_WORK_MONITOR
  chppWorkMonitorInit(&transportContext->workMonitor);
#endif

  transportContext->appContext = appContext;
  transportContext->initialized = true;

  chppPlatformLinkInit(&transportContext->linkParams);
}

void chppTransportDeinit(struct ChppTransportState *transportContext) {
  CHPP_NOT_NULL(transportContext);
  CHPP_ASSERT_LOG(transportContext->initialized,
                  "CHPP transport already deinitialized");

  chppPlatformLinkDeinit(&transportContext->linkParams);
#ifdef CHPP_ENABLE_WORK_MONITOR
  chppWorkMonitorDeinit(&transportContext->workMonitor);
#endif
  chppConditionVariableDeinit(&transportContext->resetCondVar);
  chppNotifierDeinit(&transportContext->notifier);
  chppMutexDeinit(&transportContext->mutex);

  chppClearTxDatagramQueue(transportContext);

  transportContext->initialized = false;
}

bool chppTransportWaitForResetComplete(
    struct ChppTransportState *transportContext, uint64_t timeoutMs) {
  bool success = true;
  chppMutexLock(&transportContext->mutex);
  while (success && transportContext->resetState != CHPP_RESET_STATE_NONE) {
    success = chppConditionVariableTimedWait(&transportContext->resetCondVar,
                                             &transportContext->mutex,
                                             timeoutMs * CHPP_NSEC_PER_MSEC);
  }
  chppMutexUnlock(&transportContext->mutex);
  return success;
}

bool chppRxDataCb(struct ChppTransportState *context, const uint8_t *buf,
                  size_t len) {
  CHPP_NOT_NULL(buf);
  CHPP_NOT_NULL(context);

  chppMutexLock(&context->mutex);
  if (context->rxStatus.state != CHPP_STATE_PREAMBLE &&
      chppGetCurrentTimeNs() >
          context->rxStatus.packetStartTimeNs + CHPP_TRANSPORT_RX_TIMEOUT_NS) {
    CHPP_LOGE("Packet RX timeout");
    chppAbortRxPacket(context);
  }
  chppMutexUnlock(&context->mutex);

  CHPP_LOGD("RX %" PRIuSIZE " bytes: state=%" PRIu8, len,
            context->rxStatus.state);
  uint64_t now = chppGetCurrentTimeNs();
  context->rxStatus.lastDataTimeMs = (uint32_t)(now / CHPP_NSEC_PER_MSEC);
  context->rxStatus.numTotalDataBytes += len;

  size_t consumed = 0;
  while (consumed < len) {
    chppMutexLock(&context->mutex);
    // TODO: Investigate fine-grained locking, e.g. separating variables that
    // are only relevant to a particular path.
    // Also consider removing some of the finer-grained locks altogether for
    // non-multithreaded environments with clear documentation.

    switch (context->rxStatus.state) {
      case CHPP_STATE_PREAMBLE:
        consumed +=
            chppConsumePreamble(context, &buf[consumed], len - consumed);
        break;

      case CHPP_STATE_HEADER:
        consumed += chppConsumeHeader(context, &buf[consumed], len - consumed);
        break;

      case CHPP_STATE_PAYLOAD:
        consumed += chppConsumePayload(context, &buf[consumed], len - consumed);
        break;

      case CHPP_STATE_FOOTER:
        consumed += chppConsumeFooter(context, &buf[consumed], len - consumed);
        break;

      default:
        CHPP_DEBUG_ASSERT_LOG(false, "Invalid RX state %" PRIu8,
                              context->rxStatus.state);
        chppSetRxState(context, CHPP_STATE_PREAMBLE);
    }

    chppMutexUnlock(&context->mutex);
  }

  return (context->rxStatus.state == CHPP_STATE_PREAMBLE &&
          context->rxStatus.locInState == 0);
}

void chppRxPacketCompleteCb(struct ChppTransportState *context) {
  chppMutexLock(&context->mutex);
  if (context->rxStatus.state != CHPP_STATE_PREAMBLE) {
    CHPP_LOGE("RX pkt ended early: state=%" PRIu8 " seq=%" PRIu8
              " len=%" PRIu16,
              context->rxStatus.state, context->rxHeader.seq,
              context->rxHeader.length);
    chppAbortRxPacket(context);
    chppEnqueueTxPacket(context, CHPP_TRANSPORT_ERROR_HEADER);  // NACK
  }
  chppMutexUnlock(&context->mutex);
}

bool chppEnqueueTxDatagramOrFail(struct ChppTransportState *context, void *buf,
                                 size_t len) {
  bool success = false;
  bool resetting = (context->resetState == CHPP_RESET_STATE_RESETTING);

  if (len == 0) {
    CHPP_DEBUG_ASSERT_LOG(false, "Enqueue datagram len=0!");

  } else if (resetting || !chppEnqueueTxDatagram(
                              context, CHPP_TRANSPORT_ERROR_NONE, buf, len)) {
    uint8_t *handle = buf;
    CHPP_LOGE("Resetting=%d. Discarding %" PRIuSIZE " bytes for H#%" PRIu8,
              resetting, len, *handle);

    CHPP_FREE_AND_NULLIFY(buf);

  } else {
    success = true;
  }

  return success;
}

// TODO(b/192359485): Consider removing this function, or making it more robust.
void chppEnqueueTxErrorDatagram(struct ChppTransportState *context,
                                enum ChppTransportErrorCode errorCode) {
  bool resetting = (context->resetState == CHPP_RESET_STATE_RESETTING);
  if (resetting) {
    CHPP_LOGE("Discarding app error 0x%" PRIx8 " (resetting)", errorCode);
  } else {
    switch (errorCode) {
      case CHPP_TRANSPORT_ERROR_OOM: {
        CHPP_LOGD("App layer enqueueing CHPP_TRANSPORT_ERROR_OOM");
        break;
      }
      case CHPP_TRANSPORT_ERROR_APPLAYER: {
        CHPP_LOGD("App layer enqueueing CHPP_TRANSPORT_ERROR_APPLAYER");
        break;
      }
      default: {
        // App layer should not invoke any other errors
        CHPP_DEBUG_ASSERT_LOG(false, "App enqueueing invalid err=%" PRIu8,
                              errorCode);
      }
    }
    chppEnqueueTxPacket(context, CHPP_ATTR_AND_ERROR_TO_PACKET_CODE(
                                     CHPP_TRANSPORT_ATTR_NONE, errorCode));
  }
}

uint64_t chppTransportGetTimeUntilNextDoWorkNs(
    struct ChppTransportState *context) {
  uint64_t currentTime = chppGetCurrentTimeNs();
  uint64_t nextDoWorkTime = context->appContext->nextRequestTimeoutNs;

  if (context->txStatus.hasPacketsToSend ||
      context->resetState == CHPP_RESET_STATE_RESETTING) {
    nextDoWorkTime =
        MIN(nextDoWorkTime, CHPP_TRANSPORT_TX_TIMEOUT_NS +
                                ((context->txStatus.lastTxTimeNs == 0)
                                     ? currentTime
                                     : context->txStatus.lastTxTimeNs));
  }

  CHPP_LOGD("NextDoWork=%" PRIu64 " currentTime=%" PRIu64 " delta=%" PRId64,
            nextDoWorkTime / CHPP_NSEC_PER_MSEC,
            currentTime / CHPP_NSEC_PER_MSEC,
            (nextDoWorkTime - currentTime) / (int64_t)CHPP_NSEC_PER_MSEC);

  if (nextDoWorkTime == CHPP_TIME_MAX) {
    return CHPP_TRANSPORT_TIMEOUT_INFINITE;
  } else if (nextDoWorkTime <= currentTime) {
    return CHPP_TRANSPORT_TIMEOUT_IMMEDIATE;
  } else {
    return nextDoWorkTime - currentTime;
  }
}

void chppWorkThreadStart(struct ChppTransportState *context) {
  chppTransportSendReset(context, CHPP_TRANSPORT_ATTR_RESET,
                         CHPP_TRANSPORT_ERROR_NONE);
  CHPP_LOGD("CHPP Work Thread started");

  uint32_t signals;
  do {
    uint64_t timeout = chppTransportGetTimeUntilNextDoWorkNs(context);
    if (timeout == CHPP_TRANSPORT_TIMEOUT_IMMEDIATE) {
      signals = chppNotifierGetSignal(&context->notifier);
    } else if (timeout == CHPP_TRANSPORT_TIMEOUT_INFINITE) {
      signals = chppNotifierWait(&context->notifier);
    } else {
      signals = chppNotifierTimedWait(&context->notifier, timeout);
    }

  } while (chppWorkThreadHandleSignal(context, signals));
}

bool chppWorkThreadHandleSignal(struct ChppTransportState *context,
                                uint32_t signals) {
  bool continueProcessing = false;

#ifdef CHPP_ENABLE_WORK_MONITOR
  chppWorkMonitorPreProcess(&context->workMonitor);
#endif

  if (signals & CHPP_TRANSPORT_SIGNAL_EXIT) {
    CHPP_LOGD("CHPP Work Thread terminated");
  } else {
    continueProcessing = true;
    if (signals & CHPP_TRANSPORT_SIGNAL_EVENT) {
      chppTransportDoWork(context);
    }

    if (signals == 0) {
      // Triggered by timeout

      if (chppGetCurrentTimeNs() - context->txStatus.lastTxTimeNs >=
          CHPP_TRANSPORT_TX_TIMEOUT_NS) {
        CHPP_LOGE("ACK timeout. Tx t=%" PRIu64,
                  context->txStatus.lastTxTimeNs / CHPP_NSEC_PER_MSEC);
        chppTransportDoWork(context);
      }

      if ((context->resetState == CHPP_RESET_STATE_RESETTING) &&
          (chppGetCurrentTimeNs() - context->resetTimeNs >=
           CHPP_TRANSPORT_RESET_TIMEOUT_NS)) {
        if (context->resetCount + 1 < CHPP_TRANSPORT_MAX_RESET) {
          CHPP_LOGE("RESET-ACK timeout; retrying");
          context->resetCount++;
          chppReset(context, CHPP_TRANSPORT_ATTR_RESET,
                    CHPP_TRANSPORT_ERROR_TIMEOUT);
        } else {
          CHPP_LOGE("RESET-ACK timeout; giving up");
          context->resetState = CHPP_RESET_STATE_PERMANENT_FAILURE;
          chppClearTxDatagramQueue(context);
        }
      }
    }

    if (signals & CHPP_TRANSPORT_SIGNAL_PLATFORM_MASK) {
      chppPlatformLinkDoWork(&context->linkParams,
                             signals & CHPP_TRANSPORT_SIGNAL_PLATFORM_MASK);
    }
  }

#ifdef CHPP_ENABLE_WORK_MONITOR
  chppWorkMonitorPostProcess(&context->workMonitor);
#endif

  return continueProcessing;
}

void chppWorkThreadStop(struct ChppTransportState *context) {
  chppNotifierSignal(&context->notifier, CHPP_TRANSPORT_SIGNAL_EXIT);
}

void chppLinkSendDoneCb(struct ChppPlatformLinkParameters *params,
                        enum ChppLinkErrorCode error) {
  if (error != CHPP_LINK_ERROR_NONE_SENT) {
    CHPP_LOGE("Async send failure: %" PRIu8, error);
  }

  struct ChppTransportState *context =
      container_of(params, struct ChppTransportState, linkParams);

  chppMutexLock(&context->mutex);

  context->txStatus.linkBusy = false;

  // No need to free anything as pendingTxPacket.payload is static. Likewise, we
  // keep pendingTxPacket.length to assist testing.

  chppMutexUnlock(&context->mutex);
}

void chppDatagramProcessDoneCb(struct ChppTransportState *context,
                               uint8_t *buf) {
  UNUSED_VAR(context);

  CHPP_FREE_AND_NULLIFY(buf);
}

uint8_t chppRunTransportLoopback(struct ChppTransportState *context,
                                 uint8_t *buf, size_t len) {
  UNUSED_VAR(buf);
  UNUSED_VAR(len);
  uint8_t result = CHPP_APP_ERROR_UNSUPPORTED;
  context->loopbackResult = result;

#ifdef CHPP_CLIENT_ENABLED_TRANSPORT_LOOPBACK
  result = CHPP_APP_ERROR_NONE;
  context->loopbackResult = CHPP_APP_ERROR_UNSPECIFIED;

  if (len == 0 || len > CHPP_TRANSPORT_TX_MTU_BYTES) {
    result = CHPP_APP_ERROR_INVALID_LENGTH;
    context->loopbackResult = result;

  } else if (context->txStatus.linkBusy) {
    result = CHPP_APP_ERROR_BLOCKED;
    context->loopbackResult = result;

  } else if (context->transportLoopbackData.payload != NULL) {
    result = CHPP_APP_ERROR_BUSY;
    context->loopbackResult = result;

  } else if ((context->transportLoopbackData.payload = chppMalloc(len)) ==
             NULL) {
    result = CHPP_APP_ERROR_OOM;
    context->loopbackResult = result;

  } else {
    context->transportLoopbackData.length = len;
    memcpy(context->transportLoopbackData.payload, buf, len);

    context->txStatus.linkBusy = true;
    context->pendingTxPacket.length = 0;
    memset(&context->pendingTxPacket.payload, 0, CHPP_LINK_TX_MTU_BYTES);
    context->pendingTxPacket.length +=
        chppAddPreamble(&context->pendingTxPacket.payload[0]);

    struct ChppTransportHeader *txHeader =
        (struct ChppTransportHeader *)&context->pendingTxPacket
            .payload[context->pendingTxPacket.length];
    context->pendingTxPacket.length += sizeof(*txHeader);

    txHeader->packetCode = CHPP_ATTR_AND_ERROR_TO_PACKET_CODE(
        CHPP_TRANSPORT_ATTR_LOOPBACK_REQUEST, txHeader->packetCode);

    size_t payloadLen = MIN(len, CHPP_TRANSPORT_TX_MTU_BYTES);
    txHeader->length = (uint16_t)payloadLen;
    chppAppendToPendingTxPacket(&context->pendingTxPacket, buf, payloadLen);

    chppAddFooter(&context->pendingTxPacket);

    CHPP_LOGD("Sending transport-loopback request (packet len=%" PRIuSIZE
              ", payload len=%" PRIu16 ", asked len was %" PRIuSIZE ")",
              context->pendingTxPacket.length, txHeader->length, len);
    enum ChppLinkErrorCode error = chppSendPendingPacket(context);

    if (error != CHPP_LINK_ERROR_NONE_QUEUED) {
      // Either sent synchronously or an error has occurred
      chppLinkSendDoneCb(&context->linkParams, error);

      if (error != CHPP_LINK_ERROR_NONE_SENT) {
        // An error has occurred
        CHPP_FREE_AND_NULLIFY(context->transportLoopbackData.payload);
        context->transportLoopbackData.length = 0;
        result = CHPP_APP_ERROR_UNSPECIFIED;
      }
    }
  }

  if (result != CHPP_APP_ERROR_NONE) {
    CHPP_LOGE("Trans-loopback failure: %" PRIu8, result);
  }
#endif
  return result;
}

void chppTransportSendReset(struct ChppTransportState *context,
                            enum ChppTransportPacketAttributes resetType,
                            enum ChppTransportErrorCode error) {
  // Make sure CHPP is in an initialized state
  CHPP_ASSERT_LOG((context->txDatagramQueue.pending == 0 &&
                   context->txDatagramQueue.front == 0),
                  "Not init to send reset");

  struct ChppTransportConfiguration *config =
      chppMalloc(sizeof(struct ChppTransportConfiguration));
  if (config == NULL) {
    CHPP_LOG_OOM();
  } else {
    // CHPP transport version
    config->version.major = 1;
    config->version.minor = 0;
    config->version.patch = 0;

    // Rx MTU size
    config->rxMtu = CHPP_PLATFORM_LINK_RX_MTU_BYTES;

    // Max Rx window size
    // Note: current implementation does not support a window size >1
    config->windowSize = 1;

    // Advertised transport layer (ACK) timeout
    config->timeoutInMs = CHPP_PLATFORM_TRANSPORT_TIMEOUT_MS;

    if (resetType == CHPP_TRANSPORT_ATTR_RESET_ACK) {
      CHPP_LOGD("Sending RESET-ACK");
    } else {
      CHPP_LOGD("Sending RESET");
    }

    if (resetType == CHPP_TRANSPORT_ATTR_RESET_ACK) {
      chppSetResetComplete(context);
    }

    context->resetTimeNs = chppGetCurrentTimeNs();

    chppEnqueueTxDatagram(context,
                          CHPP_ATTR_AND_ERROR_TO_PACKET_CODE(resetType, error),
                          config, sizeof(*config));
  }
}