connected-device-lib/src/com/android/car/connecteddevice/ble/BlePacketFactory.java

/*
 * 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.
 */
package com.android.car.connecteddevice.ble;

import static com.android.car.connecteddevice.util.SafeLog.loge;

import androidx.annotation.VisibleForTesting;

import com.android.car.connecteddevice.BleStreamProtos.BlePacketProto.BlePacket;
import com.android.car.protobuf.ByteString;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

/**
 * Factory for creating {@link BlePacket} protos.
 */
class BlePacketFactory {
    private static final String TAG = "BlePacketFactory";

    /**
     * The size in bytes of a {@code fixed32} field in the proto.
     */
    private static final int FIXED_32_SIZE = 4;

    /**
     * The bytes needed to encode the field number in the proto.
     *
     * <p>Since the {@link BlePacket} only has 4 fields, it will only take 1 additional byte to
     * encode.
     */
    private static final int FIELD_NUMBER_ENCODING_SIZE = 1;

    /**
     * The size in bytes of field {@code packet_number}. The proto field is a {@code fixed32}.
     */
    private static final int PACKET_NUMBER_ENCODING_SIZE =
            FIXED_32_SIZE + FIELD_NUMBER_ENCODING_SIZE;

    /**
     * Split given data if necessary to fit within the given {@code maxSize}.
     *
     * @param payload The payload to potentially split across multiple {@link BlePacket}s.
     * @param messageId The unique id for identifying message.
     * @param maxSize The maximum size of each chunk.
     * @return A list of {@link BlePacket}s.
     * @throws BlePacketFactoryException if an error occurred during the splitting of data.
     */
    static List<BlePacket> makeBlePackets(byte[] payload, int messageId, int maxSize)
            throws BlePacketFactoryException {
        List<BlePacket> blePackets = new ArrayList<>();
        int payloadSize = payload.length;
        int totalPackets = getTotalPacketNumber(messageId, payloadSize, maxSize);
        int maxPayloadSize = maxSize
                - getPacketHeaderSize(totalPackets, messageId, Math.min(payloadSize, maxSize));

        int start = 0;
        int end = Math.min(payloadSize, maxPayloadSize);
        for (int packetNum = 1; packetNum <= totalPackets; packetNum++) {
            blePackets.add(BlePacket.newBuilder()
                    .setPacketNumber(packetNum)
                    .setTotalPackets(totalPackets)
                    .setMessageId(messageId)
                    .setPayload(ByteString.copyFrom(Arrays.copyOfRange(payload, start, end)))
                    .build());
            start = end;
            end = Math.min(start + maxPayloadSize, payloadSize);
        }
        return blePackets;
    }

    /**
     * Compute the header size for the {@link BlePacket} proto in bytes. This method assumes that
     * the proto contains a payload.
     */
    @VisibleForTesting
    static int getPacketHeaderSize(int totalPackets, int messageId, int payloadSize) {
        return FIXED_32_SIZE + FIELD_NUMBER_ENCODING_SIZE
                + getEncodedSize(totalPackets) + FIELD_NUMBER_ENCODING_SIZE
                + getEncodedSize(messageId) + FIELD_NUMBER_ENCODING_SIZE
                + getEncodedSize(payloadSize) + FIELD_NUMBER_ENCODING_SIZE;
    }

    /**
     * Compute the total packets required to encode a payload of the given size.
     */
    @VisibleForTesting
    static int getTotalPacketNumber(int messageId, int payloadSize, int maxSize)
            throws BlePacketFactoryException {
        int headerSizeWithoutTotalPackets = FIXED_32_SIZE + FIELD_NUMBER_ENCODING_SIZE
                + getEncodedSize(messageId) + FIELD_NUMBER_ENCODING_SIZE
                + getEncodedSize(Math.min(payloadSize, maxSize)) + FIELD_NUMBER_ENCODING_SIZE;

        for (int value = 1; value <= PACKET_NUMBER_ENCODING_SIZE; value++) {
            int packetHeaderSize = headerSizeWithoutTotalPackets + value
                    + FIELD_NUMBER_ENCODING_SIZE;
            int maxPayloadSize = maxSize - packetHeaderSize;
            if (maxPayloadSize < 0) {
                throw new BlePacketFactoryException("Packet header size too large.");
            }
            int totalPackets = (int) Math.ceil(payloadSize / (double) maxPayloadSize);
            if (getEncodedSize(totalPackets) == value) {
                return totalPackets;
            }
        }

        loge(TAG, "Cannot get valid total packet number for message: messageId: "
                + messageId + ", payloadSize: " + payloadSize + ", maxSize: " + maxSize);
        throw new BlePacketFactoryException("No valid total packet number.");
    }

    /**
     * This method implements Protocol Buffers encoding algorithm.
     *
     * <p>Computes the number of bytes that would be needed to store a 32-bit variant.
     *
     * @param value the data that need to be encoded
     * @return the size of the encoded data
     * @see <a href="https://developers.google.com/protocol-buffers/docs/encoding#varints">
     *     Protocol Buffers Encoding</a>
     */
    private static int getEncodedSize(int value) {
        if (value < 0) {
            return 10;
        }
        if ((value & (~0 << 7)) == 0) {
            return 1;
        }
        if ((value & (~0 << 14)) == 0) {
            return 2;
        }
        if ((value & (~0 << 21)) == 0) {
            return 3;
        }
        if ((value & (~0 << 28)) == 0) {
            return 4;
        }
        return 5;
    }

    private BlePacketFactory() {}
}