1.cpp

auto computeSum(char* fileName) {
    auto fIn = open(fileName, O_RDONLY | O_BINARY, 0644);

    static constexpr size_t BUFFER_SIZE = 1 << 16;
    uint8_t buffer[BUFFER_SIZE];
    uint8_t const* pBuffer = nullptr;
    size_t bufferPos = 0;
    size_t bufferLen = 0;

    int64_t sum = 0;
    uint8_t b;
    int n = 0;
    while (bufferLen = read(fIn, buffer, BUFFER_SIZE)) {
        pBuffer = buffer;
        const uint8_t* const pBufferEnd = buffer + bufferLen;
        while (pBuffer != pBufferEnd) {
            if (*pBuffer < 128) {
                n = (n << 7) + *pBuffer;
            } else {
                n = (n << 7) + *pBuffer - 128;
                sum += n;
                n = 0;
            }
            ++pBuffer;
        }
    }
    close(fIn);
    return sum;
}