persinammon / concurrent-traffic-simulation

Concurrency practice by implementing message queue, practicing with lock guard and condition variable (pattern to avoid deadlock and data race), using r-value pass in combined with std::move

Concurrent Traffic Simulation

This project practices C++ concurrency.

I found this resource I really liked which introduced multithreading and concurrency, especially because this line -

Multithreading often carries a reputation for being difficult. Compared with other concepts in software development, one could certainly make a case for that. However, multithreading isn’t really that different from general programming. It’s just potentially more dangerous . Learning to protect against the danger, though, can allow one to implement far more powerful algorithms and programs than you could in a single threaded manner."

Or, more or less, the data races are not 100% guaranteed to show up per run. You should be able to spot them on your own.

I was very interested in the fact that, because threads do not have a copy constructor, a vector of threads needs to be populated by the move constructor and therefore by using emplace_back; also, when multiple threads are running, interleaved work can present a bug such as interrupted print statements in cout- cout is a shared resource that can be accessed in the middle of executing the same method by another thread (??).

Notable code changes made in the project:

• The basic structure of the project was already implemented- vehicles heading to destinations, communicating with an open intersection, and passing through one at a time. What was implemented was the ability for a traffic light to have a red light, and therefore pause the flow of cars. In addition to FIFO of cars being let through, the lights cycle between red and green. Every Intersection owns a TrafficLight that owns a thread that runs an infinite loop sending color messages to a message queue also owned by and wrapped by TrafficLight.
• Fleshed out a generic message queue with implementation split across TrafficLight.h and TrafficLight.cpp. Added some connecting code in Intersection, with a notable lines being L87 to L91. Every method in TrafficLight.cpp is newly implemented.
• This wasn't in the project spec at all, but I wanted to pass in a parameter to the driver that would allow CLI toggling between the two cities Paris and NYC. The data and functions were already pre-written within the driver, so all it needed was a user argument. I changed the int main() function in TrafficSimulator-Final.cpp to int main(int argc, char** argv) and used the string to toggle, with the default/for malformed input being Paris. Had to change a C-style string to an std::string.

Dependencies for Running Locally

• cmake >= 2.8
  • All OSes: click here for installation instructions
• make >= 4.1 (Linux, Mac), 3.81 (Windows)
  • Linux: make is installed by default on most Linux distros
  • Mac: install Xcode command line tools to get make
  • Windows: Click here for installation instructions
• OpenCV >= 4.1
  • The OpenCV 4.1.0 source code can be found here
• gcc/g++ >= 5.4
  • Linux: gcc / g++ is installed by default on most Linux distros
  • Mac: same deal as make - install Xcode command line tools
  • Windows: recommend using MinGW

Basic Build Instructions

Two Options -

1. Only download the executable traffic_simulation from the build folder, and run it through the ./traffic_simulation command in the same directory.

Or-

1. Clone repo to local.
2. Make a build directory in the top level directory: mkdir build && cd build
3. Compile: cmake .. && make
4. Run it through the following command: ./traffic_simulation.

To run with Paris as the background, either ./traffic_simulation or ./traffic_simulation paris. To run with NYC as the background, ./traffic_simulation nyc.