Store pointer in Neighbourhood Search grid instead of Agents#1594
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Timing comparison for the changes implemented in this PR compared to the master branch. 30-35 % speed-up in the neighbourhood search program part. The influence on the overall runtime is very small, however the memory footprint is considerably smaller. Open for discussion on which part of the PR, we want to use! 
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Looks good!
Please clean up the commits as we can merge this.
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Instead of saving agent copies on grid we moved to storing const pointers Agents are now conationed in deque no longer vector
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Currently, the Neighbourhood Search stores all agents on a 2D-grid.
This is done by storing a copy of the Agent in a vector per grid point.
Essentially, this means that all agents are stored 2 times: once in the overall vector containing the agents and once on the grid inside Neighbourhood search.
If a model or another function requests all agents within a certain range, the agents are copied again into a new vector that is returned.
In this PR, I propose to store not the agent itself on the grid but rather a pointer to that agent. This removes the additional copy of alle agents in the neighbourhood system.
To make sure that theses references are valid during an iteration cycle, the container holding the agents is changed from a
std::vectorto astd::deque. In general, the container holding the agents is now defined inGenericAgent.hppand used throughout the codebase.I also removed the
AddAgentcall to the neighbourhood system, since the neighbourhood system is updated in the iteration cycle anyhow. One possibility, where the neighbourhood system would be empty is ifAgentsInPolygonorAgentsInRangeis called, therefore a grid update is performed prior to getting the neighbours. Potentially, a state variable could be useful if these functions are called often.I also added a performance test with 15k agents in a large geometry, as well as a notebook that can be used to draw box plots for the timing results of various program components.
Optionally, I also added a function that gets the pointers of the agents in a certain range instead of a copy of the agent, I also include it in my benchmark comparison.
Preview: https://PedestrianDynamics.github.io/jupedsim/pull-requests/1594/