What is integration_steps referring to?

[kx-au]

What does the chain output info.integration_steps refer to precisely? In the documentation it says the number of steps in the full trajectory = number of integration steps taken. Say the value is N, does that mean:
(1) The current sampler position is N steps away from the previous position? (e.g. a standard HMC can take N steps to achieve the same integration length)
(2) N is the total number of integration steps required for one NUTS iteration, where the final sampled position is <N steps away from the previous position, e.g. N//2? (similar to how the standard NUTS generate the tree but only really uses half of it for the so-called "final trajectory")

On that note, what would be the number of gradient evaluations for one NUTS iteration (in terms of N)?

Thank you so much!

[rlouf]

integration_steps (which really should be named num_integration_steps) corresponds to the number of times the symplectic integrator was run building the NUTS trajectory (including the "wasted" half). So for the velocity verlet integrator (default) N would be the number of gradient evaluation.

Indeed you can see that the number returned here is the length of the full trajectory that was built.

Does that make sense? I'll open an issue to make sure this is better documented. Thank you!