About Particle Collision Algorithms in dsmc._update_velocities() Function

[heishanxiaoyao]

The "for i in range(1, N, 2):" code in Line 164 (dsmc.py) confused me, why not use a double loop to judge the collision of any particle in the box? Like this:
for i in range(N):
for j in range(i + 1, N):
p1 = permutations[offset + i ]
p2 = permutations[offset + j]
...
Sincerely hope to get your answer.