Microscopic crowd locomotion models for rustsim.

This crate implements the five microscopic pedestrian models catalogued at https://pedestriandynamics.org/models/ in pure 2-D continuous space plus a layered 2.5-D extension in the [threed] module for multi-storey environments (stations, airports, stadiums, malls).

It is called rustsim-crowd rather than rustsim-pedestrian because the same physics apply equally to cyclists, evacuees, queue-formers, and generic self-propelled agents.

Module	Model	Class	Primary references
[`social_force`]	Social Force	Force-based, 2nd order	Helbing & Molnár 1995; Helbing, Farkas & Vicsek 2000
[`collision_free_speed`]	Collision-Free Speed	Velocity-based, 1st order	Tordeux, Chraibi & Seyfried 2016
[`generalized_centrifugal_force`]	Generalized Centrifugal Force	Force-based, 2nd order	Chraibi, Seyfried & Schadschneider 2010
[`optimal_steps`]	Optimal Steps	Discrete-step utility	Seitz & Köster 2012
[`anticipation_velocity`]	Anticipation Velocity	Velocity-based, 1st order	Xu, Chraibi & Seyfried 2021
[`threed`]	Layered 2.5-D Social Force	Per-floor 2-D physics + vertical connectors	Industry standard (JuPedSim, MassMotion, Legion)

API shape

Every 2-D model shares the same three-type contract defined by the [PedestrianModel] trait:

[Pedestrian] — the shared per-agent state (position, velocity, radius, desired speed, destination).
[WallSegment] — a 2-D line segment describing a static obstacle.
Params — a model-specific parameter bundle living inside each module.

Each model exposes:

a Params struct with stable literature/engineering defaults.
explicit [calibration] helpers for deployments that must enforce a published speed-density envelope such as Weidmann (1993).
a unit struct implementing [PedestrianModel] so callers can swap models behind a trait object or generic bound.
a free step(peds, walls, params, dt) function (deprecated since 0.0.3; O(n²) reference path retained for parity tests). Production callers use step_scratch (zero-alloc) or step_with_grid (broadphase) instead.

use rustsim_crowd::prelude::*;

let mut peds = vec![Pedestrian::new(
    [0.0, 0.0],
    [0.0, 0.0],
    0.25,
    1.34,
    [10.0, 0.0],
)];
let walls: Vec<WallSegment> = Vec::new();
let params = social_force::Params::default();
// Production hot path: zero-alloc, broadphase-accelerated.
let mut scratch = Scratch::new(recommended_cell_size(
    social_force::neighbor_cutoff(&params),
));
social_force::step_scratch(&mut peds, &walls, &params, 0.05, &mut scratch);

Design constraints

Pure f64, SoA-friendly internals, no interior mutability.
step_scratch(peds, walls, params, dt, &mut scratch) is the zero-alloc hot path: allocate one [broadphase::Scratch] per simulation and reuse it tick-after-tick. step_with_grid takes a caller-owned [NeighborGrid] and allocates one Vec<[f64; 2]> per tick; step is the O(n²) reference path with no broadphase.
Determinism: every step_* variant is a deterministic function of its inputs.

rustsim-crowd 0.0.1

API shape

Design constraints