function empty_hook(args...)
    return nothing
end

function run_sim(
    dir::String;
    duration::Float64,
    snapshot_at::Float64=0.1,
    seed::Int64=42,
    n_bundle_snapshots::Int64=100,
    env_helper::Union{RL.EnvHelper,Nothing}=nothing,
)
    @assert length(dir) > 0
    @assert duration > 0
    @assert snapshot_at in 0.001:0.001:duration
    @assert seed > 0
    @assert n_bundle_snapshots >= 0

    Random.seed!(seed)

    sim_consts = JSON3.read(read("$dir/sim_consts.json", String))

    integration_steps = floor(Int64, duration / sim_consts.δt) + 1

    n_steps_before_snapshot = round(Int64, snapshot_at / sim_consts.δt)
    n_snapshots = floor(Int64, integration_steps / n_steps_before_snapshot) + 1

    @assert (n_snapshots - 1) * snapshot_at == duration

    n_bundle_snapshots = min(n_snapshots, n_bundle_snapshots)

    sim_state = JSON3.read(read("$dir/sim_state.json", String))
    n_bundles = sim_state.n_bundles

    T0::Float64 = sim_state.T
    T = T0 + duration

    if n_bundle_snapshots > 0
        save_data = true
    else
        save_data = false
    end

    @async begin
        start_datetime = now()

        run_params = (;
            # Input
            duration,
            snapshot_at,
            seed,
            n_bundle_snapshots,
            # Internal
            integration_steps,
            n_steps_before_snapshot,
            n_snapshots,
            T,
            T0,
            start_datetime,
        )

        next_bundle = n_bundles + 1

        runs_dir = "$dir/runs"

        if save_data
            write_struct_to_json(run_params, "$runs_dir/run_params_$next_bundle")
        end
    end

    bundles_dir = "$dir/bundles"
    bundle = JLD2.load_object("$bundles_dir/bundle_$n_bundles.jld2")

    particles = gen_particles(bundle, sim_consts.n_particles)

    args = (
        v₀=sim_consts.v₀,
        δt=sim_consts.δt,
        skin_r=sim_consts.skin_r,
        skin_r²=sim_consts.skin_r^2,
        n_snapshots=n_snapshots,
        c₁=4 * sim_consts.ϵ * 6 * sim_consts.σ^6 * sim_consts.δt * sim_consts.μ,
        c₂=2 * sim_consts.σ^6,
        c₃=sqrt(2 * sim_consts.D₀ * sim_consts.δt),
        c₄=sqrt(2 * sim_consts.Dᵣ * sim_consts.δt),
        v₀δt=sim_consts.v₀ * sim_consts.δt,
        μ=sim_consts.μ,
        interaction_r=sim_consts.interaction_r,
        interaction_r²=sim_consts.interaction_r^2,
        n_particles=sim_consts.n_particles,
        half_box_len=sim_consts.half_box_len,
        particles=particles,
        particles_c=[particles[i].c for i in 1:(sim_consts.n_particles)],
        verlet_lists=[
            PreVector{Int64}(sim_consts.n_particles - i) for
            i in 1:(sim_consts.n_particles - 1)
        ],
        n_bundle_snapshots=n_bundle_snapshots,
        bundle=Bundle(sim_consts.n_particles, n_bundle_snapshots),
        box=Box(
            SVector(2 * sim_consts.half_box_len, 2 * sim_consts.half_box_len),
            sim_consts.skin_r,
        ),
    )

    simulate!(
        args,
        T0,
        T,
        sim_consts.n_steps_before_verlet_list_update,
        n_steps_before_snapshot,
        n_bundles,
        dir,
        save_data,
        env_helper,
    )

    return nothing
end