struct SimConsts
    n_particles::Int64
    v₀::Float64
    δt::Float64
    packing_ratio::Float64
    μ::Float64
    Dₜ::Float64
    particle_radius::Float64
    Dᵣ::Float64
    σ::Float64
    ϵ::Float64
    interaction_r::Float64
    skin_r::Float64
    n_steps_before_verlet_list_update::Int64
    grid_n::Int64
    half_box_len::Float64
    grid_box_width::Float64
end

struct RunParams
    duration::Float64
    snapshot_at::Float64
    seed::Int64
    n_bundle_snapshots::Int64
    integration_steps::Int64
    n_steps_before_snapshot::Int64
    n_snapshots::Int64
    T::Float64
    T0::Float64
    start_datetime::Dates.DateTime
end

struct SimState
    n_bundles::Int64
    T::Float64
end

struct Bundle
    t::Vector{Float64}
    c::Matrix{SVector{2,Float64}}
    φ::Matrix{Float64}
end

function Bundle(n_particles::Int64, n_snapshots::Int64)
    return Bundle(
        Vector{Float64}(undef, n_snapshots),
        Matrix{SVector{2,Float64}}(undef, (n_particles, n_snapshots)),
        Matrix{Float64}(undef, (n_particles, n_snapshots)),
    )
end

function first_n_snapshots(bundle::Bundle, n::Int64)
    return Bundle(bundle.t[1:n], bundle.c[:, 1:n], bundle.φ[:, 1:n])
end

function save_snapshot!(
    bundle::Bundle, n_snapshot::Int64, t::Float64, particles::Vector{Particle}
)
    bundle.t[n_snapshot] = t

    @simd for p in particles
        bundle.c[p.id, n_snapshot] = p.c

        bundle.φ[p.id, n_snapshot] = p.φ
    end

    return nothing
end

function save_bundle(dir::String, bundle::Bundle, n_bundle::Int64, T::Float64)
    bundles_dir = "$dir/bundles"
    mkpath(bundles_dir)

    JLD2.save_object("$bundles_dir/bundle_$n_bundle.jld2", bundle)

    JLD2.save_object("$dir/sim_state.jld2", SimState(n_bundle, round(T; digits=3)))

    return nothing
end