Time grids

A solution stores the discretisation points at which its trajectories were computed. CTModels supports either one shared grid or one grid per component, and chooses the representation automatically.

using CTModels

pre = CTModels.PreModel()
CTModels.variable!(pre, 0)
CTModels.time!(pre; t0=0.0, tf=1.0)
CTModels.state!(pre, 2)
CTModels.control!(pre, 1)
CTModels.dynamics!(pre, (r, t, x, u, v) -> (r[1] = x[2]; r[2] = u[1]; nothing))
CTModels.objective!(pre, :min; lagrange=(t, x, u, v) -> 0.5u[1]^2)
CTModels.time_dependence!(pre; autonomous=true)
ocp = CTModels.build(pre)

One shared grid

When the four grids passed to build_solution are identical, CTModels collapses them into a UnifiedTimeGridModel to save memory:

T = collect(range(0.0, 1.0; length=101))
X = [1.0 - t/100 for t in 1:101, i in 1:2]
U = [sin(t/50) for t in 1:101, i in 1:1]
P = zeros(101, 2)

sol_u = CTModels.build_solution(ocp, T, T, T, T, X, U, Float64[], P;
    objective=0.5, iterations=10, constraints_violation=1e-6,
    message="ok", status=:optimal, successful=true)

(CTModels.time_grid_model(sol_u) isa CTModels.UnifiedTimeGridModel,
 length(CTModels.time_grid(sol_u)))

(true, 101)

One grid per component

Passing different grids for state, control, costate and path yields a MultipleTimeGridModel; each component is then queried by name through time_grid:

T_state   = collect(range(0.0, 1.0; length=101))
T_control = collect(range(0.0, 1.0; length=51))
T_costate = collect(range(0.0, 1.0; length=76))
T_path    = collect(range(0.0, 1.0; length=61))

X = [1.0 - t/100 for t in 1:101, i in 1:2]
U = [sin(t/25) for t in 1:51, i in 1:1]
P = zeros(76, 2)

sol_m = CTModels.build_solution(ocp, T_state, T_control, T_costate, T_path,
    X, U, Float64[], P;
    objective=0.5, iterations=10, constraints_violation=1e-6,
    message="ok", status=:optimal, successful=true)

(CTModels.time_grid_model(sol_m) isa CTModels.MultipleTimeGridModel,
 length(CTModels.time_grid(sol_m, :state)),
 length(CTModels.time_grid(sol_m, :control)),
 length(CTModels.time_grid(sol_m, :costate)))

(true, 101, 51, 76)

Component aliases

time_grid(sol, :states), time_grid(sol, :duals), … accept many synonyms. They are normalised to the four canonical grids (:state, :control, :costate, :path) by clean_component_symbols:

CTModels.clean_component_symbols((:states, :controls, :costate, :constraint, :duals))

(:state, :control, :costate, :path)

This is why box-constraint duals share the state/control grids, and path-constraint duals the :path grid — the mapping is centralised in one place rather than scattered across accessors.