added some doc strings. there's a new TODO of inverse dynamics but no…

… algorithsm are using this yet.

src/mjsim.jl

@@ -48,6 +48,17 @@ struct MJSim{MN, DN, S, SE, A}
     skip::Int
 end
 
+"""
+    MJSim(m::jlModel, d::jlData; skip::Integer=1)
+
+Construct an MJSim struct with a user specified jlModel and jlData. See MuJoCo.jl for more info on
+these structures.
+
+'skip' is defined as the number of times the simulation is integrated. The ctrl field in jlData is
+held as the same value for each integration steps. The effective timestep of the simulation thus becomes
+
+skip * m.opt.timestep
+"""
 function MJSim(m::jlModel, d::jlData; skip::Integer=1)
     skip > 0 || error("`skip` must be > 0")
 
@@ -97,6 +108,13 @@ function MJSim(m::jlModel, d::jlData; skip::Integer=1)
     forward!(sim)
 end
 
+"""
+    MJSim(modelpath::String, args...; kwargs...)
+
+This function automatically constructs a jlModel, jlData for a given MuJoCo XML model file path.
+
+args, kwargs allows for customized sim constructors to be created. For instance, a function may be defined for an environment that allows for dynamic model parameter randomization at load time; the fields and values can be passed in thusly.
+"""
 function MJSim(modelpath::String, args...; kwargs...)
     m = jlModel(modelpath)
     d = jlData(m)
@@ -254,13 +272,13 @@ Step the simulation by `skip` steps, where `skip` defaults to `MJSim.skip`.
 State-dependent controls (e.g. MJSim.d.{ctrl, xfrc_applied, qfrc_applied})
 should be set before calling `step!`.
 """
-function step!(sim::MJSim)
+function step!(sim::MJSim, skip::Integer=sim.skip)
     # According to MuJoCo docs order must be:
     # 1. mj_step1
     # 2. set_{ctrl, xfrc_applied, qfrc_applied}
     # 3. mj_step2
     # This implies user should call setaction! then step!
-    for _=1:sim.skip
+    for _=1:skip
         mj_step2(sim.m, sim.d)
         mj_step1(sim.m, sim.d)
     end
@@ -270,7 +288,14 @@ end
 step!(sim::MJSim, a) = (setaction!(sim, a); step!(sim))
 
 
+"""
+    zeroctrl!(sim::MJSim) = (fill!(sim.d.ctrl, zero(mjtNum)); sim)
 
+Zero out the mjData fields thats contribute to forward dynamics calculations, namely:
+   d.ctrl
+   d.qfrc_applied
+   d.xfrc_applied
+"""
 @inline zeroctrl!(sim::MJSim) = (fill!(sim.d.ctrl, zero(mjtNum)); sim)
 function zerofullctrl!(sim::MJSim)
     fill!(sim.d.ctrl, zero(mjtNum))
@@ -279,6 +304,11 @@ function zerofullctrl!(sim::MJSim)
     sim
 end
 
+"""
+    masscenter(sim::MJSim)
+
+Calculate the center of mass by summing up the Cartesian coordinates of each body.
+"""
 function masscenter(sim::MJSim)
     mcntr = zeros(SVector{3, Float64})
     mtotal = 0.0
@@ -293,15 +323,47 @@ function masscenter(sim::MJSim)
     mcntr / mtotal
 end
 
+"""
+    forward!(sim::MJSim) = (mj_forward(sim.m, sim.d); sim)
+"""
 @inline forward!(sim::MJSim) = (mj_forward(sim.m, sim.d); sim)
+"""
+    forwardskip!(sim::MJSim, skipstage::MJCore.mjtStage=MJCore.mjSTAGE_NONE, skipsensor::Bool=false)
+
+Wraps mj_forwardSkip, to allow for optimization of forward dynamics calculations by skipping appropriate calculation steps. 
+
+The enum selecting for which stages are skipped are:
+
+@cenum mjtStage::Cint begin     # computation stage
+    mjSTAGE_NONE = 0            # no computations
+    mjSTAGE_POS                 # position-dependent computations
+    mjSTAGE_VEL                 # velocity-dependent computations
+    mjSTAGE_ACC                 # acceleration/force-dependent computations
+end
+
+skipsensor == true will avoid the sensor computations.
+
+More information can be found at http://mujoco.org/book/programming.html under 'Forward Dynamics'
+"""
 @inline function forwardskip!(sim::MJSim, skipstage::MJCore.mjtStage=MJCore.mjSTAGE_NONE, skipsensor::Bool=false)
     mj_forwardSkip(sim.m, sim.d, skipstage, skipsensor)
     sim
 end
 
+#TODO inverse and inverseskip
+
+"""
+    timestep(sim::MJSim) = sim.m.opt.timestep
+"""
 timestep(sim::MJSim) = sim.m.opt.timestep
+"""
+    effective_timestep(sim::MJSim) = timestep(sim) * sim.skip
+"""
 effective_timestep(sim::MJSim) = timestep(sim) * sim.skip
+"""
+    Base.time(sim::MJSim) = sim.d.time
+"""
 Base.time(sim::MJSim) = sim.d.time
 
 _maybecopyto!(::Shapes.Length{0}, src, dest) = src
-_maybecopyto!(::Shapes.Length, src, dest) = copyto!(src, dest)
+_maybecopyto!(::Shapes.Length, src, dest) = copyto!(src, dest)