From 42f9e14c00c1016250d932367cc96b12f23fbe46 Mon Sep 17 00:00:00 2001
From: Alex <32419459+Rampagy@users.noreply.github.com>
Date: Fri, 21 Sep 2018 17:19:40 -0500
Subject: [PATCH] Updated Calculation Order (#1019)

* Updated order Calculation

Updated order of `x` and `theta` calculations so that they are no longer one timestamp behind `x_dot` and `theta_dot`.

https://github.com/openai/gym/issues/1018

* Added semi-implicit euler option

* Got implicit and standard euler mixed up

* switched default option
---
 gym/envs/classic_control/cartpole.py | 15 +++++++++++----
 1 file changed, 11 insertions(+), 4 deletions(-)

diff --git a/gym/envs/classic_control/cartpole.py b/gym/envs/classic_control/cartpole.py
index dbc9b31cb..16324ead1 100644
--- a/gym/envs/classic_control/cartpole.py
+++ b/gym/envs/classic_control/cartpole.py
@@ -62,6 +62,7 @@ class CartPoleEnv(gym.Env):
         self.polemass_length = (self.masspole * self.length)
         self.force_mag = 10.0
         self.tau = 0.02  # seconds between state updates
+        self.kinematics_integrator = 'euler'
 
         # Angle at which to fail the episode
         self.theta_threshold_radians = 12 * 2 * math.pi / 360
@@ -97,10 +98,16 @@ class CartPoleEnv(gym.Env):
         temp = (force + self.polemass_length * theta_dot * theta_dot * sintheta) / self.total_mass
         thetaacc = (self.gravity * sintheta - costheta* temp) / (self.length * (4.0/3.0 - self.masspole * costheta * costheta / self.total_mass))
         xacc  = temp - self.polemass_length * thetaacc * costheta / self.total_mass
-        x  = x + self.tau * x_dot
-        x_dot = x_dot + self.tau * xacc
-        theta = theta + self.tau * theta_dot
-        theta_dot = theta_dot + self.tau * thetaacc
+        if self.kinematics_integrator == 'euler':
+            x  = x + self.tau * x_dot
+            x_dot = x_dot + self.tau * xacc
+            theta = theta + self.tau * theta_dot
+            theta_dot = theta_dot + self.tau * thetaacc
+        else: # semi-implicit euler
+            x_dot = x_dot + self.tau * xacc
+            x  = x + self.tau * x_dot
+            theta_dot = theta_dot + self.tau * thetaacc
+            theta = theta + self.tau * theta_dot
         self.state = (x,x_dot,theta,theta_dot)
         done =  x < -self.x_threshold \
                 or x > self.x_threshold \