def finish_path(self, last_val=0):
"""
Call this at the end of a trajectory, or when one gets cut off
by an epoch ending. This looks back in the buffer to where the
trajectory started, and uses rewards and value estimates from
the whole trajectory to compute advantage estimates with GAE-Lambda,
as well as compute the rewards-to-go for each state, to use as
the targets for the value function.
The "last_val" argument should be 0 if the trajectory ended
because the agent reached a terminal state (died), and otherwise
should be V(s_T), the value function estimated for the last state.
This allows us to bootstrap the reward-to-go calculation to account
for timesteps beyond the arbitrary episode horizon (or epoch cutoff).
"""
path_slice = slice(self.path_start_idx, self.ptr)
rews = np.append(self.rew_buf[path_slice], last_val)
vals = np.append(self.val_buf[path_slice], last_val)
# the next two lines implement GAE-Lambda advantage calculation
deltas = rews[:-1] + self.gamma * vals[1:] - vals[:-1]
self.adv_buf[path_slice] = core.discount_cumsum(deltas, self.gamma * self.lam)
# the next line computes rewards-to-go, to be targets for the value function
self.ret_buf[path_slice] = core.discount_cumsum(rews, self.gamma)[:-1]
self.path_start_idx = self.ptr
def update_adv(data):
data = core.to_numpy(data)
obs, act, rew, start_ptrs, last_d, last_o, ret, adv = data['obs'], data['act'], data['rew'], data['start_ptrs'], data['last_d'], data['last_o'], data['ret'], data['adv']
# the next two lines implement GAE-Lambda advantage calculation
num_paths = start_ptrs.shape[0]-1
for i in range(num_paths):
path_slice = slice(int(start_ptrs[i]), int(start_ptrs[i+1]))
rew_path = rew[path_slice]
obs_path = obs[path_slice]
_, val_path, _ = ac.step(torch.as_tensor(obs_path, dtype=torch.float32))
if i==num_paths-1 and last_d:
_, last_val, _ = ac.step(torch.as_tensor(last_o, dtype=torch.float32))
else:
last_val = 0
rew_path = np.append(rew_path, last_val)
val_path = np.append(val_path, last_val)
# the next two lines implement GAE-Lambda advantage calculation
delta_path = rew_path[:-1] + gamma * val_path[1:] - val_path[:-1]
adv_path = core.discount_cumsum(delta_path, gamma * lam)
# the next line computes rewards-to-go, to be targets for the value function
ret_path = core.discount_cumsum(rew_path, gamma)[:-1]
delta_path = rew_path[:-1] + gamma * val_path[1:] - val_path[:-1]
adv_path = core.discount_cumsum(delta_path, gamma * lam)
ret_path = core.discount_cumsum(rew_path, gamma)[:-1]
ret[path_slice] = ret_path
adv[path_slice] = adv_path
data = core.to_torch(data)
return(data)
