diff --git a/README.md b/README.md
index 4c2a0af..8f442e2 100644
--- a/README.md
+++ b/README.md
@@ -24,6 +24,7 @@
 - [Dueling DQN](https://arxiv.org/pdf/1511.06581.pdf)
 - [Categorical DQN (C51)](https://arxiv.org/pdf/1707.06887.pdf)
 - [Quantile Regression DQN (QRDQN)](https://arxiv.org/pdf/1710.10044.pdf)
+- [Implicit Quantile Network (IQN)](https://arxiv.org/pdf/1806.06923.pdf)
 - [Policy Gradient (PG)](https://papers.nips.cc/paper/1713-policy-gradient-methods-for-reinforcement-learning-with-function-approximation.pdf)
 - [Natural Policy Gradient (NPG)](https://proceedings.neurips.cc/paper/2001/file/4b86abe48d358ecf194c56c69108433e-Paper.pdf)
 - [Advantage Actor-Critic (A2C)](https://openai.com/blog/baselines-acktr-a2c/)
diff --git a/docs/api/tianshou.policy.rst b/docs/api/tianshou.policy.rst
index 32f124f..d857943 100644
--- a/docs/api/tianshou.policy.rst
+++ b/docs/api/tianshou.policy.rst
@@ -35,6 +35,11 @@ DQN Family
    :undoc-members:
    :show-inheritance:
 
+.. autoclass:: tianshou.policy.IQNPolicy
+   :members:
+   :undoc-members:
+   :show-inheritance:
+
 On-policy
 ~~~~~~~~~
 
diff --git a/docs/index.rst b/docs/index.rst
index fe8f7d7..87189fd 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -14,6 +14,7 @@ Welcome to Tianshou!
 * :class:`~tianshou.policy.DQNPolicy` `Dueling DQN <https://arxiv.org/pdf/1511.06581.pdf>`_
 * :class:`~tianshou.policy.C51Policy` `Categorical DQN <https://arxiv.org/pdf/1707.06887.pdf>`_
 * :class:`~tianshou.policy.QRDQNPolicy` `Quantile Regression DQN <https://arxiv.org/pdf/1710.10044.pdf>`_
+* :class:`~tianshou.policy.IQNPolicy` `Implicit Quantile Network <https://arxiv.org/pdf/1806.06923.pdf>`_
 * :class:`~tianshou.policy.PGPolicy` `Policy Gradient <https://papers.nips.cc/paper/1713-policy-gradient-methods-for-reinforcement-learning-with-function-approximation.pdf>`_
 * :class:`~tianshou.policy.NPGPolicy` `Natural Policy Gradient <https://proceedings.neurips.cc/paper/2001/file/4b86abe48d358ecf194c56c69108433e-Paper.pdf>`_
 * :class:`~tianshou.policy.A2CPolicy` `Advantage Actor-Critic <https://openai.com/blog/baselines-acktr-a2c/>`_
diff --git a/examples/atari/README.md b/examples/atari/README.md
index fb45858..47c50d3 100644
--- a/examples/atari/README.md
+++ b/examples/atari/README.md
@@ -54,6 +54,20 @@ One epoch here is equal to 100,000 env step, 100 epochs stand for 10M.
 | SeaquestNoFrameskip-v4      | 5676        | ![](results/qrdqn/Seaquest_rew.png)     | `python3 atari_qrdqn.py --task "SeaquestNoFrameskip-v4"`  |
 | SpaceInvadersNoFrameskip-v4      | 938        | ![](results/qrdqn/SpaceInvader_rew.png)     | `python3 atari_qrdqn.py --task "SpaceInvadersNoFrameskip-v4"`  |
 
+# IQN (single run)
+
+One epoch here is equal to 100,000 env step, 100 epochs stand for 10M.
+
+| task                        | best reward | reward curve                          | parameters                                                   |
+| --------------------------- | ----------- | ------------------------------------- | ------------------------------------------------------------ |
+| PongNoFrameskip-v4          | 20.9        | ![](results/iqn/Pong_rew.png)         | `python3 atari_iqn.py --task "PongNoFrameskip-v4" --batch-size 64` |
+| BreakoutNoFrameskip-v4      | 578.3       | ![](results/iqn/Breakout_rew.png)     | `python3 atari_iqn.py --task "BreakoutNoFrameskip-v4" --n-step 1` |
+| EnduroNoFrameskip-v4        | 1507        | ![](results/iqn/Enduro_rew.png)       | `python3 atari_iqn.py --task "EnduroNoFrameskip-v4"`  |
+| QbertNoFrameskip-v4         | 15520       | ![](results/iqn/Qbert_rew.png)        | `python3 atari_iqn.py --task "QbertNoFrameskip-v4"`  |
+| MsPacmanNoFrameskip-v4      | 2911        | ![](results/iqn/MsPacman_rew.png)     | `python3 atari_iqn.py --task "MsPacmanNoFrameskip-v4"`  |
+| SeaquestNoFrameskip-v4      | 6236        | ![](results/iqn/Seaquest_rew.png)     | `python3 atari_iqn.py --task "SeaquestNoFrameskip-v4"`  |
+| SpaceInvadersNoFrameskip-v4 | 1370        | ![](results/iqn/SpaceInvader_rew.png) | `python3 atari_iqn.py --task "SpaceInvadersNoFrameskip-v4"`  |
+
 # BCQ
 
 To running BCQ algorithm on Atari, you need to do the following things:
diff --git a/examples/atari/atari_c51.py b/examples/atari/atari_c51.py
index 4642b87..558284f 100644
--- a/examples/atari/atari_c51.py
+++ b/examples/atari/atari_c51.py
@@ -107,7 +107,7 @@ def test_c51(args=get_args()):
         torch.save(policy.state_dict(), os.path.join(log_path, 'policy.pth'))
 
     def stop_fn(mean_rewards):
-        if env.env.spec.reward_threshold:
+        if env.spec.reward_threshold:
             return mean_rewards >= env.spec.reward_threshold
         elif 'Pong' in args.task:
             return mean_rewards >= 20
diff --git a/examples/atari/atari_dqn.py b/examples/atari/atari_dqn.py
index 00476c3..a7785c8 100644
--- a/examples/atari/atari_dqn.py
+++ b/examples/atari/atari_dqn.py
@@ -102,7 +102,7 @@ def test_dqn(args=get_args()):
         torch.save(policy.state_dict(), os.path.join(log_path, 'policy.pth'))
 
     def stop_fn(mean_rewards):
-        if env.env.spec.reward_threshold:
+        if env.spec.reward_threshold:
             return mean_rewards >= env.spec.reward_threshold
         elif 'Pong' in args.task:
             return mean_rewards >= 20
diff --git a/examples/atari/atari_iqn.py b/examples/atari/atari_iqn.py
new file mode 100644
index 0000000..ad34c8f
--- /dev/null
+++ b/examples/atari/atari_iqn.py
@@ -0,0 +1,183 @@
+import os
+import torch
+import pprint
+import argparse
+import numpy as np
+from torch.utils.tensorboard import SummaryWriter
+
+from tianshou.policy import IQNPolicy
+from tianshou.utils import BasicLogger
+from tianshou.env import SubprocVectorEnv
+from tianshou.trainer import offpolicy_trainer
+from tianshou.data import Collector, VectorReplayBuffer
+from tianshou.utils.net.discrete import ImplicitQuantileNetwork
+
+from atari_network import DQN
+from atari_wrapper import wrap_deepmind
+
+
+def get_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--task', type=str, default='PongNoFrameskip-v4')
+    parser.add_argument('--seed', type=int, default=1234)
+    parser.add_argument('--eps-test', type=float, default=0.005)
+    parser.add_argument('--eps-train', type=float, default=1.)
+    parser.add_argument('--eps-train-final', type=float, default=0.05)
+    parser.add_argument('--buffer-size', type=int, default=100000)
+    parser.add_argument('--lr', type=float, default=0.0001)
+    parser.add_argument('--gamma', type=float, default=0.99)
+    parser.add_argument('--sample-size', type=int, default=32)
+    parser.add_argument('--online-sample-size', type=int, default=8)
+    parser.add_argument('--target-sample-size', type=int, default=8)
+    parser.add_argument('--num-cosines', type=int, default=64)
+    parser.add_argument('--hidden-sizes', type=int, nargs='*', default=[512])
+    parser.add_argument('--n-step', type=int, default=3)
+    parser.add_argument('--target-update-freq', type=int, default=500)
+    parser.add_argument('--epoch', type=int, default=100)
+    parser.add_argument('--step-per-epoch', type=int, default=100000)
+    parser.add_argument('--step-per-collect', type=int, default=10)
+    parser.add_argument('--update-per-step', type=float, default=0.1)
+    parser.add_argument('--batch-size', type=int, default=32)
+    parser.add_argument('--training-num', type=int, default=10)
+    parser.add_argument('--test-num', type=int, default=10)
+    parser.add_argument('--logdir', type=str, default='log')
+    parser.add_argument('--render', type=float, default=0.)
+    parser.add_argument(
+        '--device', type=str,
+        default='cuda' if torch.cuda.is_available() else 'cpu')
+    parser.add_argument('--frames-stack', type=int, default=4)
+    parser.add_argument('--resume-path', type=str, default=None)
+    parser.add_argument('--watch', default=False, action='store_true',
+                        help='watch the play of pre-trained policy only')
+    parser.add_argument('--save-buffer-name', type=str, default=None)
+    return parser.parse_args()
+
+
+def make_atari_env(args):
+    return wrap_deepmind(args.task, frame_stack=args.frames_stack)
+
+
+def make_atari_env_watch(args):
+    return wrap_deepmind(args.task, frame_stack=args.frames_stack,
+                         episode_life=False, clip_rewards=False)
+
+
+def test_iqn(args=get_args()):
+    env = make_atari_env(args)
+    args.state_shape = env.observation_space.shape or env.observation_space.n
+    args.action_shape = env.action_space.shape or env.action_space.n
+    # should be N_FRAMES x H x W
+    print("Observations shape:", args.state_shape)
+    print("Actions shape:", args.action_shape)
+    # make environments
+    train_envs = SubprocVectorEnv([lambda: make_atari_env(args)
+                                   for _ in range(args.training_num)])
+    test_envs = SubprocVectorEnv([lambda: make_atari_env_watch(args)
+                                  for _ in range(args.test_num)])
+    # seed
+    np.random.seed(args.seed)
+    torch.manual_seed(args.seed)
+    train_envs.seed(args.seed)
+    test_envs.seed(args.seed)
+    # define model
+    feature_net = DQN(*args.state_shape, args.action_shape, args.device,
+                      features_only=True)
+    net = ImplicitQuantileNetwork(
+        feature_net, args.action_shape, args.hidden_sizes,
+        num_cosines=args.num_cosines, device=args.device
+    ).to(args.device)
+    optim = torch.optim.Adam(net.parameters(), lr=args.lr)
+    # define policy
+    policy = IQNPolicy(
+        net, optim, args.gamma, args.sample_size, args.online_sample_size,
+        args.target_sample_size, args.n_step,
+        target_update_freq=args.target_update_freq
+    ).to(args.device)
+    # load a previous policy
+    if args.resume_path:
+        policy.load_state_dict(torch.load(args.resume_path, map_location=args.device))
+        print("Loaded agent from: ", args.resume_path)
+    # replay buffer: `save_last_obs` and `stack_num` can be removed together
+    # when you have enough RAM
+    buffer = VectorReplayBuffer(
+        args.buffer_size, buffer_num=len(train_envs),
+        ignore_obs_next=True, save_only_last_obs=True, stack_num=args.frames_stack)
+    # collector
+    train_collector = Collector(policy, train_envs, buffer, exploration_noise=True)
+    test_collector = Collector(policy, test_envs, exploration_noise=True)
+    # log
+    log_path = os.path.join(args.logdir, args.task, 'iqn')
+    writer = SummaryWriter(log_path)
+    writer.add_text("args", str(args))
+    logger = BasicLogger(writer)
+
+    def save_fn(policy):
+        torch.save(policy.state_dict(), os.path.join(log_path, 'policy.pth'))
+
+    def stop_fn(mean_rewards):
+        if env.spec.reward_threshold:
+            return mean_rewards >= env.spec.reward_threshold
+        elif 'Pong' in args.task:
+            return mean_rewards >= 20
+        else:
+            return False
+
+    def train_fn(epoch, env_step):
+        # nature DQN setting, linear decay in the first 1M steps
+        if env_step <= 1e6:
+            eps = args.eps_train - env_step / 1e6 * \
+                (args.eps_train - args.eps_train_final)
+        else:
+            eps = args.eps_train_final
+        policy.set_eps(eps)
+        logger.write('train/eps', env_step, eps)
+
+    def test_fn(epoch, env_step):
+        policy.set_eps(args.eps_test)
+
+    # watch agent's performance
+    def watch():
+        print("Setup test envs ...")
+        policy.eval()
+        policy.set_eps(args.eps_test)
+        test_envs.seed(args.seed)
+        if args.save_buffer_name:
+            print(f"Generate buffer with size {args.buffer_size}")
+            buffer = VectorReplayBuffer(
+                args.buffer_size, buffer_num=len(test_envs),
+                ignore_obs_next=True, save_only_last_obs=True,
+                stack_num=args.frames_stack)
+            collector = Collector(policy, test_envs, buffer,
+                                  exploration_noise=True)
+            result = collector.collect(n_step=args.buffer_size)
+            print(f"Save buffer into {args.save_buffer_name}")
+            # Unfortunately, pickle will cause oom with 1M buffer size
+            buffer.save_hdf5(args.save_buffer_name)
+        else:
+            print("Testing agent ...")
+            test_collector.reset()
+            result = test_collector.collect(n_episode=args.test_num,
+                                            render=args.render)
+        rew = result["rews"].mean()
+        print(f'Mean reward (over {result["n/ep"]} episodes): {rew}')
+
+    if args.watch:
+        watch()
+        exit(0)
+
+    # test train_collector and start filling replay buffer
+    train_collector.collect(n_step=args.batch_size * args.training_num)
+    # trainer
+    result = offpolicy_trainer(
+        policy, train_collector, test_collector, args.epoch,
+        args.step_per_epoch, args.step_per_collect, args.test_num,
+        args.batch_size, train_fn=train_fn, test_fn=test_fn,
+        stop_fn=stop_fn, save_fn=save_fn, logger=logger,
+        update_per_step=args.update_per_step, test_in_train=False)
+
+    pprint.pprint(result)
+    watch()
+
+
+if __name__ == '__main__':
+    test_iqn(get_args())
diff --git a/examples/atari/atari_qrdqn.py b/examples/atari/atari_qrdqn.py
index 061ad6a..6677f68 100644
--- a/examples/atari/atari_qrdqn.py
+++ b/examples/atari/atari_qrdqn.py
@@ -105,7 +105,7 @@ def test_qrdqn(args=get_args()):
         torch.save(policy.state_dict(), os.path.join(log_path, 'policy.pth'))
 
     def stop_fn(mean_rewards):
-        if env.env.spec.reward_threshold:
+        if env.spec.reward_threshold:
             return mean_rewards >= env.spec.reward_threshold
         elif 'Pong' in args.task:
             return mean_rewards >= 20
diff --git a/examples/atari/results/iqn/Breakout_rew.png b/examples/atari/results/iqn/Breakout_rew.png
new file mode 100644
index 0000000..d486bf6
Binary files /dev/null and b/examples/atari/results/iqn/Breakout_rew.png differ
diff --git a/examples/atari/results/iqn/Enduro_rew.png b/examples/atari/results/iqn/Enduro_rew.png
new file mode 100644
index 0000000..6d0a8a0
Binary files /dev/null and b/examples/atari/results/iqn/Enduro_rew.png differ
diff --git a/examples/atari/results/iqn/MsPacman_rew.png b/examples/atari/results/iqn/MsPacman_rew.png
new file mode 100644
index 0000000..da9bcba
Binary files /dev/null and b/examples/atari/results/iqn/MsPacman_rew.png differ
diff --git a/examples/atari/results/iqn/Pong_rew.png b/examples/atari/results/iqn/Pong_rew.png
new file mode 100644
index 0000000..d5c91bd
Binary files /dev/null and b/examples/atari/results/iqn/Pong_rew.png differ
diff --git a/examples/atari/results/iqn/Qbert_rew.png b/examples/atari/results/iqn/Qbert_rew.png
new file mode 100644
index 0000000..749e818
Binary files /dev/null and b/examples/atari/results/iqn/Qbert_rew.png differ
diff --git a/examples/atari/results/iqn/Seaquest_rew.png b/examples/atari/results/iqn/Seaquest_rew.png
new file mode 100644
index 0000000..0b14469
Binary files /dev/null and b/examples/atari/results/iqn/Seaquest_rew.png differ
diff --git a/examples/atari/results/iqn/SpaceInvaders_rew.png b/examples/atari/results/iqn/SpaceInvaders_rew.png
new file mode 100644
index 0000000..39b67dd
Binary files /dev/null and b/examples/atari/results/iqn/SpaceInvaders_rew.png differ
diff --git a/test/discrete/test_iqn.py b/test/discrete/test_iqn.py
new file mode 100644
index 0000000..f40407f
--- /dev/null
+++ b/test/discrete/test_iqn.py
@@ -0,0 +1,149 @@
+import os
+import gym
+import torch
+import pprint
+import argparse
+import numpy as np
+from torch.utils.tensorboard import SummaryWriter
+
+from tianshou.policy import IQNPolicy
+from tianshou.utils import BasicLogger
+from tianshou.env import DummyVectorEnv
+from tianshou.utils.net.common import Net
+from tianshou.trainer import offpolicy_trainer
+from tianshou.utils.net.discrete import ImplicitQuantileNetwork
+from tianshou.data import Collector, VectorReplayBuffer, PrioritizedVectorReplayBuffer
+
+
+def get_args():
+    parser = argparse.ArgumentParser()
+    parser.add_argument('--task', type=str, default='CartPole-v0')
+    parser.add_argument('--seed', type=int, default=0)
+    parser.add_argument('--eps-test', type=float, default=0.05)
+    parser.add_argument('--eps-train', type=float, default=0.1)
+    parser.add_argument('--buffer-size', type=int, default=20000)
+    parser.add_argument('--lr', type=float, default=3e-3)
+    parser.add_argument('--gamma', type=float, default=0.9)
+    parser.add_argument('--sample-size', type=int, default=32)
+    parser.add_argument('--online-sample-size', type=int, default=8)
+    parser.add_argument('--target-sample-size', type=int, default=8)
+    parser.add_argument('--num-cosines', type=int, default=64)
+    parser.add_argument('--n-step', type=int, default=3)
+    parser.add_argument('--target-update-freq', type=int, default=320)
+    parser.add_argument('--epoch', type=int, default=10)
+    parser.add_argument('--step-per-epoch', type=int, default=10000)
+    parser.add_argument('--step-per-collect', type=int, default=10)
+    parser.add_argument('--update-per-step', type=float, default=0.1)
+    parser.add_argument('--batch-size', type=int, default=64)
+    parser.add_argument('--hidden-sizes', type=int,
+                        nargs='*', default=[64, 64, 64])
+    parser.add_argument('--training-num', type=int, default=10)
+    parser.add_argument('--test-num', type=int, default=100)
+    parser.add_argument('--logdir', type=str, default='log')
+    parser.add_argument('--render', type=float, default=0.)
+    parser.add_argument('--prioritized-replay',
+                        action="store_true", default=False)
+    parser.add_argument('--alpha', type=float, default=0.6)
+    parser.add_argument('--beta', type=float, default=0.4)
+    parser.add_argument(
+        '--device', type=str,
+        default='cuda' if torch.cuda.is_available() else 'cpu')
+    args = parser.parse_known_args()[0]
+    return args
+
+
+def test_iqn(args=get_args()):
+    env = gym.make(args.task)
+    args.state_shape = env.observation_space.shape or env.observation_space.n
+    args.action_shape = env.action_space.shape or env.action_space.n
+    # train_envs = gym.make(args.task)
+    # you can also use tianshou.env.SubprocVectorEnv
+    train_envs = DummyVectorEnv(
+        [lambda: gym.make(args.task) for _ in range(args.training_num)])
+    # test_envs = gym.make(args.task)
+    test_envs = DummyVectorEnv(
+        [lambda: gym.make(args.task) for _ in range(args.test_num)])
+    # seed
+    np.random.seed(args.seed)
+    torch.manual_seed(args.seed)
+    train_envs.seed(args.seed)
+    test_envs.seed(args.seed)
+    # model
+    feature_net = Net(args.state_shape, args.hidden_sizes[-1],
+                      hidden_sizes=args.hidden_sizes[:-1], device=args.device,
+                      softmax=False)
+    net = ImplicitQuantileNetwork(
+        feature_net, args.action_shape,
+        num_cosines=args.num_cosines, device=args.device)
+    optim = torch.optim.Adam(net.parameters(), lr=args.lr)
+    policy = IQNPolicy(
+        net, optim, args.gamma, args.sample_size, args.online_sample_size,
+        args.target_sample_size, args.n_step,
+        target_update_freq=args.target_update_freq
+    ).to(args.device)
+    # buffer
+    if args.prioritized_replay:
+        buf = PrioritizedVectorReplayBuffer(
+            args.buffer_size, buffer_num=len(train_envs),
+            alpha=args.alpha, beta=args.beta)
+    else:
+        buf = VectorReplayBuffer(args.buffer_size, buffer_num=len(train_envs))
+    # collector
+    train_collector = Collector(policy, train_envs, buf, exploration_noise=True)
+    test_collector = Collector(policy, test_envs, exploration_noise=True)
+    # policy.set_eps(1)
+    train_collector.collect(n_step=args.batch_size * args.training_num)
+    # log
+    log_path = os.path.join(args.logdir, args.task, 'iqn')
+    writer = SummaryWriter(log_path)
+    logger = BasicLogger(writer)
+
+    def save_fn(policy):
+        torch.save(policy.state_dict(), os.path.join(log_path, 'policy.pth'))
+
+    def stop_fn(mean_rewards):
+        return mean_rewards >= env.spec.reward_threshold
+
+    def train_fn(epoch, env_step):
+        # eps annnealing, just a demo
+        if env_step <= 10000:
+            policy.set_eps(args.eps_train)
+        elif env_step <= 50000:
+            eps = args.eps_train - (env_step - 10000) / \
+                40000 * (0.9 * args.eps_train)
+            policy.set_eps(eps)
+        else:
+            policy.set_eps(0.1 * args.eps_train)
+
+    def test_fn(epoch, env_step):
+        policy.set_eps(args.eps_test)
+
+    # trainer
+    result = offpolicy_trainer(
+        policy, train_collector, test_collector, args.epoch,
+        args.step_per_epoch, args.step_per_collect, args.test_num,
+        args.batch_size, train_fn=train_fn, test_fn=test_fn,
+        stop_fn=stop_fn, save_fn=save_fn, logger=logger,
+        update_per_step=args.update_per_step)
+    assert stop_fn(result['best_reward'])
+
+    if __name__ == '__main__':
+        pprint.pprint(result)
+        # Let's watch its performance!
+        env = gym.make(args.task)
+        policy.eval()
+        policy.set_eps(args.eps_test)
+        collector = Collector(policy, env)
+        result = collector.collect(n_episode=1, render=args.render)
+        rews, lens = result["rews"], result["lens"]
+        print(f"Final reward: {rews.mean()}, length: {lens.mean()}")
+
+
+def test_piqn(args=get_args()):
+    args.prioritized_replay = True
+    args.gamma = .95
+    test_iqn(args)
+
+
+if __name__ == '__main__':
+    test_iqn(get_args())
diff --git a/tianshou/policy/__init__.py b/tianshou/policy/__init__.py
index 54f0f4a..a350891 100644
--- a/tianshou/policy/__init__.py
+++ b/tianshou/policy/__init__.py
@@ -3,6 +3,7 @@ from tianshou.policy.random import RandomPolicy
 from tianshou.policy.modelfree.dqn import DQNPolicy
 from tianshou.policy.modelfree.c51 import C51Policy
 from tianshou.policy.modelfree.qrdqn import QRDQNPolicy
+from tianshou.policy.modelfree.iqn import IQNPolicy
 from tianshou.policy.modelfree.pg import PGPolicy
 from tianshou.policy.modelfree.a2c import A2CPolicy
 from tianshou.policy.modelfree.npg import NPGPolicy
@@ -26,6 +27,7 @@ __all__ = [
     "DQNPolicy",
     "C51Policy",
     "QRDQNPolicy",
+    "IQNPolicy",
     "PGPolicy",
     "A2CPolicy",
     "NPGPolicy",
diff --git a/tianshou/policy/modelfree/iqn.py b/tianshou/policy/modelfree/iqn.py
new file mode 100644
index 0000000..4c54d35
--- /dev/null
+++ b/tianshou/policy/modelfree/iqn.py
@@ -0,0 +1,105 @@
+import torch
+import numpy as np
+import torch.nn.functional as F
+from typing import Any, Dict, Optional, Union
+
+from tianshou.policy import QRDQNPolicy
+from tianshou.data import Batch, to_numpy
+
+
+class IQNPolicy(QRDQNPolicy):
+    """Implementation of Implicit Quantile Network. arXiv:1806.06923.
+
+    :param torch.nn.Module model: a model following the rules in
+        :class:`~tianshou.policy.BasePolicy`. (s -> logits)
+    :param torch.optim.Optimizer optim: a torch.optim for optimizing the model.
+    :param float discount_factor: in [0, 1].
+    :param int sample_size: the number of samples for policy evaluation.
+        Default to 32.
+    :param int online_sample_size: the number of samples for online model
+        in training. Default to 8.
+    :param int target_sample_size: the number of samples for target model
+        in training. Default to 8.
+    :param int estimation_step: the number of steps to look ahead. Default to 1.
+    :param int target_update_freq: the target network update frequency (0 if
+        you do not use the target network).
+    :param bool reward_normalization: normalize the reward to Normal(0, 1).
+        Default to False.
+
+    .. seealso::
+
+        Please refer to :class:`~tianshou.policy.QRDQNPolicy` for more detailed
+        explanation.
+    """
+
+    def __init__(
+        self,
+        model: torch.nn.Module,
+        optim: torch.optim.Optimizer,
+        discount_factor: float = 0.99,
+        sample_size: int = 32,
+        online_sample_size: int = 8,
+        target_sample_size: int = 8,
+        estimation_step: int = 1,
+        target_update_freq: int = 0,
+        reward_normalization: bool = False,
+        **kwargs: Any,
+    ) -> None:
+        super().__init__(model, optim, discount_factor, sample_size, estimation_step,
+                         target_update_freq, reward_normalization, **kwargs)
+        assert sample_size > 1, "sample_size should be greater than 1"
+        assert online_sample_size > 1, "online_sample_size should be greater than 1"
+        assert target_sample_size > 1, "target_sample_size should be greater than 1"
+        self._sample_size = sample_size  # for policy eval
+        self._online_sample_size = online_sample_size
+        self._target_sample_size = target_sample_size
+
+    def forward(
+        self,
+        batch: Batch,
+        state: Optional[Union[dict, Batch, np.ndarray]] = None,
+        model: str = "model",
+        input: str = "obs",
+        **kwargs: Any,
+    ) -> Batch:
+        if model == "model_old":
+            sample_size = self._target_sample_size
+        elif self.training:
+            sample_size = self._online_sample_size
+        else:
+            sample_size = self._sample_size
+        model = getattr(self, model)
+        obs = batch[input]
+        obs_ = obs.obs if hasattr(obs, "obs") else obs
+        (logits, taus), h = model(
+            obs_, sample_size=sample_size, state=state, info=batch.info
+        )
+        q = self.compute_q_value(logits, getattr(obs, "mask", None))
+        if not hasattr(self, "max_action_num"):
+            self.max_action_num = q.shape[1]
+        act = to_numpy(q.max(dim=1)[1])
+        return Batch(logits=logits, act=act, state=h, taus=taus)
+
+    def learn(self, batch: Batch, **kwargs: Any) -> Dict[str, float]:
+        if self._target and self._iter % self._freq == 0:
+            self.sync_weight()
+        self.optim.zero_grad()
+        weight = batch.pop("weight", 1.0)
+        out = self(batch)
+        curr_dist, taus = out.logits, out.taus
+        act = batch.act
+        curr_dist = curr_dist[np.arange(len(act)), act, :].unsqueeze(2)
+        target_dist = batch.returns.unsqueeze(1)
+        # calculate each element's difference between curr_dist and target_dist
+        u = F.smooth_l1_loss(target_dist, curr_dist, reduction="none")
+        huber_loss = (u * (
+            taus.unsqueeze(2) - (target_dist - curr_dist).detach().le(0.).float()
+        ).abs()).sum(-1).mean(1)
+        loss = (huber_loss * weight).mean()
+        # ref: https://github.com/ku2482/fqf-iqn-qrdqn.pytorch/
+        # blob/master/fqf_iqn_qrdqn/agent/qrdqn_agent.py L130
+        batch.weight = u.detach().abs().sum(-1).mean(1)  # prio-buffer
+        loss.backward()
+        self.optim.step()
+        self._iter += 1
+        return {"loss": loss.item()}
diff --git a/tianshou/utils/net/discrete.py b/tianshou/utils/net/discrete.py
index ee1294f..14cc85e 100644
--- a/tianshou/utils/net/discrete.py
+++ b/tianshou/utils/net/discrete.py
@@ -111,3 +111,94 @@ class Critic(nn.Module):
         """Mapping: s -> V(s)."""
         logits, _ = self.preprocess(s, state=kwargs.get("state", None))
         return self.last(logits)
+
+
+class CosineEmbeddingNetwork(nn.Module):
+    """Cosine embedding network for IQN. Convert a scalar in [0, 1] to a list \
+    of n-dim vectors.
+
+    :param num_cosines: the number of cosines used for the embedding.
+    :param embedding_dim: the dimension of the embedding/output.
+
+    .. note::
+
+        From https://github.com/ku2482/fqf-iqn-qrdqn.pytorch/blob/master
+        /fqf_iqn_qrdqn/network.py .
+    """
+
+    def __init__(self, num_cosines: int, embedding_dim: int) -> None:
+        super().__init__()
+        self.net = nn.Sequential(nn.Linear(num_cosines, embedding_dim), nn.ReLU())
+        self.num_cosines = num_cosines
+        self.embedding_dim = embedding_dim
+
+    def forward(self, taus: torch.Tensor) -> torch.Tensor:
+        batch_size = taus.shape[0]
+        N = taus.shape[1]
+        # Calculate i * \pi (i=1,...,N).
+        i_pi = np.pi * torch.arange(
+            start=1, end=self.num_cosines + 1, dtype=taus.dtype, device=taus.device
+        ).view(1, 1, self.num_cosines)
+        # Calculate cos(i * \pi * \tau).
+        cosines = torch.cos(taus.view(batch_size, N, 1) * i_pi).view(
+            batch_size * N, self.num_cosines
+        )
+        # Calculate embeddings of taus.
+        tau_embeddings = self.net(cosines).view(batch_size, N, self.embedding_dim)
+        return tau_embeddings
+
+
+class ImplicitQuantileNetwork(Critic):
+    """Implicit Quantile Network.
+
+    :param preprocess_net: a self-defined preprocess_net which output a
+        flattened hidden state.
+    :param int action_dim: the dimension of action space.
+    :param hidden_sizes: a sequence of int for constructing the MLP after
+        preprocess_net. Default to empty sequence (where the MLP now contains
+        only a single linear layer).
+    :param int num_cosines: the number of cosines to use for cosine embedding.
+        Default to 64.
+    :param int preprocess_net_output_dim: the output dimension of
+        preprocess_net.
+
+    .. note::
+
+        Although this class inherits Critic, it is actually a quantile Q-Network
+        with output shape (batch_size, action_dim, sample_size).
+
+        The second item of the first return value is tau vector.
+    """
+
+    def __init__(
+        self,
+        preprocess_net: nn.Module,
+        action_shape: Sequence[int],
+        hidden_sizes: Sequence[int] = (),
+        num_cosines: int = 64,
+        preprocess_net_output_dim: Optional[int] = None,
+        device: Union[str, int, torch.device] = "cpu"
+    ) -> None:
+        last_size = np.prod(action_shape)
+        super().__init__(preprocess_net, hidden_sizes, last_size,
+                         preprocess_net_output_dim, device)
+        self.input_dim = getattr(preprocess_net, "output_dim",
+                                 preprocess_net_output_dim)
+        self.embed_model = CosineEmbeddingNetwork(num_cosines,
+                                                  self.input_dim).to(device)
+
+    def forward(  # type: ignore
+        self, s: Union[np.ndarray, torch.Tensor], sample_size: int, **kwargs: Any
+    ) -> Tuple[Any, torch.Tensor]:
+        r"""Mapping: s -> Q(s, \*)."""
+        logits, h = self.preprocess(s, state=kwargs.get("state", None))
+        # Sample fractions.
+        batch_size = logits.size(0)
+        taus = torch.rand(batch_size, sample_size,
+                          dtype=logits.dtype, device=logits.device)
+        embedding = (logits.unsqueeze(1) * self.embed_model(taus)).view(
+            batch_size * sample_size, -1
+        )
+        out = self.last(embedding).view(batch_size,
+                                        sample_size, -1).transpose(1, 2)
+        return (out, taus), h