"""
.. _model-dgmg:

Tutorial: Generative models of graphs
===========================================

**Author**: `Mufei Li <https://github.com/mufeili>`_,
`Lingfan Yu <https://github.com/ylfdq1118>`_, Zheng Zhang
"""

##############################################################################
#
# In this tutorial, you learn how to train and generate one graph at
# a time. You also explore parallelism within the graph embedding operation, which is an
# essential building block. The tutorial ends with a simple optimization that
# delivers double the speed by batching across graphs.
#
# Earlier tutorials showed how embedding a graph or
# a node enables you to work on tasks such as `semi-supervised classification for nodes
# <http://docs.dgl.ai/tutorials/models/1_gcn.html#sphx-glr-tutorials-models-1-gcn-py>`__
# or `sentiment analysis
# <http://docs.dgl.ai/tutorials/models/3_tree-lstm.html#sphx-glr-tutorials-models-3-tree-lstm-py>`__.
# Wouldn't it be interesting to predict the future evolution of the graph and
# perform the analysis iteratively?
#
# To address the evolution of the graphs, you generate a variety of graph samples. In other words, you need
# **generative models** of graphs. In-addition to learning
# node and edge features, you would need to model the distribution of arbitrary graphs.
# While general generative models can model the density function explicitly and
# implicitly and generate samples at once or sequentially, you only focus
# on explicit generative models for sequential generation here. Typical applications
# include drug or materials discovery, chemical processes, or proteomics.
#
# Introduction
# --------------------
# The primitive actions of mutating a graph in Deep Graph Library (DGL) are nothing more than ``add_nodes``
# and ``add_edges``. That is, if you were to draw a circle of three nodes,
#
# .. figure:: https://user-images.githubusercontent.com/19576924/48313438-78baf000-e5f7-11e8-931e-cd00ab34fa50.gif
#    :alt:
#
# you can write the code as follows.
#

import dgl

g = dgl.DGLGraph()
g.add_nodes(1)              # Add node 0
g.add_nodes(1)              # Add node 1

# Edges in DGLGraph are directed by default.
# For undirected edges, add edges for both directions.
g.add_edges([1, 0], [0, 1]) # Add edges (1, 0), (0, 1)
g.add_nodes(1)              # Add node 2
g.add_edges([2, 1], [1, 2]) # Add edges (2, 1), (1, 2)
g.add_edges([2, 0], [0, 2]) # Add edges (2, 0), (0, 2)

#######################################################################################
# Real-world graphs are much more complex. There are many families of graphs,
# with different sizes, topologies, node types, edge types, and the possibility
# of multigraphs. Besides, a same graph can be generated in many different
# orders. Regardless, the generative process entails a few steps.
#
# - Encode a changing graph.
# - Perform actions stochastically.
# - If you are training, collect error signals and optimize the model parameters.
#
# When it comes to implementation, another important aspect is speed. How do you
# parallelize the computation, given that generating a graph is fundamentally a
# sequential process?
#
# .. note::
#
#    To be sure, this is not necessarily a hard constraint. Subgraphs can be 
#    built in parallel and then get assembled. But we
#    will restrict ourselves to the sequential processes for this tutorial.
#
#
# DGMG: The main flow
# --------------------
# For this tutorial, you use 
# `Deep Generative Models of Graphs <https://arxiv.org/abs/1803.03324>`__
# ) (DGMG) to implement a graph generative model using DGL. Its algorithmic 
# framework is general but also challenging to parallelize.
#
# .. note::
#
#    While it's possible for DGMG to handle complex graphs with typed nodes,
#    typed edges, and multigraphs, here you use a simplified version of it
#    for generating graph topologies.
#
# DGMG generates a graph by following a state machine, which is basically a
# two-level loop. Generate one node at a time and connect it to a subset of
# the existing nodes, one at a time. This is similar to language modeling. The
# generative process is an iterative one that emits one word or character or sentence
# at a time, conditioned on the sequence generated so far.
#
# At each time step, you either:
#      - Add a new node to the graph
#      - Select two existing nodes and add an edge between them
#
# .. figure:: https://user-images.githubusercontent.com/19576924/48605003-7f11e900-e9b6-11e8-8880-87362348e154.png
#    :alt:
#
# The Python code will look as follows. In fact, this is *exactly* how inference
# with DGMG is implemented in DGL.
#

def forward_inference(self):
    stop = self.add_node_and_update()
    while (not stop) and (self.g.number_of_nodes() < self.v_max + 1):
        num_trials = 0
        to_add_edge = self.add_edge_or_not()
        while to_add_edge and (num_trials < self.g.number_of_nodes() - 1):
            self.choose_dest_and_update()
            num_trials += 1
            to_add_edge = self.add_edge_or_not()
        stop = self.add_node_and_update()

    return self.g

#######################################################################################
# Assume you have a pre-trained model for generating cycles of nodes 10-20.
# How does it generate a cycle on-the-fly during inference? Use the code below
# to create an animation with your own model.
#
# ::
#
#     import torch
#     import matplotlib.animation as animation
#     import matplotlib.pyplot as plt
#     import networkx as nx
#     from copy import deepcopy
#
#     if __name__ == '__main__':
#         # pre-trained model saved with path ./model.pth
#         model = torch.load('./model.pth')
#         model.eval()
#         g = model()
#
#         src_list = g.edges()[1]
#         dest_list = g.edges()[0]
#
#         evolution = []
#
#         nx_g = nx.Graph()
#         evolution.append(deepcopy(nx_g))
#
#         for i in range(0, len(src_list), 2):
#             src = src_list[i].item()
#             dest = dest_list[i].item()
#             if src not in nx_g.nodes():
#                 nx_g.add_node(src)
#                 evolution.append(deepcopy(nx_g))
#             if dest not in nx_g.nodes():
#                 nx_g.add_node(dest)
#                 evolution.append(deepcopy(nx_g))
#             nx_g.add_edges_from([(src, dest), (dest, src)])
#             evolution.append(deepcopy(nx_g))
#
#         def animate(i):
#             ax.cla()
#             g_t = evolution[i]
#             nx.draw_circular(g_t, with_labels=True, ax=ax,
#                              node_color=['#FEBD69'] * g_t.number_of_nodes())
#
#         fig, ax = plt.subplots()
#         ani = animation.FuncAnimation(fig, animate,
#                                       frames=len(evolution),
#                                       interval=600)
#
# .. figure:: https://user-images.githubusercontent.com/19576924/48928548-2644d200-ef1b-11e8-8591-da93345382ad.gif
#    :alt:
#
# DGMG: Optimization objective
# ------------------------------
# Similar to language modeling, DGMG trains the model with *behavior cloning*,
# or *teacher forcing*. Assume for each graph there exists a sequence of
# *oracle actions* :math:`a_{1},\cdots,a_{T}` that generates it. What the model
# does is to follow these actions, compute the joint probabilities of such
# action sequences, and maximize them.
#
# By chain rule, the probability of taking :math:`a_{1},\cdots,a_{T}` is:
#
# .. math::
#
#    p(a_{1},\cdots, a_{T}) = p(a_{1})p(a_{2}|a_{1})\cdots p(a_{T}|a_{1},\cdots,a_{T-1}).\\
#
# The optimization objective is then simply the typical MLE loss:
#
# .. math::
#
#    -\log p(a_{1},\cdots,a_{T})=-\sum_{t=1}^{T}\log p(a_{t}|a_{1},\cdots, a_{t-1}).\\
#

def forward_train(self, actions):
    """
    - actions: list
        - Contains a_1, ..., a_T described above
    - self.prepare_for_train()
        - Initializes self.action_step to be 0, which will get
          incremented by 1 every time it is called.
        - Initializes objects recording log p(a_t|a_1,...a_{t-1})

    Returns
    -------
    - self.get_log_prob(): log p(a_1, ..., a_T)
    """
    self.prepare_for_train()

    stop = self.add_node_and_update(a=actions[self.action_step])
    while not stop:
        to_add_edge = self.add_edge_or_not(a=actions[self.action_step])
        while to_add_edge:
            self.choose_dest_and_update(a=actions[self.action_step])
            to_add_edge = self.add_edge_or_not(a=actions[self.action_step])
        stop = self.add_node_and_update(a=actions[self.action_step])

    return self.get_log_prob()

#######################################################################################
# The key difference between ``forward_train`` and ``forward_inference`` is
# that the training process takes oracle actions as input and returns log
# probabilities for evaluating the loss.
#
# DGMG: The implementation
# --------------------------
# The ``DGMG`` class
# ``````````````````````````
# Below you can find the skeleton code for the model. You gradually
# fill in the details for each function.
#

import torch.nn as nn


class DGMGSkeleton(nn.Module):
    def __init__(self, v_max):
        """
        Parameters
        ----------
        v_max: int
            Max number of nodes considered
        """
        super(DGMGSkeleton, self).__init__()

        # Graph configuration
        self.v_max = v_max

    def add_node_and_update(self, a=None):
        """Decide if to add a new node.
        If a new node should be added, update the graph."""
        return NotImplementedError

    def add_edge_or_not(self, a=None):
        """Decide if a new edge should be added."""
        return NotImplementedError

    def choose_dest_and_update(self, a=None):
        """Choose destination and connect it to the latest node.
        Add edges for both directions and update the graph."""
        return NotImplementedError

    def forward_train(self, actions):
        """Forward at training time. It records the probability
        of generating a ground truth graph following the actions."""
        return NotImplementedError

    def forward_inference(self):
        """Forward at inference time.
        It generates graphs on the fly."""
        return NotImplementedError

    def forward(self, actions=None):
        # The graph you will work on
        self.g = dgl.DGLGraph()

        # If there are some features for nodes and edges,
        # zero tensors will be set for those of new nodes and edges.
        self.g.set_n_initializer(dgl.frame.zero_initializer)
        self.g.set_e_initializer(dgl.frame.zero_initializer)

        if self.training:
            return self.forward_train(actions=actions)
        else:
            return self.forward_inference()

#######################################################################################
# Encoding a dynamic graph
# ``````````````````````````
# All the actions generating a graph are sampled from probability
# distributions. In order to do that, you project the structured data,
# namely the graph, onto an Euclidean space. The challenge is that such
# process, called *embedding*, needs to be repeated as the graphs mutate.
#
# Graph embedding
# ''''''''''''''''''''''''''
# Let :math:`G=(V,E)` be an arbitrary graph. Each node :math:`v` has an
# embedding vector :math:`\textbf{h}_{v} \in \mathbb{R}^{n}`. Similarly,
# the graph has an embedding vector :math:`\textbf{h}_{G} \in \mathbb{R}^{k}`.
# Typically, :math:`k > n` since a graph contains more information than
# an individual node.
#
# The graph embedding is a weighted sum of node embeddings under a linear
# transformation:
#
# .. math::
#
#    \textbf{h}_{G} =\sum_{v\in V}\text{Sigmoid}(g_m(\textbf{h}_{v}))f_{m}(\textbf{h}_{v}),\\
#
# The first term, :math:`\text{Sigmoid}(g_m(\textbf{h}_{v}))`, computes a
# gating function and can be thought of as how much the overall graph embedding
# attends on each node. The second term :math:`f_{m}:\mathbb{R}^{n}\rightarrow\mathbb{R}^{k}`
# maps the node embeddings to the space of graph embeddings.
#
# Implement graph embedding as a ``GraphEmbed`` class.
#

import torch


class GraphEmbed(nn.Module):
    def __init__(self, node_hidden_size):
        super(GraphEmbed, self).__init__()

        # Setting from the paper
        self.graph_hidden_size = 2 * node_hidden_size

        # Embed graphs
        self.node_gating = nn.Sequential(
            nn.Linear(node_hidden_size, 1),
            nn.Sigmoid()
        )
        self.node_to_graph = nn.Linear(node_hidden_size,
                                       self.graph_hidden_size)

    def forward(self, g):
        if g.number_of_nodes() == 0:
            return torch.zeros(1, self.graph_hidden_size)
        else:
            # Node features are stored as hv in ndata.
            hvs = g.ndata['hv']
            return (self.node_gating(hvs) *
                    self.node_to_graph(hvs)).sum(0, keepdim=True)

#######################################################################################
# Update node embeddings via graph propagation
# '''''''''''''''''''''''''''''''''''''''''''''
#
# The mechanism of updating node embeddings in DGMG is similar to that for
# graph convolutional networks. For a node :math:`v` in the graph, its
# neighbor :math:`u` sends a message to it with
#
# .. math::
#
#    \textbf{m}_{u\rightarrow v}=\textbf{W}_{m}\text{concat}([\textbf{h}_{v}, \textbf{h}_{u}, \textbf{x}_{u, v}]) + \textbf{b}_{m},\\
#
# where :math:`\textbf{x}_{u,v}` is the embedding of the edge between
# :math:`u` and :math:`v`.
#
# After receiving messages from all its neighbors, :math:`v` summarizes them
# with a node activation vector
#
# .. math::
#
#    \textbf{a}_{v} = \sum_{u: (u, v)\in E}\textbf{m}_{u\rightarrow v}\\
#
# and use this information to update its own feature:
#
# .. math::
#
#    \textbf{h}'_{v} = \textbf{GRU}(\textbf{h}_{v}, \textbf{a}_{v}).\\
#
# Performing all the operations above once for all nodes synchronously is
# called one round of graph propagation. The more rounds of graph propagation
# you perform, the longer distance messages travel throughout the graph.
#
# With DGL, you implement graph propagation with ``g.update_all``.
# The message notation here can be a bit confusing. Researchers can refer
# to :math:`\textbf{m}_{u\rightarrow v}` as messages, however the message function
# below only passes :math:`\text{concat}([\textbf{h}_{u}, \textbf{x}_{u, v}])`.
# The operation :math:`\textbf{W}_{m}\text{concat}([\textbf{h}_{v}, \textbf{h}_{u}, \textbf{x}_{u, v}]) + \textbf{b}_{m}`
# is then performed across all edges at once for efficiency consideration.
#

from functools import partial

class GraphProp(nn.Module):
    def __init__(self, num_prop_rounds, node_hidden_size):
        super(GraphProp, self).__init__()

        self.num_prop_rounds = num_prop_rounds

        # Setting from the paper
        self.node_activation_hidden_size = 2 * node_hidden_size

        message_funcs = []
        node_update_funcs = []
        self.reduce_funcs = []

        for t in range(num_prop_rounds):
            # input being [hv, hu, xuv]
            message_funcs.append(nn.Linear(2 * node_hidden_size + 1,
                                           self.node_activation_hidden_size))

            self.reduce_funcs.append(partial(self.dgmg_reduce, round=t))
            node_update_funcs.append(
                nn.GRUCell(self.node_activation_hidden_size,
                           node_hidden_size))

        self.message_funcs = nn.ModuleList(message_funcs)
        self.node_update_funcs = nn.ModuleList(node_update_funcs)

    def dgmg_msg(self, edges):
        """For an edge u->v, return concat([h_u, x_uv])"""
        return {'m': torch.cat([edges.src['hv'],
                                edges.data['he']],
                               dim=1)}

    def dgmg_reduce(self, nodes, round):
        hv_old = nodes.data['hv']
        m = nodes.mailbox['m']
        message = torch.cat([
            hv_old.unsqueeze(1).expand(-1, m.size(1), -1), m], dim=2)
        node_activation = (self.message_funcs[round](message)).sum(1)

        return {'a': node_activation}

    def forward(self, g):
        if g.number_of_edges() > 0:
            for t in range(self.num_prop_rounds):
                g.update_all(message_func=self.dgmg_msg,
                             reduce_func=self.reduce_funcs[t])
                g.ndata['hv'] = self.node_update_funcs[t](
                     g.ndata['a'], g.ndata['hv'])

#######################################################################################
# Actions
# ``````````````````````````
# All actions are sampled from distributions parameterized using neural networks
# and here they are in turn.
#
# Action 1: Add nodes
# ''''''''''''''''''''''''''
#
# Given the graph embedding vector :math:`\textbf{h}_{G}`, evaluate
#
# .. math::
#
#    \text{Sigmoid}(\textbf{W}_{\text{add node}}\textbf{h}_{G}+b_{\text{add node}}),\\
#
# which is then used to parametrize a Bernoulli distribution for deciding whether
# to add a new node.
#
# If a new node is to be added, initialize its feature with
#
# .. math::
#
#    \textbf{W}_{\text{init}}\text{concat}([\textbf{h}_{\text{init}} , \textbf{h}_{G}])+\textbf{b}_{\text{init}},\\
#
# where :math:`\textbf{h}_{\text{init}}` is a learnable embedding module for
# untyped nodes.
#

import torch.nn.functional as F
from torch.distributions import Bernoulli

def bernoulli_action_log_prob(logit, action):
    """Calculate the log p of an action with respect to a Bernoulli
    distribution. Use logit rather than prob for numerical stability."""
    if action == 0:
        return F.logsigmoid(-logit)
    else:
        return F.logsigmoid(logit)

class AddNode(nn.Module):
    def __init__(self, graph_embed_func, node_hidden_size):
        super(AddNode, self).__init__()

        self.graph_op = {'embed': graph_embed_func}

        self.stop = 1
        self.add_node = nn.Linear(graph_embed_func.graph_hidden_size, 1)

        # If to add a node, initialize its hv
        self.node_type_embed = nn.Embedding(1, node_hidden_size)
        self.initialize_hv = nn.Linear(node_hidden_size + \
                                       graph_embed_func.graph_hidden_size,
                                       node_hidden_size)

        self.init_node_activation = torch.zeros(1, 2 * node_hidden_size)

    def _initialize_node_repr(self, g, node_type, graph_embed):
        """Whenver a node is added, initialize its representation."""
        num_nodes = g.number_of_nodes()
        hv_init = self.initialize_hv(
            torch.cat([
                self.node_type_embed(torch.LongTensor([node_type])),
                graph_embed], dim=1))
        g.nodes[num_nodes - 1].data['hv'] = hv_init
        g.nodes[num_nodes - 1].data['a'] = self.init_node_activation

    def prepare_training(self):
        self.log_prob = []

    def forward(self, g, action=None):
        graph_embed = self.graph_op['embed'](g)

        logit = self.add_node(graph_embed)
        prob = torch.sigmoid(logit)

        if not self.training:
            action = Bernoulli(prob).sample().item()
        stop = bool(action == self.stop)

        if not stop:
            g.add_nodes(1)
            self._initialize_node_repr(g, action, graph_embed)

        if self.training:
            sample_log_prob = bernoulli_action_log_prob(logit, action)

            self.log_prob.append(sample_log_prob)

        return stop

#######################################################################################
# Action 2: Add edges
# ''''''''''''''''''''''''''
#
# Given the graph embedding vector :math:`\textbf{h}_{G}` and the node
# embedding vector :math:`\textbf{h}_{v}` for the latest node :math:`v`,
# you evaluate
#
# .. math::
#
#    \text{Sigmoid}(\textbf{W}_{\text{add edge}}\text{concat}([\textbf{h}_{G}, \textbf{h}_{v}])+b_{\text{add edge}}),\\
#
# which is then used to parametrize a Bernoulli distribution for deciding
# whether to add a new edge starting from :math:`v`.
#

class AddEdge(nn.Module):
    def __init__(self, graph_embed_func, node_hidden_size):
        super(AddEdge, self).__init__()

        self.graph_op = {'embed': graph_embed_func}
        self.add_edge = nn.Linear(graph_embed_func.graph_hidden_size + \
                                  node_hidden_size, 1)

    def prepare_training(self):
        self.log_prob = []

    def forward(self, g, action=None):
        graph_embed = self.graph_op['embed'](g)
        src_embed = g.nodes[g.number_of_nodes() - 1].data['hv']

        logit = self.add_edge(torch.cat(
            [graph_embed, src_embed], dim=1))
        prob = torch.sigmoid(logit)

        if self.training:
            sample_log_prob = bernoulli_action_log_prob(logit, action)
            self.log_prob.append(sample_log_prob)
        else:
            action = Bernoulli(prob).sample().item()

        to_add_edge = bool(action == 0)
        return to_add_edge

#######################################################################################
# Action 3: Choose a destination
# '''''''''''''''''''''''''''''''''
#
# When action 2 returns `True`, choose a destination for the
# latest node :math:`v`.
#
# For each possible destination :math:`u\in\{0, \cdots, v-1\}`, the
# probability of choosing it is given by
#
# .. math::
#
#    \frac{\text{exp}(\textbf{W}_{\text{dest}}\text{concat}([\textbf{h}_{u}, \textbf{h}_{v}])+\textbf{b}_{\text{dest}})}{\sum_{i=0}^{v-1}\text{exp}(\textbf{W}_{\text{dest}}\text{concat}([\textbf{h}_{i}, \textbf{h}_{v}])+\textbf{b}_{\text{dest}})}\\
#

from torch.distributions import Categorical

class ChooseDestAndUpdate(nn.Module):
    def __init__(self, graph_prop_func, node_hidden_size):
        super(ChooseDestAndUpdate, self).__init__()

        self.graph_op = {'prop': graph_prop_func}
        self.choose_dest = nn.Linear(2 * node_hidden_size, 1)

    def _initialize_edge_repr(self, g, src_list, dest_list):
        # For untyped edges, only add 1 to indicate its existence.
        # For multiple edge types, use a one-hot representation
        # or an embedding module.
        edge_repr = torch.ones(len(src_list), 1)
        g.edges[src_list, dest_list].data['he'] = edge_repr

    def prepare_training(self):
        self.log_prob = []

    def forward(self, g, dest):
        src = g.number_of_nodes() - 1
        possible_dests = range(src)

        src_embed_expand = g.nodes[src].data['hv'].expand(src, -1)
        possible_dests_embed = g.nodes[possible_dests].data['hv']

        dests_scores = self.choose_dest(
            torch.cat([possible_dests_embed,
                       src_embed_expand], dim=1)).view(1, -1)
        dests_probs = F.softmax(dests_scores, dim=1)

        if not self.training:
            dest = Categorical(dests_probs).sample().item()

        if not g.has_edge_between(src, dest):
            # For undirected graphs, add edges for both directions
            # so that you can perform graph propagation.
            src_list = [src, dest]
            dest_list = [dest, src]

            g.add_edges(src_list, dest_list)
            self._initialize_edge_repr(g, src_list, dest_list)

            self.graph_op['prop'](g)

        if self.training:
            if dests_probs.nelement() > 1:
                self.log_prob.append(
                    F.log_softmax(dests_scores, dim=1)[:, dest: dest + 1])

#######################################################################################
# Putting it together
# ``````````````````````````
#
# You are now ready to have a complete implementation of the model class.
#

class DGMG(DGMGSkeleton):
    def __init__(self, v_max, node_hidden_size,
                 num_prop_rounds):
        super(DGMG, self).__init__(v_max)

        # Graph embedding module
        self.graph_embed = GraphEmbed(node_hidden_size)

        # Graph propagation module
        self.graph_prop = GraphProp(num_prop_rounds,
                                    node_hidden_size)

        # Actions
        self.add_node_agent = AddNode(
            self.graph_embed, node_hidden_size)
        self.add_edge_agent = AddEdge(
            self.graph_embed, node_hidden_size)
        self.choose_dest_agent = ChooseDestAndUpdate(
            self.graph_prop, node_hidden_size)

        # Forward functions
        self.forward_train = partial(forward_train, self=self)
        self.forward_inference = partial(forward_inference, self=self)

    @property
    def action_step(self):
        old_step_count = self.step_count
        self.step_count += 1

        return old_step_count

    def prepare_for_train(self):
        self.step_count = 0

        self.add_node_agent.prepare_training()
        self.add_edge_agent.prepare_training()
        self.choose_dest_agent.prepare_training()

    def add_node_and_update(self, a=None):
        """Decide if to add a new node.
        If a new node should be added, update the graph."""

        return self.add_node_agent(self.g, a)

    def add_edge_or_not(self, a=None):
        """Decide if a new edge should be added."""

        return self.add_edge_agent(self.g, a)

    def choose_dest_and_update(self, a=None):
        """Choose destination and connect it to the latest node.
        Add edges for both directions and update the graph."""

        self.choose_dest_agent(self.g, a)

    def get_log_prob(self):
        add_node_log_p = torch.cat(self.add_node_agent.log_prob).sum()
        add_edge_log_p = torch.cat(self.add_edge_agent.log_prob).sum()
        choose_dest_log_p = torch.cat(self.choose_dest_agent.log_prob).sum()
        return add_node_log_p + add_edge_log_p + choose_dest_log_p

#######################################################################################
# Below is an animation where a graph is generated on the fly
# after every 10 batches of training for the first 400 batches. You
# can see how the model improves over time and begins generating cycles.
#
# .. figure:: https://user-images.githubusercontent.com/19576924/48929291-60fe3880-ef22-11e8-832a-fbe56656559a.gif
#    :alt:
#
# For generative models, you can evaluate performance by checking the percentage
# of valid graphs among the graphs it generates on the fly.

import torch.utils.model_zoo as model_zoo

# Download a pre-trained model state dict for generating cycles with 10-20 nodes.
state_dict = model_zoo.load_url('https://data.dgl.ai/model/dgmg_cycles-5a0c40be.pth')
model = DGMG(v_max=20, node_hidden_size=16, num_prop_rounds=2)
model.load_state_dict(state_dict)
model.eval()

def is_valid(g):
    # Check if g is a cycle having 10-20 nodes.
    def _get_previous(i, v_max):
        if i == 0:
            return v_max
        else:
            return i - 1

    def _get_next(i, v_max):
        if i == v_max:
            return 0
        else:
            return i + 1

    size = g.number_of_nodes()

    if size < 10 or size > 20:
        return False

    for node in range(size):
        neighbors = g.successors(node)

        if len(neighbors) != 2:
            return False

        if _get_previous(node, size - 1) not in neighbors:
            return False

        if _get_next(node, size - 1) not in neighbors:
            return False

    return True

num_valid = 0
for i in range(100):
    g = model()
    num_valid += is_valid(g)

del model
print('Among 100 graphs generated, {}% are valid.'.format(num_valid))

#######################################################################################
# For the complete implementation, see the `DGL DGMG example
# <https://github.com/dmlc/dgl/tree/master/examples/pytorch/dgmg>`__.
#
# Batched graph generation
# ---------------------------
#
# Speeding up DGMG is hard because each graph can be generated with a
# unique sequence of actions. One way to explore parallelism is to adopt
# asynchronous gradient descent with multiple processes. Each of them
# works on one graph at a time and the processes are loosely coordinated
# by a parameter server.
#
# DGL explores parallelism in the message-passing framework, on top of
# the framework-provided tensor operation. The earlier tutorial already
# does that in the message propagation and graph embedding phases, but
# only within one graph. For a batch of graphs, a for loop is then needed:
#
# ::
#
#     for g in g_list:
#     self.graph_prop(g)
#
# Modify the code to work on a batch of graphs at once by replacing
# these lines with the following. On CPU with a macOS, you instantly
# enjoy a six to seven-time reduction for the graph propagation part.
# ::
#
#     bg = dgl.batch(g_list)
#     self.graph_prop(bg)
#     g_list = dgl.unbatch(bg)
#
# You have already used this trick of calling ``dgl.batch`` in the
# `Tree-LSTM tutorial
# <http://docs.dgl.ai/tutorials/models/3_tree-lstm.html#sphx-glr-tutorials-models-3-tree-lstm-py>`__
# , and it is worth explaining one more time why this is so.
#
# By batching many small graphs, DGL parallels message passing on each individual
# graphs of a batch.
#
# With ``dgl.batch``, you merge ``g_{1}, ..., g_{N}`` into one single giant
# graph consisting of :math:`N` isolated small graphs. For example, if we
# have two graphs with adjacency matrices
#
# ::
#
#     [0, 1]
#     [1, 0]
#
#     [0, 1, 0]
#     [1, 0, 0]
#     [0, 1, 0]
#
# ``dgl.batch`` simply gives a graph whose adjacency matrix is
#
# ::
#
#     [0, 1, 0, 0, 0]
#     [1, 0, 0, 0, 0]
#     [0, 1, 0, 0, 0]
#     [1, 0, 0, 0, 0]
#     [0, 1, 0, 0, 0]
#
# In DGL, the message function is defined on the edges, thus batching scales
# the processing of edge user-defined functions (UDFs) linearly.
#
# The reduce UDFs or ``dgmg_reduce``, work on nodes. Each of them may
# have different numbers of incoming edges. Using ``degree bucketing``, DGL
# internally groups nodes with the same in-degrees and calls reduce UDF once
# for each group. Thus, batching also reduces number of calls to these UDFs.
#
# The modification of the node/edge features of the batched graph object
# does not take effect on the features of the original small graphs, so we
# need to replace the old graph list with the new graph list
# ``g_list = dgl.unbatch(bg)``.
#
# The complete code to the batched version can also be found in the example.
# On a testbed, you get roughly double the speed when compared to the previous implementation.
