# Copyright 2018 The TensorFlow Authors. All Rights Reserved.
#
# Licensed under the Apache License, Version 2.0 (the "License");
# you may not use this file except in compliance with the License.
# You may obtain a copy of the License at
#
#     http://www.apache.org/licenses/LICENSE-2.0
#
# Unless required by applicable law or agreed to in writing, software
# distributed under the License is distributed on an "AS IS" BASIS,
# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
# See the License for the specific language governing permissions and
# limitations under the License.
# ==============================================================================
"""Live variable analysis.

See https://en.wikipedia.org/wiki/Live_variable_analysis for a definition of
the following idioms: live variable, live in, live out, which are used
throughout this file.

This analysis attaches the following:
 * symbols that are live at the exit of control flow statements
 * symbols that are live at the entry of control flow statements

Requires activity analysis.
"""

from __future__ import absolute_import
from __future__ import division
from __future__ import print_function

import gast

from tensorflow.python.autograph.pyct import anno
from tensorflow.python.autograph.pyct import cfg
from tensorflow.python.autograph.pyct import transformer
from tensorflow.python.autograph.pyct.static_analysis import annos


class Analyzer(cfg.GraphVisitor):
  """CFG visitor that performs liveness analysis at statement level."""

  def __init__(self, graph):
    super(Analyzer, self).__init__(graph)
    # This allows communicating that nodes generate extra symbols,
    # e.g. those that a function definition closes over.
    self.extra_gen = {}

  def init_state(self, _):
    return set()

  def visit_node(self, node):
    prev_live_in = self.in_[node]

    if anno.hasanno(node.ast_node, anno.Static.SCOPE):
      node_scope = anno.getanno(node.ast_node, anno.Static.SCOPE)

      gen = node_scope.used | self.extra_gen.get(node.ast_node, frozenset())
      # TODO(mdan): verify whether composites' parents need to be added.
      # E.g. if x.y is live whether x needs to be added. Theoretically the
      # activity analysis should have both so that wouldn't be needed.
      kill = node_scope.modified

      live_out = set()
      for n in node.next:
        live_out |= self.in_[n]
      live_in = gen | (live_out - kill)

    else:
      # Nodes that don't have a scope annotation are assumed not to touch any
      # symbols.
      # This Name node below is a literal name, e.g. False
      assert isinstance(node.ast_node,
                        (gast.Name, gast.Continue, gast.Break)), type(
                            node.ast_node)
      live_in = prev_live_in
      live_out = live_in

    self.in_[node] = live_in
    self.out[node] = live_out

    # TODO(mdan): Move this to the superclass?
    return prev_live_in != live_in


class WholeTreeAnalyzer(transformer.Base):
  """Runs liveness analysis on each of the functions defined in the AST.

  If a function defined other local functions, those will have separate CFGs.
  However, dataflow analysis needs to tie up these CFGs to properly emulate the
  effect of closures. In the case of liveness, the parent function's live
  variables must account for the variables that are live at the entry of each
  subfunction. For example:

    def foo():
      # baz is live here
      def bar():
        print(baz)

  This analyzer runs liveness analysis on each individual function, accounting
  for the effect above.
  """

  def __init__(self, source_info, graphs):
    super(WholeTreeAnalyzer, self).__init__(source_info)
    self.graphs = graphs
    self.current_analyzer = None
    self.analyzers = {}

  def visit_FunctionDef(self, node):
    parent_analyzer = self.current_analyzer
    subgraph = self.graphs[node]

    # Postorder tree processing makes this a bit complicated:
    #  1. construct an analyzer object and put it on stack
    #  2. recursively walk the subtree; this will initialize the analyzer's
    #     in_ state properly (done in a block below)
    #  3. run the final analysis
    analyzer = Analyzer(subgraph)
    self.current_analyzer = analyzer
    node = self.generic_visit(node)
    analyzer.visit_reverse()

    if parent_analyzer is not None:
      # Wire the state between the two subgraphs' analyzers.
      child_in_state = analyzer.in_[subgraph.entry]
      # Exception: symbols modified in the child function are local to it
      body_scope = anno.getanno(node, annos.NodeAnno.BODY_SCOPE)
      for qn in body_scope.modified:
        # Note: a function modifying the symbol doesn't make that symbol
        # live at the function's entry. In fact when that happens it is
        # probably a case of undefined assignment, like this:
        #
        #   bar = 0
        #   def foo():
        #     print(bar)  # bar is undefined here!
        #     bar = 1
        #
        # Hence we use discard and not remove below.
        child_in_state.discard(qn)
      parent_analyzer.extra_gen[node] = frozenset(child_in_state,)

    self.analyzers[node] = analyzer
    self.current_analyzer = parent_analyzer
    return node

  def visit_nonlocal(self, node):
    raise NotImplementedError()

  def visit_global(self, node):
    raise NotImplementedError()


class Annotator(transformer.Base):
  """AST visitor that annotates each control flow block with live symbols."""

  # Note: additional nodes may be added as needed.

  def __init__(self, source_info, cross_function_analyzer):
    super(Annotator, self).__init__(source_info)
    self.cross_function_analyzer = cross_function_analyzer
    self.current_analyzer = None

  def visit_FunctionDef(self, node):
    parent_analyzer = self.current_analyzer
    self.current_analyzer = self.cross_function_analyzer.analyzers[node]

    node = self.generic_visit(node)
    self.current_analyzer = parent_analyzer
    return node

  def _block_statement_live_out(self, node):
    successors = self.current_analyzer.graph.stmt_next[node]
    stmt_live_out = set()
    for s in successors:
      stmt_live_out.update(self.current_analyzer.in_[s])
    anno.setanno(node, anno.Static.LIVE_VARS_OUT, frozenset(stmt_live_out))
    return node

  def _block_statement_live_in(self, node, entry_node):
    cfg_node = self.current_analyzer.graph.index[entry_node]
    stmt_live_in = frozenset(self.current_analyzer.in_[cfg_node])
    anno.setanno(node, anno.Static.LIVE_VARS_IN, stmt_live_in)
    return node

  def visit_If(self, node):
    node = self.generic_visit(node)
    node = self._block_statement_live_out(node)
    return self._block_statement_live_in(node, node.test)

  def visit_For(self, node):
    node = self.generic_visit(node)
    node = self._block_statement_live_out(node)
    return self._block_statement_live_in(node, node.iter)

  def visit_While(self, node):
    node = self.generic_visit(node)
    node = self._block_statement_live_out(node)
    return self._block_statement_live_in(node, node.test)


def resolve(node, source_info, graphs):
  """Resolves the live symbols at the exit of control flow statements.

  Args:
    node: ast.AST
    source_info: transformer.SourceInfo
    graphs: Dict[ast.FunctionDef, cfg.Graph]
  Returns:
    ast.AST
  """
  cross_function_analyzer = WholeTreeAnalyzer(source_info, graphs)
  node = cross_function_analyzer.visit(node)
  visitor = Annotator(source_info, cross_function_analyzer)
  node = visitor.visit(node)
  return node