text: # -*- coding: utf-8 -*-
  """
      pint.context
      ~~~~~~~~~~~~
  
      Functions and classes related to context definitions and application.
  
      :copyright: 2016 by Pint Authors, see AUTHORS for more details..
      :license: BSD, see LICENSE for more details.
  """
  
  from __future__ import division, unicode_literals, print_function, absolute_import
  
  
  import re
  from collections import defaultdict
  import weakref
  
  from .compat import ChainMap
  from .util import (ParserHelper, UnitsContainer, string_types,
                     to_units_container, SourceIterator)
  from .errors import DefinitionSyntaxError
  
  #: Regex to match the header parts of a context.
  _header_re = re.compile('@context\s*(?P<defaults>\(.*\))?\s+(?P<name>\w+)\s*(=(?P<aliases>.*))*')
  
  #: Regex to match variable names in an equation.
  _varname_re = re.compile('[A-Za-z_][A-Za-z0-9_]*')
  
  
  def _expression_to_function(eq):
      def func(ureg, value, **kwargs):
          return ureg.parse_expression(eq, value=value, **kwargs)
      return func
  
  
  class Context(object):
      """A specialized container that defines transformation functions from one
      dimension to another. Each Dimension are specified using a UnitsContainer.
      Simple transformation are given with a function taking a single parameter.
  
          >>> timedim = UnitsContainer({'[time]': 1})
          >>> spacedim = UnitsContainer({'[length]': 1})
          >>> def f(time):
          ...     'Time to length converter'
          ...     return 3. * time
          >>> c = Context()
          >>> c.add_transformation(timedim, spacedim, f)
          >>> c.transform(timedim, spacedim, 2)
          6
  
      Conversion functions may take optional keyword arguments and the context
      can have default values for these arguments.
  
          >>> def f(time, n):
          ...     'Time to length converter, n is the index of refraction of the material'
          ...     return 3. * time / n
          >>> c = Context(n=3)
          >>> c.add_transformation(timedim, spacedim, f)
          >>> c.transform(timedim, spacedim, 2)
          2
  
      """
  
      def __init__(self, name, aliases=(), defaults=None):
  
          self.name = name
          self.aliases = aliases
  
          #: Maps (src, dst) -> transformation function
          self.funcs = {}
  
          #: Maps defaults variable names to values
          self.defaults = defaults or {}
  
          #: Maps (src, dst) -> self
          #: Used as a convenience dictionary to be composed by ContextChain
          self.relation_to_context = weakref.WeakValueDictionary()
  
      @classmethod
      def from_context(cls, context, **defaults):
          """Creates a new context that shares the funcs dictionary with the
          original context. The default values are copied from the original
          context and updated with the new defaults.
  
          If defaults is empty, return the same context.
          """
          if defaults:
              newdef = dict(context.defaults, **defaults)
              c = cls(context.name, context.aliases, newdef)
              c.funcs = context.funcs
              for edge in context.funcs.keys():
                  c.relation_to_context[edge] = c
              return c
          return context
  
      @classmethod
      def from_lines(cls, lines, to_base_func=None):
          lines = SourceIterator(lines)
  
          lineno, header = next(lines)
          try:
              r = _header_re.search(header)
              name = r.groupdict()['name'].strip()
              aliases = r.groupdict()['aliases']
              if aliases:
                  aliases = tuple(a.strip() for a in r.groupdict()['aliases'].split('='))
              else:
                  aliases = ()
              defaults = r.groupdict()['defaults']
          except:
              raise DefinitionSyntaxError("Could not parse the Context header '%s'" % header,
                                          lineno=lineno)
  
          if defaults:
              def to_num(val):
                  val = complex(val)
                  if not val.imag:
                      return val.real
                  return val
  
              _txt = defaults
              try:
                  defaults = (part.split('=') for part in defaults.strip('()').split(','))
                  defaults = dict((str(k).strip(), to_num(v))
                                  for k, v in defaults)
              except (ValueError, TypeError):
                  raise DefinitionSyntaxError("Could not parse Context definition defaults: '%s'", _txt,
                                              lineno=lineno)
  
              ctx = cls(name, aliases, defaults)
          else:
              ctx = cls(name, aliases)
  
          names = set()
          for lineno, line in lines:
              try:
                  rel, eq = line.split(':')
                  names.update(_varname_re.findall(eq))
  
                  func = _expression_to_function(eq)
  
                  if '<->' in rel:
                      src, dst = (ParserHelper.from_string(s)
                                  for s in rel.split('<->'))
                      if to_base_func:
                          src = to_base_func(src)
                          dst = to_base_func(dst)
                      ctx.add_transformation(src, dst, func)
                      ctx.add_transformation(dst, src, func)
                  elif '->' in rel:
                      src, dst = (ParserHelper.from_string(s)
                                  for s in rel.split('->'))
                      if to_base_func:
                          src = to_base_func(src)
                          dst = to_base_func(dst)
                      ctx.add_transformation(src, dst, func)
                  else:
                      raise Exception
              except:
                  raise DefinitionSyntaxError("Could not parse Context %s relation '%s'" % (name, line),
                                              lineno=lineno)
  
          if defaults:
              missing_pars = set(defaults.keys()).difference(set(names))
              if missing_pars:
                  raise DefinitionSyntaxError('Context parameters {0} not found in any equation.'.format(missing_pars))
  
          return ctx
  
      def add_transformation(self, src, dst, func):
          """Add a transformation function to the context.
          """
          _key = self.__keytransform__(src, dst)
          self.funcs[_key] = func
          self.relation_to_context[_key] = self
  
      def remove_transformation(self, src, dst):
          """Add a transformation function to the context.
          """
          _key = self.__keytransform__(src, dst)
          del self.funcs[_key]
          del self.relation_to_context[_key]
  
      @staticmethod
      def __keytransform__(src, dst):
          return to_units_container(src), to_units_container(dst)
  
      def transform(self, src, dst, registry, value):
          """Transform a value.
          """
          _key = self.__keytransform__(src, dst)
          return self.funcs[_key](registry, value, **self.defaults)
  
  
  class ContextChain(ChainMap):
      """A specialized ChainMap for contexts that simplifies finding rules
      to transform from one dimension to another.
      """
  
      def __init__(self, *args, **kwargs):
          super(ContextChain, self).__init__(*args, **kwargs)
          self._graph = None
          self._contexts = []
  
      def insert_contexts(self, *contexts):
          """Insert one or more contexts in reversed order the chained map.
          (A rule in last context will take precedence)
  
          To facilitate the identification of the context with the matching rule,
          the *relation_to_context* dictionary of the context is used.
          """
          self._contexts.insert(0, contexts)
          self.maps = [ctx.relation_to_context for ctx in reversed(contexts)] + self.maps
          self._graph = None
  
      def remove_contexts(self, n):
          """Remove the last n inserted contexts from the chain.
          """
          self._contexts = self._contexts[n:]
          self.maps = self.maps[n:]
          self._graph = None
  
      @property
      def defaults(self):
          if self:
              return list(self.maps[0].values())[0].defaults
          return {}
  
      @property
      def graph(self):
          """The graph relating
          """
          if self._graph is None:
              self._graph = defaultdict(set)
              for fr_, to_ in self:
                  self._graph[fr_].add(to_)
          return self._graph
  
      def transform(self, src, dst, registry, value):
          """Transform the value, finding the rule in the chained context.
          (A rule in last context will take precedence)
  
          :raises: KeyError if the rule is not found.
          """
          return self[(src, dst)].transform(src, dst, registry, value)
  