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

from collections import defaultdict
from enum import Enum
import logging
from pathlib import Path
import re
import sys
import warnings

try:
    from collections import OrderedDict
except ImportError:
    # Backport
    from ordereddict import OrderedDict

import six

from scss.calculator import Calculator
from scss.cssdefs import _spaces_re
from scss.cssdefs import _escape_chars_re
from scss.cssdefs import _prop_split_re
from scss.errors import SassError
from scss.errors import SassBaseError
from scss.errors import SassImportError
from scss.extension import Extension
from scss.extension.core import CoreExtension
from scss.extension import NamespaceAdapterExtension
from scss.grammar import locate_blocks
from scss.rule import BlockAtRuleHeader
from scss.rule import Namespace
from scss.rule import RuleAncestry
from scss.rule import SassRule
from scss.rule import UnparsedBlock
from scss.selector import Selector
from scss.source import SourceFile
from scss.types import Arglist
from scss.types import List
from scss.types import Null
from scss.types import Number
from scss.types import String
from scss.types import Undefined
from scss.types import Url
from scss.util import normalize_var  # TODO put in...  namespace maybe?


# TODO should mention logging for the programmatic interface in the
# documentation
# TODO or have a little helper (or compiler setting) to turn it on
log = logging.getLogger(__name__)


_xcss_extends_re = re.compile(r'\s+extends\s+')


class OutputStyle(Enum):
    nested = ()
    compact = ()
    compressed = ()
    expanded = ()

    legacy = ()  # ???


class SassDeprecationWarning(UserWarning):
    # Note: DO NOT inherit from DeprecationWarning; it's turned off by default
    # in 2.7 and later!
    pass


def warn_deprecated(rule, message):
    warnings.warn(
        "{0} (at {1})".format(message, rule.file_and_line),
        SassDeprecationWarning,
        stacklevel=2,
    )


class Compiler(object):
    """A Sass compiler.  Stores settings and knows how to fire off a
    compilation.  Main entry point into compiling Sass.
    """
    def __init__(
            self, root=Path(), search_path=(),
            namespace=None, extensions=(CoreExtension,),
            import_static_css=False,
            output_style='nested', generate_source_map=False,
            live_errors=False, warn_unused_imports=False,
            ignore_parse_errors=False,
            loops_have_own_scopes=True,
            undefined_variables_fatal=True,
            super_selector='',
            ):
        """Configure a compiler.

        :param root: Directory to treat as the "project root".  Search paths
            and some custom extensions (e.g. Compass) are relative to this
            directory.  Defaults to the current directory.
        :type root: :class:`pathlib.Path`
        :param search_path: List of paths to search for ``@import``s, relative
            to ``root``.  Absolute and parent paths are allowed here, but
            ``@import`` will refuse to load files that aren't in one of the
            directories here.  Defaults to only the root.
        :type search_path: list of strings, :class:`pathlib.Path` objects, or
            something that implements a similar interface (useful for custom
            pseudo filesystems)
        """
        # TODO perhaps polite to automatically cast any string paths to Path?
        # but have to be careful since the api explicitly allows dummy objects.
        if root is None:
            self.root = None
        else:
            self.root = root.resolve()

        self.search_path = tuple(
            self.normalize_path(path)
            for path in search_path
        )

        self.extensions = []
        if namespace is not None:
            self.extensions.append(NamespaceAdapterExtension(namespace))
        for extension in extensions:
            if isinstance(extension, Extension):
                self.extensions.append(extension)
            elif (isinstance(extension, type) and
                    issubclass(extension, Extension)):
                self.extensions.append(extension())
            elif isinstance(extension, Namespace):
                self.extensions.append(
                    NamespaceAdapterExtension(extension))
            else:
                raise TypeError(
                    "Expected an Extension or Namespace, got: {0!r}"
                    .format(extension)
                )

        if import_static_css:
            self.dynamic_extensions = ('.scss', '.sass', '.css')
            self.static_extensions = ()
        else:
            self.dynamic_extensions = ('.scss', '.sass')
            self.static_extensions = ('.css',)

        self.output_style = output_style
        self.generate_source_map = generate_source_map
        self.live_errors = live_errors
        self.warn_unused_imports = warn_unused_imports
        self.ignore_parse_errors = ignore_parse_errors
        self.loops_have_own_scopes = loops_have_own_scopes
        self.undefined_variables_fatal = undefined_variables_fatal
        self.super_selector = super_selector

    def normalize_path(self, path):
        if isinstance(path, six.string_types):
            path = Path(path)
        if path.is_absolute():
            return path
        if self.root is None:
            raise IOError("Can't make absolute path when root is None")
        return self.root / path

    def make_compilation(self):
        return Compilation(self)

    def call_and_catch_errors(self, f, *args, **kwargs):
        """Call the given function with the given arguments.  If it succeeds,
        return its return value.  If it raises a :class:`scss.errors.SassError`
        and `live_errors` is turned on, return CSS containing a traceback and
        error message.
        """
        try:
            return f(*args, **kwargs)
        except SassError as e:
            if self.live_errors:
                # TODO should this setting also capture and display warnings?
                return e.to_css()
            else:
                raise

    def compile(self, *filenames):
        # TODO this doesn't spit out the compilation itself, so if you want to
        # get something out besides just the output, you have to copy this
        # method.  that sucks.
        # TODO i think the right thing is to get all the constructors out of
        # SourceFile, since it's really the compiler that knows the import
        # paths and should be consulted about this.  reconsider all this (but
        # preserve it for now, SIGH) once importers are a thing
        compilation = self.make_compilation()
        for filename in filenames:
            # TODO maybe SourceFile should not be exposed to the end user, and
            # instead Compilation should have methods for add_string etc. that
            # can call normalize_path.
            # TODO it's not possible to inject custom files into the
            # /compiler/ as persistent across compiles, nor to provide "fake"
            # imports.  do we want the former?  is the latter better suited to
            # an extension?
            source = SourceFile.from_filename(self.normalize_path(filename))
            compilation.add_source(source)
        return self.call_and_catch_errors(compilation.run)

    def compile_sources(self, *sources):
        # TODO this api is not the best please don't use it.  this all needs to
        # be vastly simplified, still, somehow.
        compilation = self.make_compilation()
        for source in sources:
            compilation.add_source(source)
        return self.call_and_catch_errors(compilation.run)

    def compile_string(self, string):
        source = SourceFile.from_string(string)
        compilation = self.make_compilation()
        compilation.add_source(source)
        return self.call_and_catch_errors(compilation.run)


def compile_file(filename, compiler_class=Compiler, **kwargs):
    """Compile a single file (provided as a :class:`pathlib.Path`), and return
    a string of CSS.

    Keyword arguments are passed along to the underlying `Compiler`.

    Note that the search path is set to the file's containing directory by
    default, unless you explicitly pass a ``search_path`` kwarg.

    :param filename: Path to the file to compile.
    :type filename: str, bytes, or :class:`pathlib.Path`
    """
    filename = Path(filename)
    if 'search_path' not in kwargs:
        kwargs['search_path'] = [filename.parent.resolve()]

    compiler = compiler_class(**kwargs)
    return compiler.compile(filename)


def compile_string(string, compiler_class=Compiler, **kwargs):
    """Compile a single string, and return a string of CSS.

    Keyword arguments are passed along to the underlying `Compiler`.
    """
    compiler = compiler_class(**kwargs)
    return compiler.compile_string(string)


class Compilation(object):
    """A single run of a compiler."""
    def __init__(self, compiler):
        self.compiler = compiler
        self.ignore_parse_errors = compiler.ignore_parse_errors

        # TODO this needs a write barrier, so assignment can't overwrite what's
        # in the original namespaces
        # TODO or maybe the extensions themselves should take care of that, so
        # it IS possible to overwrite from within sass, but only per-instance?
        self.root_namespace = Namespace.derive_from(*(
            ext.namespace for ext in compiler.extensions
            if ext.namespace
        ))

        self.sources = []
        self.source_index = {}
        self.dependency_map = defaultdict(frozenset)
        self.rules = []

    def should_scope_loop_in_rule(self, rule):
        """Return True iff a looping construct (@each, @for, @while, @if)
        should get its own scope, as is standard Sass behavior.
        """
        return rule.legacy_compiler_options.get(
            'control_scoping', self.compiler.loops_have_own_scopes)

    def add_source(self, source):
        if source.key in self.source_index:
            return self.source_index[source.key]
        self.sources.append(source)
        self.source_index[source.key] = source
        return source

    def run(self):
        # Any @import will add the source file to self.sources and infect this
        # list, so make a quick copy to insulate against that
        # TODO maybe @import should just not do that?
        for source_file in list(self.sources):
            rule = SassRule(
                source_file=source_file,
                lineno=1,

                unparsed_contents=source_file.contents,
                namespace=self.root_namespace,
            )
            self.rules.append(rule)
            self.manage_children(rule, scope=None)
            self._warn_unused_imports(rule)

        # Run through all the rules and apply @extends in a separate pass
        self.rules = self.apply_extends(self.rules)

        output, total_selectors = self.create_css(self.rules)
        if total_selectors > 65534:
            log.warning("Maximum number of supported selectors in Internet Explorer (65534) exceeded!")

        return output

    def parse_selectors(self, raw_selectors):
        """
        Parses out the old xCSS "foo extends bar" syntax.

        Returns a 2-tuple: a set of selectors, and a set of extended selectors.
        """
        # Fix tabs and spaces in selectors
        raw_selectors = _spaces_re.sub(' ', raw_selectors)

        parts = _xcss_extends_re.split(raw_selectors, 1)  # handle old xCSS extends
        if len(parts) > 1:
            unparsed_selectors, unsplit_parents = parts
            # Multiple `extends` are delimited by `&`
            unparsed_parents = unsplit_parents.split('&')
        else:
            unparsed_selectors, = parts
            unparsed_parents = ()

        selectors = Selector.parse_many(unparsed_selectors)
        parents = [Selector.parse_one(parent) for parent in unparsed_parents]

        return selectors, parents

    def _warn_unused_imports(self, rule):
        if not rule.legacy_compiler_options.get(
                'warn_unused', self.compiler.warn_unused_imports):
            return

        for name, file_and_line in rule.namespace.unused_imports():
            log.warn("Unused @import: '%s' (%s)", name, file_and_line)

    def _make_calculator(self, namespace):
        return Calculator(
            namespace,
            ignore_parse_errors=self.ignore_parse_errors,
            undefined_variables_fatal=self.compiler.undefined_variables_fatal,
        )

    # @print_timing(4)
    def manage_children(self, rule, scope):
        try:
            self._manage_children_impl(rule, scope)
        except SassBaseError as e:
            e.add_rule(rule)
            raise
        except Exception as e:
            raise SassError(e, rule=rule)

    def _manage_children_impl(self, rule, scope):
        calculator = self._make_calculator(rule.namespace)

        for c_lineno, c_property, c_codestr in locate_blocks(rule.unparsed_contents):
            block = UnparsedBlock(rule, c_lineno, c_property, c_codestr)

            ####################################################################
            # At (@) blocks
            if block.is_atrule:
                # TODO particularly wild idea: allow extensions to handle
                # unrecognized blocks, and get the pyscss stuff out of the
                # core?  even move the core stuff into the core extension?
                code = block.directive
                code = '_at_' + code.lower().replace(' ', '_')[1:]
                try:
                    method = getattr(self, code)
                except AttributeError:
                    if block.unparsed_contents is None:
                        rule.properties.append((block.prop, None))
                    elif scope is None:  # needs to have no scope to crawl down the nested rules
                        self._nest_at_rules(rule, scope, block)
                else:
                    method(calculator, rule, scope, block)

            ####################################################################
            # Properties
            elif block.unparsed_contents is None:
                self._get_properties(rule, scope, block)

            # Nested properties
            elif block.is_scope:
                if block.header.unscoped_value:
                    # Possibly deal with default unscoped value
                    self._get_properties(rule, scope, block)

                rule.unparsed_contents = block.unparsed_contents
                subscope = (scope or '') + block.header.scope + '-'
                self.manage_children(rule, subscope)

            ####################################################################
            # Nested rules
            elif scope is None:  # needs to have no scope to crawl down the nested rules
                self._nest_rules(rule, scope, block)

    def _at_warn(self, calculator, rule, scope, block):
        """
        Implements @warn
        """
        value = calculator.calculate(block.argument)
        log.warn(repr(value))

    def _at_print(self, calculator, rule, scope, block):
        """
        Implements @print
        """
        value = calculator.calculate(block.argument)
        sys.stderr.write("%s\n" % value)

    def _at_raw(self, calculator, rule, scope, block):
        """
        Implements @raw
        """
        value = calculator.calculate(block.argument)
        sys.stderr.write("%s\n" % repr(value))

    def _at_dump_context(self, calculator, rule, scope, block):
        """
        Implements @dump_context
        """
        sys.stderr.write("%s\n" % repr(rule.namespace._variables))

    def _at_dump_functions(self, calculator, rule, scope, block):
        """
        Implements @dump_functions
        """
        sys.stderr.write("%s\n" % repr(rule.namespace._functions))

    def _at_dump_mixins(self, calculator, rule, scope, block):
        """
        Implements @dump_mixins
        """
        sys.stderr.write("%s\n" % repr(rule.namespace._mixins))

    def _at_dump_imports(self, calculator, rule, scope, block):
        """
        Implements @dump_imports
        """
        sys.stderr.write("%s\n" % repr(rule.namespace._imports))

    def _at_dump_options(self, calculator, rule, scope, block):
        """
        Implements @dump_options
        """
        sys.stderr.write("%s\n" % repr(rule.options))

    def _at_debug(self, calculator, rule, scope, block):
        """
        Implements @debug
        """
        setting = block.argument.strip()
        if setting.lower() in ('1', 'true', 't', 'yes', 'y', 'on'):
            setting = True
        elif setting.lower() in ('0', 'false', 'f', 'no', 'n', 'off', 'undefined'):
            setting = False
        self.ignore_parse_errors = setting
        log.info("Debug mode is %s", 'On' if self.ignore_parse_errors else 'Off')

    def _at_pdb(self, calculator, rule, scope, block):
        """
        Implements @pdb
        """
        try:
            import ipdb as pdb
        except ImportError:
            import pdb
        pdb.set_trace()

    def _at_extend(self, calculator, rule, scope, block):
        """
        Implements @extend
        """
        from scss.selector import Selector
        selectors = calculator.apply_vars(block.argument)
        rule.extends_selectors.extend(Selector.parse_many(selectors))

    def _at_return(self, calculator, rule, scope, block):
        """
        Implements @return
        """
        # TODO should assert this only happens within a @function
        ret = calculator.calculate(block.argument)
        raise SassReturn(ret)

    # @print_timing(10)
    def _at_option(self, calculator, rule, scope, block):
        """
        Implements @option
        """
        # TODO This only actually supports "style" (which only really makes
        # sense as the first thing in a single input file) or "warn_unused"
        # (which only makes sense at file level /at best/).  Explore either
        # replacing this with a better mechanism or dropping it entirely.
        # Note also that all rules share the same underlying legacy option
        # dict, so the rules aren't even lexically scoped like you might think,
        # and @importing a file can change the compiler!  That seems totally
        # wrong.
        for option in block.argument.split(','):
            key, colon, value = option.partition(':')
            key = key.strip().lower().replace('-', '_')
            value = value.strip().lower()

            if value in ('1', 'true', 't', 'yes', 'y', 'on'):
                value = True
            elif value in ('0', 'false', 'f', 'no', 'n', 'off', 'undefined'):
                value = False
            elif not colon:
                value = True

            if key == 'compress':
                warn_deprecated(
                    rule,
                    "The 'compress' @option is deprecated.  "
                    "Please use 'style' instead."
                )
                key = 'style'
                value = 'compressed' if value else 'legacy'

            if key in ('short_colors', 'reverse_colors'):
                warn_deprecated(
                    rule,
                    "The '{0}' @option no longer has any effect."
                    .format(key),
                )
                return
            elif key == 'style':
                try:
                    OutputStyle[value]
                except KeyError:
                    raise SassError("No such output style: {0}".format(value))
            elif key in ('warn_unused', 'control_scoping'):
                # TODO deprecate control_scoping?  or add it to compiler?
                if not isinstance(value, bool):
                    raise SassError("The '{0}' @option requires a bool, not {1!r}".format(key, value))
            else:
                raise SassError("Unknown @option: {0}".format(key))

            rule.legacy_compiler_options[key] = value

    def _get_funct_def(self, rule, calculator, argument):
        funct, lpar, argstr = argument.partition('(')
        funct = calculator.do_glob_math(funct)
        funct = normalize_var(funct.strip())
        argstr = argstr.strip()

        # Parse arguments with the argspec rule
        if lpar:
            if not argstr.endswith(')'):
                raise SyntaxError("Expected ')', found end of line for %s (%s)" % (funct, rule.file_and_line))
            argstr = argstr[:-1].strip()
        else:
            # Whoops, no parens at all.  That's like calling with no arguments.
            argstr = ''

        argspec_node = calculator.parse_expression(argstr, target='goal_argspec')
        return funct, argspec_node

    def _populate_namespace_from_call(self, name, callee_namespace, mixin, args, kwargs):
        # Mutation protection
        args = list(args)
        kwargs = OrderedDict(kwargs)

        #m_params = mixin[0]
        #m_defaults = mixin[1]
        #m_codestr = mixin[2]
        pristine_callee_namespace = mixin[3]
        callee_argspec = mixin[4]
        import_key = mixin[5]

        callee_calculator = self._make_calculator(callee_namespace)

        # Populate the mixin/function's namespace with its arguments
        for var_name, node in callee_argspec.iter_def_argspec():
            if args:
                # If there are positional arguments left, use the first
                value = args.pop(0)
            elif var_name in kwargs:
                # Try keyword arguments
                value = kwargs.pop(var_name)
            elif node is not None:
                # OK, try the default argument.  Using callee_calculator means
                # that default values of arguments can refer to earlier
                # arguments' values; yes, that is how Sass works.
                value = node.evaluate(callee_calculator, divide=True)
            else:
                # TODO this should raise
                value = Undefined()

            callee_namespace.set_variable(var_name, value, local_only=True)

        if callee_argspec.slurp:
            # Slurpy var gets whatever is left
            # TODO should preserve the order of extra kwargs
            sass_kwargs = []
            for key, value in kwargs.items():
                sass_kwargs.append((String(key[1:]), value))
            callee_namespace.set_variable(
                callee_argspec.slurp.name,
                Arglist(args, sass_kwargs))
            args = []
            kwargs = {}
        elif callee_argspec.inject:
            # Callee namespace gets all the extra kwargs whether declared or
            # not
            for var_name, value in kwargs.items():
                callee_namespace.set_variable(var_name, value, local_only=True)
            kwargs = {}

        # TODO would be nice to say where the mixin/function came from
        if kwargs:
            raise NameError("%s has no such argument %s" % (name, kwargs.keys()[0]))

        if args:
            raise NameError("%s received extra arguments: %r" % (name, args))

        pristine_callee_namespace.use_import(import_key)
        return callee_namespace

    # @print_timing(10)
    def _at_function(self, calculator, rule, scope, block):
        """
        Implements @mixin and @function
        """
        if not block.argument:
            raise SyntaxError("%s requires a function name (%s)" % (block.directive, rule.file_and_line))

        funct, argspec_node = self._get_funct_def(rule, calculator, block.argument)

        defaults = {}
        new_params = []

        for var_name, default in argspec_node.iter_def_argspec():
            new_params.append(var_name)
            if default is not None:
                defaults[var_name] = default

        # TODO a function or mixin is re-parsed every time it's called; there's
        # no AST for anything but expressions  :(
        mixin = [rule.source_file, block.lineno, block.unparsed_contents, rule.namespace, argspec_node, rule.source_file]
        if block.directive == '@function':
            def _call(mixin):
                def __call(namespace, *args, **kwargs):
                    source_file = mixin[0]
                    lineno = mixin[1]
                    m_codestr = mixin[2]
                    pristine_callee_namespace = mixin[3]
                    callee_namespace = pristine_callee_namespace.derive()

                    # TODO CallOp converts Sass names to Python names, so we
                    # have to convert them back to Sass names.  would be nice
                    # to avoid this back-and-forth somehow
                    kwargs = OrderedDict(
                        (normalize_var('$' + key), value)
                        for (key, value) in kwargs.items())

                    self._populate_namespace_from_call(
                        "Function {0}".format(funct),
                        callee_namespace, mixin, args, kwargs)

                    _rule = SassRule(
                        source_file=source_file,
                        lineno=lineno,
                        unparsed_contents=m_codestr,
                        namespace=callee_namespace,

                        # rule
                        import_key=rule.import_key,
                        legacy_compiler_options=rule.legacy_compiler_options,
                        options=rule.options,
                        properties=rule.properties,
                        extends_selectors=rule.extends_selectors,
                        ancestry=rule.ancestry,
                        nested=rule.nested,
                    )
                    # TODO supposed to throw an error if there's a slurpy arg
                    # but keywords() is never called on it
                    try:
                        self.manage_children(_rule, scope)
                    except SassReturn as e:
                        return e.retval
                    else:
                        return Null()
                __call._pyscss_needs_namespace = True
                return __call
            _mixin = _call(mixin)
            _mixin.mixin = mixin
            mixin = _mixin

        if block.directive == '@mixin':
            add = rule.namespace.set_mixin
        elif block.directive == '@function':
            add = rule.namespace.set_function

        # Register the mixin for every possible arity it takes
        if argspec_node.slurp or argspec_node.inject:
            add(funct, None, mixin)
        else:
            while len(new_params):
                add(funct, len(new_params), mixin)
                param = new_params.pop()
                if param not in defaults:
                    break
            if not new_params:
                add(funct, 0, mixin)
    _at_mixin = _at_function

    # @print_timing(10)
    def _at_include(self, calculator, rule, scope, block):
        """
        Implements @include, for @mixins
        """
        caller_namespace = rule.namespace
        caller_calculator = self._make_calculator(caller_namespace)
        funct, caller_argspec = self._get_funct_def(rule, caller_calculator, block.argument)

        # Render the passed arguments, using the caller's namespace
        args, kwargs = caller_argspec.evaluate_call_args(caller_calculator)

        argc = len(args) + len(kwargs)
        try:
            mixin = caller_namespace.mixin(funct, argc)
        except KeyError:
            try:
                # TODO maybe? don't do this, once '...' works
                # Fallback to single parameter:
                mixin = caller_namespace.mixin(funct, 1)
            except KeyError:
                log.error("Mixin not found: %s:%d (%s)", funct, argc, rule.file_and_line, extra={'stack': True})
                return
            else:
                args = [List(args, use_comma=True)]
                # TODO what happens to kwargs?

        source_file = mixin[0]
        lineno = mixin[1]
        m_codestr = mixin[2]
        pristine_callee_namespace = mixin[3]
        callee_argspec = mixin[4]
        if caller_argspec.inject and callee_argspec.inject:
            # DEVIATION: Pass the ENTIRE local namespace to the mixin (yikes)
            callee_namespace = Namespace.derive_from(
                caller_namespace,
                pristine_callee_namespace)
        else:
            callee_namespace = pristine_callee_namespace.derive()

        self._populate_namespace_from_call(
            "Mixin {0}".format(funct),
            callee_namespace, mixin, args, kwargs)

        _rule = SassRule(
            # These must be file and line in which the @include occurs
            source_file=rule.source_file,
            lineno=rule.lineno,

            # These must be file and line in which the @mixin was defined
            from_source_file=source_file,
            from_lineno=lineno,

            unparsed_contents=m_codestr,
            namespace=callee_namespace,

            # rule
            import_key=rule.import_key,
            legacy_compiler_options=rule.legacy_compiler_options,
            options=rule.options,
            properties=rule.properties,
            extends_selectors=rule.extends_selectors,
            ancestry=rule.ancestry,
            nested=rule.nested,
        )

        _rule.options['@content'] = block.unparsed_contents
        self.manage_children(_rule, scope)

    # @print_timing(10)
    def _at_content(self, calculator, rule, scope, block):
        """
        Implements @content
        """
        if '@content' not in rule.options:
            log.error("Content string not found for @content (%s)", rule.file_and_line)
        rule.unparsed_contents = rule.options.pop('@content', '')
        self.manage_children(rule, scope)

    # @print_timing(10)
    def _at_import(self, calculator, rule, scope, block):
        """
        Implements @import
        Load and import mixins and functions and rules
        """
        # TODO it would be neat to opt into warning that you're using
        # values/functions from a file you didn't explicitly import
        # TODO base-level directives, like @mixin or @charset, aren't allowed
        # to be @imported into a nested block
        # TODO i'm not sure we disallow them nested in the first place
        # TODO @import is disallowed within mixins, control directives
        # TODO @import doesn't take a block -- that's probably an issue with a
        # lot of our directives

        # TODO if there's any #{}-interpolation in the AST, this should become
        # a CSS import (though in practice Ruby only even evaluates it in url()
        # -- in a string it's literal!)

        sass_paths = calculator.evaluate_expression(block.argument)
        css_imports = []

        for sass_path in sass_paths:
            # These are the rules for when an @import is interpreted as a CSS
            # import:
            if (
                    # If it's a url()
                    isinstance(sass_path, Url) or
                    # If it's not a string (including `"foo" screen`, a List)
                    not isinstance(sass_path, String) or
                    # If the filename begins with an http protocol
                    sass_path.value.startswith(('http://', 'https://')) or
                    # If the filename ends with .css
                    sass_path.value.endswith(self.compiler.static_extensions)):
                css_imports.append(sass_path.render(compress=False))
                continue

            # Should be left with a plain String
            name = sass_path.value

            source = None
            for extension in self.compiler.extensions:
                source = extension.handle_import(name, self, rule)
                if source:
                    break
            else:
                # Didn't find anything!
                raise SassImportError(name, self.compiler, rule=rule)

            source = self.add_source(source)

            if rule.namespace.has_import(source):
                # If already imported in this scope, skip
                # TODO this might not be right -- consider if you @import a
                # file at top level, then @import it inside a selector block!
                continue

            _rule = SassRule(
                source_file=source,
                lineno=block.lineno,
                unparsed_contents=source.contents,

                # rule
                legacy_compiler_options=rule.legacy_compiler_options,
                options=rule.options,
                properties=rule.properties,
                extends_selectors=rule.extends_selectors,
                ancestry=rule.ancestry,
                namespace=rule.namespace,
            )
            rule.namespace.add_import(source, rule)
            self.manage_children(_rule, scope)

        # Create a new @import rule for each import determined to be CSS
        for import_ in css_imports:
            # TODO this seems extremely janky (surely we should create an
            # actual new Rule), but the CSS rendering doesn't understand how to
            # print rules without blocks
            # TODO if this ever creates a new Rule, shuffle stuff around so
            # this is still hoisted to the top
            rule.properties.append(('@import ' + import_, None))

    # @print_timing(10)
    def _at_if(self, calculator, rule, scope, block):
        """
        Implements @if and @else if
        """
        # "@if" indicates whether any kind of `if` since the last `@else` has
        # succeeded, in which case `@else if` should be skipped
        if block.directive != '@if':
            if '@if' not in rule.options:
                raise SyntaxError("@else with no @if (%s)" % (rule.file_and_line,))
            if rule.options['@if']:
                # Last @if succeeded; stop here
                return

        condition = calculator.calculate(block.argument)
        if condition:
            inner_rule = rule.copy()
            inner_rule.unparsed_contents = block.unparsed_contents
            if not self.should_scope_loop_in_rule(inner_rule):
                # DEVIATION: Allow not creating a new namespace
                inner_rule.namespace = rule.namespace
            self.manage_children(inner_rule, scope)
        rule.options['@if'] = condition
    _at_else_if = _at_if

    # @print_timing(10)
    def _at_else(self, calculator, rule, scope, block):
        """
        Implements @else
        """
        if '@if' not in rule.options:
            log.error("@else with no @if (%s)", rule.file_and_line)
        val = rule.options.pop('@if', True)
        if not val:
            inner_rule = rule.copy()
            inner_rule.unparsed_contents = block.unparsed_contents
            inner_rule.namespace = rule.namespace  # DEVIATION: Commenting this line gives the Sass bahavior
            inner_rule.unparsed_contents = block.unparsed_contents
            self.manage_children(inner_rule, scope)

    # @print_timing(10)
    def _at_for(self, calculator, rule, scope, block):
        """
        Implements @for
        """
        var, _, name = block.argument.partition(' from ')
        frm, _, through = name.partition(' through ')
        if through:
            inclusive = True
        else:
            inclusive = False
            frm, _, through = frm.partition(' to ')
        frm = calculator.calculate(frm)
        through = calculator.calculate(through)
        try:
            frm = int(float(frm))
            through = int(float(through))
        except ValueError:
            return

        if frm > through:
            # DEVIATION: allow reversed '@for .. from .. through' (same as enumerate() and range())
            frm, through = through, frm
            rev = reversed
        else:
            rev = lambda x: x
        var = var.strip()
        var = calculator.do_glob_math(var)
        var = normalize_var(var)

        inner_rule = rule.copy()
        inner_rule.unparsed_contents = block.unparsed_contents
        if not self.should_scope_loop_in_rule(inner_rule):
            # DEVIATION: Allow not creating a new namespace
            inner_rule.namespace = rule.namespace

        if inclusive:
            through += 1
        for i in rev(range(frm, through)):
            inner_rule.namespace.set_variable(var, Number(i))
            self.manage_children(inner_rule, scope)

    # @print_timing(10)
    def _at_each(self, calculator, rule, scope, block):
        """
        Implements @each
        """
        varstring, _, valuestring = block.argument.partition(' in ')
        values = calculator.calculate(valuestring)
        if not values:
            return

        varlist = [
            normalize_var(calculator.do_glob_math(var.strip()))
            # TODO use list parsing here
            for var in varstring.split(",")
        ]

        # `@each $foo, in $bar` unpacks, but `@each $foo in $bar` does not!
        unpack = len(varlist) > 1
        if not varlist[-1]:
            varlist.pop()

        inner_rule = rule.copy()
        inner_rule.unparsed_contents = block.unparsed_contents
        if not self.should_scope_loop_in_rule(inner_rule):
            # DEVIATION: Allow not creating a new namespace
            inner_rule.namespace = rule.namespace

        for v in List.from_maybe(values):
            if unpack:
                v = List.from_maybe(v)
                for i, var in enumerate(varlist):
                    if i >= len(v):
                        value = Null()
                    else:
                        value = v[i]
                    inner_rule.namespace.set_variable(var, value)
            else:
                inner_rule.namespace.set_variable(varlist[0], v)
            self.manage_children(inner_rule, scope)

    # @print_timing(10)
    def _at_while(self, calculator, rule, scope, block):
        """
        Implements @while
        """
        first_condition = condition = calculator.calculate(block.argument)
        while condition:
            inner_rule = rule.copy()
            inner_rule.unparsed_contents = block.unparsed_contents
            if not self.should_scope_loop_in_rule(inner_rule):
                # DEVIATION: Allow not creating a new namespace
                inner_rule.namespace = rule.namespace
            self.manage_children(inner_rule, scope)
            condition = calculator.calculate(block.argument)
        rule.options['@if'] = first_condition

    # @print_timing(10)
    def _at_variables(self, calculator, rule, scope, block):
        """
        Implements @variables and @vars
        """
        warn_deprecated(
            rule,
            "@variables and @vars are deprecated.  "
            "Just assign variables at top-level.")
        _rule = rule.copy()
        _rule.unparsed_contents = block.unparsed_contents
        _rule.namespace = rule.namespace
        _rule.properties = []
        self.manage_children(_rule, scope)
    _at_vars = _at_variables

    # @print_timing(10)
    def _get_properties(self, rule, scope, block):
        """
        Implements properties and variables extraction and assignment
        """
        prop, raw_value = (_prop_split_re.split(block.prop, 1) + [None])[:2]
        if raw_value is not None:
            raw_value = raw_value.strip()

        try:
            is_var = (block.prop[len(prop)] == '=')
        except IndexError:
            is_var = False
        if is_var:
            warn_deprecated(rule, "Assignment with = is deprecated; use : instead.")
        calculator = self._make_calculator(rule.namespace)
        prop = prop.strip()
        prop = calculator.do_glob_math(prop)
        if not prop:
            return

        _prop = (scope or '') + prop
        if is_var or prop.startswith('$') and raw_value is not None:
            # Pop off any flags: !default, !global
            is_default = False
            is_global = True  # eventually sass will default this to false
            while True:
                splits = raw_value.rsplit(None, 1)
                if len(splits) < 2 or not splits[1].startswith('!'):
                    break

                raw_value, flag = splits
                if flag == '!default':
                    is_default = True
                elif flag == '!global':
                    is_global = True
                else:
                    raise ValueError("Unrecognized flag: {0}".format(flag))

            # Variable assignment
            _prop = normalize_var(_prop)
            try:
                existing_value = rule.namespace.variable(_prop)
            except KeyError:
                existing_value = None

            is_defined = existing_value is not None and not existing_value.is_null
            if is_default and is_defined:
                pass
            else:
                if is_defined and prop.startswith('$') and prop[1].isupper():
                    log.warn("Constant %r redefined", prop)

                # Variable assignment is an expression, so it always performs
                # real division
                value = calculator.calculate(raw_value, divide=True)
                rule.namespace.set_variable(
                    _prop, value, local_only=not is_global)
        else:
            # Regular property destined for output
            _prop = calculator.apply_vars(_prop)
            if raw_value is None:
                value = None
            else:
                value = calculator.calculate(raw_value)

            if value is None:
                pass
            elif isinstance(value, six.string_types):
                # TODO kill this branch
                pass
            else:
                if value.is_null:
                    return
                style = rule.legacy_compiler_options.get(
                    'style', self.compiler.output_style)
                compress = style == 'compressed'
                value = value.render(compress=compress)

            rule.properties.append((_prop, value))

    # @print_timing(10)
    def _nest_at_rules(self, rule, scope, block):
        """
        Implements @-blocks
        """
        # TODO handle @charset, probably?
        # Interpolate the current block
        # TODO this seems like it should be done in the block header.  and more
        # generally?
        calculator = self._make_calculator(rule.namespace)
        if block.header.argument:
            # TODO is this correct?  do ALL at-rules ALWAYS allow both vars and
            # interpolation?
            node = calculator.parse_vars_and_interpolations(
                block.header.argument)
            block.header.argument = node.evaluate(calculator).render()

        # TODO merge into RuleAncestry
        new_ancestry = list(rule.ancestry.headers)
        if block.directive == '@media' and new_ancestry:
            for i, header in reversed(list(enumerate(new_ancestry))):
                if header.is_selector:
                    continue
                elif header.directive == '@media':
                    new_ancestry[i] = BlockAtRuleHeader(
                        '@media',
                        "%s and %s" % (header.argument, block.argument))
                    break
                else:
                    new_ancestry.insert(i, block.header)
            else:
                new_ancestry.insert(0, block.header)
        else:
            new_ancestry.append(block.header)

        rule.descendants += 1
        new_rule = SassRule(
            source_file=rule.source_file,
            import_key=rule.import_key,
            lineno=block.lineno,
            num_header_lines=block.header.num_lines,
            unparsed_contents=block.unparsed_contents,

            legacy_compiler_options=rule.legacy_compiler_options,
            options=rule.options.copy(),
            #properties
            #extends_selectors
            ancestry=RuleAncestry(new_ancestry),

            namespace=rule.namespace.derive(),
            nested=rule.nested + 1,
        )
        self.rules.append(new_rule)
        rule.namespace.use_import(rule.source_file)
        self.manage_children(new_rule, scope)

        self._warn_unused_imports(new_rule)

    # @print_timing(10)
    def _nest_rules(self, rule, scope, block):
        """
        Implements Nested CSS rules
        """
        calculator = self._make_calculator(rule.namespace)
        raw_selectors = calculator.do_glob_math(block.prop)
        # DEVIATION: ruby sass doesn't support bare variables in selectors
        raw_selectors = calculator.apply_vars(raw_selectors)
        c_selectors, c_parents = self.parse_selectors(raw_selectors)
        if c_parents:
            warn_deprecated(
                rule,
                "The XCSS 'a extends b' syntax is deprecated.  "
                "Use 'a { @extend b; }' instead."
            )

        new_ancestry = rule.ancestry.with_nested_selectors(c_selectors)

        rule.descendants += 1
        new_rule = SassRule(
            source_file=rule.source_file,
            import_key=rule.import_key,
            lineno=block.lineno,
            num_header_lines=block.header.num_lines,
            unparsed_contents=block.unparsed_contents,

            legacy_compiler_options=rule.legacy_compiler_options,
            options=rule.options.copy(),
            #properties
            extends_selectors=c_parents,
            ancestry=new_ancestry,

            namespace=rule.namespace.derive(),
            nested=rule.nested + 1,
        )
        self.rules.append(new_rule)
        rule.namespace.use_import(rule.source_file)
        self.manage_children(new_rule, scope)

        self._warn_unused_imports(new_rule)

    # @print_timing(3)
    def apply_extends(self, rules):
        """Run through the given rules and translate all the pending @extends
        declarations into real selectors on parent rules.

        The list is modified in-place and also sorted in dependency order.
        """
        # Game plan: for each rule that has an @extend, add its selectors to
        # every rule that matches that @extend.
        # First, rig a way to find arbitrary selectors quickly.  Most selectors
        # revolve around elements, classes, and IDs, so parse those out and use
        # them as a rough key.  Ignore order and duplication for now.
        key_to_selectors = defaultdict(set)
        selector_to_rules = defaultdict(set)
        rule_selector_order = {}
        order = 0
        for rule in rules:
            for selector in rule.selectors:
                for key in selector.lookup_key():
                    key_to_selectors[key].add(selector)
                selector_to_rules[selector].add(rule)
                rule_selector_order[rule, selector] = order
                order += 1

        # Now go through all the rules with an @extends and find their parent
        # rules.
        for rule in rules:
            for selector in rule.extends_selectors:
                # This is a little dirty.  intersection isn't a class method.
                # Don't think about it too much.
                candidates = set.intersection(*(
                    key_to_selectors[key] for key in selector.lookup_key()))
                extendable_selectors = [
                    candidate for candidate in candidates
                    if candidate.is_superset_of(selector)]

                if not extendable_selectors:
                    # TODO implement !optional
                    warn_deprecated(
                        rule,
                        "Can't find any matching rules to extend {0!r} -- this "
                        "will be fatal in 2.0, unless !optional is specified!"
                        .format(selector.render()))
                    continue

                # Armed with a set of selectors that this rule can extend, do
                # some substitution and modify the appropriate parent rules.
                # One tricky bit: it's possible we're extending two selectors
                # that both exist in the same parent rule, in which case we
                # want to extend in the order the original selectors appear in
                # that rule.
                known_parents = []
                for extendable_selector in extendable_selectors:
                    parent_rules = selector_to_rules[extendable_selector]
                    for parent_rule in parent_rules:
                        if parent_rule is rule:
                            # Don't extend oneself
                            continue
                        known_parents.append(
                            (parent_rule, extendable_selector))
                # This will put our parents back in their original order
                known_parents.sort(key=rule_selector_order.__getitem__)

                for parent_rule, extendable_selector in known_parents:
                    more_parent_selectors = []

                    for rule_selector in rule.selectors:
                        more_parent_selectors.extend(
                            extendable_selector.substitute(
                                selector, rule_selector))

                    for parent in more_parent_selectors:
                        # Update indices, in case later rules try to extend
                        # this one
                        for key in parent.lookup_key():
                            key_to_selectors[key].add(parent)
                        selector_to_rules[parent].add(parent_rule)
                        rule_selector_order[parent_rule, parent] = order
                        order += 1

                    parent_rule.ancestry = (
                        parent_rule.ancestry.with_more_selectors(
                            more_parent_selectors))

        # Remove placeholder-only rules
        return [rule for rule in rules if not rule.is_pure_placeholder]

    # @print_timing(3)
    def create_css(self, rules):
        """
        Generate the final CSS string
        """
        style = rules[0].legacy_compiler_options.get(
            'style', self.compiler.output_style)
        debug_info = self.compiler.generate_source_map

        if style == 'legacy':
            sc, sp, tb, nst, srnl, nl, rnl, lnl, dbg = True, ' ', '  ', False, '', '\n', '\n', '\n', debug_info
        elif style == 'compressed':
            sc, sp, tb, nst, srnl, nl, rnl, lnl, dbg = False, '', '', False, '', '', '', '', False
        elif style == 'compact':
            sc, sp, tb, nst, srnl, nl, rnl, lnl, dbg = True, ' ', '', False, '\n', ' ', '\n', ' ', debug_info
        elif style == 'expanded':
            sc, sp, tb, nst, srnl, nl, rnl, lnl, dbg = True, ' ', '  ', False, '\n', '\n', '\n', '\n', debug_info
        else:  # if style == 'nested':
            sc, sp, tb, nst, srnl, nl, rnl, lnl, dbg = True, ' ', '  ', True, '\n', '\n', '\n', ' ', debug_info

        return self._create_css(rules, sc, sp, tb, nst, srnl, nl, rnl, lnl, dbg)

    def _textwrap(self, txt, width=70):
        if not hasattr(self, '_textwrap_wordsep_re'):
            self._textwrap_wordsep_re = re.compile(r'(?<=,)\s+')
            self._textwrap_strings_re = re.compile(r'''(["'])(?:(?!\1)[^\\]|\\.)*\1''')

        # First, remove commas from anything within strings (marking commas as \0):
        def _repl(m):
            ori = m.group(0)
            fin = ori.replace(',', '\0')
            if ori != fin:
                subs[fin] = ori
            return fin
        subs = {}
        txt = self._textwrap_strings_re.sub(_repl, txt)

        # Mark split points for word separators using (marking spaces with \1):
        txt = self._textwrap_wordsep_re.sub('\1', txt)

        # Replace all the strings back:
        for fin, ori in subs.items():
            txt = txt.replace(fin, ori)

        # Split in chunks:
        chunks = txt.split('\1')

        # Break in lines of at most long_width width appending chunks:
        ln = ''
        lines = []
        long_width = int(width * 1.2)
        for chunk in chunks:
            _ln = ln + ' ' if ln else ''
            _ln += chunk
            if len(ln) >= width or len(_ln) >= long_width:
                if ln:
                    lines.append(ln)
                _ln = chunk
            ln = _ln
        if ln:
            lines.append(ln)

        return lines

    def _create_css(self, rules, sc=True, sp=' ', tb='  ', nst=True, srnl='\n', nl='\n', rnl='\n', lnl='', debug_info=False):
        super_selector = self.compiler.super_selector
        if super_selector:
            super_selector += ' '

        skip_selectors = False

        prev_ancestry_headers = []

        total_selectors = 0

        result = ''
        dangling_property = False
        separate = False
        nesting = current_nesting = last_nesting = -1 if nst else 0
        nesting_stack = []
        for rule in rules:
            nested = rule.nested
            if nested <= 1:
                separate = True

            if nst:
                last_nesting = current_nesting
                current_nesting = nested

                delta_nesting = current_nesting - last_nesting
                if delta_nesting > 0:
                    nesting_stack += [nesting] * delta_nesting
                elif delta_nesting < 0:
                    nesting_stack = nesting_stack[:delta_nesting]
                    nesting = nesting_stack[-1]

            if rule.is_empty:
                continue

            if nst:
                nesting += 1

            ancestry = rule.ancestry
            ancestry_len = len(ancestry)

            first_mismatch = 0
            for i, (old_header, new_header) in enumerate(zip(prev_ancestry_headers, ancestry.headers)):
                if old_header != new_header:
                    first_mismatch = i
                    break

            # When sc is False, sets of properties are printed without a
            # trailing semicolon.  If the previous block isn't being closed,
            # that trailing semicolon needs adding in to separate the last
            # property from the next rule.
            if not sc and dangling_property and first_mismatch >= len(prev_ancestry_headers):
                result += ';'

            # Close blocks and outdent as necessary
            for i in range(len(prev_ancestry_headers), first_mismatch, -1):
                result += tb * (i - 1) + '}' + rnl

            # Open new blocks as necessary
            for i in range(first_mismatch, ancestry_len):
                header = ancestry.headers[i]

                if separate:
                    if result:
                        result += srnl
                    separate = False
                if debug_info:
                    def _print_debug_info(filename, lineno):
                        if debug_info == 'comments':
                            result = tb * (i + nesting) + "/* file: %s, line: %s */" % (filename, lineno) + nl
                        else:
                            filename = _escape_chars_re.sub(r'\\\1', filename)
                            result = tb * (i + nesting) + "@media -sass-debug-info{filename{font-family:file\:\/\/%s}line{font-family:\\00003%s}}" % (filename, lineno) + nl
                        return result

                    if rule.lineno and rule.source_file:
                        result += _print_debug_info(rule.source_file.path, rule.lineno)

                    if rule.from_lineno and rule.from_source_file:
                        result += _print_debug_info(rule.from_source_file.path, rule.from_lineno)

                if header.is_selector:
                    header_string = header.render(sep=',' + sp, super_selector=super_selector)
                    if nl:
                        header_string = (nl + tb * (i + nesting)).join(self._textwrap(header_string))
                else:
                    header_string = header.render()
                result += tb * (i + nesting) + header_string + sp + '{' + nl

                if header.is_selector:
                    total_selectors += 1

            prev_ancestry_headers = ancestry.headers
            dangling_property = False

            if not skip_selectors:
                result += self._print_properties(rule.properties, sc, sp, tb * (ancestry_len + nesting), nl, lnl)
                dangling_property = True

        # Close all remaining blocks
        for i in reversed(range(len(prev_ancestry_headers))):
            result += tb * i + '}' + rnl

        # Always end with a newline, even in compressed mode
        if not result.endswith('\n'):
            result += '\n'

        return (result, total_selectors)

    def _print_properties(self, properties, sc=True, sp=' ', tb='', nl='\n', lnl=' '):
        result = ''
        last_prop_index = len(properties) - 1
        for i, (name, value) in enumerate(properties):
            if value is None:
                prop = name
            elif value:
                if nl:
                    value = (nl + tb + tb).join(self._textwrap(value))
                prop = name + ':' + sp + value
            else:
                # Empty string means there's supposed to be a value but it
                # evaluated to nothing; skip this
                # TODO interacts poorly with last_prop_index
                continue

            if i == last_prop_index:
                if sc:
                    result += tb + prop + ';' + lnl
                else:
                    result += tb + prop + lnl
            else:
                result += tb + prop + ';' + nl
        return result


class SassReturn(SassBaseError):
    """Special control-flow exception used to hop up the stack from a Sass
    function's ``@return``.
    """
    def __init__(self, retval):
        super(SassReturn, self).__init__()
        self.retval = retval

    def __str__(self):
        return "Returning {0!r}".format(self.retval)
