"""Fluent AST helpers.

The functions defined in this module offer a shorthand for defining common AST
nodes.

They take a string argument and immediately return a corresponding AST node.
(As opposed to Transforms which are AST nodes on their own and only return the
migrated AST nodes when they are evaluated by a MigrationContext.) """


from fluent.syntax import FluentParser, ast as FTL
from fluent.syntax.visitor import Transformer
from .transforms import Transform, CONCAT, COPY, COPY_PATTERN
from .errors import NotSupportedError, InvalidTransformError


def VARIABLE_REFERENCE(name):
    """Create an ExternalArgument expression."""

    return FTL.VariableReference(
        id=FTL.Identifier(name)
    )


def MESSAGE_REFERENCE(name):
    """Create a MessageReference expression.

    If the passed name contains a `.`, we're generating
    a message reference with an attribute.
    """
    if '.' in name:
        name, attribute = name.split('.')
        attribute = FTL.Identifier(attribute)
    else:
        attribute = None

    return FTL.MessageReference(
        id=FTL.Identifier(name),
        attribute=attribute,
    )


def TERM_REFERENCE(name):
    """Create a TermReference expression."""

    return FTL.TermReference(
        id=FTL.Identifier(name)
    )


class IntoTranforms(Transformer):
    IMPLICIT_TRANSFORMS = ("CONCAT",)
    FORBIDDEN_TRANSFORMS = ("PLURALS", "REPLACE", "REPLACE_IN_TEXT")

    def __init__(self, substitutions):
        self.substitutions = substitutions

    def visit_Junk(self, node):
        anno = node.annotations[0]
        raise InvalidTransformError(
            "Transform contains parse error: {}, at {}".format(
                anno.message, anno.span.start))

    def visit_FunctionReference(self, node):
        name = node.id.name
        if name in self.IMPLICIT_TRANSFORMS:
            raise NotSupportedError(
                "{} may not be used with transforms_from(). It runs "
                "implicitly on all Patterns anyways.".format(name))
        if name in self.FORBIDDEN_TRANSFORMS:
            raise NotSupportedError(
                "{} may not be used with transforms_from(). It requires "
                "additional logic in Python code.".format(name))
        if name in ('COPY', 'COPY_PATTERN'):
            args = (
                self.into_argument(arg) for arg in node.arguments.positional
            )
            kwargs = {
                arg.name.name: self.into_argument(arg.value)
                for arg in node.arguments.named}
            if name == 'COPY':
                return COPY(*args, **kwargs)
            return COPY_PATTERN(*args, **kwargs)
        return self.generic_visit(node)

    def visit_Placeable(self, node):
        """If the expression is a Transform, replace this Placeable
        with the Transform it's holding.
        Transforms evaluate to Patterns, which are flattened as
        elements of Patterns in Transform.pattern_of, but only
        one level deep.
        """
        node = self.generic_visit(node)
        if isinstance(node.expression, Transform):
            return node.expression
        return node

    def visit_Pattern(self, node):
        """Replace the Pattern with CONCAT which is more accepting of its
        elements. CONCAT takes PatternElements, Expressions and other
        Patterns (e.g. returned from evaluating transforms).
        """
        node = self.generic_visit(node)
        return CONCAT(*node.elements)

    def into_argument(self, node):
        """Convert AST node into an argument to migration transforms."""
        if isinstance(node, FTL.StringLiteral):
            # Special cases for booleans which don't exist in Fluent.
            if node.value == "True":
                return True
            if node.value == "False":
                return False
            return node.value
        if isinstance(node, FTL.MessageReference):
            try:
                return self.substitutions[node.id.name]
            except KeyError:
                raise InvalidTransformError(
                    "Unknown substitution in COPY: {}".format(
                        node.id.name))
        else:
            raise InvalidTransformError(
                "Invalid argument passed to COPY: {}".format(
                    type(node).__name__))


def transforms_from(ftl, **substitutions):
    """Parse FTL code into a list of Message nodes with Transforms.

    The FTL may use a fabricated COPY function inside of placeables which
    will be converted into actual COPY migration transform.

        new-key = Hardcoded text { COPY("filepath.dtd", "string.key") }

    For convenience, COPY may also refer to transforms_from's keyword
    arguments via the MessageReference syntax:

        transforms_from(\"""
        new-key = Hardcoded text { COPY(file_dtd, "string.key") }
        \""", file_dtd="very/long/path/to/a/file.dtd")

    """

    parser = FluentParser(with_spans=False)
    resource = parser.parse(ftl)
    return IntoTranforms(substitutions).visit(resource).body
