import fluent.syntax.ast as FTL

from .errors import SkipTransform
from .util import get_message, get_transform


def merge_resource(ctx, reference, current, transforms, in_changeset):
    """Transform legacy translations into FTL.

    Use the `reference` FTL AST as a template.  For each en-US string in the
    reference, first check for an existing translation in the current FTL
    `localization` and use it if it's present; then if the string has
    a transform defined in the migration specification and if it's in the
    currently processed changeset, evaluate the transform.
    """

    def merge_body(body):
        return [
            entry
            for entry in map(merge_entry, body)
            if entry is not None
        ]

    def merge_entry(entry):
        # All standalone comments will be merged.
        if isinstance(entry, FTL.BaseComment):
            return entry

        # Ignore Junk
        if isinstance(entry, FTL.Junk):
            return None

        ident = entry.id.name

        # If the message is present in the existing localization, we add it to
        # the resulting resource.  This ensures consecutive merges don't remove
        # translations but rather create supersets of them.
        existing = get_message(current.body, ident)
        if existing is not None:
            return existing

        transform = get_transform(transforms, ident)

        # Make sure this message is supposed to be migrated as part of the
        # current changeset.
        if transform is not None and in_changeset(ident):
            if transform.comment is None:
                transform.comment = entry.comment
            try:
                return ctx.evaluate(transform)
            except SkipTransform:
                return None

    body = merge_body(reference.body)
    return FTL.Resource(body)
