def _find_name_version_sep(egg_info, canonical_name):
    # type: (str, str) -> int
    """Find the separator's index based on the package's canonical name.

    `egg_info` must be an egg info string for the given package, and
    `canonical_name` must be the package's canonical name.

    This function is needed since the canonicalized name does not necessarily
    have the same length as the egg info's name part. An example::

    >>> egg_info = 'foo__bar-1.0'
    >>> canonical_name = 'foo-bar'
    >>> _find_name_version_sep(egg_info, canonical_name)
    8
    """
    # Project name and version must be separated by one single dash. Find all
    # occurrences of dashes; if the string in front of it matches the canonical
    # name, this is the one separating the name and version parts.
    for i, c in enumerate(egg_info):
        if c != "-":
            continue
        if canonicalize_name(egg_info[:i]) == canonical_name:
            return i
    raise ValueError("{} does not match {}".format(egg_info, canonical_name))