import os


def scm_find_files(path, scm_files, scm_dirs):
    """ setuptools compatible file finder that follows symlinks

    - path: the root directory from which to search
    - scm_files: set of scm controlled files
    - scm_dirs: set of scm controlled directories

    scm_files and scm_dirs must be absolute with symlinks resolved (realpath),
    with normalized case (normcase)

    Spec here: http://setuptools.readthedocs.io/en/latest/setuptools.html#\
        adding-support-for-revision-control-systems
    """
    realpath = os.path.normcase(os.path.realpath(path))
    seen = set()
    res = []
    for dirpath, dirnames, filenames in os.walk(realpath, followlinks=True):
        # dirpath with symlinks resolved
        realdirpath = os.path.normcase(os.path.realpath(dirpath))

        def _link_not_in_scm(n):
            fn = os.path.join(realdirpath, os.path.normcase(n))
            return os.path.islink(fn) and fn not in scm_files

        if realdirpath not in scm_dirs:
            # directory not in scm, don't walk it's content
            dirnames[:] = []
            continue
        if os.path.islink(dirpath) and \
                not os.path.relpath(realdirpath, realpath).startswith(os.pardir):
            # a symlink to a directory not outside path:
            # we keep it in the result and don't walk its content
            res.append(
                os.path.join(path, os.path.relpath(dirpath, path)))
            dirnames[:] = []
            continue
        if realdirpath in seen:
            # symlink loop protection
            dirnames[:] = []
            continue
        dirnames[:] = [dn for dn in dirnames if not _link_not_in_scm(dn)]
        for filename in filenames:
            if _link_not_in_scm(filename):
                continue
            # dirpath + filename with symlinks preserved
            fullfilename = os.path.join(dirpath, filename)
            if os.path.normcase(os.path.realpath(fullfilename)) in scm_files:
                res.append(
                    os.path.join(path, os.path.relpath(fullfilename, path)))
        seen.add(realdirpath)
    return res

import os


def scm_find_files(path, scm_files, scm_dirs):
    """ setuptools compatible file finder that follows symlinks

    - path: the root directory from which to search
    - scm_files: set of scm controlled files and symlinks
      (including symlinks to directories)
    - scm_dirs: set of scm controlled directories

    scm_files and scm_dirs must be absolute with symlinks resolved (realpath),
    with normalized case (normcase)

    Spec here: http://setuptools.readthedocs.io/en/latest/setuptools.html#\
        adding-support-for-revision-control-systems
    """
    realpath = os.path.normcase(os.path.realpath(path))
    seen = set()
    res = []
    for dirpath, dirnames, filenames in os.walk(realpath, followlinks=True):
        # dirpath with symlinks resolved
        realdirpath = os.path.normcase(os.path.realpath(dirpath))

        def _link_not_in_scm(n):
            fn = os.path.join(realdirpath, os.path.normcase(n))
            return os.path.islink(fn) and fn not in scm_files

        if realdirpath not in scm_dirs:
            # directory not in scm, don't walk it's content
            dirnames[:] = []
            continue
        if os.path.islink(dirpath) and \
                not os.path.relpath(realdirpath, realpath).startswith(os.pardir):
            # a symlink to a directory not outside path:
            # we keep it in the result and don't walk its content
            res.append(
                os.path.join(path, os.path.relpath(dirpath, path)))
            dirnames[:] = []
            continue
        if realdirpath in seen:
            # symlink loop protection
            dirnames[:] = []
            continue
        dirnames[:] = [dn for dn in dirnames if not _link_not_in_scm(dn)]
        for filename in filenames:
            if _link_not_in_scm(filename):
                continue
            # dirpath + filename with symlinks preserved
            fullfilename = os.path.join(dirpath, filename)
            if os.path.normcase(os.path.realpath(fullfilename)) in scm_files:
                res.append(
                    os.path.join(path, os.path.relpath(fullfilename, path)))
        seen.add(realdirpath)
    return res

Improve docstring