import datetime
import os
import subprocess


VERSION = (1, 5, 0, 'alpha', 0)


def get_version(version=None):
    """Derives a PEP386-compliant version number from VERSION."""
    if version is None:
        version = VERSION
    assert len(version) == 5
    assert version[3] in ('alpha', 'beta', 'rc', 'final')

    # Now build the two parts of the version number:
    # main = X.Y[.Z]
    # sub = .devN - for pre-alpha releases
    #     | {a|b|c}N - for alpha, beta and rc releases

    parts = 2 if version[2] == 0 else 3
    main = '.'.join(str(x) for x in version[:parts])

    sub = ''
    if version[3] == 'alpha' and version[4] == 0:
        git_changeset = get_git_changeset()
        if git_changeset:
            sub = '.dev%s' % git_changeset

    elif version[3] != 'final':
        mapping = {'alpha': 'a', 'beta': 'b', 'rc': 'c'}
        sub = mapping[version[3]] + str(version[4])

    return main + sub


def get_git_changeset():
    """Returns a numeric identifier of the latest git changeset.

    The result is the UTC timestamp of the changeset in YYYYMMDDHHMMSS format.
    This value isn't guaranteed to be unique but collisions are very unlikely,
    so it's sufficient for generating the development version numbers.
    """
    repo_dir = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
    git_show = subprocess.Popen('git show --pretty=format:%ct --quiet HEAD',
            stdout=subprocess.PIPE, stderr=subprocess.PIPE,
            shell=True, cwd=repo_dir, universal_newlines=True)
    timestamp = git_show.communicate()[0].partition('\n')[0]
    try:
        timestamp = datetime.datetime.utcfromtimestamp(int(timestamp))
    except ValueError:
        return None
    return timestamp.strftime('%Y%m%d%H%M%S')