Which epoch are we in?
    Return integer (>=0) on success

def get_epoch_number( block_height ):
    """
    Which epoch are we in?
    Return integer (>=0) on success
    """
    global EPOCHS

    if block_height <= EPOCHS[0]['end_block']:
        return 0

    for i in xrange(1, len(EPOCHS)):
        if EPOCHS[i-1]['end_block'] < block_height and (block_height <= EPOCHS[i]['end_block'] or EPOCHS[i]['end_block'] == EPOCH_NOW):
            return i

    # should never happen 
    log.error("FATAL: No epoch for %s" % block_height)
    os.abort()