import re


_UNLOAD_SQL_TEMPLATE = """
    UNLOAD ('{query}')
    TO '{s3_prefix}'
    CREDENTIALS '{creds}'
    {options}"""

_AWS_CREDS_TEMPLATE = 'aws_access_key_id={aws_access_key};' \
    'aws_secret_access_key={aws_access_secret}'

_DESCRIBE_TABLE_SQL_TEMPLATE = """
    SELECT column_name, is_nullable, data_type, column_default,
           character_maximum_length, character_octet_length,
           numeric_precision, numeric_precision_radix, numeric_scale,
           datetime_precision
    FROM information_schema.columns
    WHERE table_schema = '{schema}' and table_name = '{table}'
    ORDER BY ordinal_position"""


def _escape_single_quotes(query):
    """Escape unescaped single quotes with backslashes.

    If single quotes already escaped, do nothing.

    Escaping of single quotes is required because of syntax of UNLOAD command:
    UNLOAD ('SELECT \'\' AS some_column, ...')

    Args:
        query (str): SQL SELECT query for using in UNLOAD operation.

    Returns:
        str: SQL SELECT query containing only escaped single quotes.
    """
    return re.sub(r"(?<!\\)'", r"\'", query)


def _reduce_whitespace(in_str):
    """Reduces all whitespace chunks to single spaces.

    Args:
        in_str (str): input string

    Returns:
        str: string containing single spaces
    """

    return ' '.join(in_str.split())


def unload_sql(**kwargs):
    """Forms Redshift UNLOAD directive SQL

    (see http://docs.aws.amazon.com/redshift/latest/dg/r_UNLOAD.html)

    Kwargs:
        query (str): a SELECT query; its results are unloaded
        s3_prefix (str): the full path, including bucket name,
            to the S3 location to write the output file objects
        aws_access_key (str): aws access key
        aws_access_secret(str): aws access secret key
        options (str): options portion of the sql literal

    Return:
        str: UNLOAD SQL
    """

    kwargs.setdefault('creds', _AWS_CREDS_TEMPLATE.format(**kwargs))

    kwargs['query'] = _escape_single_quotes(kwargs['query'])

    sql = _UNLOAD_SQL_TEMPLATE.format(**kwargs)

    return _reduce_whitespace(sql)


def describe_table_sql(schema, table):
    """Forms the 'describe' query for columns of a Redshift table.

    Selects information relevant to ordinality, datatype and nullability of all
    columns in a table.  Can be used for schema validation and translation.

    Args:
        schema (str): schema name
        table (str): table name

    Returns:
        str: DESCRIBE sql
    """

    sql = _DESCRIBE_TABLE_SQL_TEMPLATE.format(schema=schema, table=table)

    return _reduce_whitespace(sql)