"""Utility functions for getting Snowflake tables meta."""

from collections import namedtuple

from garcon_contrib.snowflake import garcon_snowflake

from snowflake_etl.conf import config

SFTable = namedtuple('SFTable', ['db', 'schema', 'name'])


def format_meta(sf_table, columns):
    """Format columns meta.

    Args:
        sf_table (SFTable): target table to format schema for.
        columns (list(tuple)): columns meta as fetched by DESC.

    Returns:
        dict: json-serializable dict
    """
    def to_bool(x): return x and x == 'Y'

    def _id(x): return x

    meta_field_names = (
        'column_name', 'data_type', 'kind', 'is_nullable',
        'column_default', 'primary_key?', 'unique_key?',
        'check', 'expression', 'comment')

    assert (len(meta_field_names) ==
            len(columns[0])), 'Number of column\'s fields do not match'

    # name -> value mapper
    fields_to_dump = {
        'column_name': _id,
        'data_type': _id,
        'is_nullable': to_bool,
        'column_default': _id
    }
    fmt_columns = [
        dict(
            (name, fields_to_dump[name](value))
            for name, value in zip(meta_field_names, column)
            if name in fields_to_dump)
        for column in columns
    ]

    return {
        'database': sf_table.db,
        'schema': sf_table.schema,
        'table': sf_table.name,
        'columns': fmt_columns
    }


def fetch_meta(sf_table):
    """Fetch a table columns meta.

    Args:
        sf_table (SFTable): target table to fetch schema for.

    Returns:
        list(tuple): columns meta.
    """
    sql = config.SF_DEFAULT_QUERIES['describe_table'].format(
        db=sf_table.db, schema=sf_table.schema, table=sf_table.name)

    meta = garcon_snowflake.execute_with_py_conn(
        sql, garcon_snowflake.FetchEnum.ALL,
        sf_config=config.SF_CONFIG)['results']

    return meta


def get_table_schema(db, schema, table):
    """Get Snowflake table schema.

    Args:
        db (str): table's database.
        schema (str): table's schema.
        table (str): table itself.

    Returns:
        dict: table schema.
    """
    sf_table = SFTable(db, schema, table)
    meta = fetch_meta(sf_table)

    return format_meta(sf_table, meta)