#!/usr/bin/env python3
"""Flatten the REST Countries v3.1 dataset into a clean, flat table.

Usage:
    python3 extract_countries.py <source_json> <output_csv>

Source (raw, from the REST Countries repository):
    https://raw.githubusercontent.com/restcountries/restcountries/master/src/main/resources/countriesV3.1.json
The JSON is an array of 250 country records; this script flattens every record
to one CSV row with the fixed 14-column contract below.

Columns (order):
    name_common | name_official | alpha2 | alpha3 | numeric |
    region | subregion | capital | population | area_km2 |
    currencies | languages | tlds | flag
"""
import csv
import json
import sys


def as_text(value):
    """Empty string for None/empty, plain str otherwise."""
    if value is None or value == '':
        return ''
    return str(value)


def list_join(value):
    """Join a list of strings with ' | '; empty string for None."""
    if value is None:
        return ''
    return ' | '.join(str(item) for item in value)


def currency_codes(currencies):
    if not isinstance(currencies, dict):
        return ''
    return ' | '.join(sorted(currencies.keys()))


def language_codes(languages):
    if not isinstance(languages, dict):
        return ''
    return ' | '.join(sorted(languages.keys()))


def flatten(record):
    name = record.get('name') or {}
    return [
        as_text(name.get('common')),                 # name_common
        as_text(name.get('official')),               # name_official
        as_text(record.get('cca2')),                 # alpha2
        as_text(record.get('cca3')),                 # alpha3
        as_text(record.get('ccn3')),                 # numeric
        as_text(record.get('region')),               # region
        as_text(record.get('subregion')),            # subregion
        list_join(record.get('capital')),            # capital
        as_text(record.get('population')),           # population
        as_text(record.get('area')),                 # area_km2
        currency_codes(record.get('currencies')),    # currencies
        language_codes(record.get('languages')),     # languages
        list_join(record.get('tld')),                # tlds
        as_text(record.get('flag')),                 # flag
    ]


HEADERS = [
    'name_common', 'name_official', 'alpha2', 'alpha3', 'numeric',
    'region', 'subregion', 'capital', 'population', 'area_km2',
    'currencies', 'languages', 'tlds', 'flag',
]


def main():
    if len(sys.argv) != 3:
        sys.stderr.write('usage: extract_countries.py <source_json> <output_csv>\n')
        return 2
    source_path, out_path = sys.argv[1], sys.argv[2]
    try:
        with open(source_path, encoding='utf-8') as fh:
            records = json.load(fh)
    except (OSError, ValueError) as exc:
        sys.stderr.write('cannot read source %s: %s\n' % (source_path, exc))
        return 1
    if not isinstance(records, list):
        sys.stderr.write('source JSON is not a list\n')
        return 1
    with open(out_path, 'w', encoding='utf-8', newline='') as fh:
        writer = csv.writer(fh, lineterminator='\n')
        writer.writerow(HEADERS)
        for record in records:
            writer.writerow(flatten(record))
    print('records_in: %d' % len(records))
    print('wrote: %s' % out_path)
    return 0


if __name__ == '__main__':
    sys.exit(main())