halkyon/tool/maps/japan_prefectures.py

#!/usr/bin/env python3
"""Builds the app's map of Japan's prefectures from Natural Earth.

Natural Earth's vector data is in the public domain. This script downloads
its Admin 1 (states, provinces) layer at 1:10m, release v5.1.2, checks the
download against the hash recorded below, keeps Japan's 47 prefectures, and
reduces them to a compact outline for display (outlines.py): Douglas-Peucker
simplification (0.006 degrees), islands too small to see dropped (each
prefecture keeps its largest), coordinates rounded to 4 decimals. The result
is an approximation, not survey-accurate data.

Each region is named by its ISO 3166-2 code (JP-01 ... JP-47), with Natural
Earth's English and Japanese names.

Run from the repository root with Python 3 and no other packages:

    python3 tool/maps/japan_prefectures.py

It rewrites halkyon_learn_flutter/assets/maps/japan_prefectures.geojson.
"""

import json
import sys
from pathlib import Path

sys.path.insert(0, str(Path(__file__).parent))
from outlines import download, polygons_of, reduce  # noqa: E402

SOURCE = (
    "https://raw.githubusercontent.com/nvkelso/natural-earth-vector/"
    "v5.1.2/geojson/ne_10m_admin_1_states_provinces.geojson"
)
SOURCE_SHA256 = "22d0e3ad85eb3e27f17cabf8ba2d50e554fbc27a87796ff891d958185da62fb5"
TARGET = Path("halkyon_learn_flutter/assets/maps/japan_prefectures.geojson")

TOLERANCE = 0.006  # degrees
MIN_AREA = 0.0005  # square degrees; smaller islands are dropped
DECIMALS = 4


def main():
    source = json.loads(download(SOURCE, SOURCE_SHA256))
    features = []
    for feature in source["features"]:
        properties = feature["properties"]
        if properties.get("adm0_a3") != "JPN":
            continue
        features.append({
            "type": "Feature",
            "properties": {
                "code": properties["iso_3166_2"],
                "name": properties["name"],
                "name_ja": properties["name_ja"],
            },
            "geometry": reduce(polygons_of(feature["geometry"]), TOLERANCE,
                               MIN_AREA, DECIMALS),
        })
    features.sort(key=lambda feature: feature["properties"]["code"])
    codes = [feature["properties"]["code"] for feature in features]
    if codes != [f"JP-{n:02d}" for n in range(1, 48)]:
        sys.exit(f"Expected the 47 prefectures JP-01 ... JP-47, got {codes}")
    TARGET.write_text(json.dumps(
        {"type": "FeatureCollection", "features": features},
        ensure_ascii=False, separators=(",", ":")) + "\n", encoding="utf-8")
    print(f"Wrote {TARGET} ({TARGET.stat().st_size} bytes)", file=sys.stderr)


if __name__ == "__main__":
    main()