"""A source fetched once into the cache, and a table written as the loader admits it."""
import re
import sys
import time
import urllib.request
from pathlib import Path


def fetch(source, cache, name, magic=b"", data=None, headers=None):
    """The bytes of a URL, downloaded into cache/name once, or of a local file."""
    if not re.match(r"^https?://", source):
        return Path(source).read_bytes()
    path = Path(cache) / name
    for attempt in range(1, 6):
        if path.exists():
            return path.read_bytes()
        req = urllib.request.Request(source, data=data, headers={"User-Agent": "fejkdata data-import", **(headers or {})})
        try:
            with urllib.request.urlopen(req, timeout=600) as r:
                body = r.read()
        except OSError:
            body = b""
        if body.startswith(magic) and b"Request Rejected" not in body[:512]:
            path.parent.mkdir(parents=True, exist_ok=True)
            path.write_bytes(body)
        else:
            time.sleep(10 * attempt)
    sys.exit(f"{source}: no valid download in 5 attempts")


def write(path, columns, rows):
    """Write the rows as a TSV; every cell must be non-empty and free of tabs, newlines and braces."""
    lines = ["\t".join(columns)]
    for row in rows:
        cells = [str(row[c]) for c in columns]
        assert all(cells) and not any(re.search(r"[\t\n{}]", c) for c in cells), row
        lines.append("\t".join(cells))
    Path(path).write_text("\n".join(lines) + "\n", encoding="utf-8")
    print(f"{path}: {len(lines) - 1} rows", file=sys.stderr)