#!/usr/bin/env python3
"""Send the newest changed CHANGELOG.md entry for a Drone main-branch push."""
import argparse
import json
import os
from pathlib import Path
import re
import subprocess
import sys
import urllib.error
import urllib.parse
import urllib.request

ROOT = Path(__file__).resolve().parents[1]
CHANGELOG = 'CHANGELOG.md'


class ChangelogError(Exception):
    pass


class NoRedirect(urllib.request.HTTPRedirectHandler):
    def redirect_request(self, req, fp, code, msg, headers, newurl):
        return None


def sections(text):
    """Entries start at level-two headings; level-three headings stay inside."""
    headings = list(re.finditer(r'^## ([^\r\n]+)\r?$', text, re.MULTILINE))
    result = []
    for index, heading in enumerate(headings):
        end = headings[index + 1].start() if index + 1 < len(headings) else len(text)
        title = heading[1].strip()
        content = text[heading.end():end].strip().replace('\r\n', '\n')
        if title in [item[0] for item in result]:
            raise ChangelogError('Changelog entry headings must be unique.')
        result.append((title, content))
    return result


def latest_changes(before, after):
    old, new = sections(before), sections(after)
    if not new or not new[0][1]:
        raise ChangelogError('CHANGELOG.md needs a nonempty newest ## entry.')
    latest = new[0]
    if old and (latest == old[0] or latest[0] in [title for title, _ in old[1:]]):
        return None  # Unchanged newest entry, or removal/reordering of old entries.
    version_note = ''
    preamble = after.split('\n## ', 1)[0]
    versions = re.findall(r'^Latest published Workshop version: \*\*(\d+\.\d+\.\d+)\*\*\r?$',
                          preamble, re.MULTILINE)
    if 'Latest published Workshop version:' in preamble:
        if len(versions) != 1 or any(int(part) > 32000 for part in versions[0].split('.')):
            raise ChangelogError('Invalid Workshop version record in CHANGELOG.md.')
        version_note = 'Last published Workshop version: **' + versions[0] + '**\n\n'
    message = '**' + latest[0] + '**\n\n' + version_note + latest[1]
    if len(message.encode('utf-16-le')) // 2 > 2000:
        raise ChangelogError('Newest changelog entry exceeds Discord’s 2000-character limit.')
    return message


def git(repo, *args):
    result = subprocess.run(['git', *args], cwd=repo, text=True,
                            stdout=subprocess.PIPE, stderr=subprocess.PIPE)
    if result.returncode:
        raise ChangelogError('Required Git history is unavailable; no changelog sent.')
    return result.stdout


def commit(value):
    if not re.fullmatch(r'[0-9a-fA-F]{40}|[0-9a-fA-F]{64}', value or ''):
        raise ChangelogError('Drone must provide complete before/after commit IDs.')
    return value.lower()


def at_revision(repo, sha):
    git(repo, 'cat-file', '-e', sha + '^{commit}')
    if not git(repo, 'ls-tree', '--name-only', sha, '--', CHANGELOG).strip():
        return ''
    return git(repo, 'show', sha + ':' + CHANGELOG)


def prepare(repo, env):
    if env.get('DRONE_BUILD_EVENT') != 'push' or env.get('DRONE_COMMIT_BRANCH') != 'main':
        return None
    before = commit(env.get('DRONE_COMMIT_BEFORE'))
    after = commit(env.get('DRONE_COMMIT_SHA'))
    if git(repo, 'rev-parse', 'HEAD').strip() != after:
        raise ChangelogError('Checkout does not match the pushed commit; no changelog sent.')
    previous = ''
    if set(before) != {'0'}:
        try:
            git(repo, 'cat-file', '-e', before + '^{commit}')
        except ChangelogError:
            # A force-push may leave the before commit outside the cloned history.
            git(repo, 'fetch', '--no-tags', 'origin', before)
        previous = at_revision(repo, before)
    return latest_changes(previous, at_revision(repo, after))


def send(message, webhook):
    parsed = urllib.parse.urlsplit(webhook)
    if (parsed.scheme != 'https' or parsed.netloc != 'discord.com'
            or not re.fullmatch(r'/api/webhooks/[0-9]+/[A-Za-z0-9_-]+', parsed.path)
            or parsed.query or parsed.fragment):
        raise ChangelogError('Set the Drone secret discord_webhook_url to a Discord webhook URL.')
    payload = json.dumps({'content': message, 'allowed_mentions': {'parse': []}}).encode()
    request = urllib.request.Request(webhook + '?wait=true', data=payload, headers={
        'Content-Type': 'application/json', 'User-Agent': 'EscapistsReintroduced-Changelog/1.0'})
    # Never retry automatically: a lost response may still mean Discord accepted it.
    try:
        with urllib.request.build_opener(NoRedirect()).open(request, timeout=30) as response:
            result = json.load(response)
        if not isinstance(result, dict) or not result.get('id'):
            raise ValueError('Missing confirmation')
    except urllib.error.HTTPError as error:
        raise ChangelogError('Discord returned HTTP ' + str(error.code)
                             + '; inspect delivery before rerunning.') from None
    except (urllib.error.URLError, OSError, ValueError):
        raise ChangelogError('Discord delivery could not be confirmed; inspect Discord before rerunning.') from None


def main(argv=None):
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument('--dry-run', action='store_true', help='Print the selected entry without sending it')
    args = parser.parse_args(argv)
    try:
        message = prepare(ROOT, os.environ)
        if message is None:
            print('No new changelog entry to send.')
        elif args.dry_run:
            print(message)
        else:
            send(message, os.environ.get('DISCORD_WEBHOOK_URL', '').strip())
            print('Discord changelog delivered.')
    except (ChangelogError, ValueError) as error:
        # URL parsing errors may contain secrets; only our own messages are safe.
        print(str(error) if isinstance(error, ChangelogError) else 'Invalid webhook configuration.', file=sys.stderr)
        return 1
    return 0


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