#!/usr/bin/env python3
"""Generate a deterministic Keep-a-Changelog draft from a Git repository."""
from __future__ import annotations
import argparse
import re
import subprocess
from collections import defaultdict
from pathlib import Path

GROUPS = {
    'feat': 'Added', 'fix': 'Fixed', 'perf': 'Changed', 'refactor': 'Changed',
    'docs': 'Documentation', 'test': 'Testing', 'build': 'Build', 'ci': 'CI',
    'chore': 'Maintenance', 'revert': 'Reverted',
}
COMMIT = re.compile(r'^(?P<type>[a-zA-Z]+)(?:\((?P<scope>[^)]+)\))?(?P<breaking>!)?:\s*(?P<text>.+)$')

def parse(subject: str) -> tuple[str, str]:
    match = COMMIT.match(subject.strip())
    if not match:
        return 'Other', subject.strip()
    category = GROUPS.get(match.group('type').lower(), 'Other')
    scope = match.group('scope')
    text = match.group('text').strip()
    if scope:
        text = f'**{scope}:** {text}'
    if match.group('breaking'):
        text = f'**BREAKING:** {text}'
    return category, text

def render(subjects: list[str], title: str = 'Unreleased') -> str:
    grouped: dict[str, list[str]] = defaultdict(list)
    seen: set[tuple[str, str]] = set()
    for subject in subjects:
        category, text = parse(subject)
        key = (category, text.casefold())
        if text and key not in seen:
            seen.add(key)
            grouped[category].append(text)
    order = list(dict.fromkeys(GROUPS.values())) + ['Other']
    lines = [f'## [{title}]', '']
    for category in order:
        entries = grouped.get(category, [])
        if not entries:
            continue
        lines.extend([f'### {category}', ''])
        lines.extend(f'- {entry}' for entry in entries)
        lines.append('')
    if len(lines) == 2:
        lines.extend(['_No changes found._', ''])
    return '\n'.join(lines).rstrip() + '\n'

def git_subjects(repo: Path, since: str | None, until: str) -> list[str]:
    revision = f'{since}..{until}' if since else until
    proc = subprocess.run(
        ['git', '-C', str(repo), 'log', '--no-merges', '--format=%s', revision],
        check=True, text=True, capture_output=True,
    )
    return [line for line in proc.stdout.splitlines() if line.strip()]

def main() -> None:
    parser = argparse.ArgumentParser(description=__doc__)
    parser.add_argument('--repo', type=Path, default=Path('.'))
    parser.add_argument('--since', help='Start tag or commit, excluded')
    parser.add_argument('--until', default='HEAD', help='End revision, included')
    parser.add_argument('--title', default='Unreleased')
    parser.add_argument('--output', type=Path)
    args = parser.parse_args()
    output = render(git_subjects(args.repo, args.since, args.until), args.title)
    if args.output:
        args.output.write_text(output, encoding='utf-8')
    else:
        print(output, end='')

if __name__ == '__main__':
    main()