"""How far behind origin/main the running build is.

Hits Gitea's compare endpoint with a short per-SHA cache. Network errors
return None and the footer just hides the status — the SHA itself still
renders, so a flaky git host never breaks the page.
"""
import logging
import time

import requests


_GITEA_REPO_API = "https://git.moritz.run/api/v1/repos/moritz/lazyflat"
_CACHE_TTL = 60.0
_HTTP_TIMEOUT = 1.5
_cache: dict[str, tuple[int, float]] = {}

logger = logging.getLogger("web.version")


def commits_behind_main(sha: str) -> int | None:
    """Return how many commits `sha` is behind origin/main, or None on error/dev."""
    if not sha or sha == "dev":
        return None
    now = time.monotonic()
    cached = _cache.get(sha)
    if cached and (now - cached[1]) < _CACHE_TTL:
        return cached[0]
    try:
        r = requests.get(f"{_GITEA_REPO_API}/compare/{sha}...main", timeout=_HTTP_TIMEOUT)
        r.raise_for_status()
        behind = int(r.json().get("total_commits", 0))
    except (requests.RequestException, ValueError, KeyError) as e:
        logger.debug("commits_behind_main fetch failed: %s", e)
        return cached[0] if cached else None
    _cache[sha] = (behind, now)
    return behind