"""System timezone detection for local-day activity totals.

Provides timezone detection (TZ env, /etc/localtime) and offset
computation used by local-day derivation. All functions are
stateless and safe to call from the collector and TUI reader.
"""
import os
from datetime import datetime, timezone
from pathlib import Path


def detect_system_tz() -> str:
    """Detect the system timezone name.

    Resolution order:
    1. ``TZ`` environment variable (may be empty or ":UTC")
    2. Symlink target of ``/etc/localtime``
    3. Falls back to ``"UTC"``

    Returns a POSIX timezone name like ``"Asia/Kolkata"`` or ``"UTC"``.
    """
    tz = os.environ.get("TZ", "").strip()
    if tz:
        return tz.lstrip(":")

    localtime = Path("/etc/localtime")
    if localtime.is_symlink():
        target_path = Path(os.readlink(str(localtime)))
        if not target_path.is_absolute():
            target_path = localtime.parent / target_path
        target = target_path.resolve().as_posix()
        marker = "/zoneinfo/"
        marker_index = target.find(marker)
        if marker_index >= 0:
            return target[marker_index + len(marker):]
        return target

    return "UTC"


def get_tz_offset_str(dt: datetime, tz_name: str) -> str:
    """UTC offset as ``+HH:MM`` or ``-HH:MM`` string for *dt* in *tz_name*."""
    from zoneinfo import ZoneInfo
    local_tz = ZoneInfo(tz_name)
    local_dt = dt.astimezone(local_tz)
    offset = local_dt.utcoffset()
    total_seconds = int(offset.total_seconds())
    sign = "+" if total_seconds >= 0 else "-"
    total_seconds = abs(total_seconds)
    hours = total_seconds // 3600
    minutes = (total_seconds % 3600) // 60
    return "%s%02d:%02d" % (sign, hours, minutes)