"""cache.json I/O.

This module owns the on-disk cache contract: where the file lives,
what the header looks like, and how mismatches are handled. The
contract is the two-tier `cache_schema` + `id_rules` model from
docs/CACHE_CONTRACT.md (extension repo).

  cache_schema       on-disk shape. Mismatch -> force rebuild.
  id_rules           integer; bumps when extraction rules change.
                     Mismatch -> mark stale, allow lazy re-extract.
  id_rules_signature sha256 over canonical rule text (see
                     rcjav.ids.current_rules_signature). Belt-and-
                     braces drift check that catches a forgotten
                     `id_rules` bump.

Legacy users on `version: 3` get an in-place header upgrade with no
forced rescan; the cache is marked as `id_rules: 0` so it shows up
as "stale by rules" until they Re-extract IDs.
"""
from __future__ import annotations

import json
import os
import sys
import time
from datetime import datetime
from pathlib import Path


# Lives next to rc-jav.py at the repo root.
CACHE_PATH = Path(__file__).resolve().parents[1] / "cache.json"
CACHE_STALE_HOURS = 24

# Two-tier version contract (see docs/CACHE_CONTRACT.md):
CACHE_SCHEMA_VERSION = 1   # on-disk shape; bump = force rebuild
ID_RULES_VERSION = 1       # extraction rules; bump = mark stale (lazy re-extract)

# Legacy alias preserved for any external caller that still imports it.
# Maps to CACHE_SCHEMA_VERSION + ID_RULES_VERSION under the new contract.
CACHE_VERSION = 3


def _fresh_cache(signature: str = "unknown") -> dict:
    return {
        "cache_schema": CACHE_SCHEMA_VERSION,
        "id_rules": ID_RULES_VERSION,
        "id_rules_signature": signature,
        "remotes": {},
    }


def _migrate_legacy_v3(data: dict) -> dict:
    """Translate a legacy `version: 3` cache to the new header in place.

    Sets `id_rules: 0` so the cache reads as "stale by rules" — user
    sees the new amber state and can opt into a fast Re-extract without
    a rclone re-scan.
    """
    return {
        "cache_schema": CACHE_SCHEMA_VERSION,
        "id_rules": 0,
        "id_rules_signature": "legacy",
        "remotes": data.get("remotes", {}),
    }


def load_cache(current_signature: str | None = None) -> dict:
    """Read and (if necessary) migrate cache.json.

    `current_signature` is the value of `rcjav.ids.current_rules_signature()`
    captured by the caller. It's only stamped into the header when this
    function has to mint a *fresh* cache; when migrating legacy data we
    deliberately stamp `"legacy"` so the cache reads as stale-by-rules.
    """
    fresh_sig = current_signature or "unknown"

    if not CACHE_PATH.exists():
        return _fresh_cache(fresh_sig)

    try:
        data = json.loads(CACHE_PATH.read_text(encoding="utf-8"))
    except (json.JSONDecodeError, OSError):
        return _fresh_cache(fresh_sig)

    if not isinstance(data, dict) or not isinstance(data.get("remotes"), dict):
        return _fresh_cache(fresh_sig)

    # Legacy header: { "version": 3, "remotes": {...} } — migrate in place.
    if "version" in data and "cache_schema" not in data:
        if data.get("version") == 3:
            return _migrate_legacy_v3(data)
        sys.stderr.write(
            f"[warn] unknown legacy cache version {data.get('version')!r}; "
            f"rebuilding.\n"
        )
        return _fresh_cache(fresh_sig)

    # New header: validate schema. Mismatch = force rebuild (per contract).
    if data.get("cache_schema") != CACHE_SCHEMA_VERSION:
        sys.stderr.write(
            f"[warn] cache_schema mismatch (got {data.get('cache_schema')!r}, "
            f"expected {CACHE_SCHEMA_VERSION}); forcing full rescan.\n"
        )
        return _fresh_cache(fresh_sig)

    return data


def cache_state(cache: dict, current_signature: str) -> str:
    """Classify a cache dict against the live rule set.

    Returns one of: "fresh", "stale_by_rules", "schema_mismatch".

    "schema_mismatch" should normally never reach the caller — load_cache
    already rebuilds. It's reported for diagnostics flows that read
    cache.json directly without going through load_cache.
    """
    if cache.get("cache_schema") != CACHE_SCHEMA_VERSION:
        return "schema_mismatch"
    rules_match = cache.get("id_rules") == ID_RULES_VERSION
    sig_match = cache.get("id_rules_signature") == current_signature
    return "fresh" if (rules_match and sig_match) else "stale_by_rules"


def stamp_current_rules(cache: dict, current_signature: str) -> None:
    """Stamp `id_rules` and `id_rules_signature` to current values in place.

    Use after a successful re-extract or full scan completes against the
    live rule set.
    """
    cache["id_rules"] = ID_RULES_VERSION
    cache["id_rules_signature"] = current_signature


def save_cache(cache: dict) -> None:
    # Write to a sibling tmp file then atomically replace, so a killed mid-write
    # (Ctrl-C, power loss, concurrent --scan) can't leave a half-written
    # cache.json — load_cache would otherwise see invalid JSON and fall back to
    # an empty cache, forcing a full re-scan.
    tmp = CACHE_PATH.with_suffix(CACHE_PATH.suffix + ".tmp")
    tmp.write_text(json.dumps(cache, indent=2), encoding="utf-8")
    try:
        os.replace(tmp, CACHE_PATH)
    except PermissionError:
        # Windows: destination may be briefly locked by antivirus or a concurrent reader.
        time.sleep(0.5)
        os.replace(tmp, CACHE_PATH)


def cache_age_hours(scanned_at: str) -> float | None:
    try:
        dt = datetime.fromisoformat(scanned_at.replace("Z", "+00:00"))
    except ValueError:
        return None
    now = datetime.now(dt.tzinfo) if dt.tzinfo else datetime.now()
    return (now - dt).total_seconds() / 3600.0


def fmt_age(hours: float) -> str:
    if hours < 1:
        return f"{int(hours * 60)}m"
    if hours < 24:
        return f"{hours:.1f}h"
    return f"{hours / 24:.1f}d"