"""Per-student persistence in a private Hugging Face Dataset repo.

The container filesystem is ephemeral — it resets whenever the Space restarts —
so anything a student saves has to leave the container. A private Dataset repo is
free, unlimited, versioned, and has no inactivity pause, which suits a beta.

Layout, one file per student:

    students/{student_id}/profile.json
    students/{student_id}/syllabi.json

**One file per student, never a shared mutable file.** A dataset repo is git, not
a database: there is no row locking, so two writers on one file either clobber
each other or lose a race. Per-student files make that impossible between
students, and `write_json` retries on the remaining same-student case.

Every write is a network commit (hundreds of ms to seconds), so callers should
debounce rather than save on every keystroke — see `app/store.js`.

Local development: with no `HF_TOKEN` set, reads and writes fall back to a
git-ignored `.data/` directory so the app runs with no credentials at all.
"""
from __future__ import annotations

import json
import logging
import os
import time
from pathlib import Path

log = logging.getLogger("foresight.storage")

DATASET_REPO = os.environ.get("FORESIGHT_DATASET_REPO", "umangchaudhry/foresight")
LOCAL_DIR = Path(os.environ.get("FORESIGHT_LOCAL_DATA_DIR", ".data"))
WRITE_ATTEMPTS = 3


def token() -> str | None:
    """HF write token. Must be set as a *Space secret* for deployment —
    a GitHub Actions secret only covers pushing the code to the Space."""
    return os.environ.get("HF_TOKEN") or os.environ.get("HUGGING_FACE_HUB_TOKEN")


def using_hub() -> bool:
    return bool(token())


def student_path(student_id: str, name: str) -> str:
    return f"students/{student_id}/{name}"


# --- local fallback ---------------------------------------------------------
def _local_read(path: str):
    f = LOCAL_DIR / path
    if not f.exists():
        return None
    return json.loads(f.read_text(encoding="utf-8"))


def _local_write(path: str, payload: dict) -> None:
    f = LOCAL_DIR / path
    f.parent.mkdir(parents=True, exist_ok=True)
    f.write_text(json.dumps(payload, indent=2, ensure_ascii=False), encoding="utf-8")


# --- hub -------------------------------------------------------------------
def _hub_read(path: str):
    from huggingface_hub import hf_hub_download
    from huggingface_hub.errors import EntryNotFoundError, RepositoryNotFoundError

    try:
        local = hf_hub_download(
            repo_id=DATASET_REPO, filename=path, repo_type="dataset",
            token=token(), force_download=True,
        )
    except (EntryNotFoundError, RepositoryNotFoundError):
        return None
    except Exception as err:                       # 404s surface in several shapes
        if "404" in str(err) or "not found" in str(err).lower():
            return None
        raise
    return json.loads(Path(local).read_text(encoding="utf-8"))


def _hub_write(path: str, payload: dict, message: str) -> None:
    from huggingface_hub import HfApi

    body = json.dumps(payload, indent=2, ensure_ascii=False).encode("utf-8")
    api = HfApi(token=token())
    last: Exception | None = None
    for attempt in range(1, WRITE_ATTEMPTS + 1):
        try:
            api.upload_file(
                path_or_fileobj=body, path_in_repo=path,
                repo_id=DATASET_REPO, repo_type="dataset",
                commit_message=message,
            )
            return
        except Exception as err:
            # A concurrent commit to the same file loses the race; retry on a
            # fresh head rather than dropping the student's save.
            last = err
            if attempt < WRITE_ATTEMPTS:
                time.sleep(0.5 * attempt)
    raise RuntimeError(f"failed to write {path} after {WRITE_ATTEMPTS} attempts: {last}")


# --- public API ------------------------------------------------------------
def read_json(path: str):
    """Stored JSON at `path`, or None if it doesn't exist yet."""
    if using_hub():
        return _hub_read(path)
    return _local_read(path)


def write_json(path: str, payload: dict, message: str | None = None) -> None:
    if using_hub():
        _hub_write(path, payload, message or f"update {path}")
    else:
        log.warning("HF_TOKEN unset — writing %s to %s instead of the dataset repo", path, LOCAL_DIR)
        _local_write(path, payload)


def describe() -> dict:
    """Where data is going, for /healthz and startup logging."""
    return {
        "backend": "huggingface-dataset" if using_hub() else "local-directory",
        "dataset_repo": DATASET_REPO if using_hub() else None,
        "local_dir": None if using_hub() else str(LOCAL_DIR),
    }