"""Configuration loader for ReMDM MiniHack.

Loads YAML configs with deep-merge and CLI override support,
following the Craftax config pattern.
"""

from __future__ import annotations

import contextlib
import logging
import os
import secrets
from datetime import UTC, datetime
from pathlib import Path
from types import SimpleNamespace

import yaml

logger = logging.getLogger(__name__)


_PROJECT_ROOT = Path(__file__).resolve().parent.parent


def deep_merge(base: dict, override: dict) -> dict:
    """Recursively merge *override* into *base* (mutates *base*).

    Args:
        base: Base dictionary to merge into.
        override: Dictionary whose values take precedence.

    Returns:
        The merged dictionary (same object as *base*).
    """
    for key, value in override.items():
        if key in base and isinstance(base[key], dict) and isinstance(value, dict):
            deep_merge(base[key], value)
        else:
            base[key] = value
    return base


# Valid config keys that do not appear in defaults.yaml: `device` is
# auto-selected at load time and serialised into checkpoint config snapshots.
_RUN_KEYS = {"device"}

# Keys used by earlier code versions that survive in released checkpoint
# config snapshots (e.g. config_iter600.yaml on the HF Hub). Accepted so the
# documented snapshot-evaluation workflow keeps working; nothing reads them.

def validate_keys(
    keys, allowed: set[str], source: str, valid_source: str = "configs/defaults.yaml"
) -> None:
    """Reject unknown config keys instead of silently ignoring them.

    Args:
        keys: Keys to check.
        allowed: The full set of valid config keys.
        source: Label for the error message (file path or 'override').
        valid_source: Where the caller's valid keys are defined.

    Raises:
        KeyError: If any key is not a known config key.
    """
    unknown = sorted(set(keys) - allowed)
    if unknown:
        raise KeyError(
            f"Unknown config key(s) {unknown} in {source}. "
            f"Valid keys are defined in {valid_source}."
        )


def parse_overrides(pairs: list[str]) -> dict[str, str]:
    """Split ``KEY=VALUE`` CLI strings into a dict.

    Args:
        pairs: Raw ``--override`` arguments.

    Returns:
        Mapping of key to raw (uncast) string value.

    Raises:
        ValueError: If an argument is not of the form ``KEY=VALUE``.
    """
    overrides: dict[str, str] = {}
    for item in pairs:
        if "=" not in item:
            raise ValueError(f"--override expects KEY=VALUE, got '{item}'")
        key, value = item.split("=", 1)
        overrides[key] = value
    return overrides


def cast_override(key: str, raw: str, current) -> object:
    """Cast a CLI override string to the type of the current config value.

    Args:
        key: Config key being overridden.
        raw: Raw string from the command line.
        current: Current (default or config-file) value, used for typing.

    Returns:
        Parsed Python value.

    Raises:
        TypeError: If the value cannot be interpreted as the key's type.
    """
    if isinstance(current, str):
        return raw

    try:
        value = yaml.safe_load(raw)
    except yaml.YAMLError:
        value = raw

    if current is None or value is None:
        return value

    # YAML 1.1 reads '1e-4' as a string; accept scientific notation for
    # numeric keys.
    if (
        isinstance(current, (int, float))
        and not isinstance(current, bool)
        and isinstance(value, str)
    ):
        with contextlib.suppress(ValueError):
            value = float(value)

    if isinstance(current, bool):
        if not isinstance(value, bool):
            raise TypeError(f"'{key}' expects a boolean, got '{raw}'")
        return value
    if isinstance(current, int):
        if isinstance(value, bool) or not isinstance(value, (int, float)):
            raise TypeError(f"'{key}' expects an integer, got '{raw}'")
        if isinstance(value, float):
            if not value.is_integer():
                raise TypeError(f"'{key}' expects an integer, got '{raw}'")
            value = int(value)
        return value
    if isinstance(current, float):
        if isinstance(value, bool) or not isinstance(value, (int, float)):
            raise TypeError(f"'{key}' expects a number, got '{raw}'")
        return float(value)
    if isinstance(current, list):
        if not isinstance(value, list):
            raise TypeError(f"'{key}' expects a list, got '{raw}'")
        return value
    return value


def load_config(
    config_path: str | None = None,
    cli_overrides: dict | None = None,
) -> SimpleNamespace:
    """Load configuration from YAML with optional overrides.

    1. Load ``configs/defaults.yaml``, the shared paper recipe.
    2. Deep-merge *config_path* on top (skipped if it is the defaults file
       itself). Presets are a single layer: they never inherit from each
       other, so a key a preset does not restate comes from the recipe.
    3. Apply *cli_overrides* key=value pairs.
    4. Auto-select device (``cuda`` if available, else ``cpu``; honour
       ``DEVICE`` env-var).
    5. Validate invariants.

    Args:
        config_path: Path to a YAML file merged on top of defaults.
            ``None`` uses defaults only.
        cli_overrides: ``{key: value}`` pairs applied last.

    Returns:
        A ``SimpleNamespace`` containing all hyperparameters.

    Raises:
        AssertionError: If ``mask_token != action_dim`` or
            ``pad_token != action_dim + 1``.
    """
    if cli_overrides is None:
        cli_overrides = {}

    defaults_path = _PROJECT_ROOT / "configs" / "defaults.yaml"
    with open(defaults_path) as fh:
        cfg = yaml.safe_load(fh)

    allowed = set(cfg) | _RUN_KEYS

    if config_path is not None:
        config_path_resolved = Path(config_path)
        if not config_path_resolved.is_absolute():
            config_path_resolved = _PROJECT_ROOT / config_path_resolved
        if config_path_resolved.resolve() != defaults_path.resolve():
            with open(config_path_resolved) as fh:
                overrides = yaml.safe_load(fh) or {}
            validate_keys(overrides, allowed, str(config_path))
            deep_merge(cfg, overrides)

    validate_keys(cli_overrides, allowed, "--override")
    for key, value in cli_overrides.items():
        if isinstance(value, str):
            value = cast_override(key, value, cfg.get(key))
        cfg[key] = value

    # Device selection
    env_device = os.environ.get("DEVICE")
    if env_device:
        cfg["device"] = env_device
    elif "device" not in cfg:
        try:
            import torch

            cfg["device"] = "cuda" if torch.cuda.is_available() else "cpu"
        except ImportError:
            cfg["device"] = "cpu"

    ns = SimpleNamespace(**cfg)

    # Validation
    assert ns.mask_token == ns.action_dim, (
        f"mask_token ({ns.mask_token}) must equal action_dim ({ns.action_dim})"
    )
    assert ns.pad_token == ns.action_dim + 1, (
        f"pad_token ({ns.pad_token}) must equal action_dim + 1 ({ns.action_dim + 1})"
    )

    return ns


def make_run_dir(cfg: SimpleNamespace, tag: str = "run") -> Path:
    """Create a unique run subdirectory under ``cfg.checkpoint_dir``.

    Generates a directory named ``{tag}_{YYYYMMDD}_{HHMMSS}_{hex4}``
    to prevent concurrent runs from overwriting each other's
    checkpoints. Updates ``cfg.checkpoint_dir`` in place.

    Args:
        cfg: Config namespace (``checkpoint_dir`` is mutated).
        tag: Prefix for the directory name (e.g. ``"dagger"``,
            ``"offline"``).

    Returns:
        The created directory path.
    """
    ts = datetime.now(UTC).strftime("%Y%m%d_%H%M%S")
    suffix = secrets.token_hex(2)
    run_dir = Path(cfg.checkpoint_dir).resolve() / f"{tag}_{ts}_{suffix}"
    run_dir.mkdir(parents=True, exist_ok=True)
    cfg.checkpoint_dir = str(run_dir)
    logger.info("Checkpoint directory: %s", run_dir)
    return run_dir