"""Session Scratchpad — ephemeral key-value notes per session.

Provides fast, JSON-backed scratch storage that lives for a session
and can be promoted to durable palace memory.

Storage: ~/.hermes/scratchpad/{session_id}.json

Refs: Epic #367, Sub-issue #372
"""

from __future__ import annotations

import json
import os
import subprocess
import time
from pathlib import Path
from typing import Any, Optional

# ---------------------------------------------------------------------------
# Constants
# ---------------------------------------------------------------------------

SCRATCHPAD_DIR = Path.home() / ".hermes" / "scratchpad"
MEMPALACE_BIN = "/Library/Frameworks/Python.framework/Versions/3.12/bin/mempalace"


# ---------------------------------------------------------------------------
# Internal helpers
# ---------------------------------------------------------------------------

def _scratch_path(session_id: str) -> Path:
    """Return the JSON file path for a given session."""
    # Sanitize session_id to prevent path traversal
    safe_id = "".join(c for c in session_id if c.isalnum() or c in "-_")
    if not safe_id:
        safe_id = "unnamed"
    return SCRATCHPAD_DIR / f"{safe_id}.json"


def _load(session_id: str) -> dict:
    """Load scratchpad data, returning empty dict on failure."""
    path = _scratch_path(session_id)
    try:
        if path.exists():
            return json.loads(path.read_text(encoding="utf-8"))
    except (OSError, json.JSONDecodeError):
        pass
    return {}


def _save(session_id: str, data: dict) -> None:
    """Persist scratchpad data to disk."""
    SCRATCHPAD_DIR.mkdir(parents=True, exist_ok=True)
    path = _scratch_path(session_id)
    path.write_text(json.dumps(data, indent=2, default=str), encoding="utf-8")


# ---------------------------------------------------------------------------
# Public API
# ---------------------------------------------------------------------------

def write_scratch(session_id: str, key: str, value: Any) -> None:
    """Write a note to the session scratchpad.

    Args:
        session_id: Current session identifier.
        key: Note key (string).
        value: Note value (any JSON-serializable type).
    """
    data = _load(session_id)
    data[key] = {
        "value": value,
        "written_at": time.strftime("%Y-%m-%d %H:%M:%S"),
    }
    _save(session_id, data)


def read_scratch(session_id: str, key: Optional[str] = None) -> dict:
    """Read session scratchpad (all keys or one).

    Args:
        session_id: Current session identifier.
        key: Optional specific key. If None, returns all entries.

    Returns:
        dict — either {key: {value, written_at}} or the full scratchpad.
    """
    data = _load(session_id)
    if key is not None:
        entry = data.get(key)
        return {key: entry} if entry else {}
    return data


def delete_scratch(session_id: str, key: str) -> bool:
    """Remove a single key from the scratchpad.

    Returns True if the key existed and was removed.
    """
    data = _load(session_id)
    if key in data:
        del data[key]
        _save(session_id, data)
        return True
    return False


def list_sessions() -> list[str]:
    """List all session IDs that have scratchpad files."""
    try:
        if SCRATCHPAD_DIR.exists():
            return [
                f.stem
                for f in SCRATCHPAD_DIR.iterdir()
                if f.suffix == ".json" and f.is_file()
            ]
    except OSError:
        pass
    return []


def promote_to_palace(
    session_id: str,
    key: str,
    room: str = "general",
    drawer: Optional[str] = None,
) -> bool:
    """Move a scratchpad note to durable palace memory.

    Uses the mempalace CLI to store the note in the specified room.
    Removes the note from the scratchpad after successful promotion.

    Args:
        session_id: Session containing the note.
        key: Scratchpad key to promote.
        room: Palace room name (default: 'general').
        drawer: Optional drawer name within the room. Defaults to key.

    Returns:
        True if promotion succeeded, False otherwise.
    """
    data = _load(session_id)
    entry = data.get(key)
    if not entry:
        return False

    value = entry.get("value", entry) if isinstance(entry, dict) else entry
    content = json.dumps(value, default=str) if not isinstance(value, str) else value

    try:
        bin_path = MEMPALACE_BIN if os.path.exists(MEMPALACE_BIN) else "mempalace"
        target_drawer = drawer or key
        result = subprocess.run(
            [bin_path, "store", room, target_drawer, content],
            capture_output=True,
            text=True,
            timeout=10,
        )
        if result.returncode == 0:
            # Remove from scratchpad after successful promotion
            del data[key]
            _save(session_id, data)
            return True
    except (FileNotFoundError, subprocess.TimeoutExpired, OSError):
        # mempalace CLI not available — degrade gracefully
        pass

    return False


def clear_session(session_id: str) -> bool:
    """Delete the entire scratchpad for a session.

    Returns True if the file existed and was removed.
    """
    path = _scratch_path(session_id)
    try:
        if path.exists():
            path.unlink()
            return True
    except OSError:
        pass
    return False