from __future__ import annotations

import uuid
from pathlib import Path

PARTIAL_NAME = "upload.partial"


def new_session_dir(base: Path) -> tuple[str, Path]:
    session_id = uuid.uuid4().hex
    d = base / session_id
    d.mkdir(parents=True, exist_ok=True)
    return session_id, d


def partial_path(session_dir: Path) -> Path:
    return session_dir / PARTIAL_NAME


def write_chunk(session_dir: Path, data: bytes, offset: int) -> int:
    p = partial_path(session_dir)
    if offset < 0:
        raise ValueError("offset must be >= 0")
    current = p.stat().st_size if p.is_file() else 0
    if offset != current:
        raise ValueError(
            f"Offset mismatch: expected {current} for append-only upload, got {offset}"
        )
    with open(p, "ab") as f:
        f.write(data)
    return offset + len(data)


def finalize_rename(session_dir: Path, final_name: str) -> Path:
    src = partial_path(session_dir)
    if not src.is_file() or src.stat().st_size == 0:
        raise FileNotFoundError("No uploaded data to finalize")
    dst = session_dir / final_name
    if dst.exists():
        dst.unlink()
    src.rename(dst)
    return dst