"""System prompt assembly from a list of source files.

The gateway holds no prompt text: an agent names its granules (paths from
``config.py``) and :func:`assemble` concatenates them in that order, so the
result is byte-for-byte identical for every session of the same agent as
long as the files are. Each granule's hash is logged at assembly so a
drifted prompt can be traced to the file that changed.
"""

from __future__ import annotations

import hashlib
import logging
from pathlib import Path
from typing import TYPE_CHECKING

if TYPE_CHECKING:
    from collections.abc import Iterable

__all__ = ["assemble"]

_log = logging.getLogger("beaver_gateway.core.prompt")


def assemble(sources: Iterable[str | Path]) -> str:
    parts: list[str] = []
    for source in sources:
        path = Path(source)
        text = path.read_text(encoding="utf-8").strip()
        digest = hashlib.sha256(text.encode("utf-8")).hexdigest()[:12]
        _log.info("prompt granule %s sha=%s bytes=%d", path, digest, len(text))
        if text:
            parts.append(text)
    return "\n\n".join(parts) + "\n"