"""Stable RFC 9457-style problem responses for public API failures."""

from __future__ import annotations

from dataclasses import dataclass
from typing import Any

from fastapi import Request
from fastapi.responses import JSONResponse

PROBLEM_MEDIA_TYPE = "application/problem+json"
PROBLEM_BASE = "https://geological-rag.local/problems"


@dataclass(frozen=True, slots=True)
class ApiProblem(Exception):
    """An intentionally public and sanitized application failure."""

    status: int
    code: str
    title: str
    detail: str

    def __post_init__(self) -> None:
        Exception.__init__(self, self.code)


def problem_payload(
    *,
    status: int,
    code: str,
    title: str,
    detail: str,
    trace_id: str,
    field_errors: list[dict[str, Any]] | None = None,
) -> dict[str, Any]:
    """Build the only public error envelope used by formal product routes."""

    return {
        "type": f"{PROBLEM_BASE}/{code.lower().replace('_', '-')}",
        "title": title,
        "status": status,
        "code": code,
        "detail": detail,
        "trace_id": trace_id,
        "field_errors": field_errors or [],
    }


def api_problem_handler(request: Request, exc: ApiProblem) -> JSONResponse:
    trace_id = str(getattr(request.state, "trace_id", "unavailable"))
    return JSONResponse(
        status_code=exc.status,
        media_type=PROBLEM_MEDIA_TYPE,
        content=problem_payload(
            status=exc.status,
            code=exc.code,
            title=exc.title,
            detail=exc.detail,
            trace_id=trace_id,
        ),
    )