"""Request validation shared by every slice. Errors carry a status, a fix-it message, and the field."""

from flask import request


class ApiError(Exception):
    def __init__(self, status: int, message: str, field: str | None = None):
        super().__init__(message)
        self.status = status
        self.message = message
        self.field = field


def json_body(allowed: set[str]) -> dict:
    body = request.get_json(silent=True)
    if not isinstance(body, dict):
        raise ApiError(400, "Request body must be a JSON object")
    unknown = sorted(set(body) - allowed)
    if unknown:
        raise ApiError(
            400,
            f"Unknown field(s): {', '.join(unknown)}. Allowed: {', '.join(sorted(allowed))}",
            unknown[0],
        )
    return body


def require_utf8(value: str, field: str) -> None:
    try:
        value.encode("utf-8")
    except UnicodeEncodeError:
        raise ApiError(
            400,
            f"{field} contains unpaired surrogate characters; send valid UTF-8 text",
            field,
        ) from None


def text(body: dict, field: str, max_len: int) -> str:
    value = body.get(field)
    if not isinstance(value, str) or not value.strip():
        raise ApiError(400, f"{field} is required and must be non-blank text", field)
    value = value.strip()
    if len(value) > max_len:
        raise ApiError(400, f"{field} must be at most {max_len} characters", field)
    if "\x00" in value:
        raise ApiError(400, f"{field} must not contain NUL characters", field)
    require_utf8(value, field)
    return value


def integer(body: dict, field: str, low: int, high: int) -> int:
    value = body.get(field)
    if isinstance(value, bool) or not isinstance(value, int):
        raise ApiError(400, f"{field} must be a whole number", field)
    if not low <= value <= high:
        raise ApiError(400, f"{field} must be between {low} and {high}", field)
    return value