from __future__ import annotations

import os
from collections.abc import Mapping
from dataclasses import dataclass
from urllib.parse import urlparse


class HostAgentConfigurationError(ValueError):
    """Raised when Host Agent process configuration is invalid."""


@dataclass(frozen=True)
class HostAgentConfig:
    control_plane_url: str
    host_id: str
    token: str
    heartbeat_interval_seconds: float = 30.0
    poll_timeout_seconds: float = 20.0
    retry_backoff_seconds: float = 1.0
    max_retry_backoff_seconds: float = 30.0


def load_host_agent_config(
    env: Mapping[str, str] | None = None,
) -> HostAgentConfig:
    values = os.environ if env is None else env
    control_plane_url = values.get(
        "HOST_AGENT_CONTROL_PLANE_URL",
        "http://127.0.0.1:8001",
    ).strip().rstrip("/")
    parsed_url = urlparse(control_plane_url)
    if parsed_url.scheme not in {"http", "https"} or not parsed_url.netloc:
        raise HostAgentConfigurationError(
            "HOST_AGENT_CONTROL_PLANE_URL must be an HTTP(S) URL"
        )

    host_id = values.get("HOST_AGENT_HOST_ID", "").strip()
    if not host_id:
        raise HostAgentConfigurationError("HOST_AGENT_HOST_ID is required")
    token = values.get("HOST_AGENT_TOKEN", "").strip()
    if not token:
        raise HostAgentConfigurationError("HOST_AGENT_TOKEN is required")

    config = HostAgentConfig(
        control_plane_url=control_plane_url,
        host_id=host_id,
        token=token,
        heartbeat_interval_seconds=_positive_float(
            values,
            "HOST_AGENT_HEARTBEAT_INTERVAL_SECONDS",
            30.0,
        ),
        poll_timeout_seconds=_positive_float(
            values,
            "HOST_AGENT_POLL_TIMEOUT_SECONDS",
            20.0,
        ),
        retry_backoff_seconds=_positive_float(
            values,
            "HOST_AGENT_RETRY_BACKOFF_SECONDS",
            1.0,
        ),
        max_retry_backoff_seconds=_positive_float(
            values,
            "HOST_AGENT_MAX_RETRY_BACKOFF_SECONDS",
            30.0,
        ),
    )
    if config.max_retry_backoff_seconds < config.retry_backoff_seconds:
        raise HostAgentConfigurationError(
            "maximum retry backoff must not be less than initial backoff"
        )
    return config


def _positive_float(
    values: Mapping[str, str],
    name: str,
    default: float,
) -> float:
    raw_value = values.get(name)
    if raw_value is None:
        return default
    try:
        value = float(raw_value)
    except ValueError as exc:
        raise HostAgentConfigurationError(f"{name} must be a number") from exc
    if value <= 0:
        raise HostAgentConfigurationError(f"{name} must be greater than zero")
    return value