"""Session-scoped outbound proxy routing."""
from __future__ import annotations

import base64
from collections.abc import Mapping
from urllib.parse import quote, urlsplit, urlunsplit


def routed_proxy(user_proxy: str, egress_proxy: str) -> str:
    """Route an optional user proxy through the deployment egress gateway.

    Args:
        user_proxy: User-selected upstream HTTP, HTTPS, or SOCKS proxy.
        egress_proxy: Deployment-managed HTTP egress gateway.

    Returns:
        A proxy URL suitable for ordinary HTTP clients. When both values are
        present, the upstream URL is encoded as gateway credentials so the
        gateway can establish the second proxy hop.
    """
    user_proxy = user_proxy.strip()
    egress_proxy = egress_proxy.strip()
    if not egress_proxy:
        return user_proxy
    if not user_proxy:
        return egress_proxy
    if urlsplit(user_proxy).scheme not in {"http", "https"}:
        raise ValueError("the deployment egress gateway supports HTTP(S) upstream proxies only")
    parsed = urlsplit(egress_proxy)
    if parsed.scheme not in {"http", "https"} or not parsed.hostname:
        raise ValueError("SRH_EGRESS_PROXY must be an HTTP or HTTPS URL")
    token = base64.urlsafe_b64encode(user_proxy.encode()).decode().rstrip("=")
    host = parsed.hostname
    if ":" in host:
        host = f"[{host}]"
    if parsed.port:
        host += f":{parsed.port}"
    netloc = f"{quote('srh.' + token, safe='')}:x@{host}"
    return urlunsplit((parsed.scheme, netloc, parsed.path, parsed.query, parsed.fragment))


def proxy_environment(
    environment: Mapping[str, str], *, user_proxy: str, egress_proxy: str,
) -> dict[str, str]:
    """Return a session environment containing the effective outbound proxy."""
    result = dict(environment)
    for name in ("http_proxy", "HTTP_PROXY", "https_proxy", "HTTPS_PROXY"):
        result.pop(name, None)
    effective = routed_proxy(user_proxy, egress_proxy)
    if effective:
        result.update({"HTTP_PROXY": effective, "HTTPS_PROXY": effective})
    return result