import { NextResponse } from "next/server"

const MAX_ICS_BYTES = 2 * 1024 * 1024

function isAllowedIcsUrl(raw: string): boolean {
  try {
    const url = new URL(raw)
    if (url.protocol !== "http:" && url.protocol !== "https:") return false
    const host = url.hostname.toLowerCase()
    if (host === "localhost" || host === "127.0.0.1" || host === "::1") {
      return false
    }
    return true
  } catch {
    return false
  }
}

export async function GET(request: Request) {
  const url = new URL(request.url).searchParams.get("url")?.trim()
  if (!url || !isAllowedIcsUrl(url)) {
    return NextResponse.json({ error: "invalid_url" }, { status: 400 })
  }

  try {
    const res = await fetch(url, {
      headers: { Accept: "text/calendar, text/plain, */*" },
      redirect: "follow",
      cache: "no-store",
    })
    if (!res.ok) {
      return NextResponse.json({ error: "fetch_failed" }, { status: 502 })
    }

    const contentType = res.headers.get("content-type") ?? ""
    if (
      contentType &&
      !contentType.includes("text/calendar") &&
      !contentType.includes("text/plain") &&
      !contentType.includes("application/octet-stream")
    ) {
      return NextResponse.json({ error: "unsupported_content_type" }, { status: 415 })
    }

    const buffer = await res.arrayBuffer()
    if (buffer.byteLength > MAX_ICS_BYTES) {
      return NextResponse.json({ error: "payload_too_large" }, { status: 413 })
    }

    return new NextResponse(new TextDecoder("utf-8").decode(buffer), {
      status: 200,
      headers: {
        "Content-Type": "text/calendar; charset=utf-8",
        "Cache-Control": "private, max-age=300",
      },
    })
  } catch {
    return NextResponse.json({ error: "fetch_failed" }, { status: 502 })
  }
}