// author: simon-pierre boucher <contact@spboucher.ai>
export interface EscalationInput {
  readonly status: number;
  readonly contentType: string;
  readonly body: string;
}

export type TierDecision =
  | { readonly action: "proceed" }
  | { readonly action: "escalate"; readonly reason: string }
  | { readonly action: "non-html"; readonly reason: string };

type Severity = "decisive" | "strong" | "moderate";
interface Signal {
  readonly severity: Severity;
  readonly reason: string;
}

const CHALLENGE_MARKERS = [
  "challenge-platform",
  "cf-browser-verification",
  "_Incapsula_",
  "px-captcha",
  "datadome",
];

const EMPTY_ROOT_RE = /<[^>]+\bid=["']?(root|__next|app)["']?[^>]*>\s*<\/[a-z]+>/i;
const NOSCRIPT_JS_RE = /<noscript[^>]*>[\s\S]*?(enable\s+javascript|javascript\s+is\s+required)[\s\S]*?<\/noscript>/i;
const TITLE_RE = /<title[^>]*>\s*(just a moment|attention required|access denied)/i;
const HREF_RE = /<a\s[^>]*href\s*=/i;

function isHtml(contentType: string): boolean {
  const ct = contentType.toLowerCase();
  return ct.includes("text/html") || ct.includes("application/xhtml+xml") || ct === "";
}

function textHtmlRatio(html: string): number {
  if (html.length === 0) return 0;
  const text = html
    .replace(/<script[\s\S]*?<\/script>/gi, "")
    .replace(/<style[\s\S]*?<\/style>/gi, "")
    .replace(/<[^>]+>/g, "")
    .replace(/\s+/g, " ")
    .trim();
  return text.length / html.length;
}

/**
 * Decide whether a Tier 0 response should escalate to Tier 1 (§3.4). Pure, no I/O.
 * Decisive signals escalate on their own; two or more strong/moderate signals
 * escalate together. Non-HTML content types route to §9 instead of escalating.
 */
export function shouldEscalate(input: EscalationInput): TierDecision {
  if (!isHtml(input.contentType)) {
    return { action: "non-html", reason: `content-type:${input.contentType || "unknown"}` };
  }

  const signals: Signal[] = [];
  const body = input.body;
  const bytes = Buffer.byteLength(body, "utf8");

  if (input.status === 403 || input.status === 429 || input.status === 503) {
    signals.push({ severity: "decisive", reason: `status:${input.status}` });
  }

  if (bytes < 2048 && EMPTY_ROOT_RE.test(body)) {
    signals.push({ severity: "decisive", reason: "empty-spa-root" });
  }

  for (const marker of CHALLENGE_MARKERS) {
    if (body.includes(marker)) {
      signals.push({ severity: "decisive", reason: `challenge:${marker}` });
      break;
    }
  }

  if (TITLE_RE.test(body)) {
    signals.push({ severity: "decisive", reason: "challenge-title" });
  }

  if (textHtmlRatio(body) < 0.05) {
    signals.push({ severity: "strong", reason: "low-text-ratio" });
  }

  if (NOSCRIPT_JS_RE.test(body)) {
    signals.push({ severity: "strong", reason: "noscript-js-required" });
  }

  if (bytes > 10_240 && !HREF_RE.test(body)) {
    signals.push({ severity: "moderate", reason: "no-links" });
  }

  const decisive = signals.find((s) => s.severity === "decisive");
  if (decisive) {
    return { action: "escalate", reason: signals.map((s) => s.reason).join(",") };
  }

  const weak = signals.filter((s) => s.severity === "strong" || s.severity === "moderate");
  if (weak.length >= 2) {
    return { action: "escalate", reason: weak.map((s) => s.reason).join(",") };
  }

  return { action: "proceed" };
}