// Pure URL construction. No browser APIs, so node:test imports this directly.
//
// Every ChatGPT parameter below is undocumented by OpenAI and has broken before.
// This module's contract is only that it emits a well-formed URL; whether
// ChatGPT honours model / temporary-chat / hints is out of our control, which is
// why each is independently omittable and every label is hint-strength.
//
// The root path is mandatory: chatgpt.com/search began returning 404 in
// mid-2025, and the root form is what OpenAI's own extension uses.

const BASE = "https://chatgpt.com/";

export const MAX_URL_CHARS = 8000;

// Fraction of the kept text within which a whitespace cut is preferred.
const WHITESPACE_WINDOW = 0.9;

function applyTemplate(template, query) {
  if (query === "") {
    return "";
  }
  const tpl = typeof template === "string" ? template : "";
  if (tpl.includes("{query}")) {
    return tpl.replaceAll("{query}", query);
  }
  const trimmed = tpl.trim();
  return trimmed === "" ? query : `${trimmed} ${query}`;
}

function addOptionalParams(url, settings) {
  const model = typeof settings.model === "string" ? settings.model.trim() : "";
  if (model !== "") {
    url.searchParams.set("model", model);
  }
  if (settings.temporaryChat) {
    url.searchParams.set("temporary-chat", "true");
  }
  if (settings.webSearch) {
    url.searchParams.set("hints", "search");
  }
  return url;
}

function lastWhitespaceIndex(text) {
  for (let i = text.length - 1; i >= 0; i -= 1) {
    if (/\s/.test(text[i])) {
      return i;
    }
  }
  return -1;
}

// Returns the longest prefix of `prompt` whose finished URL fits the budget.
// Slicing happens by CODE POINT on the decoded string, so a surrogate pair can
// never be split and a percent-escape can never be severed (encoding happens
// afterwards, via URLSearchParams).
function fitToBudget(prompt, settings) {
  const codePoints = Array.from(prompt);

  const urlLengthFor = (count) => {
    const url = new URL(BASE);
    url.searchParams.set("q", codePoints.slice(0, count).join(""));
    addOptionalParams(url, settings);
    return url.href.length;
  };

  if (urlLengthFor(codePoints.length) <= MAX_URL_CHARS) {
    return prompt;
  }

  let low = 0;
  let high = codePoints.length;
  while (low < high) {
    const mid = Math.floor((low + high + 1) / 2);
    if (urlLengthFor(mid) <= MAX_URL_CHARS) {
      low = mid;
    } else {
      high = mid - 1;
    }
  }

  const kept = codePoints.slice(0, low).join("");
  const boundary = Math.floor(kept.length * WHITESPACE_WINDOW);
  const cut = lastWhitespaceIndex(kept);
  return cut >= boundary && cut > 0 ? kept.slice(0, cut) : kept;
}

export function buildChatGptUrl(settings, rawQuery) {
  const query = String(rawQuery ?? "").trim();
  const prompt = fitToBudget(applyTemplate(settings.promptTemplate, query), settings);

  // q first, matching the most-cited working examples. Total serialized length
  // is order-independent, so measuring without q above is sound.
  const url = new URL(BASE);
  if (prompt !== "") {
    url.searchParams.set("q", prompt);
  }
  addOptionalParams(url, settings);
  return url.href;
}