// Claude plan limits for the claude-usage agent (issue #50).
//
// One GET to Anthropic's (undocumented) OAuth usage endpoint, with the access
// token Claude Code already stored in ~/.claude/.credentials.json. Returns the
// `limits` block of the POST /claude-usage contract (schemaVersion 1), minus
// the "last good values" fallback, which state.js owns.
//
// Properties that are load-bearing and easy to break:
//
//   * THE CREDENTIALS FILE IS READ, NEVER WRITTEN. Refresh tokens are single
//     use: refreshing here would log Claude Code out. An expired token means
//     `token_expired` and no call at all. The file is only ever passed to
//     fs.readFileSync (no flag), and __tests__/agent-claude-usage.test.js
//     spies on every writing function of node:fs to prove it.
//   * THE BEARER ONLY GOES TO USAGE_URL. The same process also holds a
//     configurable push URL; the OAuth token must never follow it. The URL is
//     a constant (no environment variable changes it) and redirects are
//     refused (`redirect: "error"`), so the token cannot be bounced elsewhere.
//   * `detail` IS AN ENUM. It comes from DETAILS below and nothing else, never
//     from an error message or a response body: both can carry upstream text,
//     and this value ends up in the admin dashboard.
//   * ONE CALL, NO RETRY. The endpoint rate-limits hard and sends no
//     Retry-After; a failed call waits for the next cron beat (15 min).

const fs = require("node:fs");
const os = require("node:os");
const path = require("node:path");

const USAGE_URL = "https://api.anthropic.com/api/oauth/usage";
const ANTHROPIC_BETA = "oauth-2025-04-20";
const LIMITS_TIMEOUT_MS = 10000;
// Far beyond the ~2 KB the endpoint answers. Counted in bytes while reading:
// past it the stream is cancelled, neither read further nor parsed (#54).
const MAX_RESPONSE_BYTES = 256 * 1024;
const MAX_ITEMS = 12;
const MAX_STRING = 128;

const DETAILS = {
  NO_CREDENTIALS: "no_credentials",
  PARSE_ERROR: "parse_error",
  NO_VERSION: "no_version",
  TIMEOUT: "timeout",
  NETWORK: "network",
  BAD_RESPONSE: "bad_response",
};

// Same shape as the server sanitizer's timestamp check: a value outside it
// would be dropped there, so it is normalized (or nulled) here instead.
const ISO_TIMESTAMP_RE = /^\d{4}-\d{2}-\d{2}T\d{2}:\d{2}:\d{2}(\.\d{1,9})?(Z|[+-]\d{2}:\d{2})$/;

function defaultCredentialsPath(home = os.homedir()) {
  return path.join(home, ".claude", ".credentials.json");
}

function isPlainObject(value) {
  return typeof value === "object" && value !== null && !Array.isArray(value);
}

function failure(status, detail) {
  const out = { status, fetchedAt: null, items: [] };
  if (detail) out.detail = detail;
  return out;
}

function cleanString(value) {
  return typeof value === "string" ? value.slice(0, MAX_STRING) : null;
}

// The contract wants an integer 0-100. Upstream sends a number that can be
// fractional (utilization) and could overshoot 100 once a limit is exceeded.
function cleanPercent(value) {
  if (typeof value !== "number" || !Number.isFinite(value)) return undefined;
  return Math.min(100, Math.max(0, Math.round(value)));
}

// ISO string kept as sent when it already has the contract's shape; any other
// parseable string, or an epoch number (seconds or milliseconds), is
// re-rendered with toISOString; anything else is null.
function cleanResetsAt(value) {
  if (typeof value === "string") {
    if (value.length <= MAX_STRING && ISO_TIMESTAMP_RE.test(value) && !Number.isNaN(Date.parse(value))) {
      return value;
    }
    const ms = Date.parse(value);
    return Number.isNaN(ms) ? null : new Date(ms).toISOString();
  }
  if (typeof value === "number" && Number.isFinite(value) && value > 0) {
    const ms = value < 1e11 ? value * 1000 : value;
    const date = new Date(ms);
    return Number.isNaN(date.getTime()) ? null : date.toISOString();
  }
  return null;
}

function item({ kind, group, percent, severity, resetsAt, model }) {
  const out = { kind, group };
  if (percent !== undefined) out.percent = percent;
  if (typeof severity === "string") out.severity = severity;
  out.resetsAt = resetsAt;
  out.model = model;
  return out;
}

// `limits[]` entries: { kind, group, percent, severity, resets_at,
// scope: { model: { display_name } } | null, is_active }. Built field by field,
// so no upstream key reaches the snapshot unless it is listed here.
function itemsFromLimits(list) {
  const items = [];
  for (const entry of list) {
    if (items.length >= MAX_ITEMS) break;
    if (!isPlainObject(entry) || typeof entry.kind !== "string" || !entry.kind) continue;
    const scope = isPlainObject(entry.scope) ? entry.scope : null;
    const model = scope && isPlainObject(scope.model) ? cleanString(scope.model.display_name) : null;
    items.push(
      item({
        kind: entry.kind.slice(0, MAX_STRING),
        group: typeof entry.group === "string" ? entry.group.slice(0, MAX_STRING) : entry.kind.slice(0, MAX_STRING),
        percent: cleanPercent(entry.percent),
        severity: cleanString(entry.severity),
        resetsAt: cleanResetsAt(entry.resets_at),
        model,
      }),
    );
  }
  return items;
}

// Older shape: five_hour / seven_day { utilization, resets_at }.
function itemsFromWindows(body) {
  const items = [];
  const windows = [
    ["five_hour", "session", "session"],
    ["seven_day", "weekly_all", "weekly"],
  ];
  for (const [key, kind, group] of windows) {
    const w = body[key];
    if (!isPlainObject(w)) continue;
    const percent = cleanPercent(w.utilization);
    if (percent === undefined) continue;
    items.push(item({ kind, group, percent, severity: null, resetsAt: cleanResetsAt(w.resets_at), model: null }));
  }
  return items;
}

function parseUsage(body) {
  if (!isPlainObject(body)) return [];
  if (Array.isArray(body.limits)) {
    const items = itemsFromLimits(body.limits);
    if (items.length) return items;
  }
  return itemsFromWindows(body);
}

// Read the OAuth credentials. Never throws, never logs the content.
function readCredentials(credentialsPath, readFile) {
  let raw;
  try {
    raw = readFile(credentialsPath);
  } catch (err) {
    if (err && err.code === "ENOENT") return { detail: DETAILS.NO_CREDENTIALS };
    return { detail: DETAILS.PARSE_ERROR };
  }
  let parsed;
  try {
    // Claude Code rewrites this file in place: a half-written JSON is a normal
    // occurrence, and the next beat will read the finished one.
    parsed = JSON.parse(String(raw));
  } catch {
    return { detail: DETAILS.PARSE_ERROR };
  }
  const oauth = isPlainObject(parsed) && isPlainObject(parsed.claudeAiOauth) ? parsed.claudeAiOauth : null;
  if (!oauth || typeof oauth.accessToken !== "string" || !oauth.accessToken) {
    return { detail: DETAILS.NO_CREDENTIALS };
  }
  return { accessToken: oauth.accessToken, expiresAt: oauth.expiresAt };
}

async function readBody(res) {
  if (!res.body || typeof res.body.getReader !== "function") throw new Error("no body");
  const reader = res.body.getReader();
  const chunks = [];
  let total = 0;
  for (;;) {
    // A timeout surfaces here as a TimeoutError/AbortError, mapped by the caller.
    const { done, value } = await reader.read();
    if (done) break;
    total += value.byteLength;
    if (total > MAX_RESPONSE_BYTES) {
      reader.cancel().catch(() => {});
      throw new Error("too large");
    }
    chunks.push(value);
  }
  return JSON.parse(Buffer.concat(chunks).toString("utf-8"));
}

function discardBody(res) {
  try {
    if (res && res.body && typeof res.body.cancel === "function") res.body.cancel().catch(() => {});
  } catch {
    // nothing to release
  }
}

/**
 * Fetch the plan limits once.
 *
 * @param {object}   opts
 * @param {object}   [opts.env]               process environment (only CLAUDE_USAGE_LIMITS is read)
 * @param {string}   [opts.credentialsPath]   ~/.claude/.credentials.json by default
 * @param {Function} [opts.readFile]          (path) => string, read-only — injectable for tests
 * @param {Function} [opts.fetch]             fetch implementation — injectable for tests
 * @param {string|null} opts.claudeCodeVersion version for the User-Agent (from the transcripts)
 * @param {number}   [opts.now]               epoch ms
 * @param {number}   [opts.timeoutMs]
 * @returns {Promise<{status: string, detail?: string, fetchedAt: string|null, items: object[]}>}
 */
async function fetchLimits({
  env = process.env,
  credentialsPath = defaultCredentialsPath(),
  readFile = (file) => fs.readFileSync(file, "utf-8"),
  fetch: fetchImpl = globalThis.fetch,
  claudeCodeVersion = null,
  now = Date.now(),
  timeoutMs = LIMITS_TIMEOUT_MS,
} = {}) {
  if (String(env.CLAUDE_USAGE_LIMITS || "").trim().toLowerCase() === "off") {
    return failure("disabled");
  }

  const creds = readCredentials(credentialsPath, readFile);
  if (creds.detail) return failure("unavailable", creds.detail);

  // Never refreshed: the refresh token is single use and belongs to Claude
  // Code. A missing expiresAt is left to the server to judge (a 401 then).
  if (typeof creds.expiresAt === "number" && Number.isFinite(creds.expiresAt) && creds.expiresAt <= now) {
    return failure("token_expired");
  }

  if (typeof claudeCodeVersion !== "string" || !claudeCodeVersion) {
    return failure("unavailable", DETAILS.NO_VERSION);
  }

  let res;
  try {
    res = await fetchImpl(USAGE_URL, {
      method: "GET",
      headers: {
        Authorization: `Bearer ${creds.accessToken}`,
        "anthropic-beta": ANTHROPIC_BETA,
        "User-Agent": `claude-code/${claudeCodeVersion}`,
        Accept: "application/json",
      },
      redirect: "error",
      signal: AbortSignal.timeout(timeoutMs),
    });
  } catch (err) {
    const name = err && err.name;
    return failure("unavailable", name === "TimeoutError" || name === "AbortError" ? DETAILS.TIMEOUT : DETAILS.NETWORK);
  }

  const status = res && Number.isInteger(res.status) ? res.status : 0;
  if (status < 200 || status > 299) {
    discardBody(res);
    return failure("http_error", status >= 100 && status <= 599 ? `http_${status}` : DETAILS.BAD_RESPONSE);
  }

  let body;
  try {
    body = await readBody(res);
  } catch (err) {
    const name = err && err.name;
    if (name === "TimeoutError" || name === "AbortError") return failure("unavailable", DETAILS.TIMEOUT);
    return failure("unavailable", DETAILS.BAD_RESPONSE);
  }

  const items = parseUsage(body);
  if (!items.length) return failure("unavailable", DETAILS.BAD_RESPONSE);
  return { status: "ok", fetchedAt: new Date(now).toISOString(), items };
}

module.exports = {
  fetchLimits,
  parseUsage,
  defaultCredentialsPath,
  USAGE_URL,
  ANTHROPIC_BETA,
  LIMITS_TIMEOUT_MS,
  DETAILS,
};