/**
 * FRED (Federal Reserve Economic Data) CSV fetcher via curl.exe
 *
 * Why curl.exe instead of axios:
 *   Node.js axios → FRED's Akamai CDN times out on SP500, DGS2, DGS10 series.
 *   curl.exe → same URL returns data in ~1s.
 *
 * Why date range params (cosd/coed):
 *   Full DGS2/DGS10 datasets (daily from 1962) are huge → Akamai times out.
 *   Limiting to recent years solves this. All calculations only need ≤3 years.
 *
 * Root cause: Akamai CDN differentiates by TLS fingerprint AND URL pattern.
 */

const { execFile } = require('child_process')

const FRED_BASE = 'https://fred.stlouisfed.org/graph/fredgraph.csv'

/**
 * Fetch FRED CSV data using curl.exe with a date range limit
 * @param {string} seriesId - FRED series ID (e.g. 'SP500', 'FEDFUNDS')
 * @param {object} [opts]
 * @param {number} [opts.timeout=20] - curl --max-time seconds
 * @param {number} [opts.lookbackYears=3] - years to look back from today
 * @param {string} [opts.coed] - custom observation end date YYYY-MM-DD (default today)
 * @param {string} [opts.cosd] - custom observation start date YYYY-MM-DD (default coed - lookbackYears)
 * @returns {Promise<string>} raw CSV text
 */
const fetchFredCurl = (seriesId, opts = {}) => {
  const timeout = opts.timeout || 20
  const lookbackYears = opts.lookbackYears || 3
  const now = new Date()
  const coed = opts.coed || now.toISOString().slice(0, 10)
  const cosd = opts.cosd || (new Date(now.getTime() - lookbackYears * 365 * 24 * 60 * 60 * 1000)).toISOString().slice(0, 10)
  const url = `${FRED_BASE}?id=${encodeURIComponent(seriesId)}&cosd=${cosd}&coed=${coed}`

  return new Promise((resolve, reject) => {
    execFile('curl.exe', ['-s', '--max-time', String(timeout), url], {
      encoding: 'utf8',
      timeout: (timeout + 10) * 1000,
      maxBuffer: 10 * 1024 * 1024,
      windowsHide: true
    }, (err, stdout, stderr) => {
      if (err) {
        const msg = stderr ? stderr.trim() : err.message
        return reject(new Error(`curl failed for ${seriesId}: ${msg}`))
      }
      resolve(stdout)
    })
  })
}

module.exports = { fetchFredCurl }