const crypto = require('crypto')

function base64UrlDecode (input) {
  const normalized = String(input).replace(/-/g, '+').replace(/_/g, '/')
  const padded = normalized + '='.repeat((4 - (normalized.length % 4)) % 4)
  return Buffer.from(padded, 'base64')
}

/**
 * Verifies + decodes a Meta `signed_request` (used by the Instagram deauthorize
 * and data-deletion callbacks). Returns the decoded payload, or null if the
 * signature doesn't match / the value is malformed — callers must treat null
 * as "reject the request", never fall back to trusting the unverified payload.
 */
function parseSignedRequest (signedRequest, appSecret) {
  if (!signedRequest || typeof signedRequest !== 'string' || !appSecret) return null

  const parts = signedRequest.split('.')
  if (parts.length !== 2) return null
  const [encodedSig, encodedPayload] = parts

  let providedSig
  try {
    providedSig = base64UrlDecode(encodedSig)
  } catch {
    return null
  }

  const expectedSig = crypto.createHmac('sha256', appSecret).update(encodedPayload).digest()
  if (providedSig.length !== expectedSig.length || !crypto.timingSafeEqual(providedSig, expectedSig)) {
    return null
  }

  try {
    const data = JSON.parse(base64UrlDecode(encodedPayload).toString('utf8'))
    if (data.algorithm !== 'HMAC-SHA256') return null
    return data
  } catch {
    return null
  }
}

module.exports = { parseSignedRequest }