//
//  Created by Mingliang Chen on 17/8/1.
//  illuspas[a]gmail.com
//  Copyright (c) 2018 Nodemedia. All rights reserved.
//
const Crypto = require("crypto")

const MESSAGE_FORMAT_0 = 0
const MESSAGE_FORMAT_1 = 1
const MESSAGE_FORMAT_2 = 2

const RTMP_SIG_SIZE = 1536
const SHA256DL = 32

const RandomCrud = Buffer.from([
  0xf0, 0xee, 0xc2, 0x4a, 0x80, 0x68, 0xbe, 0xe8,
  0x2e, 0x00, 0xd0, 0xd1, 0x02, 0x9e, 0x7e, 0x57,
  0x6e, 0xec, 0x5d, 0x2d, 0x29, 0x80, 0x6f, 0xab,
  0x93, 0xb8, 0xe6, 0x36, 0xcf, 0xeb, 0x31, 0xae
])

const GenuineFMSConst = "Genuine Adobe Flash Media Server 001"
const GenuineFMSConstCrud = Buffer.concat([Buffer.from(GenuineFMSConst, "utf8"), RandomCrud])

const GenuineFPConst = "Genuine Adobe Flash Player 001"
const GenuineFPConstCrud = Buffer.concat([Buffer.from(GenuineFPConst, "utf8"), RandomCrud])

function calcHmac(data, key) {
  let hmac = Crypto.createHmac("sha256", key)
  hmac.update(data)
  return hmac.digest()
}

function GetClientGenuineConstDigestOffset(buf) {
  let offset = buf[0] + buf[1] + buf[2] + buf[3]
  offset = (offset % 728) + 12
  return offset
}

function GetServerGenuineConstDigestOffset(buf) {
  let offset = buf[0] + buf[1] + buf[2] + buf[3]
  offset = (offset % 728) + 776
  return offset
}

function detectClientMessageFormat(clientsig) {
  let computedSignature, msg, providedSignature, sdl
  sdl = GetServerGenuineConstDigestOffset(clientsig.slice(772, 776))
  msg = Buffer.concat([clientsig.slice(0, sdl), clientsig.slice(sdl + SHA256DL)], 1504)
  computedSignature = calcHmac(msg, GenuineFPConst)
  providedSignature = clientsig.slice(sdl, sdl + SHA256DL)
  if (computedSignature.equals(providedSignature)) {
    return MESSAGE_FORMAT_2
  }
  sdl = GetClientGenuineConstDigestOffset(clientsig.slice(8, 12))
  msg = Buffer.concat([clientsig.slice(0, sdl), clientsig.slice(sdl + SHA256DL)], 1504)
  computedSignature = calcHmac(msg, GenuineFPConst)
  providedSignature = clientsig.slice(sdl, sdl + SHA256DL)
  if (computedSignature.equals(providedSignature)) {
    return MESSAGE_FORMAT_1
  }
  return MESSAGE_FORMAT_0
}

function generateS1(messageFormat) {
  let randomBytes = Crypto.randomBytes(RTMP_SIG_SIZE - 8)
  let handshakeBytes = Buffer.concat([Buffer.from([0, 0, 0, 0, 1, 2, 3, 4]), randomBytes], RTMP_SIG_SIZE)

  let serverDigestOffset
  if (messageFormat === 1) {
    serverDigestOffset = GetClientGenuineConstDigestOffset(handshakeBytes.slice(8, 12))
  } else {
    serverDigestOffset = GetServerGenuineConstDigestOffset(handshakeBytes.slice(772, 776))
  }

  let msg = Buffer.concat([handshakeBytes.slice(0, serverDigestOffset), handshakeBytes.slice(serverDigestOffset + SHA256DL)], RTMP_SIG_SIZE - SHA256DL)
  let hash = calcHmac(msg, GenuineFMSConst)
  hash.copy(handshakeBytes, serverDigestOffset, 0, 32)
  return handshakeBytes
}

function generateS2(messageFormat, clientsig, callback) {
  let randomBytes = Crypto.randomBytes(RTMP_SIG_SIZE - 32)
  let challengeKeyOffset
  if (messageFormat === 1) {
    challengeKeyOffset = GetClientGenuineConstDigestOffset(clientsig.slice(8, 12))
  } else {
    challengeKeyOffset = GetServerGenuineConstDigestOffset(clientsig.slice(772, 776))
  }
  let challengeKey = clientsig.slice(challengeKeyOffset, challengeKeyOffset + 32)
  let hash = calcHmac(challengeKey, GenuineFMSConstCrud)
  let signature = calcHmac(randomBytes, hash)
  let s2Bytes = Buffer.concat([randomBytes, signature], RTMP_SIG_SIZE)
  return s2Bytes
}

function generateS0S1S2(clientsig) {
  let clientType = Buffer.alloc(1, 3)
  let messageFormat = detectClientMessageFormat(clientsig)
  let allBytes
  if (messageFormat === MESSAGE_FORMAT_0) {
    //    Logger.debug("[rtmp handshake] using simple handshake.")
    allBytes = Buffer.concat([clientType, clientsig, clientsig])
  } else {
    //    Logger.debug("[rtmp handshake] using complex handshake.")
    allBytes = Buffer.concat([clientType, generateS1(messageFormat), generateS2(messageFormat, clientsig)])
  }
  return allBytes
}

module.exports = { generateS0S1S2 }