string SHA256(string str)
{
    unsigned char hashRaw[32];
    char buf[128] = "";
    SHA256_CTX sha256;
    SHA256_Init(&sha256);
    SHA256_Update(&sha256, str.c_str(), str.size());
    SHA256_Final(hashRaw, &sha256);
    for (int i = 0; i < 32; ++i)
        sprintf((buf + (i << 1)), "%02x", hashRaw[i]);
    return string(buf);
}

int maxLength, cnt;
char tmp[10240];
vector<strHash> s;

void dfs(int depth = 0)
{
    if (depth >= maxLength)
        return;
    for (int i = 1; i < 256; ++i)
    {
        tmp[depth] = i;
        s.push_back(strHash{string(str), SHA256(string(str))});
        dfs(depth + 1);
        tmp[depth] = 0;
    }
}

/**
 * private: 產生未經 Hash 的字串列表
 *
 * @param {int} maxLen 最多要產生幾個字串？
 * @param {int} range 字元的範圍，預設為 256 即 ASCII 字元範圍。
 * @return 字串列表
 */
function _genStrList (maxLen, range = 256) {
  let strings

  if (strings.length <= 0) {
    for (let charCode = 0; charCode != range; charCode++) {
      if (strings.length >= maxLen) return strings
      strings.push(String.fromCharCode(charCode))
    }
  }

  while (true) {
    for (let str of strings) {
      for (let charCode = 0; charCode != range; charCode++) {
        if (strings.length >= maxLen) return strings
        strings.push(str + String.fromCharCode(charCode))
      }
    }
  }
}

/**
 * 產生彩虹表
 *
 * @param {int} len 最多要產生幾個 Hash 字串？預設為 65536 個。
 * @param {int} range 字元的範圍，預設為 256 即 ASCII 字元範圍。
 * @return {map} 彩虹表
 */
function genHashlist (len = 65536, range = 256) {
  console.log("-> 📖 Generating Strings...")
  const strings = _genStrList(len, range)
  console.log("-> \n🆗 Generated Strings.")

  console.log("-> 🖩 Hashing Strings...")
  let hashlist = new Map()
  const crypto = require('crypto')

  for (const rawStr of strings) {
    // 預設的 Hash 方式是 SHA256
    const hash = crypto.createHash('sha256')
    hash.update(rawStr, 'UTF-8')
    const hashVal = hash.digest('hex').toString()
    hashlist.set(hashVal, {
      original: rawStr
    })

    console.log("-> 🖩 Hashing Strings, Now: " + hashlist.keys().length)
  }

  console.log("-> 🆗 Hashed Strings.")
  return hashlist
}