葛立恒数的最后n位

Graham's Number's last digits.js

// https://www.zhihu.com/question/372790433/answer/2873612363
// a 代表基数，m 代表被余数, t初始值表示m中0的个数(即因子5的个数)
function my_tetration_mod (a, t, m) {
  if (m === 2n) return a % 2n
  let phi_m // phi(m) 为欧拉函数，表示小于m且与m互质的自然数个数
  if (t > 0) {
    phi_m = m * 2n / 5n
  } else {
    phi_m = m / 2n
  }
  const pow = my_tetration_mod(a, t - 1, phi_m) + phi_m
  return pow_mod(a, pow, m)
}
const digits = 100
console.log(my_tetration_mod(3n, digits, BigInt(`1${'0'.repeat(digits)}`))) // 9404248265018193851562535796399618993967905496638003222348723967018485186439059104575627262464195387n

// https://gist.github.com/krzkaczor/0bdba0ee9555659ae5fe
// console.log(pow_mod(3n, 587n, 1000n)) // 387
function pow_mod (base, pow, divisor) {
  let result = 1n
  let x = base % divisor
  while (pow > 0n) {
    if (pow % 2n === 1n) {
      result *= x
      result %= divisor
    }
    pow /= 2n
    x *= x
    x %= divisor
  }
  return result
}