code: func Log(x int64, base int) int {
  	assert2(base > 1, "base must be > 1")
  	ret := 0
  	for x >= int64(base) {
  		x /= int64(base)
  		ret++
  	}
  	return ret
  }
docs: // util/MathUtil.java
  /* Returns x <= 0 ? 0 : floor(log(x) ? log(base)) */