[BUG] The checkedAdd method will not return maxInt64 in case of overflow

[tangwenqiang]

Description

The checkedAdd will return (maxInt64 - 2) instead of maxInt64 as the comment saying.

(A possible fix could be: ((naiveSum >> 63) ^ 1) => int64((uint64(naiveSum)>>63)^1))

To Reproduce

Steps to reproduce the behavior:
run this test:

func TestCheckedAdd(t *testing.T) {
	var a, b int64
	a = math.MaxInt64
	b = 23
	
	fmt.Println(checkedAdd(a, b))  // output is: 9223372036854775805 
}

Expected behavior

return maxInt64 when overflow.

[maypok86]

Hi, yeah, it looks like I made a mistake in two places at once.

It really should have been an unsigned shift. I tried to test the correctness of this function using Ratio but there was an incident with float64 rounding.

hits := math.MaxInt64
requests := math.MaxInt64 - 2
result := float64(hits)/float64(requests)
fmt.Println(result)

Here the result will be exactly 1.0 :(

I'll probably add a fix to the v2.0 release because users are unlikely to be able to notice this bug.