Test for an integer overflow

[JohnGB]

Firstly, I realise that the spec allows integer overflows, and I'm not suggesting that should change.  

However I need to check to see if an overflow has occurred in some code, but I'm not sure how to do this.

So given a (simplified) function signature of  func Add(x, y int64) int64 , what would be the best way to check whether an overflow has occurred?

[JohnGB]

Typo by me.  I meant a function signature of:

func Multiply(x, y int64) int64

[James Wendel]

Does this answer it?
http://stackoverflow.com/questions/199333/best-way-to-detect-integer-overflow-in-c-c

[Rob Pike]

There's likely better code than this, but I've tested this version
thoroughly and it's straightfoward.

func mulOverflows(a, b uint64) bool {
if a <= 1 || b <= 1 {
return false
}
c := a * b
return c/b != a
}

const mostNegative = -(mostPositive + 1)
const mostPositive = 1<<63 - 1

func signedMulOverflows(a, b int64) bool {
if a == 0 || b == 0 || a == 1 || b == 1 {
return false
}
if a == mostNegative || b == mostNegative {
return true
}
c := a * b
return c/b != a
}

[John Beckett]

@Rob: That's an elegant way of solving the problem, thanks.

@James: Being lazy, I understood Rob's code better than the StackOverflow answer, so I'll go with that.  But thanks for the suggestion.

}

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