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The following piece of code while it looks innocent actually does not compile
return int64(0.5 * float64((1<<n)-1))
It produces the following error
exp.go:22:28: invalid operation: 1 << n (shift of type float64)
What is happening here?
This is because when you do float64(1<<n) the first instinct of golang’s compiler is to convert the 1 into float64. So its equivalent to float64(1) << n which obviously would not work because you can’t shift floats.
The solution is to rewrite it into two statements
pow2minus1 := (1<<n) - 1
return int64(0.5 * float64(pow2minus1)