Golang怪异big.Int行为，位数很少

Question

I am trying to calculate the GCD of two numbers via the Euclidean method, and noticed weird behavior. Although I am passing the value of the number to the function, the numbers are modified by the function for numbers with low number of bits (usually less than 64). Could anyone clarify if this is a mistake on my behalf or is it expected behavior? You can check the code in the playground here: Golang playground example

I thought the expected behavior is that no values should be changed by passing them to my functions. Thanks for any help, I couldn't find any other occurrences like this in the last day of searching.

Answer 1

big.Int is a struct containing nat, which is defined as type nat []Word so, while you are indeed copying the big.Int, the slice inside it is effectively a shallow copy (underlying storage for slice is same)

For why the bigger bit widths don't exhibit the problem, not sure.

Possibly it requires a reallocation somewhere and that creates a new []Word thereby leaving the original in tact.

This is also probably why the stdlib generally returns *big.Int, since a value isn't really copyable

here's a version on play that properly copies the big.Int values. The key difference being:

a := new(big.Int).Set(x)
b := new(big.Int).Set(y)

(and using pointers in general as values aren't particularly useful)