How Can I Efficiently Check if a big.Int is 0 or 1 in Go?

Testing for 0 in a big.Int

When working with big.Ints, there are times when you need to test for a value of 0. The standard method for this is to compare the big.Int with zero using Cmp(zero) == 0. However, this can be a slow operation, especially if you are looking for a quick and efficient way to test for 0.

Fortunately, there are faster methods available. One way is to use the Int.Bits() function, which returns a slice of bytes representing the raw bytes of the big.Int's representation. In the case of 0, the slice will be empty. By checking the length of the slice, you can quickly determine if the big.Int is 0.

import "math/big"

func isZero(i *big.Int) bool {
    return len(i.Bits()) == 0
}

Alternatively, you can use the Int.BitLen() function, which returns the bit length of the big.Int. For 0, the bit length will be 0.

import "math/big"

func isZero(i *big.Int) bool {
    return i.BitLen() == 0
}

These methods are significantly faster than comparing the big.Int with zero using Cmp(). Benchmarks show that they are over 20 times faster in the case of testing for 0 and 10 times faster for testing for 1.

Testing for 1 in a big.Int

Using a similar approach, you can also create a function to test for a value of 1 in a big.Int. This function would check if the slice returned by Int.Bits() has a length of 1 and the only element is 1. Additionally, it would check that the sign of the big.Int is positive.

import "math/big"

func isOne(i *big.Int) bool {
    bits := i.Bits()
    return len(bits) == 1 && bits[0] == 1 && i.Sign() > 0
}

This method is also significantly faster than comparing the big.Int with a big.Int representing 1 using Cmp(). Benchmarks show that it is over 10 times faster.

By utilizing these faster methods for testing if a big.Int is 0 or 1, you can significantly improve the performance of your code.

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