How Can I Implement Custom Key Equality for Go Maps?

Custom Key Equality for Go Maps

When creating a map in Go, you can use any type as a key as long as it implements the comparable interface. By default, Go uses the builtin equality operator (==) for comparing keys. However, there may be instances where you want to define your own equality criteria.

Example

Consider the following custom key type:

type Key struct {
    a *int
}

To compare two instances of Key, you want to use your own Equal function:

func Equal(x Key, y Key) bool {
    return *x.a == *y.a
}

Workaround

Unfortunately, Go doesn't allow you to specify custom hashing or equality functions for map keys. Instead, you can use the following workaround:

1. Derive a Key Attribute: Instead of using the Key struct directly as a key, derive an attribute (e.g., integer or string) that serves as the identity for the key. Ensure that collisions only occur for semantically identical keys.
2. Implement Hash Function: Create a method on Key that calculates the derived attribute and returns it as an integer (for hashing).

func (k *Key) HashKey() int {
    return *(*k).a
}

Example Usage

Using the example above, the map would be:

k1, k2 := Key{intPtr(1)}, Key{intPtr(2)}
m := map[int]string{}
m[k1.HashKey()] = "one"
m[k2.HashKey()] = "two"

Precautions

Remember that this approach relies on the immutability of the derived key attribute. If any fields in the Key struct change, the identity of the key changes and it will not work as expected.

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