proto3 has lists of value types: https://protobuf.dev/programming-guides/proto3/#scalar

Inside of them, fixed64 is a special type I’d like to check & test at here.

From its description:

Always eight bytes. More efficient than uint64 if values are often greater than 256.

Though it mentioned can be applied to C++ uint64, it seems also works for int64.

As it mentioned:

fixed32 is compatible with sfixed32, and fixed64 with sfixed64.

Now, here is one question, how will it be look like if we assign int64 to fixed64? To be more specifically, positive int64 will mapped to same value in fixed64, how about negative int64 numbers?

==>

The “ZigZag” encoding instead of two’s complement to encode negative integers. Positive integers n are encoded as 2 * n (the even numbers), while negative integers -n are encoded as 2 * n + 1 (the odd numbers). The encoding thus “zig-zags” between positive and negative numbers.

Using some bit tricks, it’s cheap to convert n into its ZigZag representation:

((n + n) ^ -(n < 0)) - (n < 0)

References

• https://protobuf.dev/programming-guides/proto3/