Mapping long bit arrays to a lookup table

Given a set of n (n < 1000) unique bit arrays (for example c++ std::bitset class template) of length l (l > 64), which is the simplest mapping to store such bit arrays in a lookup table L with elements from 0 to n-1?

What I want to achieve is the following:

L["000010001 ... 00101010"] = 0
L["111000000 ... 01000100"] = 1
...
L["001101100 ... 01010111"] = n-1

The bit arrays, if converted to decimals, are not ordered.

I am currently using std::unordered_map<std::bitset<81>, int> and std::unordered_map::find but I have a feeling that there is a faster way to do it.