[milkshakes]

O(1) means constant time. Redis sets are interesting, because there are a few possible implementations under the hood, but the typical case winds up implementing it as a hash table. Hash tables have constant time lookup.

Think of it this way (this isn't literally what happens, but it's close).

1) Take the url you're looking for. Run it through a hash function. This takes an (amortized) constant amount of time.

2) Now you have an index to check. (the return value from the hash function). So index into your actual table, and check to see what's stored there. If there's a value stored there, then the url is a member of the set. This also takes a constant amount of time.

Does this help?

[genwin]

> So index into your actual table, and check to see what's stored there

But that check isn't a constant time lookup. The lookup time can vary. (Analogously, a lookup in a phone book can vary in time; we can't necessarily go to the exact spot the first time.) So the total time for both steps must vary as well. I think.

[milkshakes]

I think where you're getting confused is that you're conceptualizing this like a search problem, where you compare values and inspect each member to see if it matches the target.

That's not what's going on here. Instead, you use the value as in input into a function that tells you where to look for it, then you look, to see if it's there.

If it's not there, it won't be anywhere else, so you don't have to keep looking. Things get interesting with collisions but that's a subject for another time.

[genwin]

OK, thanks, I'm starting to understand. Good explanation. I quit being lazy and searched around too, to see that it's possible.