Add Size method on cache

[nlachfr]

Closes #40

[Yiling-J]

@nlachfr qlen is the window size of each shard(window is a very small buffer used in W-TinyLFU, you can read the paper for more detail), not the map size. Also don't forget to add unit test in cache_test.go, you can create a new test case or update TestCost.

[nlachfr]

My bad, I only had a quick look at internals, I thought it was the overall shard cost since it is updated during insertion
Is there an overall state somewhere which would avoid iterating over every entries ?

[Yiling-J]

@nlachfr The good news is yes, but there are a few things to consider. Firstly, it's not 100% accurate. Secondly, you may need some knowledge of TinyLFU. Theine has two parts: a shared map for storing data, and a policy for determining what to keep and remove when the cache is full.
The policy part is the tlfu.go file, you will find a TinyLFU struct includes an SLRU field. SLRU has two linked lists, each with a 'len' field that represents the cost sum of items in the list. However, it's not 100% accurate due to two reasons:

1. There are several window buffers (qlen).
2. The data map communicates with the policy using a buffer, meaning an item might exist in both the map and the buffer, but not in the policy.

[nlachfr]

OK, in that case, what do you think I should try to implement ? A function that loops over every item or one that return an approximate of the current state of the cache ? Or both ?

For a bit more context, I need this info in order to provide some metrics on cache usage. I have added wrappers around Set / Delete operations for collecting this, but it would be must simpler to have it available directly. In this scenario, having an approximate of size usage would be reasonable.

[Yiling-J]

@nlachfr I think approximate is good enough, but the method name should show that clearly, something like EstimatedSize. And to get that value, you can sum the qlen in all shards and two linked list len. Also add a race test: main goroutine randomly read/write cache and goroutine 2 call your new method periodically.

[nlachfr]

@Yiling-J So, this MR adds two methods on Cache and LoadingCache:

• Size() int which returns the size of the cache (iterates over every entry)
• EstimatedSize() int which returns an approximate of the cache size (sum of qlen + list len)

As you guessed, the EstimatedSize one cause some race issues, so I just changed the len int field of List[K, V] to an atomic.Int64

[Yiling-J]

Looks fine to me. Could you update the README to include the new APIs?

[Yiling-J]

@nlachfr Looks good, Thanks for the contribution🙏