Solved Efficient File Reading

Consider this:
I have a considerably large file, composed of more than 1000 lines separated by carriage returns.
There are a few lines which have certain keywords, for the purposes of this thread I will refer to it as "key".

I want to retrieve the last line that contains "key". I am currently using a BufferedReader to go line by line until the end of the file, which is has a dismal efficiency of around O(N), ignoring string contains() procedures.

Trying to retrieve the last line 50 times is already taking an extensive toll on the server, around 5-10 seconds of utter lag.

My question is: could I read the file with the big O as close to O(1) as possible?

 

Do it 1 time.

 

@teej107
Excuse me? I need to search it an unknown number of times, for different keys each time.

 

@ShowbizLocket61 ah so it's at different parts of the file? Do it at the beginning in the onEnable.

 

@teej107
What do you mean? The file updates at erratic times.

And who wants to lag out the server on enable?

The point of this thread is to make the searching more efficient, not to avoid it altogether.

 

@timtower
The file has a fixed location, and I have no control over it whatsoever. Only reading.

 

@timtower
CSV

 

If I wanted to make this more efficient, I would tell you to use a different formatting for the file.
"Lagging" out the server in the onEnable is the best time to do so because rarely are players playing when you restart or reload plugins. Whenever you update the file, update the values you have saved in the code.

 

@teej107
The file is not mine; I have no control over it; I can only read it. I also have no way of telling when it updates.

 

Who's file is it then?

 

@teej107
'nother plugin's.

 

@timtower
No, I can't.
Could we stay on topic, please?

The point of this thread is to find a way to make the searching more efficient, not to avoid it.

 

If you know, when the file updates (I mean, you pretty much can gues, aren't you?), then only do the file reading then, or as suggested above, use an Async task

 

Have you tried hooking into the plugin?

 

@ShowbizLocket61 Apache Commons has a class, ReversedLinesFileReader. It is very similar to a BufferedReader, but it starts with the last line rather than the first. You can get the last line using this class.

And it's packaged with Bukkit!

EDIT: Oh you want the last line containing 'key'. Nevermind then.

 

@ShowbizLocket61

Why is O(n) dismal? And you made the same mistake again. String searching is still O(n*m), regardless, this isn't "dismal," especially with such a small file.

Why wouldn't you try to avoid the operation instead of just making it "more efficient?" That seems the most practical method for achieving your flawed idea of "efficient and space saving as possible" or whatever that was.

There's no reason you can't ask the other author to add an API, or add it yourself by recompiling it or adding introspection.

And how do you propose that being possible? You still have to read over the entire file, and that is by necessity O(n), there's no way to avoid that. That being said, there's no reason this should be taking 5-10 seconds, it should be taking on that order of time. Post your code, there has to be a pretty egregious error in it for that to be the case.

How certain are you of this? Because that's not true. In the least.

https://docs.oracle.com/javase/tutorial/essential/io/notification.html

 

@Mrs. bwfctower
Thanks for that! It'll be useful since the target line would almost always be closer to the end than the beginning.

@mythbusterma
I said, O(n) disregarding string operations. I don't want to try to avoid the operation, because that operation is the only way to do it. I was asking for you all to answer the question in the context and factors provided by the question itself.

Anyhow, I posted this thread for help and ideas, not a heated argument like so many of the threads on this forum have became. If Mrs. bwfctower had not suggested the reversed reader and if mythbusterma had not put out that Watch Service link, I would have definitely regretted posting.

Even though my problem is not exactly solved, I will mark it as so because I am just tired of these thinly veiled attacks popping up around the forums.

 

@ShowbizLocket61

I'm not attacking you, I'm pointing out flaws in your logic. At not point did insult you, your abilities, or your code.

 

I know that feeling, I can hardly give good-habit advice without some noob coming along and trying to say it's wrong.