It really depends upon why you are indexing the file?

And how you are going to use that index?

And, just how large are these files? 3 upper case chars == only 17576 unique values.

And why are you going to keep 24 bytes of each line? If that is enough to satisfy your needs for each line, then why do you need the index. If not, you are going to have to seek the file and read the line to get the rest, so why keep any at all?

Your description: " I am only interested in the first 3 characters." and "keeping only the first unique 24 bytes" is confusing.

Examine what is said, not who speaks -- Silence betokens consent -- Love the truth but pardon error.
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In the absence of evidence, opinion is indistinguishable from prejudice.
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In reply to Re: index first 24 bytes of every line in file by BrowserUk
in thread index first 24 bits of every line in file by mhearse

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