If I were faced with this challenge, I would re-use a technique from my days as a C programmer, and read a chunk of N bytes into a buffer from the file. I'd search my way through the buffer, stopping when I went past the half way point.

I'd then shift the buffer and where I'm searching in the buffer down by N/2 bytes, read another N/2 bytes into the now free half of the buffer, and continue until the pattern was found or the file is exhausted.

I can't provide any working code at this time (paying work intervenes) but that should get you going.

--t. alex
but my friends call me T.