This program helps to find repeated strings of digits in Pi (or another saved constant).

I used the python program download_pi.py to download lots of digits from the internet.

These need to be unzipped,
for FILE in *.zip; do unzip $FILE; done

Then the program can be compiled and run with
gcc -o pi_repeat.exe -O3 -m64 pi_repeat.c

Run with 
pi_repeat.exe 10
where 10 is the number of digits to be searched for.

Constants K and B and the upper limit 15,000,000,000 in the program may need to be tweaked.
The hash table has 10**K entries.
The linked list holds B entries.
The upper limit says how far to search.
The program reads in sections of the constant and stores into a hash table of linked lists.
When it has used B slots, it switches to scanning the rest of the constant with the current table.
If it finds a match, then it prints the temporary best guess and goes back and rechecks all
shorter sequences to check that this is the first repeat.