freq - A CLI for counting occurrences

freq counts the number of non-overlapping substrings in a file or stdin.

It was written when grep -F <PATTERN> | wc -l was found to be too slow for counting strings in multi-GB log files. freq is also typically faster than wc -l for counting the lines in a file.

Depending on the exact inputs used, freq is usually IO-bound. It uses several tricks to increase performance:

• Input is read in a separate thread, and aggressively buffered.
• The bytecount crate is used for single-character patterns.
• The memchr crate (specifically memchr::memmem) is used for processing longer patterns.