what's the easiest way to do file compression and multi-dimensional arrays

Answer 1

List the fields in the data structure you are storing in the file...

Date/Time fields can be stored in 6 bytes: month, day year, hour, minute, sec... one byte each;

the current year is 20, last year was 19, etc.

Account numbers can be compressed to binary coded decimals...

Currency amounts, depending on their size can be stored as an integer and 1 byte for the change amount.