Bonus: The Last Batch Is Smaller

Tools: GCC, Make

Goal

Make the encryptor from 03-file-encryptor accept any number of files: 57 files in batches of 10 are five full batches and one of 7, and six threads. Afterwards you will be able to split n items into batches of k when k does not divide n, which is the usual case.

Background

03-file-encryptor refused a file count that was not a multiple of the batch size. The honest version rounds the number of threads up, and gives the last thread what is left. Two things change, and nothing else: how many threads there are, and how many files the last batch has.

The shipped encryptor.c is the skeleton of 03-file-encryptor with two TODOs added. Start from your finished encryptor.c from that exercise, and copy the two new TODO comments into it, or paste your four finished pieces into the shipped file; either way, the first four TODOs are the ones you already did.

Your Task

1. TODO 5 — the number of threads: nfiles / batch_size, rounded up, in integer arithmetic. Think about what happens with 57 and 10, with 60 and 10, and with 3 and 10.
2. TODO 6 — the size of each batch: batch_size for all but the last, and for the last, whatever is left. Write it so that it is right for every batch, not as a special case for the last index.

Build & Run

make
./gen_files.sh
./encryptor secret 10 data/*.bin

gen_files.sh here makes 57 files by default; ./gen_files.sh 64 makes 64, for the case the core exercise handled.

Check Your Work

• Six threads, and the last one reports 7 files, file-51.bin .. file-57.bin.
• The same two checks as the core exercise: after one pass every file differs from its copy in data.orig/, after two passes diff -r data data.orig is silent. The file to watch is file-57.bin.
• Batch sizes 1, 7, 19, 56, 57 and 100 on the 57 files. Predict the number of threads and the size of the last batch for each before running.
• With ./gen_files.sh 64 and a batch size of 8, the program must behave exactly as the core exercise did: eight full batches, no extra thread.