Diffcount: Count bit and byte differences between files

Diffcount is an application for counting the bit and byte differences between two files, or between one file and a constant value.

Features

• Arbitrary offsets can be set for each input file.
• A maximum compare length can be specified to limit the amount of compared data.
• Designed to be reasonably fast with large files.

Installation

Diffcount relies on popcnt intrinsic functions, using the POPCNT instruction that was introduced at about the same time as SSE4. You should signal to gcc that your processor supports the POPCNT instruction, either with an appropriate -march= option, for example:

gcc -march=broadwell -O3 -o diffcount diffcount.c

or, more generically, with -mpopcnt:

gcc -mpopcnt -O3 -o diffcount diffcount.c

Compiling with optimizations is highly encouraged, as they provide significant performance improvements.

Usage

The user runs:

diffcount [-ch] [-n len] file1 file2/const [seek1 [seek2]]

with the command line arguments:

• -c: compare file to constant byte value
• -h: print help
• -n: specify a maximum number of bytes to compare
• seek1: offset for file1
• seek2: offset for file2

In constant mode, a constant byte value should be specified in place of file2. In constant mode, specifying seek2 has no effect.