difftac

#!/bin/sh
# difftac (part of ossobv/vcutil) // wdoekes/2014,2017 // Public Domain
#
# Takes a unified diff and reverses it.
#
# Usage:
#
#     difftac some/patch.diff > some/reversedpatch.diff
#
# Bugs:
# - only works on patches that operate on a single file
# - does not cope with leading "garbage"/comments before the patch
#
# Original author: Lie Ryan (2010)
# Source: http://stackoverflow.com/questions/3902388/
#                permanently-reversing-a-patch-file/3902431#3902431
# Slightly improved by wdoekes.
#
swap() {
    sed -e "s/^$1/PPP/;s/^$2/$1/;s/^PPP/$2/"
}
file_header() {
    head -2 "$1" | tac | swap +++ ---
}
fix_chunk_header() {
    sed -e 's/@@ -\([0-9]\+,[0-9]\+\) +\([0-9]\+,[0-9]\+\) @@/@@ -\2 +\1 @@/' 
}
fix_lines() {
    swap + -
}

# Take filename or file from stdin
file="$1"
if test -z "$file"; then
    file="`mktemp`"
    cat > "$file"
fi

# Check whether our bugs might affect it
if tail -n +2 "$file" | grep -q '^--- '; then
    echo >&2
    echo '*** Leading data or multiple files found. ***' >&2
    echo '*** Diff may become corrupt! ***' >&2
    echo >&2
fi

file_header "$file"
tail "$file" -n +3 | fix_chunk_header | fix_lines

# If we used stdin, we remove the tempfile
if test -z "$1"; then
    rm "$file"
fi