#!/bin/bash
set -e

script=$(basename $0)

function usage() {
	cat << EOU
Usage: $script start end expr

  Frob patches.

  This tiny script renames "git format-patch" patches by executing 'expr'
  on the number that prefix the patch file, but only if the patch file name
  starts with a number in ['start','end'].

Examples:
  $ ls *patch
  0008-Super-patch.patch
  0009-Mega-patch.patch
  $ $script 8 9 -7
  $ ls *patch
  0001-Super-patch.patch
  0002-Mega-patch.patch

  $ ls *patch
  0117-Super-patch.patch
  0118-Mega-patch.patch
  $ $script 117 118 +900 -17
  $ ls *patch
  1000-Super-patch.patch
  1001-Mega-patch.patch
EOU
}

[ $# -ge 3 ] || {
	usage
	exit 1
}

start=$1
end=$2
shift 2
op=$*

for i in $(seq $start $end); do
	prefix=$(printf "%04d" $i)
	files="$files $(ls $prefix-*.patch)"
done

for f in $files; do
	prefix=${f:0:4}
	base=${f#$prefix-}

	rank=$((10#$prefix))	# don't interpret the leading 0 as base 8
	((n=$rank $op))

	new_prefix=$(printf "%04d" $n)
	mv $prefix-$base $new_prefix-$base
done