#! /usr/bin/env python

# This script is for taking a directory full of music files that start with a
# track number and changing the number of each file by the specified positive
# or negative amount. Useful for taking mp3 files from a 2-disc set and
# putting them in order into a single directory.
#
# Distributed under the terms of the Gnu General Public License. See
# www.gnu.org for more details.


import sys, os, re

#------------------------------------------------------------------------------#
def init():
    global num, action

    if (len(sys.argv) < 2) or (sys.argv[1] in ('-h', '--help')):
        show_usage()

    # get the num and figure out if we're adding or subtracting
    num = sys.argv[1]
    action_re = re.compile("^(\+|-)+(\d+)$")
    m = action_re.match(num)
    if m is None:
        sys.exit("n must be a number, preceded with + or -")
    action = ("+", "-")[m.group(1) == "-"]
    num = int(m.group(2))

    if len(sys.argv) > 2:
        dir = sys.argv[2]
    else:
        dir = '.'

    if not os.path.isdir(dir):
        sys.exit(dir + " is not a directory. shame on you.")

    if dir != '.':
        os.chdir(dir)
#------------------------------------------------------------------------------#
def main():
    # loop through the files and rename them if appropriate
    files = os.listdir('.')

    file_re = re.compile("^(\d{1,})(.*)")

    for file in files:
        if not os.path.isfile(file):
            continue
        m = file_re.match(file)
        if m is None:
            continue

        tracknum = int(m.group(1))
        if action is "+":
            tracknum = tracknum + num
        elif action is "-":
            tracknum = tracknum - num

        # keep them from going to negative track numbers
        if tracknum < 0:
            sys.exit("this would yield negative track numbers. quitting.")

        tracknum = repr(tracknum)
        if len(tracknum) == 1:
            tracknum = "0" + tracknum

        newfile = tracknum + m.group(2)
        os.rename(file, newfile)
        print "renamed " + file + " to " + newfile
#------------------------------------------------------------------------------#
def show_usage():
    usage = "usage: tracknum (+|-)n [directory]"
    sys.exit(usage)
#------------------------------------------------------------------------------#
init()
main()