#!/usr/bin/env python

"""
safe_write.py - HTTP cache-friendly file writing

safe_write allows you to use a process to generate a static file (usually, run 
by cron or at), while only changing the file on disk if the data has changed.
This means that the file modification time changes only when the data changes;
because this is what most Web servers use to generate the Last-Modified HTTP
response header, this will result in better caching.

safe_write can be used as a function in your program, or the library can be
used as a command-line filter;

% generate_some_data | safe_write.py targetfile.html
"""

__license__ = """
Copyright (c) 2004 Mark Nottingham <mnot@pobox.com>

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
"""

__version__ = '0.61'

import sys, sha


def safe_write(file, data):
	"""
	Write the data to the file only if it is different. This makes for 
	better caching, since it won't change the Last-Modified date unless 
	necessary.
	"""
	
	try:
		fh = open(file, 'r')
		cur_data = sha.new(fh.read()).digest()
		fh.close()
	except IOError:
		cur_data = None
	if cur_data != sha.new(data).digest():
		fh = open(file, 'w')
		fh.write(data)
		fh.close()


def usage(msg):
    sys.stderr.write("""\
%s
USAGE: %s filename
       where filename is the target filename for writing. 
       The data stream is read from STDIN.
""" % (msg, sys.argv[0]))


if __name__ == '__main__':
    try:
        safe_write(sys.argv[1], sys.stdin.read())
    except Exception, msg:
        usage(msg)