#!/usr/bin/env python
#
# Walk through a pair of textfiles looking for where they begin to differ.
# May be useful for comparing logs when regression tests break.
#
# This file is Copyright (c) 2010 by the GPSD project
# BSD terms apply: see the file COPYING in the distribution root for details.

import sys

class BufferedFile(file):
      def __init__(self, name):
            file.__init__(self, name)
            self.linebuffer = []
            self.lineno = 0
      def readline(self):
            self.lineno += 1
            if self.linebuffer:
                  return self.linebuffer.pop()
            else:
                  return file.readline(self)
      def pushback(self, line):
            self.lineno -= 1
            self.linebuffer.append(line)
      def peek(self):
            return self.linebuffer[-1]

def eatspan(f1, f2):
      consumed = 0
      while True:
            line1 = f1.readline()
            line2 = f2.readline()
            if line1 and line2 and line1 == line2:
                  consumed += 1
                  continue
            f1.pushback(line1)
            f2.pushback(line2)
            return consumed

if __name__ == "__main__":
      f1 = BufferedFile(sys.argv[1])
      f2 = BufferedFile(sys.argv[2])

      eaten = eatspan(f1, f2)
      print "First %d lines match" % eaten
      print `f1.peek()`
      print `f2.peek()`