#!/usr/bin/env python

# Reverse an XTC trajectory
#
# (c)2011 Tsjerk A. Wassenaar
#   University of Groningen
#

from struct import unpack
import sys
import os

def i(x): return sum([ord(x[j])<<(24-j*8) for j in range(4)])

def strseek(stream,s,bufsize=10000):
    v = len(s)
    x = stream.read(bufsize)
    n = 0
    while len(x) >= v:
        m = x.find(s)
        if m > -1:
            # If we found the tag, the position is the total length 
            # read plus the index at which the tag was found 
            n += m
            yield n
            # Now we strip the part up to and including the tag
            x = x[m+v:]    
            n += v
        elif len(x) > v:
            # If the tag was not found, strip the string, keeping only
            # the last v-1 characters, as these could combine with the
            # next part to be read in.
            # n is incremented with the number of characters taken from
            # the current string x (len(x)-v+1)
            n += len(x)-v+1
            x = x[1-v:]
        if len(x) <= v:
            x += stream.read(bufsize)

# Get the tag to identify the start of a frame
f   = open(sys.argv[1])
tag = f.read(8)                     # Tag: magic number and number of atoms
n   = 92 + i(f.read(84)[-4:])       # Size of frame in bytes
f.close()

# Find all positions at which the tag is found and the frame lengths
frames  = [ i for i in strseek(open(sys.argv[1]),tag) ]
nf      = len(frames)
lengths = [ j-i for i,j in zip(frames,frames[1:]+[nf]) ] 

# Reverse the lists
frames.reverse()
lengths.reverse()

# Write the frames in reversed order
f = open(sys.argv[1])
o = open(sys.argv[2],"w")
for pos,bytes in zip(frames,lengths):
    f.seek(pos)
    o.write(f.read(bytes))