Convert endianness for ACE format cross sections

convert_ace_endian.py

#!/usr/bin/python

import sys
from struct import pack, unpack

# Get name of original ACE file
filename = sys.argv[1]

# Open unconverted and converted files
aceOld = open(filename, 'rb')
aceNew = open(filename + '_conv', 'wb')

endianOld = '<'    # little endian
endianNew = '>'    # big endian
recl_length = 4096 # Record length in bytes
entries = 512      # Number of entries per record 

while True:
    # Check for EOF
    start = aceOld.tell()
    if aceOld.read(1) == '':
        break
    aceOld.seek(start)

    # Header block
    headerSpec = '10sdd10s70s10s'
    header = unpack(endianOld + headerSpec, aceOld.read(116))
    aceNew.write(pack(endianNew + headerSpec, *header))
    print(header[0])

    # ZAID combinations
    zaid = unpack(endianOld + 16*'id', aceOld.read(192))
    aceNew.write(pack(endianNew + 16*'id', *zaid))

    # NXS array
    nxs = unpack(endianOld + '16i', aceOld.read(64))
    aceNew.write(pack(endianNew + '16i', *nxs))

    # JXS array
    jxs = unpack(endianOld + '32i', aceOld.read(128))
    aceNew.write(pack(endianNew + '32i', *jxs))

    # Copy bytes between end of JXS and start of XSS on next record
    n_bytes = start + recl_length - aceOld.tell()
    data = aceOld.read(n_bytes)
    aceNew.write(data)

    # XSS array
    length = nxs[0]
    xss = unpack(endianOld + '{0}d'.format(length), aceOld.read(length*8))
    aceNew.write(pack(endianNew + '{0}d'.format(length), *xss))

    # Copy remaining bytes between end of XSS and beginning of next nuclide
    n_records = (length + entries - 1)/entries
    n_bytes = start + recl_length*(n_records + 1) - aceOld.tell()
    data = aceOld.read(n_bytes)
    aceNew.write(data)