import struct
import sys
import os
import random
from pathlib import Path

def index_binpack(file):
    print('Indexing...')
    index = []
    offset = 0
    report_every = 100
    prev_mib = -report_every
    while file.peek():
        chunk_header = file.read(8)
        assert chunk_header[0:4] == b'BINP'
        size = struct.unpack('<I', chunk_header[4:])[0]
        file.seek(size, os.SEEK_CUR)
        index.append((offset, size + 8))
        offset += size + 8

        mib = offset // 1024 // 1024
        if mib // 100 != prev_mib // 100:
            print('Indexed {} MiB'.format(mib))
            prev_mib = mib

    return index

def copy_binpack_indexed(in_file, index, out_file):
    print('Copying...')
    total_size = 0
    report_every = 100
    prev_mib = -report_every
    for offset, size in index:
        in_file.seek(offset, os.SEEK_SET)
        data = in_file.read(size)
        assert len(data) == size
        out_file.write(data)

        total_size += size
        mib = total_size // 1024 // 1024
        if mib // 100 != prev_mib // 100:
            print('Copied {} MiB'.format(mib))
            prev_mib = mib

def main():
    if len(sys.argv) < 3:
        print('Usage: python shuffle_binpack.py infile outfile')
        return

    in_filename = sys.argv[1]
    out_filename = sys.argv[2]

    if (Path(out_filename).exists()):
        print('Output path already exists. Please specify a path to a file that does not exist.')
        return

    in_file = open(in_filename, 'rb')
    out_file = open(out_filename, 'wb')

    index = index_binpack(in_file)
    print('Shuffling...')
    random.shuffle(index)

    copy_binpack_indexed(in_file, index, out_file)

    in_file.close()
    out_file.close()

main()