A fine question. I'm uncertain what assumptions I can make about the data files, as I don't control the code which generates them. Each individual data file looks like:

# time              data
0.000000          99.537
1.000000         100.273
2.000000          98.169
3.000000         105.835
4.000000          93.013
5.000000          96.145
6.000000          87.040
7.000000          97.764
8.000000          97.811
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I have to join the data files based on the time point. I can probably assume that the time points will be ordered and the same in each file, and also a fixed column width that, worst case, I can calculate per file. I cannot assume the timepoints will always be integers. I was being conservative when I wrote this, but now it seems to be my bottleneck. I am sending an email to the other developer asking what guarantees we can work out about it.