Mimick tac with python.

[Jussi Piitulainen]

Christian Gollwitzer writes:

> Am 30.01.16 um 05:58 schrieb Random832:
>> On Fri, Jan 29, 2016, at 23:46, Hongyi Zhao wrote:
>>> awk '{a[NR]=$0} END {while (NR) print a[NR--]}' input_file
>>> perl -e 'print reverse<>' input_file
>>
>> Well, both of those read the whole file into memory - tac is sometimes
>> smarter than that, but that makes for a more complex program.
>
> Now I'm curious. How is it possible to output the first line as last
> again if not by remembering it from the every beginning? How could tac
> be implemented other than sucking up everything into memory?

It may be possible to map the data into virtual memory so that the
program sees it as an array of bytes. The data is paged in when
accessed. The program just scans the array backwards, looking for
end-of-line characters. I believe they can be identified reliably, as
bytes, even in a backward scan of UTF-8-encoded data.

The data needs to be in a file. The keywords are something like "memory
mapping" and "mmap". I've only experimented with this briefly once in
Julia, so I don't really know more.

Oh. There's https://docs.python.org/3/library/mmap.html in Python.