lcw: Estimate the number of lines in a file.
==============================================
lcw is like ``wc -l`` but faster, less precise, and equally accurate. ::

    usage: lcw [-h] [--sample-size N] [--page-size PAGE_SIZE] [--just-ml] file

    Estimate how many lines are in a file.

    positional arguments:
      file

    optional arguments:
      -h, --help            show this help message and exit
      --sample-size N, -n N
                            How many pages to count (default: 1000)
      --page-size PAGE_SIZE, -p PAGE_SIZE
                            Size of an observation (default: 16384)
      --just-ml, -j         Only print the maximum likelihood estimate (default:
                            False)

Speed
--------
It's faster than ``wc -l`` on big files. ::

    $ wc -c big-file.csv
     1071895374 big-file.csv

    $ time lcw big-file.csv
    2386238 ± 22903 lines (99% confidence)

    real    0m0.172s
    user    0m0.140s
    sys     0m0.027s

    $ time wc -l big-file.csv
     2388430 big-file.csv

    real    0m1.379s
    user    0m1.170s
    sys     0m0.197s

Math
------
lcw uses elementary statistics to perform unbiased estimates of the
number of lines in a file. It takes a random sample of "pages" within
the file and counts how many newlines are in each page.

It multiplies the average count by the number of pages in the file
in order to get its best guess at the number of lines in the file
(the maximum likelihood estimate) and then computes a 99% normal
confidence interval, applying a finite population correction for the
estimate the standard deviation of sample totals.

Tuning
--------
It is best to use the page size that your storage medium uses;
modern storage media read entire pages at once, so using a page size
that is too small will be bad for performance.

The sample size is set with ``-n``, and typical rules of thumb say
that this should be at least 20 for the confidence level to be valid.
The page size is set with ``-p`` and should be something like
2048, 4096, 8192, or 16384.
