This code kata simulates a graphite pencil.

Python 2.7 or 3.4+. Assuming running on Mac.

This project uses the Python package pytest for testing. Ensure that the Python package installer pip is installed beforehand.

Test are split between the three major functions of the application: write, erase, edit. I implemented each function after writing each test file to practice more Test Driven Development.

Run make init to install pytest. Then run make test to run all tests.

Open the Python interpreter and enter

from stationary import Pencil, Paper

Initialize a new Pencil and Paper and you're all set. Details below.

Pencil(point_dur, length, eraser_dur)

point_dur, length, and eraser_dur are ints representing the point durability, length, and eraser durability of the Pencil instance. Defaults are set to 10, 5, and 24 respectively.

Enter pencil.write(text, paper) to write some text on a paper instance. Writing decreases point durability, and once durability and length are used up, the pencil no longer writes. Sharpening is done automatically when durability is insufficient and length is greater than zero. Capital letters cost 2 durabilities, white space costs 0, and all other characters cost 1.

Enter pencil.erase(text, paper) to erase the last matching pattern of text from a paper instance. Erasing decreases eraser durability, and once that is spent, the pencil no longer erases.

Enter pencil.edit(text, paper, start) to write over a paper instance's text with some other text at starting index start. There were some likely intentional ambiguities in the specs, so I made some key assumptions with my implementation:

• If an edit goes over the length of the paper (e.g. see test_edit_collision_out_of_bounds in test_edit.py), the pencil attempts to edit whatever space is available on the paper before giving up. Doing so does not raise an exception.
• Edits can still technically be writes, so the same rules on point durability and length for writing apply here. If a pencil's total durability is used up, it can no longer write new characters or @'s. If there's no need write new characters or @'s there is no degradation.
• While editing over a whitespace with a non-whitespace character means the latter is written, editing a whitespace over a non-whitespace doesn't make sense. That is considered a collision and so the @ is written over the non-whitespace character.
• Writing a collision (@) still degrades the pencil. Following convention above, the degradation cost is still one. This created some interesting test cases where a pencil couldn't write in a capital, but a @ was fine. (See test_edit_collision_and_point_degradation in test_edit.py).

Paper(text)

Paper is simply an abstraction for the string to write/erase/edit. The default string is the empty string.