Held: Thursday, 13 February 2014

Back to Outline 03 - The Bash Shell - A Refresher. On to Outline 05 - Learning Vi.

Summary

We consider how we might write a program that "cleans up" a typescript file created by script.

Related Pages

• EBoard (Source) (HTML) (PDF)

Overview

• Background.
• Analysis.
• Algorithm.
• Inputs.
• Implementation.

Administrivia

• Apologies for not being able to be in class this week. I had an unexpected obligation.
• Colin will run class today. Treat him well.
• I also won't be in class next week - I have to coach at the middle school math competition.
• I'm likely to be sending out the homework in a day or two.

Background

Why did Sam give you this assignment?

• To help you think about dealing with funky characters in input.
• To help you think about dealing with real input. For example, how do you decide how long lines should be?
• To help you think about writing small programs that cooperate with others.
• To encourage you to look at tools that let you analyze text files.

Reflection

• What are the complexities of input?
  • We'll look at the question in two ways.
• What are the things the user does that will cause complexity in the shell?
  • Arrow keys
  • Backspace
  • Ctrl-U
  • What else?
• What do you see in the file that corresponds to these actions?
  • A lot of backspace characters
  • The weird triplet of esc [ K
  • What else?
• How can you tell what characters are in the typescript?
  • od -a typescript
• How do you figure out what characters those are?
  • man ascii
• What should should you do with each of those?
  • Backspace: Have a buffer that you remove characters from.
  • Weird triplet: Remove (?)
  • What else?
• How big should the buffer be?
  • Determine empirically?
  • LINE_MAX is supposed to be in <limits.h> (see examples/linemax.c)
    • It's really in /usr/include/x86_64-linux-gnu/bits/posix2_lim.h
    • I think lines can be longer than that.
  • Use dynamic arrays. (Okay, use malloced arrays and expand if you need more characters.)
• What complications are we ignoring?
  • Interactive things in the terminal, such as a vi session.
  • And yes, it's okay to ignore those.
  • (But if you have time, you should think about it.)

Algorithm

Given that analysis, what should our algorithm look like?

while input characters remain
    ch = next character
    switch ch
      case backspace:
          remove the last character in the buffer
      case 'n':
          print the buffer, reset
      case '\r'
          ignore the character
      case escape
          read two more characters, which will almost be [ and something
          ignore them
      default
          add the character to the buffer
print whatever remains in the buffer

Build Inputs

• Work in groups of three or four or so students.
• Create a few typescripts that you think will be challenging to a program.
  • Challenging should not be "use vi".
  • Try long lines, lots of erased text (through different forms), partial erasures, more.
• See how well any submitted programs work on your inputs.
• Upload them to the repo for further testing.

Write Programs

• Work in whatever group size you think is reasonable. I'd recommend three, but you can choose.
• Now that you've thought about the general algorithm