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The challenge is to manipulate strings using basic Vim-like commands

Input:

  • A string of text to be transformed. The input alphabet is [a-z].
  • A string of legal commands

Output:

  • The transformed string

There are a few ideas you should understand:

The cursor is represented as a location between 2 characters. | will be used to represent the cursor(s) in this explanation.

  • |foobar - The cursor at beginning of string
  • foo|bar - Cursor in the middle of the string
  • foobar| - Cursor at the end of the string

A selection is a a group of characters between 2 cursors.

  • |foobar| - The selection is foobar
  • |foo|bar - The selection is foo
  • foo|bar| - the selection is bar
  • |foobar - The selection is the empy string (both cursors at same location)

A selection is formed from the previous cursor position and the result of the current command.

Using the command > on |foo would make the selection |f|oo.

Commands can be described as functions. C<selection> is a command that has a selection parameter. The next command provides the selection.

  • D> deletes the character to the right of the cursor. f|oo -> f|o
  • DNa deletes from the cursor to the next a character f|oobar -> f|ar
  • Etc.

As you can see, commands "associate to the right"

  • DNa is parsed as (D(Na))
  • DDNa is parsed as (D(D(Na)))
  • Etc.

The insert mode command I extends until the next command.

  • DIbar> is parsed as (D(Ibar))(>).

These are the commands:

  • > - Move cursor right. > on foo|bar -> foob|ar
  • < - Move cursor left. < on foo|bar -> fo|obar
  • B - Move cursor to the beginning of the string
  • E - Move cursor to the end of the string
  • N<char> - Move cursor to place before next <char>. Na on |foobar -> foob|ar
  • P<char> - Move cursor to place after previous <char>. Po on foobar| -> foo|bar
  • I<string> - Enter/exit insert mode. Characters in <string> are inserted at the current cursor position. <string> extends until the next command. The cursor is moved to end of insertion <string>. Iquz on foo|bar -> fooquz|bar
  • D<selection> - delete characters in <selection>. f|ooba|r -> f|r. The output selection from this command is the empty string.

Notes:

  • The cursor cannot move out of bounds. B<<< is the same as B. Nx is the same as E if no x can be found.
  • The cursor location always starts at the beginning of the string
  • The cursor should not be outputted
  • Commands are always uppercase, characters are lowercase.

Restrictions:

  • No text editor scripts like Vimscript

Winning criteria:

  • It's code golf. Lowest character count wins.

Examples:

Input:

  • foobar
  • >>><<>If

Output:

  • fofobar

Input:

  • foobar
  • >>>DNEDPf

Output:

  • f

Input:

  • foobar
  • >>>DDDDIbazD<Iquz>DPfDII

Output:

  • far
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  • \$\begingroup\$ "<string> extends until the next command." But how do we know that it's a command rather than part of the string? \$\endgroup\$ – Peter Taylor Jan 21 '12 at 21:59
  • \$\begingroup\$ @PeterTaylor Commands are uppercase, characters are lower. \$\endgroup\$ – Pubby Jan 21 '12 at 22:04
  • \$\begingroup\$ I'm starting to think that this question might be a bit too complicated for a code-golf tag. \$\endgroup\$ – Mr. Llama Jan 23 '12 at 15:47
  • 2
    \$\begingroup\$ The first example is like teaching a 4th grader how to multiply; the second example is like trying to teach that 4th grader algebra, and the third example is like trying to teach that 4th grader calculus. \$\endgroup\$ – Dr. Wily's Apprentice Jan 24 '12 at 15:14
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Haskell, 371 characters

z l b n i=p where
 p(a:c@ ~(q:u))|Just x<-a`lookup`zip"<>BEDNP"[l#c,m l#c,b#c,m b#c,z(d$k id)(\k _->k"")h(k f)c,m(n q)#u,n q#u]=x
 p('I':q:c)|q>'`'=i q c;p(_:c)=f c;p""=(++).r
d f k(q:x)=k x.f q;d f k""=k"";k&x=k"".(r x++);h q=(.snd.s q);n q k=(\(a,b)->k b.(r a++)).s q
x%c=f c""x;f=z(d(:))(&)n$ \q c->f('I':c).(q:);m f=flip.f.flip;k=const;r=reverse;(#)=(.f);s=span.(/=)

This defines a function % which returns the result of applying the string of commands to the original string.

Example usage:

> "foobar" % ">>>DNEDPf"
"f"
> "foobar" % ">>>DDDDIbazD<Iquz>DPfDII"
"far"
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