up vote 10 down vote favorite

Sometimes a long absolute path, in e.g. a command-line parameter to a linux tool, can be shortened, using current working directory as reference:

$ pwd
/home/heh

$ cat /home/heh/mydir/myfile
my stuff

$ cat mydir/myfile
my stuff

In this challenge, you should make a function or a program that receives two parameters:

  1. Absolute path, using the linux format (starts with /)
  2. Current directory, using the same format

The output is the shorter of the following:

  • Input 1 unchanged
  • Relative path that refers to the same file/directory as the absolute path

Fine points:

  • If your operating system is compatible with linux, you can use the system's current directory instead of receiving it as input
  • You can assume the inputs contain only alphanumeric characters (and path separators)
  • You can assume the input absolute path doesn't have a path separator / at the end
  • You can assume the input current directory has a path separator / at the end
  • You cannot assume that the absolute path refers to an existing file, or that any part of it is an accessible directory; however, the current directory can be assumed valid
  • You can assume there are no symlinks anywhere near either path - because I don't want to require any special way of dealing with symlinks
  • No need to support the case where either of the inputs is the root directory
  • "The current directory" should be output as . (an empty string is not valid)

Test cases (input1, input2, output):

/home/user/mydir/myfile
/home/user
mydir/myfile

/var/users/admin/secret/passwd
/var/users/joe/hack
../../admin/secret/passwd

/home/user/myfile
/tmp/someplace
/home/user/myfile

/dir1/dir2
/dir1/dir2/dir3/dir4
../..

/dir1/dir2
/dir1/dir2
.
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"You can assume the input current directory has a path separator / at the end". However, in your examples, this is not the case. – Shaggy 12 hours ago
1  
I like it this way, but some people like it the other way – anatolyg 12 hours ago
    
Closely related. – AdmBorkBork 12 hours ago
    
What should happen if absolute and relative path have the same length? – Dennis♦ 10 hours ago
    
If same length, either output is good. – anatolyg 9 hours ago
up vote 5 down vote

JavaScript (ES6), 107 bytes

Takes the absolute path a and the current path c in currying syntax (a)(c).

a=>c=>(A=a.split`/`,s='',c.split`/`.map(d=>!s&A[0]==d?A.shift():s+='../'),s=s+A.join`/`||'.')[a.length]?a:s

Test cases

let f =

a=>c=>(A=a.split`/`,s='',c.split`/`.map(d=>!s&A[0]==d?A.shift():s+='../'),s=s+A.join`/`||'.')[a.length]?a:s

console.log(f
  ('/home/user/mydir/myfile')
  ('/home/user')
);

console.log(f
  ('/var/users/admin/secret/passwd')
  ('/var/users/joe/hack')
);

console.log(f
  ('/home/user/myfile')
  ('/tmp/someplace')
);

console.log(f
  ('/dir1/dir2')
  ('/dir1/dir2/dir3/dir4')
);

console.log(f
  ('/dir1/dir2')
  ('/dir1/dir2')
);

share|improve this answer
    
A very nice trick with [a.length] ! May I borrow it to improve my Node.js answer ? – zeppelin 7 hours ago
    
@zeppelin Sure. Go for it! – Arnauld 7 hours ago
up vote 3 down vote

Retina, 85 bytes

^(..+)(.*;)\1
%$2
(%?)(.*);(.*)
$1$3;$2
(\w+)(?=.*;)
..
%;/

;
/
.*//
/
%/|%|/$

^$
.

Try it online!

Will golf further in the morning

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up vote 2 down vote

Zsh + realpath, 58 bytes

r=`realpath -m --relative-to=$*`
(($#2<$#r))&&r=$2
echo $r

Try it online!

Bash version, 62 bytes

r=`realpath -m --relative-to=$*`
((${#2}<${#r}))&&r=$2
echo $r

Try it online!

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up vote 2 down vote

ES6 (Node.js REPL), 56, 54, 46 bytes

  • Borrowed the [f.length] trick from @Arnauld's answer, -6 bytes
  • Use the current directory instead of an explicit directory parameter, -2 bytes
  • Removed superfluous parentheses, -2 bytes

Golfed

f=>(r=path.relative(".",f))[f.length]?f:r||"."

Test

> F=f=>(r=path.relative(".",f))[f.length]?f:r||"."
[Function: F]

> F("/home/user/mydir/myfile")
'mydir/myfile'

> F("/var/users/admin/secret/passwd")
'../../admin/secret/passwd'

> F("/home/user/myfile")
'/home/user/myfile'

> F("/dir1/dir2")
'../..'

> F("/dir1/dir2")
'.'
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up vote 2 down vote

Julia 0.5, 32 bytes

!,~=relpath,endof
t->~t<~!t?t:!t

This uses the current working directory as base and cannot be tested on TIO at the moment.

Example run

Warning: This will alter your file system.

$ sudo julia --quiet
julia> !,~=relpath,endof
(relpath,endof)

julia> shorten = t->~t<~!t?t:!t
(::#1) (generic function with 1 method)

julia> test("/home/user/mydir/myfile","/home/user")
"mydir/myfile"

julia> test("/var/users/admin/secret/passwd","/var/users/joe/hack")
"../../admin/secret/passwd"

julia> test("/home/user/myfile","/tmp/someplace")
"/home/user/myfile"

julia> test("/dir1/dir2","/dir1/dir2/dir3/dir4")
"../.."

julia> test("/dir1/dir2","/dir1/dir2")
"."

Alternate version, 35 bytes (dyadic)

^,~=relpath,endof
t-b=~t<~t^b?t:t^b

This takes the base directory as input, so it can be tested without modifying the file system.

Try it online!

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up vote 0 down vote

Python 2, 135 bytes

i=0
a,c=[j.split('/')for j in input()]
if a==c:a=[]
while a[:i+1]==c[:i+1]:i+=1
print'/'.join([['..']*len(c[i:]),['']][i<2]+a[i:])or'.'

Try it Online!

Kind of long, but I wanted to do a solution without built-in path functions.

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