Shorten an absolute path

Question

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
.

Answer 1

JavaScript (ES6), 107 106 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+=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+=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')
);

Answer 2

Retina, 85 83 82 bytes

1 byte saved thanks to @MartinEnder

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

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

^$
.

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Answer 3

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> function test(target,base)
       mkpath(base)
       cd(base)
       shorten(target)
       end
test (generic function with 1 method)
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.

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Answer 4

ES6 (Node.js REPL), 56, 54, 46, 45 bytes

• Use empty string, instead of "." to denote the current directory (on input), -1 byte
• 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")
'.'

Answer 5

Python 2, 135 144 bytes

i=0
a,c=input()
b,d=a.split('/')*(a!=c),c.split('/')
while b[:i+1]==d[:i+1]:i+=1
print'.'[i:]or min('/'.join(['..']*len(d[i:])+b[i:]),a,key=len)

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

Edit: 9 bytes added to account for test case provided by Nathan Merrill

Answer 6

Zsh + realpath, 58 bytes

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

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Bash version, 62 bytes

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

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Answer 7

Python 3 – 53 bytes

Using os.path:

import os
lambda x:min(x,os.path.relpath(x),key=len)

Full program (61 bytes):

import os
x=input();print(min(x,os.path.relpath(x),key=len))

Answer 8

PHP, 204 Bytes

[,$l,$p]=$argv;$z=($d=array_diff_assoc)($y=($w=explode)("/",$p),$x=$w("/",$l));$m=str_pad("",3*count($z)-1,"../");$j=join("/",$r=$d($x,$y));echo$l!=$p?strlen($u=$m&&$j?"$m/$j":$m.$j)<strlen($l)?$u:$l:".";

Testcases

Expanded

[,$l,$p]=$argv;
$z=($d=array_diff_assoc)($y=($w=explode)("/",$p),$x=$w("/",$l));
$m=str_pad("",3*count($z)-1,"../");
$j=join("/",$r=$d($x,$y));
echo$l!=$p
    ?strlen($u=$m&&$j?"$m/$j":$m.$j)<strlen($l)
      ?$u
      :$l
    :".";

if an Output ../../ instead of ../.. is allowed it can be shorten to 175 Bytes

[,$l,$p]=$argv;$z=($d=array_diff_assoc)($y=($w=explode)("/",$p),$x=$w("/",$l));echo$l!=$p?strlen($m=str_pad("",3*count($z),"../").join("/",$r=$d($x,$y)))<strlen($l)?$m:$l:".";

Answer 9

C# - 66 bytes

Using a .NET builtin and forcing to be a valid path:

(f,t)=>f==t?".":new Uri("/"+t).MakeRelativeUri(new Uri("/"+f))+"";

Where f,t and output are string.

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