# Longest path on a 2d plane

You are provided a set of arbitary, unique, 2d, integer Cartesian coordinates: e.g. [(0,0), (0,1), (1,0)]

Find the longest path possible from this set of coordinates, with the restriction that a coordinate can be "visited" only once. (And you don't "come back" to the coordinate you started at).

Important:

You cannot "pass over" a coordinate or around it. For instance, in the last note example (Rectangle), you cannot move from D to A without visiting C (which may be a revisit, invalidating the length thus found). This was pointed out by @FryAmTheEggman.

Function Input: Array of 2d Cartesian Coordinates
Function Output: Maximum length only
Winner: Shortest code wins, no holds barred (Not the most space-time efficient)

Examples

1: In this case shown above, the longest path with no coordinate "visited" twice is A -> B -> O (or O-B-A, or B-A-O), and the path length is sqrt(2) + 1 = 2.414

2: In this case shown above, the longest path with no coordinate "visited" twice is A-B-O-C (and obviously C-O-B-A, O-C-A-B etc.), and for the unit square shown, it calculates to sqrt(2) + sqrt(2) + 1 = 3.828.

Note: Here's an additional test case which isn't as trivial as the two previous examples. This is a rectangle formed from 6 coordinates:

Here, the longest path is: A -> E -> C -> O -> D -> B, which is 8.7147
(max possible diagonals walked and no edges traversed)

• Here's a very similar question, albeit with different scoring. – Geobits Feb 19 '16 at 14:40
• @Geobits Agreed, but I'd not say "very", having gone through the problem description there. And for that matter, any min/max path problem is essentially some flavor of your usual graph suspects. I'm interested in a byte saving solution here. – BluePill Feb 19 '16 at 14:43
• @Fatalize Done. It's 8.7147. – BluePill Feb 19 '16 at 15:13
• By the way: Welcome to PPCG! – Fatalize Feb 19 '16 at 15:14
• @Fatalize Thank you! (Actually I've been an observer here for a while, just got active and into the whole thing starting today). :) – BluePill Feb 19 '16 at 15:15

# Pyth, 10510310092 86 bytes

V.pQK0FktlNJ.a[@Nk@Nhk)FdlNI&!qdk&!qdhkq+.a[@Nk@Nd).a[@Nd@Nhk)J=K.n5B)=K+KJ)IgKZ=ZK))Z

Z = 0 - value of longest path
Q = eval(input())

V.pQ         for N in permutations(Q):
K0           K = 0 - value of current path
FktlN        for k in len(N) - 1:
J.a          set J = distance of
[@Nk                 Q[k] and Q[k+1]
@Nhk)
FdlN         for d in len(N):
I&                 if d != k && d != (k + 1)
!qdk
&!qdhk
q+                and if sum of
.a                   distance Q[k] and Q[d]
[@Nk
@Nd)
.a                   distance Q[d] and Q[k+1]
[@Nd
@Nhk)
J                    are equal to J then
=K.n5              set K to -Infinity
B                  and break loop
( it means that we passed over point )
)                   end of two if statements
=K+KJ                  K+=J add distance to our length
)                      end of for
IgKZ                   if K >= Z - if we found same or better path
=ZK                  Z = K       set it to out max variable
))                     end of two for statements
Z                      output value of longest path


Try it here!

# Mathematica, 139 bytes

Max[Tr@BlockMap[If[1##&@@(Im[#/#2]&@@@Outer[#/Abs@#&[#-#2]&,l~Complement~#,#])==0,-∞,Abs[{1,-1}.#]]&,#,2,1]&/@Permutations[l=#+I#2&@@@#]]&


Test case

%[{{0,0},{0,1},{1,0},{1,1},{2,0},{2,1}}]
(* 3 Sqrt[2]+2 Sqrt[5] *)

%//N
(* 8.71478 *)


# Perl, 341322 318 bytes

sub f{@g=map{$_<10?"0$_":$_}0..$#_;$"=',';@l=grep{"@g"eq join$",sort/../g}glob"{@g}"x(@i=@_);map{@c=/../g;$s=0;$v=1;for$k(1..$#c){$s+=$D=d($k-1,$k);$_!=$k&&$_!=$k-1&&$D==d($_,$k)+d($_,$k-1)and$v=0 for 0..$#c}$m=$s if$m<$s&&$v}@l;$m}sub d{@a=@{$i[$c[$_[0]]]};@b=@{$i[$c[$_[1]]]};sqrt(($a[0]-$b[0])**2+($a[1]-$b[1])**2)}  The code supports up to a 100 points. Since it produces all possible point permutations, 100 points would require at least 3.7×10134 yottabytes of memory (12 points would use 1.8Gb). Commented: sub f { @g = map {$_<10 ? "0$_" :$_ } 0..$#_; # generate fixed-width path indices$" = ',';                               # set $LIST_SEPARATOR to comma for glob @l = grep { # only iterate paths with unique points "@g" eq join$", sort /../g         # compare sorted indices with unique indices
} glob "{@g}" x (@i=@_);                # produce all permutations of path indices
# and save @_ in @i for sub d
map {
@c = /../g;                         # unpack the path indices
$s=0; # total path length$v=1;                               # validity flag
for $k (1..$#c) {                   # iterate path
$s += # sum path length$D = d( $k-1,$k );         # line distance

$_!=$k && $_!=$k-1            # except for the current line,
&& $D == d($_, $k ) # if the point is on the line, + d($_, $k-1 ) and$v = 0                    # then reset it's validity
for 0 .. $#c # iterate path again to check all points }$m=$s if$m<$s &&$v                # update maximum path length
} @l;
$m # return the max } sub d { @a = @{$i[$c[$_[0]]] };                # resolve the index $_[0] to the first coord @b = @{$i[$c[$_[1]]] };                # idem for $_[1] sqrt( ($a[0] - $b[0])**2 + ($a[1] - $b[1])**2 ) }  TestCases: print f( [0,1], [0,0], [1,0] ),$/;        $m=0; # reset max for next call print f( [0,0], [0,1], [1,0], [1,1] ),$/; $m=0; print f( [0,0], [0,1], [0,2] ),$/;        $m=0; print f( [0,0], [0,1], [0,2], [1,0], [1,1], [1,2]),$/;          $m=0;  • 322 bytes: save 19 by not resetting $", and some inlining
• 318 bytes: save 4 by reducing max nr of coords to 100.