Draw a network of nodes

Question

There is a network of up to 26 nodes (named A to Z or a to z as per your wish). Every pair of nodes may be connected or disconnected. A node may be connected to at most 4 other nodes. Your task is to draw the network in a 2D diagram. Input will be given such that this task is possible (see more constraints in output section).

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Format

Input

• Pairs of letters (A to Z or a to z as per your wish). They are not sorted in any order.
• Optional - number of pairs

Output

• An ASCII drawing that shows the actual links between the nodes. Nodes are given by a to z or A to Z. Use - for horizontal links and | for vertical links. Links may be of any (non-zero) length but they should be straight horizontal/vertical lines that do not bend. Spaces can be added provided they don't disfigure the picture.

You may not use built-ins that help in layout of the graph. Other graph-related built-ins may be allowed (though solutions without built-ins would be more appreciated). Shortest code wins.

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Sample data

Input

A B
B F
B L
F K
L K
K R
K S
R P
S J
S P
J A
T V
V N

Output

A - B - F   T - V
|   |   |       |
|   L - K - R   N
|       |   |
J ----- S - P

Input

H C
G H
A B
B F
B C
F G
C D
D A

Output

A - B ----- F
|   |       |
D - C - H - G

Answer 1

CJam, 142

You didn't ask for an optimal, deterministic or fast solution, so here you go:

qN%Sf%::c_s_&:Aff#:E;{A{;[DmrDmr]2f*}%:C;E{Cf=~.-:*}%0m1{E{Cf=$~1$.-_:g:T\:+,1>\ff*\f.+T0="-|"=f+~}%CA.++:L2f<__&=!}?}gS25*a25*L{~2$4$=\tt}/N*

Try it online

This generates random coordinates for each letter and tests if the layout is acceptable (edge letters lining up and no intersections), until it is. It gets mind-numbingly slow as you add more edges.

The two D letters in the code specify the maximum x and y coordinates; I picked D (=13) because I think it should be sufficient for all cases, feel free to prove me wrong. But you can change them to other values to speed up the program, e.g. the 2nd example should finish within a minute or two if you use 3 and 4 instead.

Answer 2

C, 813 bytes

#include<map>
#include<set>
#include<cstdlib>
typedef int I;typedef char*C;I a,t,s,h;struct p{I x,y;}j,k;std::map<I,std::set<I>>l;std::map<I,p>g;C m,M="  |-";I L(I n,C q,C Q){j=g[n],h=1;for(I o:l[n])if(g.find(o)!=g.end())if(!(a=(k=g[o]).y==j.y)&&k.x^j.x)h=0;else for(I x=j.x,y=j.y,e=k.y*s+k.x,b,d=(k.x<j.x||k.y<j.y)?-1:1;a?x+=d:y+=d,(b=y*s+x)^e;m[b]=q[a])if(m[b]^Q[a]){h=0;break;}}I N(){for(auto i:g)for(I n:l[i.first])if(g.find(n)==g.end())return n;for(auto i:l)if(g.find(a=i.first)==g.end())return a;exit(puts(m));}I f(){for(I n=N(),x,y=0,b;y<s;y+=2)for(x=0;x<s;x+=2)m[b=y*s+x]==*M?g[n]={x,y},m[b]=n,L(n,M+2,M),h&&f(),L(n,M,M+2),m[b]=*M,g.erase(n):0;}I main(I c,C*v){for(;--c;l[a].insert(s),l[s].insert(a))a=v[c][0],s=v[c][1];t=l.size(),s=t|1;memset(m=(C)calloc(s,s),*M,s*s-1);for(a=1;a<s;++a)m[a*s-1]=10;f();}

Takes input as commandline arguments, e.g.:

./network AB BF BL FK LK KR KS RP SJ SP JA TV VN

Nowhere near competitive with aditsu's answer by size, but much more efficient!

This will brute-force all possible solutions, but will quickly recognise failure as it goes. For the two test cases, it finishes almost immediately, and it seems to only take a few seconds on more awkward inputs. It also has no limitation to the accepted node names (though you can't name one space, | or -) and has no limit on the number of nodes (as long as all names fit in a byte, so the practical limit is 252 nodes, and it will get slow long before reaching that many).

There's plenty of scope for speeding this up; a lot of short-circuit exiting was lost to the golfing, and there's parts which could be moved out of hot-loops. Also some observations of symmetry can dramatically reduce the positioning of the first 2 nodes, among other things.

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Breakdown:

#include<map>
#include<set>
#include<cstdlib>
typedef int I;
typedef char*C;
I a,t,s,h;                // Variables shared between functions
struct p{I x,y;}          // Coord datatype
j,k;                      // Temporary coord references
std::map<I,std::set<I>>l; // Bidirectional multimap of node links
std::map<I,p>g;           // Map of nodes to positions
C m,                      // Rendered grid
M="  |-";                 // Lookup table for output characters

// Line rendering function
// Sets h to 1 if all lines are drawn successfully, or 0 if there is a blocker
I L(I n,C q,C Q){
  j=g[n],h=1;
  for(I o:l[n])                  // For each connection to the current node
    if(g.find(o)!=g.end())       // If the target node has been positioned
      if(!(a=(k=g[o]).y==j.y)&&k.x^j.x)h=0; // Fail if the nodes are not aligned
      else
        for(I x=j.x,y=j.y,             // Loop from node to target
          e=k.y*s+k.x,
          b,d=(k.x<j.x||k.y<j.y)?-1:1;
          a?x+=d:y+=d,(b=y*s+x)^e;
          m[b]=q[a])                   // Render character (| or -)
          if(m[b]^Q[a]){h=0;break;}    // Abort if cell is not blank
}

// Next node selection: finds the next connected node to try,
// or the next unconnected node if the current connected set is complete.
// Displays the result and exits if the entire graph has been rendered.
I N(){
  for(auto i:g)for(I n:l[i.first])  // Search for a connected node...
    if(g.find(n)==g.end())return n; // ...and return the first available
  for(auto i:l)                     // Else search for an unconnected node...
    if(g.find(a=i.first)==g.end())
      return a;                     // ...and return the first available
  exit(puts(m));                    // Else draw the grid to screen and stop
}

// Recursive brute-force implementation
I f(){
  for(I n=N(),x,y=0,b;y<s;y+=2) // Loop through all grid positions
    for(x=0;x<s;x+=2)
      m[b=y*s+x]==*M            // If the current position is available
       ?g[n]={x,y},             // Record the location for this node
        m[b]=n,                 // Render the node
        L(n,M+2,M),             // Render lines to connected nodes
        h&&f(),                 // If line rendering succeeded, recurse
        L(n,M,M+2),             // Un-render lines
        m[b]=*M,g.erase(n)      // Un-render node
       :0;
}

// Input parsing and grid setup
I main(I c,C*v){
  // Parse all inputs into a bidirectional multi-map
  for(;--c;l[a].insert(s),l[s].insert(a))a=v[c][0],s=v[c][1];
  t=l.size(),s=t|1; // Calculate a grid size
  // Allocate memory for the grid and render newlines
  memset(m=(C)calloc(s,s),*M,s*s-1);
  for(a=1;a<s;++a)m[a*s-1]=10;
  f(); // Begin recursive solving
}