Shortest route through a one-way system

Question

My home town, Rhyl, has a one-way traffic system which seems to have been designed to keep people away from their destination for as long as possible. Your task, should you choose to attempt it, is to produce a program to give the shortest route through such a traffic system.

Input

Input will be on STDIN, and will be a list of start and end points followed by a blank line followed by a list of queries, as follows:

A B
B A
B C
C D
D C

A D
C A
B A

Each road can only be travelled in the direction(s) given, so, in the above example the road A - B is a two way street whereas B - C is a one-way street from B to C. Travelling from C to B is prohibited.

Start and end points will all be represented by a single upper-case letter.

Output

Output should be the shortest route (measured by number of points visited) from the given start point to the given end point for each query received. If no such route exists, output a blank line. If more than one shortest route exists, output the first when sorting all shortest routes lexicographically.

For the above example the output would be:

A B C D

B A

Test Scripts

As before I'm providing tests for this task based on scripts written by Joey and Ventero:-

• Bash
• PowerShell

and also tests and expected output for anyone who can't use the above scripts

• Plain text

Usage: ./test [your program and its arguments]

Rewards

All answers which have obviously had some attempt at golfing that meet the spec and pass all the tests will get my upvote. The shortest working answer by 26/01/2012 will be accepted.

Answer 1

Haskell, 223 207 characters

main=interact$unlines.f.break null.map words.lines
s%[f,t]=[[f]]#t where[]#_="";x#t|y@(_:_)<-[z|z<-x,last z==t]=unwords$minimum y|1<3=s&x#t
s&z=[x++[b]|x<-z,[a,b]<-s,last x==a,notElem b x];f(s,_:q)=map(s%)q

Answer 2

Python (2.x), 382 369 358 338 323 318 characters

All tips and comments welcome!

import os;u=str.split;l=u(os.read(0,1e9),'\n')
p,g,c=l.index(''),{},map;o=g.setdefault
def f(g,s,e,q=[]):q=q+[s];y=([],[q])[s==e];[c(y.append,f(g,n,e,q))for n in set(o(s,[]))-set(q)];return y
for k,v in c(u,l[:p]):o(k,[]);g[k]+=v
for w,e in c(u,l[p+1:]):h=sorted(f(g,w,e));print''if not h else' '.join(min(h,key=len))

Should pass the tests in this form. Feed input via stdin, e.g. python shortestroute.py < test.txt.

Answer 3

C:450,437,404,390 characters

#include<stdio.h>
#include <string.h>
r[99][99],p[99],q[99],m[99],i,j,x,y,s;
char t[9],e[9999];
F(k)
{
    m[k]^s?r[p[k]=q[i]][k]?m[q[j++]=k]=s:0:0;
    if(++k<99)F(k);
}
f()
{
    F(0);
    if(++i^j)f();
}
P(o)
{
    if(p[o])P(p[o]);
    *t=m[o]^s?0:o;
    strcat(e,t);
}
w()
{
    gets(t);
    r[*t][t[2]]=*t?w():0;
}
W()
{
    gets(t);
    x=*t,x?y=t[j=2],s=x+y*99,m[q[t[2]=i=p[x]=0]=x]=s,f(),P(y),strcat(e,"\n"),W():0; 
}
main()
{
    w();
    W();
    puts(e);
}