Biker mice

(*

 Revision: Dec 22 2012 New age dawn! :-D

    -Minor corrections made inside the comments for clarity and readability.
    -A not used variable detected and deleted.

 For example if you want to obtain the "natural" permutations for base-13, this
 is for the decimal digits and a,b,c notice that c=12 regarded as digit in the
 conventional notation, but since we started here to count from zero, each latin
 alphabet letter have its conventional value +1 when it is used for counts.

 Then follow the usual instructions given below (please read them), and call:

  --------------------------------------------------------[Linux Shell example]

  gandalf@rivendell@sh$ permworep c

  ----------------------------------------------------------------------------

  Written on my host machine:

  * 32-bit Intel Atom with 160Gb Hdd 1Gb Ram,
  * Slackware GNU Linux distro version 14
  * Free Pascal Compiler 2.6.2

  Formerly was http://pastebin.com/ZCLc63XG... all bogus bugs apparently fixed.

*)

program PermutationsWithoutRepetitions;

(*

  USAGE:
  -----

   i] Compile this sourcecode and place the executable in a proper location.
      (It is strongly suggested to use the FPC compiler. Please visit:
      freepascal.org)

  ii] Create a empty file called "permworep_idata.txt" and fill it just with
      a sigle zero. Save it in the same directory for the executable mentioned
      in [i] and ensure that there will be granted read-write permissions
      for this program during its execution.

 iii] For obtaining the 10! permutations for the first 10 non-negative integers
      enumerated starting from zero, call the program passing "9" as its unique
      parameter.

      (This is due zero is the first digit in every positional number system)

      Instead of using double byte numbers like 11 or 16, use the letter
      representing it as digit in a base greater than the decimal. The program
      is theoretically limited up to 36 digits enumerated from zero.

  iv] Depending on the resources available during the execution, this program
      might not work properly as expected, being interrumped or crashing for
      example when used to computed sets of permutations greater than 10 ("a"
      letter).

 Written by R. J. Cano, <aallggoorriitthhmmuuss@gmail.com> on Dec 9th 2012.

 Observation: There exists better known algorithms/methods for computing
 pemutations. The present program is merely another one more among the possible
 implementations for doing such jobs.

 See for example the entry A055881 at OEIS.org, specifically the comment
 by Joerg Arndt.

 The purpose on writing this program was to serve as ilustration for the
 algorithm known as Steinhaus-Jhonson-Trotter, and additionally about how
 to do these thing using the hard disk instead the primary memory.

 Important: The present program is released under the terms
            of the GNU General Public License 3.0 and it goes
            without any warranty whatsoever.

*)

type
 my_string=string[15];
 (* Mem. saving measure: Limit only 16 bytes per string.
    Up to the first 15! permutations only *)

const
 basename:string='permworep';
 digit:array [0..35] of char=
   ('0','1','2','3','4','5','6','7','8','9',
    'a','b','c','d','e','f','g','h','i','j',
    'k','l','m','n','o','p','q','r','s','t',
    'u','v','w','x','y','z');
var
 f_input,
 f_output : text;
 input,
 output,
 request:my_string;
 size_i,
 size_o,
 index_a,
 index_b:byte;
 index_c,
 dir:shortint;

function f(x:char):byte;
var ans:byte;
begin
  if (x in ['a'..'z']) then begin
    ans:=10+ord(x)-ord('a');
  end else if (x in ['0'..'9']) then begin
    ans:=ord(x)-ord('0');
  end else begin
    ans:= 255;
  end;
  f:=ans;
end;

begin
 if (paramcount = 1) then begin
  request:=paramstr(1);
  size_o:=f(request[length(request)])+1;
  assign(f_input,basename+'_idata.txt');
  assign(f_output,basename+'_odata.txt');
  reset(f_input);
  readln(f_input,input);
  close(f_input);
  size_i:=f(input[length(input)])+1;
  while (size_o > size_i) do begin
   dir:=-1;
   assign(f_input,basename+'_idata.txt');
   assign(f_output,basename+'_odata.txt');
   reset(f_input);
   rewrite(f_output);
   while (not eof(f_input)) do begin
     readln(f_input,input);
     for index_a:= 0 to size_i do begin
       output:='';
       index_c:=-1;
       for index_b:= 0 to size_i do begin
         if ((dir=-1) and (index_a+index_b=size_i)) then begin
           output:=output+digit[size_i];
         end else if ((dir=1) and (index_a=index_b)) then begin
           output:=output+digit[size_i];
         end else begin
           if (index_c=size_i-1) then index_c:=-1;
           index_c:=index_c+1;
           output:=output+input[index_c+1];
         end;
       end;
       writeln(f_output,output);
     end;
     dir:= dir*(-1);
   end;
   close(f_input);
   close(f_output);
   erase(f_input);
   assign(f_input,'tmp');
   rename(f_output,basename+'_idata.txt');
   size_i:= size_i + 1;
  end;
 end;
end.