program lander;

const
   instrfile      = 'LANDINST.DAT';
   maxrand        = 32767;
   timeinc        = 0.1;
   slightly       = 0.1;
   moderately     = 0.2;
   very           = 0.3;
   impossible     = 0.5;
   certainty      = 1.0;
   minmeteors     = 2.0;
   maxmeteors     = 7.0;
   eachmisschance = 0.8;
   minavoid       = 10;
   landingheight  = 0.1;
   crashlandspeed = 6.0;

type
   answers = set of 'A'..'Z';
   str80 = string[80];

var
   speed, height, gravity : real;
   maxlandingspeed        : real;
   chance, misschance     : real;
   burn, fuel             : integer;
   nummeteors             : integer;
   inp                    : string[10];
   ranout                 : boolean;

(*
 * returns a random real number between lowlimit and highlimit
 *)

function rand(lowlimit, highlimit : real) : real;

begin
   rand := lowlimit + (highlimit - lowlimit + 1) * random;
end;

(*
 * returns true if a random number, weighted by the difficulty level, is
 * less than the argument
 *)

function unlucky(percentchance : real) : boolean;

begin
   unlucky := rand(0.0, 1.0 - chance) <= percentchance;
end;

(*
 * converts a numerical ascii character to its corresponding integral value
 *)

function asciitoint(digit : char) : integer;

begin
   asciitoint := ord(digit) - ord('0');
end;

(*
 * returns true if the argument is a numerical digit
 *)

function isdigit(ch : char) : boolean;

begin
   isdigit := (ch >= '0') and (ch <= '9');
end;

(*
 * convert determines the numerical value of the digits ina string. it returns
 * the integer and updates the string to now contain whatever was after the
 * end of the number. if the string is blank, the value is zero; if a number
 * is not found it returns -1.
 *)

function convert(numstring : str80) : integer;

var
   intvalue       : integer;
   position       : integer;
   digitsfound    : integer;
   endofnumber    : boolean;
   notinteresting : boolean;

begin
   intvalue := 0;
   digitsfound := 0;
   position := 1;
   endofnumber := false;
   notinteresting := true;
   if length(numstring) > 0 then
      begin
      while (position < length(numstring)) and notinteresting do
         if numstring[position] = ' ' then
            position := position + 1
         else
            notinteresting := false;
      while (position <= length(numstring)) and (not endofnumber) do
         if isdigit(numstring[position]) then
            begin
            intvalue := intvalue * 10 + asciitoint(numstring[position]);
            position := position + 1;
            digitsfound := digitsfound + 1;
            end
         else
            endofnumber := true;
      if digitsfound = 0 then
         intvalue := -1;
      end;
   convert := intvalue;
end;

(*
 * asks a question with a single character answer. if the response is in the
 * set 'answers', the letter is returned. otherwise the string ifbad is
 * printed and the question is asked again.
 *)

function ask(question, ifbad : str80; responses : answers) : char;

var
   bad : boolean;
   ch  : char;

begin
   bad := true;
   repeat
      write(question);
      readln(ch);
      writeln;
      if ch in responses then
         bad := false
      else
         writeln(ifbad);
   until not bad;
   ask := ch;
end;

(*
 * prints out the instructions from a file instrfile
 *)

procedure instructions;

var
   instruct : text;
   ch       : char;
   str      : string[80];

begin
   assign(instruct,instrfile);
   reset(instruct);
   while not eof(instruct) do
      begin
      readln(instruct,str);
      writeln(str);
      end;
end;

procedure startup;

begin
   if ask('Do you want instructions? ','Please answer Y or N',['Y','N']) = 'Y' then
      instructions;
   randomize;
end;

(*
 * ask the player for the difficulty level of the next landing
 *)

procedure getdifficulty;

var
   level : char;

begin
   level := ask('Level of difficulty? ','B:Beginner, E:Expert, N:Navigator, A:Astronaut',
                 ['B','E','N','A']);
   case level of
      'B' : chance := slightly;
      'E' : chance := moderately;
      'N' : chance := very;
      'A' : chance := impossible;
   end;
end;

(*
 * variables that must be re-set each time a new landing is attempted
 *)

procedure startgame;

begin
   getdifficulty;
   gravity := rand(9.0 + chance, 11.0 + chance);
   height  := rand(1.0 + chance, 2.0 + chance) * 100.0;
   speed   := rand(0.0, 100.0 * chance) + 30.0;
   fuel    := round(50.0 * rand(3.0 - chance, 4.0 - chance));
   maxlandingspeed := crashlandspeed - 10.0 * chance;
   misschance := certainty;
   ranout := false;
end;

(*
 * tell player his height, speed and direction
 *)

procedure writestatus;

begin
   writeln;
   write('You are ');
   if speed > 0.0 then
      write('falling')
   else
      write('rising');
   writeln(' from a height of ',height:1:1);
   writeln('meters at ',abs(speed):1:1,' m/s.');
   if not ranout then
      writeln('There are ',fuel:1,' liters of fuel left.');
   ranout := fuel = 0;
end;

(*
 * determines if there are any meteors, and if so, how many
 *)

procedure lookformeteors;

var
   eachrock : integer;

begin
   misschance := certainty;
   if unlucky(0.1) then
      begin
      nummeteors := round(rand(minmeteors,maxmeteors + 10.0 * chance));
      for eachrock := 1 to nummeteors do
         misschance := misschance * eachmisschance;
      writeln('We are on a collision course with ',nummeteors:1);
      write('meteors.');
      if ranout then
         writeln
      else
         begin
         writeln('If we do not use more than ',minavoid:1,'liters of fuel in the');
         writeln('next second, there is a ',round(100.0*(1.0-misschance)),
                 ' % probability that we will be');
         writeln('hit. If more is used it will be only 10 %.');
         end;
   end;
end;

(*
 * asks the player for the amount of fuel to use in the next time period.
 *)

procedure getburn;

const
   fuelprompt = 'Units of fuel : ';

begin
   repeat
      write(fuelprompt);
      readln(inp);
      burn := convert(inp);
      if burn > fuel then
         writeln('There isn''t that much fuel left.')
      else if burn < 0 then
         writeln('I don''t think that''s possible.');
   until (burn <= fuel) and (burn >= 0);
end;

(*
 * figure out the craft's new speed according to the laws of physics
 *)

procedure updatestatus;

var
   deltat : integer;

begin
   fuel := fuel - burn;
   deltat := 0;
   repeat
      deltat := deltat + 1;
      height := height - speed * timeinc - (gravity - burn) * 0.5 * sqr(timeinc);
      speed := speed + (gravity - burn) * timeinc;
   until (deltat = trunc(1 / timeinc)) or (height <= landingheight);
end;

(*
 * all the procedures that make a turn
 *)

procedure doaturn;

begin
   writestatus;
   lookformeteors;
   if fuel > 0 then
      getburn
   else
      burn := 0;
   updatestatus;
end;

(*
 * the course has changed: meteors have a lower chance of hitting
 *)

function coursechanged : boolean;

begin
   if unlucky(0.1) then
      begin
      writeln('Despite the precautionary measures taken, the ship was destroyed.');
      coursechanged := true;
      end
   else
      begin
      writeln('Your prudent actions saved the ship from the menacing meteors!');
      coursechanged := false;
      end
end;

function coursesame: boolean;

begin
   if not unlucky(1.0 - misschance) then
      begin
      writeln('Your piloting skills have steered you through the center of the swarm!');
      coursesame := false;
      end
   else if ranout then
      begin
      writeln('What a pity .. your craft was demolished by meteors before it could');
      writeln('be vaporized on contact with the surface.');
      coursesame := true;
      end
   else
      begin
      writeln('Your pointless gambling has destroyed the ship, you foolish plebe!');
      coursesame := true;
      end;
end;

(*
 * figures if any meteors (if there were any) managed to hit the ship.
 * different messages are printed depending on the thrust of the last turn.
 *)


function anyhit : boolean;

begin
   if (misschance = certainty) or (height <= landingheight) then
      anyhit := false
   else if burn > minavoid then
      anyhit := coursechanged
   else
      anyhit := coursesame;
end;

(*
 * returns true if the ship has come close enough to the ground that we can
 * say it has landed. could crash or touch down safely.
 *)

function landed : boolean;

begin
   if height < landingheight then
      begin
      speed := abs(speed);
      if speed < maxlandingspeed then
         write('We have landed safely')
      else
         write('We have crashed');
      writeln(' at a speed of ',speed:1:1,' meters/second.');
      landed := true;
      end
   else
      landed := false;
end;


begin
   startup;
   repeat
      startgame;
      repeat
         doaturn;
      until landed or anyhit;
   until ask('Again? ','Y or N',['Y','N']) = 'N';
   writeln('Bye!');
end.