-- three versions of fibonacci function in Ada 95

-- to run this program on a system (linux or cygwin) with gnat ada95 installed:
-- type "gnatmake fibs.adb" then "./fibs".


with Text_IO, Ada.Integer_Text_IO;
use Text_IO, Ada.Integer_Text_IO;
with Ada.Strings.Unbounded; use Ada.Strings.Unbounded;
with Ada.Float_Text_IO; use Ada.Float_Text_IO;

-- Most basic Ada code segments are enclosed in begin and end. Sometimes,
-- such as with function/procedure bodies, "do" or "is" or "loop" is
-- used in place of "begin". They correspond to { and } in other
-- languages.  Every code segment is logically made up of a declaration
-- segment and a body with actual code (in C++/Java declarations and
-- code can be mixed, but not in Ada).

-- The basic format of an Ada procedure is:

--  procedure name is
--        declarations ...  (variables, local functions, datatypes)
--  begin
--         body of procedure
--  end name;

-- Procedures can have in (default), out and in out parameters.  Functions
-- may only have in parameters.  In params cannot be changed.

--  There's no "main" procedure but the outermost procedure (fibs here)
--  should match the filename


-- warning: Ada is NOT case sensitive

procedure fibs is

function Fib1(N : Integer) return Integer is
  begin
     if (N<2) then return 1;
       else return (Fib1(N-1) + Fib1(N-2));
     end if;
  end Fib1;


  function Fib2(N : Integer) return Integer is

     -- declaring a local function within fib2:
     function F(N : Integer; A:Integer; B:Integer) return Integer is
       begin
          if (N<2) then return B; else return F(N-1,B,A+B);
          end if;
       end F;
     --
  begin
     return F(N,1,1);
  end Fib2;


  -- in the third version, we use a while loop together with an "out"
  -- parameter to return the value:
  procedure Fib3(N:Integer; Answer : out Integer) is
     -- local vars:
     A : Integer;
     B : Integer;
     I : Integer;
     Temp : Integer;
  begin
     A := 1;
     B := 1;    -- note the use of ":=" for assignment, "=" means "==" in ada
     I := N;
     while (I>1) loop
        Temp := B;
        B := A+B;
        A := Temp;
        I := I-1;
     end loop;
     Answer := B;    -- puts result in out parameter
  end Fib3;

  -- another thing to note: why is the variable I needed?  Why not just
  -- use N?  Because Ada is trying to impose a stricter style of programming
  -- on you.  You cannot change the values of "in" parameters.

  X, Y : Integer;   -- utility vars
begin   -- this is the main body of fibs
   Put("The 5th Fibonacci number is "); Put(Fib1(5),Width=>4); New_Line;
   Put("With tail recursion it's also"); Put(Fib2(5),Width=>4); New_Line;
   Fib3(5,X);
   Put("Without recursion it's still "); Put(X,Width=>4); New_Line;
end;