with Ada.Text_IO; -- Import input-output package
 
procedure Hello is
begin
   -- Put_Line prints text with a trailing newline
   Ada.Text_IO.Put_Line ("Hello, World!");
end Hello;

procedure Variables is
   -- Declarations go in this section before the 'begin' block
   Age : Integer := 25;
   Pi  : constant Float := 3.14159;
   
   -- Custom Integer Type with strict range constraints
   type Day_Of_Month is new Integer range 1 .. 31;
   Today : Day_Of_Month := 28;
   
   -- Subtype (retains compatibility with parent type but checks ranges)
   subtype Working_Hours is Integer range 0 .. 8;
   Shift : Working_Hours := 8;
begin
   -- Today := Today + 10; -- Will raise Constraint_Error at runtime if value exceeds 31!
   null; -- Keyword indicating no action (empty statement)
end Variables;

if Score >= 90 then
   Ada.Text_IO.Put_Line ("Grade A");
elsif Score >= 80 then
   Ada.Text_IO.Put_Line ("Grade B");
else
   Ada.Text_IO.Put_Line ("Grade F");
end if;
 
-- Case statement (Must be exhaustive!)
case Score is
   when 90 .. 100 =>
      Ada.Text_IO.Put_Line ("Excellent");
   when 80 .. 89 =>
      Ada.Text_IO.Put_Line ("Good");
   when others =>
      Ada.Text_IO.Put_Line ("Other");
end case;

-- 1. Standard Counted Loop (Range is inclusive)
for I in 1 .. 5 loop
   Ada.Text_IO.Put_Line ("Index: " & Integer'Image (I));
end loop;
 
   -- Image attribute converts integer to printable string
   
-- 2. While Loop
while Counter < 3 loop
   Counter := Counter + 1;
end loop;
 
-- 3. Infinite loop with Exit statement
loop
   Counter := Counter - 1;
   exit when Counter = 0; -- Break loop condition
end loop;

-- 1. Procedure definition
procedure Calculate (X : in Float; Y : in Float; Result : out Float) is
begin
   Result := X + Y;
end Calculate;
 
-- 2. Function definition
function Square (Val : in Float) return Float is
begin
   return Val * Val;
end Square;
 
-- Parameter Modes:
-- in      : Read-only (default if unspecified)
-- out     : Write-only (cannot read original value inside)
-- in out  : Read-write (can both read and modify value)

-- math_utils.ads
package Math_Utils is
   -- Public functions
   function Double (X : in Integer) return Integer;
   
private
   -- Hidden elements
   Secret_Multiplier : constant Integer := 2;
end Math_Utils;

-- math_utils.adb
package body Math_Utils is
   function Double (X : in Integer) return Integer is
   begin
      return X * Secret_Multiplier;
   end Double;
end Math_Utils;

-- Task specification
task type Worker is
   entry Start (ID : in Integer); -- Entry point
end Worker;
 
-- Task body (execution thread)
task body Worker is
   Local_ID : Integer;
begin
   accept Start (ID : in Integer) do
      Local_ID := ID; -- Rendezvous point (blocks until caller hits entry)
   end Start;
   
   Ada.Text_IO.Put_Line ("Worker running: " & Integer'Image (Local_ID));
end Worker;

protected Counter is
   procedure Increment;
   function Get return Integer;
private
   Val : Integer := 0;
end Counter;
 
protected body Counter is
   procedure Increment is
   begin
      Val := Val + 1; -- Exclusive write lock
   end Increment;
   
   function Get return Integer is
   begin
      return Val; -- Shared read lock
   end Get;
end Counter;

-- Generic procedure template
generic
   type Element is private; -- generic type argument
procedure Swap (X, Y : in out Element);
 
procedure Swap (X, Y : in out Element) is
   Temp : Element;
begin
   Temp := X;
   X := Y;
   Y := Temp;
end Swap;
 
-- Instantiation:
-- procedure Swap_Int is new Swap (Integer);

# Compile Ada program (handles spec and body dependency linking)
gnatmake hello.adb
 
# Run executable
./hello