PASCAL MACRO COMPILER BIT ARRAY MACROS

PASCAL MACRO COMPILER
Sample Bit Array Program

This sample program illustates how the bit array macros can be used. The bit array macros are included using the %I include directive. The Init_Bit macro procedure is used to define the bit masks. The Clear_Bit and Set_Bit macro functions are used to initialize the bit array, and to set individual bits. The Test_Bit macro procedure is used to test the bits.

Notice, in particular, the section of code

    begin
      ct := ct + 1;
      Set_Bit (target, n);		{No.  Mark it occupied}
    end

which is used as an argument of the Test_Bit procedure. It is possible to use this block of code as a procedure argument because Set_Bit has been declared as a macro function. The entire begin-end block is a code expression, and therefore may contain macro variables and macro functions.

The sample program estimates how many darts can be thrown at random into a target that has been divided into 100 equally probable cells before any cell is hit twice. The generated Pascal program calculates the average of 250 simulations.

The generated Pascal program is shown following the macro program. The source code for the bit array macros can be seen at Bit Array Macros.

%Macro	BitSample;

{%I pas\bitarray.mac}

%Begin	{BitSample}
Program  Darts;

Var
  rep: integer; 			{Repeat counter}
  n: integer;				{Loop index}
  ct: integer;				{Hit count}
  target: array[0..12] of byte; 	{Target in byte form}
  total,avg: real;
%Init_bit;

Begin  {Darts}
Randomize;
total := 0;
for  rep := 1 to 250  do begin
%;
  for  n := 0 to 100-1	do		{Clear the target}
    Clear_Bit (target, n);

  ct := 0;
  repeat
    n := random (100);			{Throw a random dart}
    %Test_Bit (target, n, break,	{Is the cell already occupied?}
    begin
      ct := ct + 1;
      Set_Bit (target, n);		{No.  Mark it occupied}
    end);
  until  false;
  total := total + ct;
end;
avg := total / 250;
writeln ('Average is ', avg:4:2, ' darts');
End.  {Darts}
%End.  {BitSample}

This is the Pascal program that is generated by the Pascal Macro Compiler from the macro program shown above.

Program  Darts;

Var
  rep: integer; 			{Repeat counter}
  n: integer;				{Loop index}
  ct: integer;				{Hit count}
  target: array[0..12] of byte; 	{Target in byte form}
  total,avg: real;
Var
  byte_index, bit_index: integer;	{Variables needed for bit arrays}
Const
  mask_bit: array[0..7] of byte 	{Masks for bit arrays}
    = (1, 2, 4, 8, 16, 32, 64, 128);
  unmask_bit: array[0..7] of byte
    = (254, 253, 251, 247, 239, 223, 191, 127);

Begin  {Darts}
Randomize;
total := 0;
for  rep := 1 to 250  do begin
  for  n := 0 to 100-1	do		{Clear the target}
    begin
  bit_index := ( n) and 7 ;
  byte_index := ( n) shr 3 ;
  target[byte_index] := target[byte_index] and unmask_bit[bit_index] ;
end;

  ct := 0;
  repeat
    n := random (100);			{Throw a random dart}
begin
bit_index := ( n) and 7;
byte_index := ( n) shr 3;
if  (target[byte_index] and mask_bit[bit_index]) <> 0
    then   break
    else	{Is the cell already occupied?}
    begin
      ct := ct + 1;
      begin
  bit_index := ( n) and 7 ;
  byte_index := ( n) shr 3 ;
  target[byte_index] := target[byte_index] or mask_bit[bit_index] ;
end;		{No.  Mark it occupied}
    end;
end;
  until  false;
  total := total + ct;
end;
avg := total / 250;
writeln ('Average is ', avg:4:2, ' darts');
End.  {Darts}