program FindNthSetBit32;
{
findNthSetBit32:
----------------
Given:
- x : a 32-bit unsigned integer
- n : which set bit to find (1-based index)
Returns:
A 32-bit mask with ONLY the Nth set bit of x turned on.
If n is larger than the number of set bits, returns 0.
Algorithm:
- Count trailing zeros to locate the lowest set bit.
- Remove that bit using x := x and (x - 1).
- When we reach the Nth one, return 1 shl index.
Notes:
- Pascal does not have a built-in ctz(), so we implement
a small efficient function that scans from LSB upward.
}
{$mode objfpc} { Enable modern FreePascal features }
type
UInt32 = Cardinal;
{ Count trailing zeros (ctz) for a 32-bit integer.
Returns the index (0–31) of the lowest set bit.
Assumes x <> 0. }
function CountTrailingZeros(x: UInt32): Integer;
var
i: Integer;
begin
i := 0;
while (x and 1) = 0 do
begin
x := x shr 1;
Inc(i);
end;
CountTrailingZeros := i;
end;
{ Find the Nth set bit (1-based) in a 32-bit integer.
Returns a mask with ONLY that bit set, or 0 if not enough bits. }
function FindNthSetBit32(x: UInt32; n: UInt32): UInt32;
var
index: Integer;
begin
while x <> 0 do
begin
{ Locate lowest set bit }
index := CountTrailingZeros(x);
{ If this is the Nth set bit, return mask }
Dec(n);
if n = 0 then
begin
FindNthSetBit32 := UInt32(1) shl index;
Exit;
end;
{ Remove lowest set bit }
x := x and (x - 1);
end;
{ Fewer than n set bits }
FindNthSetBit32 := 0;
end;
{ Print a 32-bit binary representation }
function ToBinary32(x: UInt32): String;
var
i: Integer;
begin
Result := '';
for i := 31 downto 0 do
Result := Result + Chr(Ord('0') + ((x shr i) and 1));
end;
var
value: UInt32;
n: UInt32;
resultMask: UInt32;
begin
value := %00001101001101101100100010100000;
n := 4;
resultMask := FindNthSetBit32(value, n);
Writeln('Input value: ', ToBinary32(value));
Writeln('N = ', n);
Writeln('Result mask: ', ToBinary32(resultMask));
end.
{
run:
Input value: 00001101001101101100100010100000
N = 4
Result mask: 00000000000000000100000000000000
}
