Counting sort

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The counting sort is an integer sorting algorithm.

Features

  • Very fast
  • Only small integers

unit UCountingSortExtra.pas

unit UCountingSortExtra;

interface

type
  // data type
  TItemCountingSort=integer;

// sort order
function IsAscendingCountingSort: boolean; inline;

implementation

// sort order
function IsAscendingCountingSort: boolean; inline;
begin
   result := true;
end;

end.


unit UCountingSort.pas

unit UCountingSort;

interface

uses
  UCountingSortExtra;

// sorting function
procedure CountingSort( var a: array of TItemCountingSort );

implementation


procedure CountingSort( var a: array of TItemCountingSort );
var
  min, max : TItemCountingSort;
  count_a  : array of integer;
  i, j, z  : integer;
begin
  min := high( a );
  max := min;
  for i := low( a ) to high( a ) do
    begin
      if a[ i ] < min then min := a[ i ];
      if a[ i ] > max then max := a[ i ];
    end;
  SetLength( count_a, max - min + 1);
  for i := 0 to ( max - min ) do count_a[ i ] := 0;
  for i := low( a ) to high( a ) do
    count_a[ a[ i ] - min ] := count_a[ a[ i ] - min ] + 1;
  if IsAscendingCountingSort then z:= low( a ) else z := high( a );
  for i := min to max do
     for j := 0 to ( count_a[ i - min ] - 1 ) do
       begin
         a[ z ] := i;
         if IsAscendingCountingSort then inc( z ) else dec( z );
       end;
end;

end.

Example of the use

uses
  UCountingSort

  ...

var

  a: array[0..100] of integer;


begin

  ...

  CountingSort( a );