Array sort
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A typical task is to sort an array. One of the problems is that an array could be of any type. Not a simple type, but more complicated types.
Basic Version
Prior to introducing Generics into Freepascal, sorting an array of arbitrary type could be achieve via untyped variables (implicit Pointers).
- No generics involved
- works on any version of FPC (or Delphi)
- any array data type
Source Code
unit anysort;
{$ifdef fpc}{$mode delphi}{$H+}{$endif}
interface
type
TCompareFunc = function (const elem1, elem2): Integer;
procedure AnySort(var Arr; Count: Integer; Stride: Integer; CompareFunc: TCompareFunc);
implementation
type
TByteArray = array [Word] of byte;
PByteArray = ^TByteArray;
procedure AnyQuickSort(var Arr; idxL, idxH: Integer;
Stride: Integer; CompareFunc: TCompareFunc; var SwapBuf, MedBuf);
var
ls,hs : Integer;
li,hi : Integer;
mi : Integer;
ms : Integer;
pb : PByteArray;
begin
pb:=@Arr;
li:=idxL;
hi:=idxH;
mi:=(li+hi) div 2;
ls:=li*Stride;
hs:=hi*Stride;
ms:=mi*Stride;
Move(pb[ms], medBuf, Stride);
repeat
while CompareFunc( pb[ls], medBuf) < 0 do begin
inc(ls, Stride);
inc(li);
end;
while CompareFunc( medBuf, pb[hs] ) < 0 do begin
dec(hs, Stride);
dec(hi);
end;
if ls <= hs then begin
Move(pb[ls], SwapBuf, Stride);
Move(pb[hs], pb[ls], Stride);
Move(SwapBuf, pb[hs], Stride);
// begin fix 11/11/2021: update ms if the reference point is moved
if li=mi then ms:=hs;
if hi=mi then ms:=ls;
// end fix
inc(ls, Stride); inc(li);
dec(hs, Stride); dec(hi);
end;
until ls>hs;
if hi>idxL then AnyQuickSort(Arr, idxL, hi, Stride, CompareFunc, SwapBuf, MedBuf);
if li<idxH then AnyQuickSort(Arr, li, idxH, Stride, CompareFunc, SwapBuf, MedBuf);
end;
procedure AnySort(var Arr; Count: Integer; Stride: Integer; CompareFunc: TCompareFunc);
var
buf: array of byte;
begin
if Count <= 1 then Exit; // should be more than 1 to be sortable
SetLength(buf, Stride*2);
AnyQuickSort(Arr, 0, Count-1, Stride, compareFunc, buf[0], buf[Stride]);
end;
end.
Use Sample
Here's an example on how to use AnySort() function to sort an array of Integer
interface
uses AnySort;
procedure SortArrayInteger(var arr: array of Integer; count: Integer);
implementation
function CompareInt(const d1,d2): integer;
var
i1 : integer absolute d1;
i2 : integer absolute d2;
begin
if i1=i2 then Result:=0
else if i1<i2 then Result:=-1
else Result:=1;
end;
procedure SortArrayInteger(var arr: array of Integer; count: Integer);
begin
anysort.AnySort(arr, Count, sizeof(Integer), @CompareInt);
end;
Generics Version
- FPC-only.
- Uses generic with static comparer.
- Comparer is a simple less predicate.
- Essentially an Introsort, a QuickSort that falls back to a simple algorithm for small enough subarrays.
- Bypasses managed types handling to greatly speed up sorting arrays of reference-counted types; won't work with custom Copy/AddRef operators.
- Protected against
- O(N²) time (falls back to heap sort);
- O(N) recursion depth; maximum recursion depth is ⌈bitsizeof(SizeUint) - log₂ SelectionThreshold⌉.
Source Code
{$mode objfpc} {$coperators on}
unit AnySort2;
interface
type
// Comparer should provide function Less(const a, b: Elem): boolean.
generic Sorter<Elem, Comparer> = class
type
pElem = ^Elem;
class procedure Sort(p: pElem; count: SizeUint); static;
class procedure Sort(var a: array of Elem); static;
private const
SelectionThreshold = 12;
HeapArity = 4;
type
SwapTemp = array[0 .. sizeof(Elem) - 1] of byte;
class procedure SelectionSort(p: pElem; count: SizeUint); static;
class function Median(p: pElem; count: SizeUint): pElem; static;
class procedure QSort(p: pElem; count, reasonable: SizeUint); static;
class procedure HeapReplacePessimistic(p: pElem; count, id: SizeUint; const x: SwapTemp); static;
class procedure HeapSort(p: pElem; count: SizeUint); static;
end;
generic ComparerLessOp<Elem> = class
class function Less(const a, b: Elem): boolean; static; inline;
end;
generic SorterLessOp<Elem> = class(specialize Sorter<Elem, specialize ComparerLessOp<Elem>>) end;
implementation
class procedure Sorter.Sort(p: pElem; count: SizeUint);
begin
QSort(p, count, count);
end;
class procedure Sorter.Sort(var a: array of Elem);
begin
Sort(pElem(a), length(a));
end;
class procedure Sorter.SelectionSort(p: pElem; count: SizeUint);
var
i, j, imin: SizeInt;
t: SwapTemp;
begin
for i := 0 to SizeInt(count) - 2 do
begin
imin := i;
for j := i + 1 to SizeInt(count) - 1 do
if Comparer.Less(p[j], p[imin]) then imin := j;
t := SwapTemp(p[i]); SwapTemp(p[i]) := SwapTemp(p[imin]); SwapTemp(p[imin]) := t;
end;
end;
class function Sorter.Median(p: pElem; count: SizeUint): pElem;
var
a, b, c, t: pElem;
begin
a := p;
b := p + count div 2;
c := p + SizeUint(count - 1);
if Comparer.Less(b^, a^) then begin t := a; a := b; b := t; end;
if Comparer.Less(c^, b^) then begin t := c; c := b; b := t; end;
if Comparer.Less(b^, a^) then result := a else result := b;
end;
// see MSVC implementation of std::sort
class procedure Sorter.QSort(p: pElem; count, reasonable: SizeUint);
var
L, R: SizeInt;
t, pivot: SwapTemp;
begin
while (count > SelectionThreshold) and (reasonable > 0) do
begin
reasonable := reasonable div 2 + reasonable div 4;
pivot := SwapTemp(Median(p, count)^);
R := 0;
L := count - 1;
repeat
while Comparer.Less(p[R], Elem(pivot)) do inc(R);
while Comparer.Less(Elem(pivot), p[L]) do dec(L);
if R <= L then
begin
t := SwapTemp(p[R]); SwapTemp(p[R]) := SwapTemp(p[L]); SwapTemp(p[L]) := t;
inc(R);
dec(L);
end;
until R > L;
// [0 .. L], [R .. count - 1]
// possible edge cases are L = -1 or R = count !
if SizeInt(count) - R <= L then
begin
QSort(p + R, SizeInt(count) - R, reasonable); // QSort calls with count = 0 or count = 1 are safe.
count := L + 1;
end else
begin
QSort(p, L + 1, reasonable);
p += R;
count -= R;
end;
end;
if count > SelectionThreshold then
HeapSort(p, count)
else if count >= 2 then
SelectionSort(p, count);
end;
// HeapReplacePessimistic(p, count, id, x)
//
// is equivalent to
//
// p[id] := x;
// id := HeapDownThoroughly(..., id);
// HeapUp(..., id)
//
// where HeapDownThoroughly doesn't stop at correct position, instead shifting the element all the way down, saving one compare at each level.
// HeapUp then bubbles the element up to the correct position.
// See Python's 'heapq' for justification over simple HeapDown.
//
// Careful with 'x' passed by reference and pointing into 'p'!
class procedure Sorter.HeapReplacePessimistic(p: pElem; count, id: SizeUint; const x: SwapTemp);
var
child, bestChild, lastChild, parent, start: SizeUint;
begin
start := id;
repeat
child := HeapArity * id; // childs of 'id' are p[child + 1] ... p[child + HeapArity].
bestChild := child + 1;
if bestChild >= count then break;
lastChild := child + HeapArity;
if lastChild >= count then lastChild := count - 1;
for child := child + 2 to lastChild do
if Comparer.Less(p[bestChild], p[child]) then bestChild := child;
SwapTemp(p[id]) := SwapTemp(p[bestChild]);
id := bestChild;
until false;
while id > start do
begin
parent := (id - 1) div HeapArity;
if not Comparer.Less(p[parent], Elem(x)) then break;
SwapTemp(p[id]) := SwapTemp(p[parent]);
id := parent;
end;
SwapTemp(p[id]) := x;
end;
class procedure Sorter.HeapSort(p: pElem; count: SizeUint);
var
i: SizeInt;
t: SwapTemp;
begin
for i := SizeInt((count + (HeapArity - 2)) div HeapArity) - 1 downto 0 do
begin
t := SwapTemp(p[i]);
HeapReplacePessimistic(p, count, i, t);
end;
for i := SizeInt(count) - 1 downto 1 do
begin
t := SwapTemp(p[i]);
SwapTemp(p[i]) := SwapTemp(p[0]);
HeapReplacePessimistic(p, i, 0, t);
end;
end;
class function ComparerLessOp.Less(const a, b: Elem): boolean;
begin
result := a < b;
end;
end.
Use Sample
Usage:
{$mode objfpc}
uses AnySort2;
var
a: array of integer;
begin
a := specialize TArray<integer>.Create(111, 555, 888, 777, 333, 444, 666);
specialize SorterLessOp<integer>.Sort(a);
end.