delphispring4d

Spring4D cannot sort an ordered dictionary

I need a dictionary that can be sorted. I think Spring4D TOrderedDictionary is a class implementing that but I cannot make it work: sorting doesn't work.

I build a small test program showing the issue I have:

program Spring4DOrderedDictionaryTest;
{$APPTYPE CONSOLE}
{$R *.res}
uses
    System.SysUtils,
    System.Generics.Collections,
    System.Generics.Defaults,
    Spring.Collections;

type
    TKey   = TPair<Double, Integer>;
    TValue = Integer;

    TEqComparer = class(TEqualityComparer<TKey>, IEqualityComparer<TKey>)
        function Equals(const Left, Right: TKey): Boolean; reintroduce;
        function GetHashCode(const Value: TKey): Integer; reintroduce;
    end;

    TAComparer = class(TComparer<TPair<TPair<Double, Integer>, Integer>>, IComparer<TPair<TKey, TValue>>)
        function Compare(const Left, Right: TPair<TKey, Integer>): Integer; override;
    end;

function TEqComparer.Equals(const Left, Right: TKey): Boolean;
begin
    Result := Abs(Left.Key - Right.Key) < 1E-9;
    if Result then
        Result := Left.Value = Right.Value;
end;


function TEqComparer.GetHashCode(const Value: TKey): Integer;
begin
    Result := Round(Value.Key * 1E6) + Value.Value;
end;

function TAComparer.Compare(const Left, Right: TPair<TKey, Integer>): Integer;
begin
    if Abs(Left.Key.Key - Right.Key.Key) < 1E-9 then begin
        if Left.Key.Value < Right.Key.Value then
            Result := -1
        else if Left.Key.Value > Right.Key.Value then
            Result := 1
        else
            Result := 0;
    end
    else begin
        if Left.Key.Key < Right.Key.Key then
            Result := -1
        else if Left.Key.Key > Right.Key.Key then
            Result := 1
        else
            Result := 0;
    end;
end;

const
    SEG_START = 0;
    SEG_END   = 1;
var
    Events      : IOrderedDictionary<TKey, TValue>;
    Event       : TPair<TKey, TValue>;
    EqComparer  : IEqualityComparer<TKey>;
    AComparer   : IComparer<TPair<TKey, TValue>>;
begin
    EqComparer := TEqComparer.Create;
    AComparer  := TAComparer.Create;
    Events     := TCollections.CreateOrderedDictionary<TKey, TValue>;//(EqComparer);

    // Put some data into the dictionary
    Events.Add(TKey.Create(-7.41, SEG_START), 0);
    Events.Add(TKey.Create(-1.3,  SEG_END  ), 0);
    Events.Add(TKey.Create(-4.0,  SEG_START), 1);
    Events.Add(TKey.Create(-4.21, SEG_END  ), 1);
    Events.Add(TKey.Create(-4.92, SEG_START), 2);
    Events.Add(TKey.Create(-4.26, SEG_END  ), 2);
    Events.Add(TKey.Create(-4.55, SEG_START), 3);
    Events.Add(TKey.Create(-2.54, SEG_END  ), 3);
    Events.Add(TKey.Create(-3.70, SEG_START), 4);
    Events.Add(TKey.Create(-3.70, SEG_END  ), 4);

    // Sort the dictionary
    Events.Ordered(AComparer);

    // Show the values in dictionary (Should be sorted -7.41 first and -1.3 last)    
    for Event in Events do
        WriteLn(Format('  %5.2f %d %d', [Event.Key.Key, Event.Key.Value, Event.Value]));

    ReadLn;
end.

The code above display the values in the add order, not the order that - I think - AComparer should create.

I tried to create the OrderedDictionary using an equality comparer or not: with no change.

Any help appreciated.

Solution

  • First you have to know that usually the order of items in a dictionary is undefined as it depends on the underlying data structure - usually it's a hashtable so the order of items while deterministic appears "random" or in no particular order.

    Now there are certain dictionaries in spring4d that make a guarantee on item order.

    In 1.2.2 (the currently latest released version) there are generally three ways to create dictionaries:

    TCollections.CreateDictionary // internally is just a wrapper around the TDictionary from System.Generics.Collections
TCollections.CreateSortedDictionary // internally uses a red/black tree which makes items ordered - when not explicitly passed it uses the default comparer for the key type
TCollections.CreateOrderedDictionary // internally uses an additional list to preserve items in order they were added to the dictionary

    In the upcoming 2.0 (currently develop branch) there are two ways:

    TCollections.CreateDictionary // internally uses a new implementation based on a hashtable which also preserves the order of addition
TCollections.CreateSortedDictionary // same as in 1.2.2 this uses a red/black tree to store items based on the IComparer<TKey>

    Second you have to know that the Ordered method comes from IEnumerable<T> and does not modify the collection it is being called on but returns a new IEnumerable<T> which represents the items in the order determined by the comparer you passed to that function.

    So you have two options: