Added MultiIndexEnumerator

AdventOfCode.lpi

@@ -157,6 +157,10 @@
         <Filename Value="UCommon.pas"/>
         <IsPartOfProject Value="True"/>
       </Unit>
+      <Unit>
+        <Filename Value="UMultiIndexEnumerator.pas"/>
+        <IsPartOfProject Value="True"/>
+      </Unit>
     </Units>
   </ProjectOptions>
   <CompilerOptions>

UMultiIndexEnumerator.pas

@@ -0,0 +1,160 @@
+{
+  Solutions to the Advent Of Code.
+  Copyright (C) 2024  Stefan Müller
+
+  This program is free software: you can redistribute it and/or modify it under
+  the terms of the GNU General Public License as published by the Free Software
+  Foundation, either version 3 of the License, or (at your option) any later
+  version.
+
+  This program is distributed in the hope that it will be useful, but WITHOUT
+  ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
+  FOR A PARTICULAR PURPOSE.  See the GNU General Public License for more details.
+
+  You should have received a copy of the GNU General Public License along with
+  this program.  If not, see <http://www.gnu.org/licenses/>.
+}
+
+unit UMultiIndexEnumerator;
+
+{$mode ObjFPC}{$H+}
+
+interface
+
+uses
+  Classes, SysUtils;
+
+type
+  TIndexArray = array of Integer;
+
+  TIndexValidationResult = (ivrValid, ivrSkip, ivrBacktrack);
+
+  TEnumerableMultiIndexStrategy = class;
+
+  { TMultiIndexEnumerator }
+
+  TMultiIndexEnumerator = class(TInterfacedObject, specialize IEnumerator<TIndexArray>)
+  private
+    FStrategy: TEnumerableMultiIndexStrategy;
+    FCurrent: TIndexArray;
+    FMustInit: Boolean;
+    function UpdateArray(const AInit: Boolean): Boolean;
+  public
+    constructor Create(const AStrategy: TEnumerableMultiIndexStrategy);
+    function GetCurrent: TIndexArray;
+    function MoveNext: Boolean;
+    procedure Reset;
+    property Current: TIndexArray read GetCurrent;
+  end;
+
+  { TEnumerableMultiIndexStrategy }
+
+  TEnumerableMultiIndexStrategy = class(TInterfacedObject, specialize IEnumerable<TIndexArray>)
+  public
+    function GetEnumerator: specialize IEnumerator<TIndexArray>;
+    function GetCardinality: Integer; virtual; abstract;
+    function TryGetStartIndexValue(constref ACurrentIndexArray: TIndexArray; const ACurrentIndex: Integer;
+      out AStartIndexValue: Integer): Boolean; virtual; abstract;
+    function ValidateIndexValue(constref ACurrentIndexArray: TIndexArray; const ACurrentIndex: Integer):
+      TIndexValidationResult; virtual; abstract;
+  end;
+
+implementation
+
+{ TMultiIndexEnumerator }
+
+function TMultiIndexEnumerator.UpdateArray(const AInit: Boolean): Boolean;
+var
+  i, initialized: Integer;
+  r: TIndexValidationResult;
+begin
+  if AInit then
+  begin
+    i := 0;
+    initialized := -1;
+  end
+  else begin
+    i := Length(FCurrent) - 1;
+    initialized := i;
+  end;
+
+  while i < Length(FCurrent) do
+  begin
+    if initialized < i then
+    begin
+      // Checks whether start index value can be set, and backtracks or aborts if not.
+      if not FStrategy.TryGetStartIndexValue(FCurrent, i, FCurrent[i]) then
+        if i > 0 then
+        begin
+          Dec(i);
+          Continue;
+        end
+        else begin
+          Result := False;
+          Exit;
+        end
+    end
+    else
+      // Sets next candidate for current index value.
+      Inc(FCurrent[i]);
+
+    // Checks if current index value is valid, and increases it until it is, or backtracks or aborts if so indicated.
+    while True do
+    begin
+      r := FStrategy.ValidateIndexValue(FCurrent, i);
+      case r of
+        ivrValid: begin
+          initialized := i;
+          Inc(i);
+          Break;
+        end;
+        ivrSkip:
+          Inc(FCurrent[i]);
+        ivrBacktrack:
+          if i > 0 then
+          begin
+            Dec(i);
+            Break;
+          end
+          else begin
+            Result := False;
+            Exit;
+          end;
+      end;
+    end;
+  end;
+  Result := True;
+end;
+
+constructor TMultiIndexEnumerator.Create(const AStrategy: TEnumerableMultiIndexStrategy);
+begin
+  FStrategy := AStrategy;
+  SetLength(FCurrent, FStrategy.GetCardinality);
+  Reset;
+end;
+
+function TMultiIndexEnumerator.GetCurrent: TIndexArray;
+begin
+  Result := FCurrent;
+end;
+
+function TMultiIndexEnumerator.MoveNext: Boolean;
+begin
+  Result := UpdateArray(FMustInit);
+  FMustInit := False;
+end;
+
+procedure TMultiIndexEnumerator.Reset;
+begin
+  FMustInit := True;
+end;
+
+{ TEnumerableMultiIndexStrategy }
+
+function TEnumerableMultiIndexStrategy.GetEnumerator: specialize IEnumerator<TIndexArray>;
+begin
+  Result := TMultiIndexEnumerator.Create(Self);
+end;
+
+end.
+