AdventOfCode2023/solvers/UParabolicReflectorDish.pas

{
  Solutions to the Advent Of Code.
  Copyright (C) 2023  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 UParabolicReflectorDish;

{$mode ObjFPC}{$H+}

interface

uses
  Classes, SysUtils, Generics.Collections, USolver;

const
  CRoundRockChar = 'O';
  CCubeRockChar = '#';
  CEmptyChar = '.';
  CMaxSpinCount = 1000000000;

type

  { TRockPile }

  TRockPile = class
  private
    FStart, FLength: Integer;
  public
    constructor Create(const AStart: Integer);
    procedure AddRock;
    function Any: Boolean;
    procedure SetStart(const AStart: Integer);
    function CalcWeight(const ALineCount: Integer): Integer;
  end;

  TRockPiles = specialize TObjectList<TRockPile>;

  { TPlatform }

  TPlatform = class
  private
    FLines: TStringList;
    procedure TiltNorth;
    procedure TiltSouth;
    procedure TiltWest;
    procedure TiltEast;
    function IsEqualTo(const FOther: TStringList): Boolean;
    procedure SwapRockLocation(const AColumn, ARockLineIndex, AEmptyLineIndex: Integer);
  public
    constructor Create;
    destructor Destroy; override;
    procedure Add(const ALine: string);
    procedure Spin;
    function CalcWeight: Integer;
  end;

  { TParabolicReflectorDish }

  TParabolicReflectorDish = class(TSolver)
  private
    FLineIndex: Integer;
    FActivePiles, FFinishedPiles: TRockPiles;
    FPlatform: TPlatform;
  public
    constructor Create;
    destructor Destroy; override;
    procedure ProcessDataLine(const ALine: string); override;
    procedure Finish; override;
    function GetDataFileName: string; override;
    function GetPuzzleName: string; override;
  end;

implementation

{ TRockPile }

constructor TRockPile.Create(const AStart: Integer);
begin
  FStart := AStart;
  FLength := 0;
end;

procedure TRockPile.AddRock;
begin
  Inc(FLength);
end;

function TRockPile.Any: Boolean;
begin
  Result := (FLength > 0);
end;

procedure TRockPile.SetStart(const AStart: Integer);
begin
  FStart := AStart;
end;

function TRockPile.CalcWeight(const ALineCount: Integer): Integer;
begin
  Result := FLength * (2 * (ALineCount - FStart) - FLength + 1) div 2;
end;

{ TPlatform }

procedure TPlatform.TiltNorth;
var
  i, j, k: Integer;
begin
  for i := 0 to FLines.Count - 1 do
    for j := 1 to Length(FLines[i]) do
      if FLines[i][j] = CRoundRockChar then
      begin
        k := i - 1;
        while (k >= 0) and (FLines[k][j] = CEmptyChar) do
          Dec(k);
        Inc(k);
        if k < i then
          SwapRockLocation(j, i, k);
      end;
end;

procedure TPlatform.TiltSouth;
var
  i, j, k: Integer;
begin
  for i := FLines.Count - 1 downto 0 do
    for j := 1 to Length(FLines[i]) do
      if FLines[i][j] = CRoundRockChar then
      begin
        k := i + 1;
        while (k < FLines.Count) and (FLines[k][j] = CEmptyChar) do
          Inc(k);
        Dec(k);
        if k > i then
          SwapRockLocation(j, i, k);
      end;
end;

procedure TPlatform.TiltWest;
var
  i, j, k: Integer;
  s: string;
begin
  for i := 0 to FLines.Count - 1 do
  begin
    s := FLines[i];
    k := 1;
    for j := 1 to Length(s) do
    begin
      case s[j] of
        CEmptyChar:
          if k <= 0 then
            k := j;
        CRoundRockChar: begin
          if (k > 0) and (k < j) then
          begin
            s[k] := CRoundRockChar;
            s[j] := CEmptyChar;
            Inc(k);
          end
          else
            k := 0;
        end;
        CCubeRockChar: k := 0;
      end;
    end;
    FLines[i] := s;
  end;
end;

procedure TPlatform.TiltEast;
var
  i, j, k: Integer;
  s: string;
begin
  for i := 0 to FLines.Count - 1 do
  begin
    s := FLines[i];
    k := Length(s) + 1;
    for j := Length(s) downto 1 do
    begin
      case s[j] of
        CEmptyChar:
          if k > Length(s) then
            k := j;
        CRoundRockChar: begin
          if (k <= Length(s)) and (j < k) then
          begin
            s[k] := CRoundRockChar;
            s[j] := CEmptyChar;
            Dec(k);
          end
          else
            k := Length(s) + 1;
        end;
        CCubeRockChar: k := Length(s) + 1;
      end;
    end;
    FLines[i] := s;
  end;
end;

function TPlatform.IsEqualTo(const FOther: TStringList): Boolean;
var
  i: Integer;
begin
  if FLines.Count = FOther.Count then
  begin
    Result := True;
    for i := 0 to FLines.Count - 1 do
      if FLines[i] <> FOther[i] then
      begin
        Result := False;
        Exit;
      end;
  end
  else
    Result := False;
end;

procedure TPlatform.SwapRockLocation(const AColumn, ARockLineIndex, AEmptyLineIndex: Integer);
var
  s: string;
begin
  s := FLines[ARockLineIndex];
  s[AColumn] := CEmptyChar;
  FLines[ARockLineIndex] := s;
  s := FLines[AEmptyLineIndex];
  s[AColumn] := CRoundRockChar;
  FLines[AEmptyLineIndex] := s;
end;

constructor TPlatform.Create;
begin
  FLines := TStringList.Create;
end;

destructor TPlatform.Destroy;
begin
  FLines.Free;
  inherited Destroy;
end;

procedure TPlatform.Add(const ALine: string);
begin
  FLines.Add(ALine);
end;

procedure TPlatform.Spin;
var
  i, j, x: Integer;
  match: Boolean;
  history: specialize TObjectList<TStringList>;
begin
  // Intializes history of platform rock configurations.
  history := specialize TObjectList<TStringList>.Create;
  history.Add(TStringList.Create);
  history[0].AddStrings(FLines);

  // Performs spins until a configuration from the history is encountered again.
  for i := 1 to CMaxSpinCount do
  begin
    TiltNorth;
    TiltWest;
    TiltSouth;
    TiltEast;

    // Searches history for the current configuration.
    j := 0;
    match := False;
    while (j < history.Count) and not match do
    begin
      match := IsEqualTo(history[j]);
      if not match then
        Inc(j);
    end;

    if match then
    begin
      x := CMaxSpinCount mod (i - j);
      while x < j do
        Inc(x, i - j);
      FLines.Free;
      FLines := history.ExtractIndex(x);
      Break;
    end
    else begin
      history.Add(TStringList.Create);
      history[i].AddStrings(FLines);
    end;
  end;
  history.Free;
end;

function TPlatform.CalcWeight: Integer;
var
  i, j, len, count: Integer;
begin
  Result := 0;
  len := Length(FLines[0]);
  for i := 0 to FLines.Count - 1 do
  begin
    count := 0;
    for j := 1 to len do
      if FLines[i][j] = CRoundRockChar then
        Inc(count);
    Inc(Result, count * (FLines.Count - i));
  end;
end;

{ TParabolicReflectorDish }

constructor TParabolicReflectorDish.Create;
begin
  FLineIndex := 0;
  FActivePiles := TRockPiles.Create;
  FFinishedPiles := TRockPiles.Create;
  FPlatform := TPlatform.Create;
end;

destructor TParabolicReflectorDish.Destroy;
begin
  FActivePiles.Free;
  FFinishedPiles.Free;
  FPlatform.Free;
  inherited Destroy;
end;

procedure TParabolicReflectorDish.ProcessDataLine(const ALine: string);
var
  i: Integer;
begin
  Inc(FLineIndex);

  // Initializes the list of active piles, one per column.
  if FActivePiles.Count = 0 then
  begin
    FActivePiles.Count := Length(ALine);
    for i:= 0 to FActivePiles.Count - 1 do
      FActivePiles[i] := TRockPile.Create(0);
  end;

  // Updates the active piles from the current line.
  for i := 1 to Length(ALine) do
  begin
    case ALine[i] of
      CRoundRockChar: FActivePiles[i - 1].AddRock;
      CCubeRockChar:
        if FActivePiles[i - 1].Any then
        begin
          FFinishedPiles.Add(FActivePiles.ExtractIndex(i - 1));
          FActivePiles.Insert(i - 1, TRockPile.Create(FLineIndex));
        end
        else
          FActivePiles[i - 1].SetStart(FLineIndex);
    end;
  end;

  FPlatform.Add(ALine);
end;

procedure TParabolicReflectorDish.Finish;
var
  pile: TRockPile;
begin
  for pile in FFinishedPiles do
    Inc(FPart1, pile.CalcWeight(FLineIndex));
  for pile in FActivePiles do
    Inc(FPart1, pile.CalcWeight(FLineIndex));

  // Spins the platform and weighs the rocks.
  FPlatform.Spin;
  FPart2 := FPlatform.CalcWeight;
end;

function TParabolicReflectorDish.GetDataFileName: string;
begin
  Result := 'parabolic_reflector_dish.txt';
end;

function TParabolicReflectorDish.GetPuzzleName: string;
begin
  Result := 'Day 14: Parabolic Reflector Dish';
end;

end.