{
  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 UFloorWillBeLava;

{$mode ObjFPC}{$H+}

interface

uses
  Classes, SysUtils, Generics.Collections, USolver;

type

  { TBeam }

  TBeam = record
    Position, Direction: TPoint;
  end;

  TEnergyState = (esNone, esWestOrHorizontal, esEastOrVertical, esBoth);

  { TTransition }

  TTransition = record
    IncomingDirection, OutgoingDirection, SplitDirection: TPoint;
    Tile: Char;
    EnergyChange: TEnergyState;
  end;

  { TFloorWillBeLava }

  TFloorWillBeLava = class(TSolver)
  private
    FLines: TStringList;
    function IsBeamOutOfBounds(constref ABeam: TBeam; const AWidth, AHeight: Integer): Boolean;
    function GetTile(constref APosition: TPoint): Char;
    function GetNewBeam(constref APosition, ADirection: TPoint): TBeam;
  public
    constructor Create;
    destructor Destroy; override;
    procedure ProcessDataLine(const ALine: string); override;
    procedure Finish; override;
    function GetDataFileName: string; override;
    function GetPuzzleName: string; override;
  end;

const
  CNoDirection: TPoint = (X: 0; Y: 0);
  CEmptyChar = '.';
  CStartingBeam: TBeam = (Position: (X: 0; Y: 0); Direction: (X: 1; Y: 0));
  CTransitions: array of TTransition = (
    (IncomingDirection: (X: 1; Y: 0); OutgoingDirection: (X: 0; Y: -1); SplitDirection: (X: 0; Y: 0); Tile: '/';
      EnergyChange: esWestOrHorizontal),
    (IncomingDirection: (X: 0; Y: 1); OutgoingDirection: (X: -1; Y: 0); SplitDirection: (X: 0; Y: 0); Tile: '/';
      EnergyChange: esWestOrHorizontal),
    (IncomingDirection: (X: -1; Y: 0); OutgoingDirection: (X: 0; Y: 1); SplitDirection: (X: 0; Y: 0); Tile: '/';
      EnergyChange: esEastOrVertical),
    (IncomingDirection: (X: 0; Y: -1); OutgoingDirection: (X: 1; Y: 0); SplitDirection: (X: 0; Y: 0); Tile: '/';
      EnergyChange: esEastOrVertical),
    (IncomingDirection: (X: 1; Y: 0); OutgoingDirection: (X: 0; Y: 1); SplitDirection: (X: 0; Y: 0); Tile: '\';
      EnergyChange: esWestOrHorizontal),
    (IncomingDirection: (X: 0; Y: 1); OutgoingDirection: (X: 1; Y: 0); SplitDirection: (X: 0; Y: 0); Tile: '\';
      EnergyChange: esEastOrVertical),
    (IncomingDirection: (X: -1; Y: 0); OutgoingDirection: (X: 0; Y: -1); SplitDirection: (X: 0; Y: 0); Tile: '\';
      EnergyChange: esEastOrVertical),
    (IncomingDirection: (X: 0; Y: -1); OutgoingDirection: (X: -1; Y: 0); SplitDirection: (X: 0; Y: 0); Tile: '\';
      EnergyChange: esWestOrHorizontal),
    (IncomingDirection: (X: 1; Y: 0); OutgoingDirection: (X: 0; Y: -1); SplitDirection: (X: 0; Y: 1); Tile: '|';
      EnergyChange: esBoth),
    (IncomingDirection: (X: -1; Y: 0); OutgoingDirection: (X: 0; Y: -1); SplitDirection: (X: 0; Y: 1); Tile: '|';
      EnergyChange: esBoth),
    (IncomingDirection: (X: 0; Y: 1); OutgoingDirection: (X: -1; Y: 0); SplitDirection: (X: 1; Y: 0); Tile: '-';
      EnergyChange: esBoth),
    (IncomingDirection: (X: 0; Y: -1); OutgoingDirection: (X: -1; Y: 0); SplitDirection: (X: 1; Y: 0); Tile: '-';
      EnergyChange: esBoth)
  );

implementation

{ TFloorWillBeLava }

function TFloorWillBeLava.IsBeamOutOfBounds(constref ABeam: TBeam; const AWidth, AHeight: Integer): Boolean;
begin
  Result := (ABeam.Position.X < 0) or (ABeam.Position.X >= AWidth)
    or (ABeam.Position.Y < 0) or (ABeam.Position.Y >= AHeight);
end;

function TFloorWillBeLava.GetTile(constref APosition: TPoint): Char;
begin
  Result := FLines[APosition.Y][APosition.X + 1];
end;

function TFloorWillBeLava.GetNewBeam(constref APosition, ADirection: TPoint): TBeam;
begin
  Result.Position := APosition;
  Result.Direction := ADirection;
end;

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

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

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

procedure TFloorWillBeLava.Finish;
var
  energyMap: array of array of TEnergyState;
  width, height, i, j: Integer;
  done: Boolean;
  stack: specialize TStack<TBeam>;
  beam: TBeam;
  transition: TTransition;
  energyChange: TEnergyState;
  s: string;
begin
  // Initializes energy map.
  width := Length(FLines[0]);
  height := FLines.Count;
  SetLength(energyMap, width, height);
  for i := 0 to width - 1 do
    for j := 0 to height - 1 do
      energyMap[i, j] := esNone;

  // Starts beam.
  done := False;
  beam := CStartingBeam;
  stack := specialize TStack<TBeam>.Create;

  repeat
    // Processes the current beam.
    if IsBeamOutOfBounds(beam, width, height) then
      done := True
    else begin
      if beam.Direction.X <> 0 then
        energyChange := esWestOrHorizontal
      else
        energyChange := esEastOrVertical;

      if GetTile(beam.Position) <> CEmptyChar then
      begin
        // Checks the current position for direction changes and splits.
        for transition in CTransitions do
          if (transition.IncomingDirection = beam.Direction) and (transition.Tile = GetTile(beam.Position)) then
          begin
            if transition.SplitDirection <> CNoDirection then
              stack.Push(GetNewBeam(beam.Position + transition.SplitDirection, transition.SplitDirection));
            beam.Direction := transition.OutgoingDirection;
            energyChange := transition.EnergyChange;
            Break;
          end;
      end;

      // Energizes the current position.
      case energyMap[beam.Position.X, beam.Position.Y] of
        esNone: energyMap[beam.Position.X, beam.Position.Y] := energyChange;
        esWestOrHorizontal:
          if energyChange = esEastOrVertical then
            energyMap[beam.Position.X, beam.Position.Y] := esBoth
          else
            done := True;
        esEastOrVertical:
          if energyChange = esWestOrHorizontal then
            energyMap[beam.Position.X, beam.Position.Y] := esBoth
          else
            done := True;
        esBoth: done := True;
      end;

      // Moves the beam.
      beam.Position := beam.Position + beam.Direction;
    end;

    if done and (stack.Count > 0) then
    begin
      // Starts the next beam that was split earlier.
      done := False;
      beam := stack.Pop;
    end;
  until done;

  stack.Free;

  // Counts energized tiles.
  for i := 0 to width - 1 do
    for j := 0 to height - 1 do
      if energyMap[i, j] <> esNone then
        Inc(FPart1);
end;

function TFloorWillBeLava.GetDataFileName: string;
begin
  Result := 'floor_will_be_lava.txt';
end;

function TFloorWillBeLava.GetPuzzleName: string;
begin
  Result := 'Day 16: The Floor Will Be Lava';
end;

end.