{
  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 UHauntedWasteland;

{$mode ObjFPC}{$H+}

interface

uses
  Classes, SysUtils, Generics.Collections, USolver;

type

  { TNode }

  TNode = class
  private
    FName: string;
    FLeft, FRight: TNode;
  public
    constructor Create(const AName: string);
    property Name: string read FName;
    procedure AddNeighbors(constref ALeft, ARight: TNode);
    function TryStep(const ADirection: Char; out ONewLocation: TNode): Boolean;
  end;

  TNodes = specialize TObjectList<TNode>;

  { THauntedWasteland }

  THauntedWasteland = class(TSolver)
  private
    FDirections: string;
    FNextDirectionIndex: Integer;
    FNodes, FUnparsedNodes: TNodes;
    FCurrentNode, FTargetNode: TNode;
    procedure ProcessNode(const ALine: string);
    procedure DoSteps;
  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

{ TNode }

constructor TNode.Create(const AName: string);
begin
  FName := AName;
  FLeft := nil;
  FRight := nil;
end;

procedure TNode.AddNeighbors(constref ALeft, ARight: TNode);
begin
  FLeft := ALeft;
  FRight := ARight;
end;

function TNode.TryStep(const ADirection: Char; out ONewLocation: TNode): Boolean;
begin
  ONewLocation := nil;
  Result := False;
  case ADirection of
    'L':
      if FLeft <> nil then
      begin
        ONewLocation := FLeft;
        Result := True;
      end;
    'R':
      if FRight <> nil then
      begin
        ONewLocation := FRight;
        Result := True;
      end;
  end;
end;

{ THauntedWasteland }

procedure THauntedWasteland.ProcessNode(const ALine: string);
var
  name, left, right: string;
  i: Integer;
  parsingNode, leftNode, rightNode: TNode;
begin
  name := LeftStr(ALine, 3);
  left := Copy(ALine, 8, 3);
  right := Copy(ALine, 13, 3);

  // Tries to find the new node in unparsed nodes.
  i := 0;
  parsingNode := nil;
  while (parsingNode = nil) and (i < FUnparsedNodes.Count) do
  begin
    if FUnparsedNodes[i].Name = name then
    begin
      // New node was already encountered and is now parsed.
      parsingNode := FUnparsedNodes[i];
      FUnparsedNodes.Delete(i);
    end;
    Inc(i);
  end;

  // Adds new node to list of all nodes. We do this before looking for leftNode and rightNode because the node might
  // reference itself.
  if parsingNode = nil then
  begin
    parsingNode := TNode.Create(name);
    FNodes.Add(parsingNode);
  end;

  // Tries to find leftNode and rightNode.
  i := 0;
  leftNode := nil;
  rightNode := nil;
  while ((leftNode = nil) or (rightNode = nil)) and (i < FNodes.Count) do
  begin
    if (leftNode = nil) and (FNodes[i].Name = left) then
    begin
      leftNode := FNodes[i];
    end;
    if (rightNode = nil) and (FNodes[i].Name = right) then
    begin
      rightNode := FNodes[i];
    end;
    Inc(i);
  end;

  // Adds leftNode and/or rightNode to list of all nodes and unparsed nodes.
  if leftNode = nil then
  begin
    leftNode := TNode.Create(left);
    FNodes.Add(leftNode);
    FUnparsedNodes.Add(leftNode);
  end;
  if rightNode = nil then
  begin
    if right = left then
      // This happens only if both left and right nodes are new.
      rightNode := leftNode
    else begin
      rightNode := TNode.Create(right);
      FNodes.Add(rightNode);
      FUnparsedNodes.Add(rightNode);
    end;
  end;

  parsingNode.AddNeighbors(leftNode, rightNode);

  // Checks for start and end nodes.
  if name = 'ZZZ' then
    FTargetNode := parsingNode
  else if name = 'AAA' then
    FCurrentNode := parsingNode;
end;

procedure THauntedWasteland.DoSteps;
var
  new: TNode;
begin
  if FCurrentNode <> nil then
  begin
    while (FCurrentNode <> FTargetNode) and FCurrentNode.TryStep(FDirections[FNextDirectionIndex], new) do
    begin
      FCurrentNode := new;
      Inc(FNextDirectionIndex);
      if FNextDirectionIndex > Length(FDirections) then
        FNextDirectionIndex := 1;
      Inc(FPart1);
    end;
  end;
end;

constructor THauntedWasteland.Create;
begin
  FNextDirectionIndex := 1;
  FNodes := TNodes.Create;
  FUnparsedNodes := TNodes.Create(False);
  FCurrentNode := nil;
  FTargetNode := nil;
end;

destructor THauntedWasteland.Destroy;
begin
  FNodes.Free;
  FUnparsedNodes.Free;
  inherited Destroy;
end;

procedure THauntedWasteland.ProcessDataLine(const ALine: string);
begin
  if FDirections = '' then
    FDirections := ALine
  else if (ALine <> '') and ((FTargetNode = nil) or (FCurrentNode <> FTargetNode)) then
  begin
    ProcessNode(ALine);
    DoSteps;
  end;
end;

procedure THauntedWasteland.Finish;
begin

end;

function THauntedWasteland.GetDataFileName: string;
begin
  Result := 'haunted_wasteland.txt';
end;

function THauntedWasteland.GetPuzzleName: string;
begin
  Result := 'Day 8: Haunted Wasteland';
end;

end.