Publicaciones anteriores sobre este tema: Algoritmo Minimax en teoría de juegos Función de evaluación en la teoría de juegos Tic-Tac-Toe AI: encontrar el movimiento óptimo Poda Alfa-Beta .
Zobrist Hashing es una función hash que se usa ampliamente en juegos de mesa de 2 jugadores. Es la función hash más común utilizada en la tabla de transposición. Las tablas de transposición básicamente almacenan los valores evaluados de estados anteriores del tablero de modo que, si se vuelven a encontrar, simplemente recuperamos el valor almacenado de la tabla de transposición. Cubriremos las tablas de transposición en un artículo posterior. En este artículo tomaremos el ejemplo de un tablero de ajedrez e implementaremos una función hash para ello.
Pseudocódigo:
// A matrix with random numbers initialized once Table[#ofBoardCells][#ofPieces] // Returns Zobrist hash function for current conf- // iguration of board. function findhash(board): hash = 0 for each cell on the board : if cell is not empty : piece = board[cell] hash ^= table[cell][piece] return hash
Explicación :
La idea detrás de Zobrist Hashing es que para un estado determinado del tablero, si hay una pieza en una celda determinada, usamos el número aleatorio de esa pieza de la celda correspondiente en la tabla.
Si hay más bits en el número aleatorio, menor será la probabilidad de que se produzca una colisión de hash. Por lo tanto, los números de 64 bits se utilizan comúnmente como estándar y es muy poco probable que se produzca una colisión hash con números tan grandes. La tabla debe inicializarse sólo una vez durante la ejecución del programa.
Además, la razón por la que Zobrist Hashing se usa ampliamente en juegos de mesa es porque cuando un jugador hace un movimiento no es necesario volver a calcular el valor de hash desde cero. Debido a la naturaleza de la operación XOR, podemos simplemente usar algunas operaciones XOR para recalcular el valor hash.
Implementación:
Intentaremos encontrar un valor hash para la configuración de placa dada.
// A program to illustrate Zobrist Hashing Algorithm #include
// A Java program to illustrate Zobrist Hashing Algorithm import java.util.*; public class GFG { public static List<List<List<Integer>>> ZobristTable = new ArrayList<>(); // mt19937 mt(01234567); public static void initialise() { for (int i = 0; i < 8; i++) { ZobristTable.add(new ArrayList<>()); for (int j = 0; j < 8; j++) { ZobristTable.get(i).add(new ArrayList<>()); for (int k = 0; k < 12; k++) { ZobristTable.get(i).get(j).add(0); } } } } // Generates a Random number from 0 to 2^64-1 public static long randomLong() { long min = 0L; long max = (long) Math.pow(2 64); Random rnd = new Random(); return rnd.nextLong() * (max - min) + min; } // This function associates each piece with a number public static int indexOf(char piece) { if (piece=='P') return 0; if (piece=='N') return 1; if (piece=='B') return 2; if (piece=='R') return 3; if (piece=='Q') return 4; if (piece=='K') return 5; if (piece=='p') return 6; if (piece=='n') return 7; if (piece=='b') return 8; if (piece=='r') return 9; if (piece=='q') return 10; if (piece=='k') return 11; else return -1; } // Initializes the table public static void initTable() { for (int i = 0; i < 8; i++) { for (int j = 0; j < 8; j++) { for (int k = 0; k < 12; k++) { Random rnd = new Random(); ZobristTable.get(i).get(j).set(k (int) randomLong()); } } } } // Computes the hash value of a given board public static int computeHash(List<List<Character>> board) { int h = 0; for (int i = 0; i < 8; i++) { for (int j = 0; j < 8; j++) { if (board.get(i).get(j) != '-') { int piece = indexOf(board.get(i).get(j)); h ^= ZobristTable.get(i).get(j).get(piece); } } } return h; } public static void main(String[] args) { // Main Function // Uppercase letters are white pieces // Lowercase letters are black pieces List<List<Character>> board = new ArrayList<>(); board.add(new ArrayList<>(Arrays.asList('-' '-' '-' 'K' '-' '-' '-' '-'))); board.add(new ArrayList<>(Arrays.asList('-' 'R' '-' '-' '-' '-' 'Q' '-'))); board.add(new ArrayList<>(Arrays.asList('-' '-' '-' 'K' '-' '-' '-' '-'))); board.add(new ArrayList<>(Arrays.asList('-' '-' '-' '-' '-' '-' '-' '-'))); board.add(new ArrayList<>(Arrays.asList('-' 'P' '-' '-' '-' '-' 'p' '-'))); board.add(new ArrayList<>(Arrays.asList('-' '-' '-' '-' '-' 'p' '-' '-'))); board.add(new ArrayList<>(Arrays.asList('-' '-' '-' '-' '-' '-' '-' '-'))); board.add(new ArrayList<>(Arrays.asList('p' '-' '-' '-' 'b' '-' '-' 'q'))); board.add(new ArrayList<>(Arrays.asList('-' '-' '-' '-' 'n' '-' '-' 'k'))); initialise(); initTable(); int hashValue = computeHash(board); System.out.println('The hash value is : ' + hashValue); // Move the white king to the left char piece = board.get(0).get(3); board.get(0).set(3 '-'); hashValue ^= ZobristTable.get(0).get(3).get(indexOf(piece)); board.get(0).set(2 piece); hashValue ^= ZobristTable.get(0).get(2).get(indexOf(piece)); System.out.println('The new hash value is : ' + hashValue); // Undo the white king move piece = board.get(0).get(2); board.get(0).set(2 '-'); hashValue ^= ZobristTable.get(0).get(2).get(indexOf(piece)); board.get(0).set(3 piece); hashValue ^= ZobristTable.get(0).get(3).get(indexOf(piece)); System.out.println('The new hash value is : ' + hashValue); } } // This code is contributed by Vaibhav
# A program to illustrate Zobrist Hashing Algorithm # python code implementation import random # mt19937 mt(01234567); # Generates a Random number from 0 to 2^64-1 def randomInt(): min = 0 max = pow(2 64) return random.randint(min max) # This function associates each piece with # a number def indexOf(piece): if (piece=='P'): return 0 elif (piece=='N'): return 1 elif (piece=='B'): return 2 elif (piece=='R'): return 3 elif (piece=='Q'): return 4 elif (piece=='K'): return 5 elif (piece=='p'): return 6 elif (piece=='n'): return 7 elif (piece=='b'): return 8 elif (piece=='r'): return 9 elif (piece=='q'): return 10 elif (piece=='k'): return 11 else: return -1 # Initializes the table def initTable(): # 8x8x12 array ZobristTable = [[[randomInt() for k in range(12)] for j in range(8)] for i in range(8)] return ZobristTable # Computes the hash value of a given board def computeHash(board ZobristTable): h = 0 for i in range(8): for j in range(8): if (board[i][j] != '-'): piece = indexOf(board[i][j]) h ^= ZobristTable[i][j][piece] return h # Main Function # Uppercase letters are white pieces # Lowercase letters are black pieces board = [ '---K----' '-R----Q-' '--------' '-P----p-' '-----p--' '--------' 'p---b--q' '----n--k' ] ZobristTable = initTable() hashValue = computeHash(board ZobristTable) print('The hash value is : ' + str(hashValue)) #Move the white king to the left piece = board[0][3] board[0] = list(board[0]) board[0][3] = '-' board[0] = ''.join(board[0]) hashValue ^= ZobristTable[0][3][indexOf(piece)] board[0] = list(board[0]) board[0][2] = piece board[0] = ''.join(board[0]) hashValue ^= ZobristTable[0][2][indexOf(piece)] print('The new hash value is : ' + str(hashValue)) # Undo the white king move piece = board[0][2] board[0] = list(board[0]) board[0][2] = '-' board[0] = ''.join(board[0]) hashValue ^= ZobristTable[0][2][indexOf(piece)] board[0] = list(board[0]) board[0][3] = piece board[0] = ''.join(board[0]) hashValue ^= ZobristTable[0][3][indexOf(piece)] print('The old hash value is : ' + str(hashValue))
using System; using System.Collections; using System.Collections.Generic; using System.Linq; // C# program for the above approach // A program to illustrate Zobrist Hashing Algorithm // javascript code implementation class HelloWorld { public static List<List<List<int>>> ZobristTable = new List<List<List<int>>>(); // mt19937 mt(01234567); public static void initialise(){ for(int i = 0; i < 8; i++){ ZobristTable.Add(new List<List<int>>()); for(int j = 0; j < 8; j++){ ZobristTable[i].Add(new List<int>()); for(int k = 0; k < 12; k++){ ZobristTable[i][j].Add(0); } } } } // Generates a Random number from 0 to 2^64-1 public static int randomInt() { int min = 0; int max = (int)Math.Pow(2 64); Random rnd = new Random(); return rnd.Next() * (max - min) + min; } // This function associates each piece with // a number public static int indexOf(char piece) { if (piece=='P') return 0; if (piece=='N') return 1; if (piece=='B') return 2; if (piece=='R') return 3; if (piece=='Q') return 4; if (piece=='K') return 5; if (piece=='p') return 6; if (piece=='n') return 7; if (piece=='b') return 8; if (piece=='r') return 9; if (piece=='q') return 10; if (piece=='k') return 11; else return -1; } // Initializes the table public static void initTable() { for (int i = 0; i<8; i++){ for(int j = 0; j < 8; j++){ for(int k = 0; k < 12; k++){ Random rnd = new Random(); ZobristTable[i][j][k] = rnd.Next(); } } } } // Computes the hash value of a given board public static int computeHash(List<List<char>> board) { int h = 0; for (int i = 0; i<8; i++) { for (int j = 0; j<8; j++) { if (board[i][j]!='-') { int piece = indexOf(board[i][j]); h ^= ZobristTable[i][j][piece]; } } } return h; } static void Main() { // Main Function // Uppercase letters are white pieces // Lowercase letters are black pieces List<List<char>> board = new List<List<char>>(); board.Add(new List<char>(){'-' '-' '-' 'K' '-' '-' '-' '-'}); board.Add(new List<char>(){'-' 'R' '-' '-' '-' '-' 'Q' '-'}); board.Add(new List<char>(){'-' '-' '-' 'K' '-' '-' '-' '-'}); board.Add(new List<char>(){'-' '-' '-' '-' '-' '-' '-' '-'}); board.Add(new List<char>(){'-' 'P' '-' '-' '-' '-' 'p' '-'}); board.Add(new List<char>(){'-' '-' '-' '-' '-' 'p' '-' '-' }); board.Add(new List<char>(){'-' '-' '-' '-' '-' '-' '-' '-'}); board.Add(new List<char>(){'p' '-' '-' '-' 'b' '-' '-' 'q'}); board.Add(new List<char>(){'-' '-' '-' '-' 'n' '-' '-' 'k'}); initialise(); initTable(); int hashValue = computeHash(board); Console.WriteLine('The hash value is : ' + hashValue); //Move the white king to the left char piece = board[0][3]; board[0][3] = '-'; hashValue ^= ZobristTable[0][3][indexOf(piece)]; board[0][2] = piece; hashValue ^= ZobristTable[0][2][indexOf(piece)]; Console.WriteLine('The new hash value is : ' + hashValue); // Undo the white king move piece = board[0][2]; board[0][2] = '-'; hashValue ^= ZobristTable[0][2][indexOf(piece)]; board[0][3] = piece; hashValue ^= ZobristTable[0][3][indexOf(piece)]; Console.WriteLine('The old hash value is : ' + hashValue); } } // The code is contributed by Nidhi goel.
// A program to illustrate Zobrist Hashing Algorithm // javascript code implementation let ZobristTable = new Array(8); for(let i = 0; i < 8; i++){ ZobristTable[i] = new Array(8); } for(let i = 0; i < 8; i++){ for(let j = 0; j < 8; j++){ ZobristTable[i][j] = new Array(12); } } // mt19937 mt(01234567); // Generates a Random number from 0 to 2^64-1 function randomInt() { let min = 0; let max = Math.pow(2 64); return Math.floor(Math.random() * (max - min) + min); } // This function associates each piece with // a number function indexOf(piece) { if (piece=='P') return 0; if (piece=='N') return 1; if (piece=='B') return 2; if (piece=='R') return 3; if (piece=='Q') return 4; if (piece=='K') return 5; if (piece=='p') return 6; if (piece=='n') return 7; if (piece=='b') return 8; if (piece=='r') return 9; if (piece=='q') return 10; if (piece=='k') return 11; else return -1; } // Initializes the table function initTable() { for (let i = 0; i<8; i++) for (let j = 0; j<8; j++) for (let k = 0; k<12; k++){ ZobristTable[i][j][k] = randomInt(); } } // Computes the hash value of a given board function computeHash(board) { let h = 0; for (let i = 0; i<8; i++) { for (let j = 0; j<8; j++) { if (board[i][j]!='-') { let piece = indexOf(board[i][j]); h ^= ZobristTable[i][j][piece]; } } } return h; } // Main Function // Uppercase letters are white pieces // Lowercase letters are black pieces let board = [ '---K----' '-R----Q-' '--------' '-P----p-' '-----p--' '--------' 'p---b--q' '----n--k' ]; initTable(); let hashValue = computeHash(board); console.log('The hash value is : ' + hashValue); //Move the white king to the left let piece = board[0][3]; board[0] = board[0].split(''); board[0][3] = '-'; board[0] = board[0].join(''); hashValue ^= ZobristTable[0][3][indexOf(piece)]; board[0] = board[0].split(''); board[0][2] = piece; board[0] = board[0].join(''); hashValue ^= ZobristTable[0][2][indexOf(piece)]; console.log('The new hash value is : ' + hashValue); // Undo the white king move piece = board[0][2]; board[0] = board[0].split(''); board[0][2] = '-'; board[0] = board[0].join(''); hashValue ^= ZobristTable[0][2][indexOf(piece)]; board[0][3] = piece; hashValue ^= ZobristTable[0][3][indexOf(piece)]; console.log('The old hash value is : ' + hashValue); // The code is contributed by Nidhi goel.
Producción :
The hash value is : 14226429382419125366 The new hash value is : 15124945578233295113 The old hash value is : 14226429382419125366