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====== Chess - Programming - Zobrist Hashing - A program to illustrate the Zobrist Hashing Algorithm ======

<code cpp>
// A program to illustrate Zobrist Hashing Algorithm 
#include <bits/stdc++.h> 
using namespace std; 
  
unsigned long long int ZobristTable[8][8][12]; 
mt19937 mt(01234567); 
  
// Generates a Random number from 0 to 2^64-1.
unsigned long long int randomInt() 
{ 
  uniform_int_distribution<unsigned long long int> dist(0, UINT64_MAX); 
  return dist(mt); 
} 

  
// This function associates each piece with a number.
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.
void initTable() 
{ 
  for (int i=0; i<8; i++) 
    for (int j=0; j<8; j++) 
     for (int k=0; k<12; k++) 
        ZobristTable[i][j][k] = randomInt(); 
} 

  
// Computes the hash value of a given board.
unsigned long long int computeHash(char board[8][8]) 
{ 
  unsigned long long 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; 
} 
  
  
// Main Function 
int main() 
{ 
  // Uppercase letters are white pieces.
  // Lowercase letters are black pieces.
  char board[8][8] = 
  { 
    "---K----", 
    "-R----Q-", 
    "--------", 
    "-P----p-", 
    "-----p--", 
    "--------", 
    "p---b--q", 
    "----n--k"
  }; 
  
  initTable(); 
  
  unsigned long long int hashValue = computeHash(board); 
  printf("The hash value is     : %llu", 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)]; 
  
  printf("The new hash vlaue is : %llu", 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)]; 
  
  printf("The old hash vlaue is : %llu", hashValue); 
  
  return 0; 
} 
</code>

returns:

<code>
The hash value is     : 14226429382419125366
The new hash value is : 15124945578233295113
The old hash value is : 14226429382419125366
</code>

----

===== References =====

https://tutorialspoint.dev/algorithm/game-theory/minimax-algorithm-in-game-theory-set-5-zobrist-hashing